I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged, and a 12v outlet on the exterior wall and probably one on the inisde also. Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger. I'll need some sort of weatherproof box for exterior wall outlet of course. I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

Jon Parker wrote:

Does that all make sense?
Even assuming you've got a 'free' 12V battery, what you spend on 3-way
switch, 2x sockets and a weatherproof enclosure will probably cost more
than occasionally running the car and plugging the air pump into the
cigar lighter ...

On 22/04/2020 13:44, Andy Burns wrote:
Jon Parker wrote:

Does that all make sense?
Even assuming you've got a 'free' 12V battery, what you spend on 3-way
switch, 2x sockets and a weatherproof enclosure will probably cost more
than occasionally running the car and plugging the air pump into the
cigar lighter ...

You are assuming the because the OP has a garage that he also has a car.

On 22/04/2020 13:39, Jon Parker wrote:
I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged, and a 12v outlet on the exterior wall and probably one on the inisde also. Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger. I'll need some sort of weatherproof box for exterior wall outlet of course. I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

Because you don't have mains out there? You may find you need a larger
solar panel than you might expect to keep the battery charged,
particularly in the winter. Don't expect one of the 12 inch square ones
to do much for you. DAMHIK.

I used 12 volt batteries to provide lighting to an off-grid stables, and
also run electric fencers when required. Have one of those little 12
volt LED voltmeters plumbed into the system (on a switch) so that it is
easy to keep an eye on the battery state of charge.

On Wednesday, April 22, 2020 at 1:45:05 PM UTC+1, Andy Burns wrote:
Jon Parker wrote:

Does that all make sense?
Even assuming you've got a 'free' 12V battery, what you spend on 3-way
switch, 2x sockets and a weatherproof enclosure will probably cost more
than occasionally running the car and plugging the air pump into the
cigar lighter ...

Not overly bothered about cost. Just something I've been meaning to get round to, and suddenly I seem to have a lot of free time...

Maybe not a car battery, perhaps a motorcycle or similar size.

Andrew wrote:

You are assuming the because the OP has a garage that he also has a car.

Well, he did mention pumping up car-tyres.

It happens that Jon Parker formulated :
I intend to use a 3-position switch to enable me to easily switch the battery
positive to the solar charger, or over to the 12v outlets, and to prevent the
outlet from receiving power when the battery is connected to the charger.
I'll need some sort of weatherproof box for exterior wall outlet of course. I
expect I'llneed to put a suitable fuse in the positive line, around 6 inches
from the terminal, 10 or 20 amp should be fine I think.

There is no obvious need to switch the power between charge and the
sockets?

If I were considering doing this, I would be installing a mains powered
12v (13.8v) SMPSU - do you not have any mains power in the garage?

On 22/04/2020 13:39, Jon Parker wrote:
I'm considering building a basic 12v power system for my garage
comprising a car battery, a solar panel on the outside to keep it
charged, and a 12v outlet on the exterior wall and probably one on
the inisde also. Going to be used for running the air pump mainly
when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the
battery positive to the solar charger, or over to the 12v outlets,
and to prevent the outlet from receiving power when the battery is
connected to the charger. I'll need some sort of weatherproof box for
exterior wall outlet of course. I expect I'llneed to put a suitable
fuse in the positive line, around 6 inches from the terminal, 10 or
20 amp should be fine I think.

Does that all make sense? Any suggestions?

Do the sums very carefully before spending any money! Unless you are
seriously off grid it will almost certainly be cheaper and much less
hassle to buy a mains charger and a pair of SLA's to use in rotation.
Solar panels work in summer but allow batteries to be ruined in winter.

That is the solution I opted for my greenhouse watering system and for
powering my telescopes. The batteries last about 5 years or so.

You don't need to switch the solar panel out of circuit. They are quite
high impedance and tolerant of a dead short across them. Battery voltage
will be a bit high in full sun but most stuff for nominal 12v lead acid
use is expecting about 13.8v anyway.

You will need a schottky diode in series with the solar panel (unless it
already has one) to stop leakage at night. Be sure to include a low
voltage automotive fuse very close to one terminal. And make sure that
nothing can ever short out across the battery terminals.

--
Regards,
Martin Brown

Andy Burns wrote:
Jon Parker wrote:

Does that all make sense?
Even assuming you've got a 'free' 12V battery, what you spend on 3-way
switch, 2x sockets and a weatherproof enclosure will probably cost more
than occasionally running the car and plugging the air pump into the
cigar lighter ...

Or running mains power to the garage (if not there already) and
getting a 12v power supply.

--
Chris Green
·

On Wednesday, 22 April 2020 13:39:29 UTC+1, Jon Parker wrote:
I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged, and a 12v outlet on the exterior wall and probably one on the inisde also. Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger. I'll need some sort of weatherproof box for exterior wall outlet of course. I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

Get rid of the switch then it'll work. Just connect it all together. Battery fuse should be chosen to suit the wiring in use. If you use 2.5mm T&E you can use 30A fusing - you could even re-use an old Wylex fuse box if you want.

On Wed, 22 Apr 2020 05:39:26 -0700 (PDT), Jon Parker
wrote:

I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged,

I have that here for some 12V flouros in the workshop.

and a 12v outlet on the exterior wall and probably one on the inisde also.

Anything 'exterior' is going to need to be weatherproof to avoid
corrosion. It 'might' be better to just leave it at internal and have
a temporary extension if required?

Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

Ok.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger.

Why (or are you talking of an additional 'charger')?

I'll need some sort of weatherproof box for exterior wall outlet of course.

See above.

I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

It needs to be a bit more than the worse case of the load and less
than the capacity of the cable. ;-)

Does that all make sense?

Sort of.

Any suggestions?

Yes. The problem with the above solution is that there is no
protection for the charge, or the battery being over-discharged. So,
I'd be tempted to use a charge controller that is sufficiently rated
to cover the load and the solar panel.

That way, you won't cook the battery during a run of sunny days
(depending on the size of the panel and battery) or run it past say
50% discharge whilst pumping up that big pool on the first sunny day
after a run of dull ones.

eBay has quite a selection and a PWM or better, MPPT model of the load
of peak load ratings you mention aren't that expensive.

Just make sure you use reasonably heavy cables on the output / battery
connections (and the solar panel if you have the option).

At 12V, every ohm counts. ;-)

Cheers, T i m

On Wed, 22 Apr 2020 14:28:53 +0100, Martin Brown
wrote:

snip

Do the sums very carefully before spending any money!

+1 ;-)

Unless you are
seriously off grid it will almost certainly be cheaper and much less
hassle to buy a mains charger and a pair of SLA's to use in rotation.

I've done that ...

Solar panels work in summer but allow batteries to be ruined in winter.

But isn't the outcome a function of all the components?

eg, If the OP's use is likely mostly in the summer (except tyre
inflation probably but that's rarely a sustained load), and that's
when there is the most sun, if he had a big enough panel connected
though a charge controller to a good enough battery, why wouldn't that
be as good as all those solar powered roadside signs that seem to be
able to cope all year round?

That is the solution I opted for my greenhouse watering system

And would that typically run every day, all year round?

and for
powering my telescopes.

That should be easy if you are anywhere near a big city in the UK and
need a cleat mild night when it's not raining! ;-(

The batteries last about 5 years or so.

Whilst I'm not suggesting that a pair of 'portable' batteries wouldn't
also be a viable solution, it might not be as convenient (money / spec
being no object) than a decent solar solution?

I think they key here *is* having the right spec gear and so insuring
you have way more (charge specifically) capacity than you typically
need, including in the winter.

Cheers, T i m

On 22/04/2020 15:54, T i m wrote:
On Wed, 22 Apr 2020 14:28:53 +0100, Martin Brown
wrote:

snip

Do the sums very carefully before spending any money!

+1 ;-)

Unless you are
seriously off grid it will almost certainly be cheaper and much less
hassle to buy a mains charger and a pair of SLA's to use in rotation.

I've done that ...

Solar panels work in summer but allow batteries to be ruined in winter.

But isn't the outcome a function of all the components?

You need a very big solar panel and a smart charge controller to protect
the battery from overcharging if you want something to do real work.

eg, If the OP's use is likely mostly in the summer (except tyre
inflation probably but that's rarely a sustained load), and that's
when there is the most sun, if he had a big enough panel connected
though a charge controller to a good enough battery, why wouldn't that
be as good as all those solar powered roadside signs that seem to be
able to cope all year round?

Those solar powered "please go round the dangerous bend" signs are fine
in midsummer but invariably dead in the water on cold frosty winters
mornings and trash a set of batteries every winter. There is one not far
from me on a sharp bend where a car entered someone's living room.

https://www.gazettelive.co.uk/news/l...-house-3702602

It was midsummer so the sign would have been working.

--
Regards,
Martin Brown

On Wed, 22 Apr 2020 16:03:34 +0100, Martin Brown
wrote:

On 22/04/2020 15:54, T i m wrote:
On Wed, 22 Apr 2020 14:28:53 +0100, Martin Brown
wrote:

snip

Do the sums very carefully before spending any money!

+1 ;-)

Unless you are
seriously off grid it will almost certainly be cheaper and much less
hassle to buy a mains charger and a pair of SLA's to use in rotation.

I've done that ...

Solar panels work in summer but allow batteries to be ruined in winter.

But isn't the outcome a function of all the components?

You need a very big solar panel and a smart charge controller to protect
the battery from overcharging if you want something to do real work.

I'm not sure about 'very big' but you certainly need 'big enough under
the worst conditions'. ;-)

eg, If the OP's use is likely mostly in the summer (except tyre
inflation probably but that's rarely a sustained load), and that's
when there is the most sun, if he had a big enough panel connected
though a charge controller to a good enough battery, why wouldn't that
be as good as all those solar powered roadside signs that seem to be
able to cope all year round?

Those solar powered "please go round the dangerous bend" signs are fine
in midsummer but invariably dead in the water on cold frosty winters
mornings and trash a set of batteries every winter.

Really? Can you cite any evidence for this being a general not local /
specific issue please?

There is one not far
from me on a sharp bend where a car entered someone's living room.

https://www.gazettelive.co.uk/news/l...-house-3702602

It was midsummer so the sign would have been working.

Erm ... so nothing to do with the sign?

And irrespective, *if* such signs were failing in the winter then I
would say they were poorly designed?

We were using the 'two sealed LA batteries being cycled on a mains
charger' solution to power an electric fence around a rabbit hutch.
They were 17Ah batteries and we were probably having to swap them out
every week.

We 'upgraded' that to a 5W solar panel and PWM controller and
depending on the weather, the run time went up to about 3 weeks per
battery. Also, should the battery voltage drop below a preset level,
the output would be disconnected, protecting the battery from over
discharge.

And this was with the panel being put in a very poor location (where
it only saw the sun for a short slot in the morning) and the load
being on *every day* (well night, typically dusk to dawn, that could
be quite a few hours mind winter).

The OP's worst case usage pattern is likely to reflect the best
sunshine (pumping up paddling pools). Cycle tyres typically get the
most air in the spring and car tyres could be at any time, but only
for relatively short bursts (excuse the pun). ;-)

Cheers, T i m

On 22/04/2020 16:35, T i m wrote:
On Wed, 22 Apr 2020 16:03:34 +0100, Martin Brown
wrote:

On 22/04/2020 15:54, T i m wrote:
On Wed, 22 Apr 2020 14:28:53 +0100, Martin Brown
wrote:

snip

Do the sums very carefully before spending any money!

+1 ;-)

Unless you are
seriously off grid it will almost certainly be cheaper and much less
hassle to buy a mains charger and a pair of SLA's to use in rotation.

I've done that ...

Solar panels work in summer but allow batteries to be ruined in winter.

But isn't the outcome a function of all the components?

You need a very big solar panel and a smart charge controller to protect
the battery from overcharging if you want something to do real work.

I'm not sure about 'very big' but you certainly need 'big enough under
the worst conditions'. ;-)

eg, If the OP's use is likely mostly in the summer (except tyre
inflation probably but that's rarely a sustained load), and that's
when there is the most sun, if he had a big enough panel connected
though a charge controller to a good enough battery, why wouldn't that
be as good as all those solar powered roadside signs that seem to be
able to cope all year round?

Those solar powered "please go round the dangerous bend" signs are fine
in midsummer but invariably dead in the water on cold frosty winters
mornings and trash a set of batteries every winter.

Really? Can you cite any evidence for this being a general not local /
specific issue please?

It is a problem with the UK's high latitude and low sun in winter so
probably applies to the entire country. I expect the signs work fine at
lower latitudes that get some decent amount of winter sunshine.

There is one not far
from me on a sharp bend where a car entered someone's living room.

https://www.gazettelive.co.uk/news/l...-house-3702602

It was midsummer so the sign would have been working.

Erm ... so nothing to do with the sign?

And irrespective, *if* such signs were failing in the winter then I
would say they were poorly designed?

+1 but it reflects how they fail round here. North Yorkshire isn't
really all that sunny in winter and fogs linger in the vale of York.

We were using the 'two sealed LA batteries being cycled on a mains
charger' solution to power an electric fence around a rabbit hutch.
They were 17Ah batteries and we were probably having to swap them out
every week.

We 'upgraded' that to a 5W solar panel and PWM controller and
depending on the weather, the run time went up to about 3 weeks per
battery. Also, should the battery voltage drop below a preset level,
the output would be disconnected, protecting the battery from over
discharge.

And this was with the panel being put in a very poor location (where
it only saw the sun for a short slot in the morning) and the load
being on *every day* (well night, typically dusk to dawn, that could
be quite a few hours mind winter).

The OP's worst case usage pattern is likely to reflect the best
sunshine (pumping up paddling pools). Cycle tyres typically get the
most air in the spring and car tyres could be at any time, but only
for relatively short bursts (excuse the pun). ;-)

Cheers, T i m

If he only uses it in the summer then it might be a cost no object
solution to the alternative of straight battery swap.

--
Regards,
Martin Brown

On Wed, 22 Apr 2020 17:20:06 +0100, Martin Brown
wrote:

snip

Those solar powered "please go round the dangerous bend" signs are fine
in midsummer but invariably dead in the water on cold frosty winters
mornings and trash a set of batteries every winter.

Really? Can you cite any evidence for this being a general not local /
specific issue please?

It is a problem with the UK's high latitude and low sun in winter so
probably applies to the entire country.

Sure, I appreciate we aren't ideally placed for the best solar
harvesting but we do get some. As long as that's more than something
is using (overall) then it can be a viable solution, especially in
non-grid situations.

I expect the signs work fine at
lower latitudes that get some decent amount of winter sunshine.

I expect they work most places they are designed for.

snip

And irrespective, *if* such signs were failing in the winter then I
would say they were poorly designed?

+1 but it reflects how they fail round here. North Yorkshire isn't
really all that sunny in winter and fogs linger in the vale of York.

Understood. So, I can see how a generic sign tested on average UK sun
exposure might fail under specific circumstances.

snip

The OP's worst case usage pattern is likely to reflect the best
sunshine (pumping up paddling pools). Cycle tyres typically get the
most air in the spring and car tyres could be at any time, but only
for relatively short bursts (excuse the pun). ;-)


If he only uses it in the summer then it might be a cost no object
solution to the alternative of straight battery swap.

If he doesn't actually need power down there for most of the year and
is intending to use solar anyway then it needs to be usable when he
needs it. So that could be (as suggested elsewhere) just a small panel
and matching battery (nothing else). Not optimal but could be
sufficient (depending on usage pattern).

If he only needs it very infrequently he might as well take the
battery down there as required (that can be charged indoors)?

Cheers, T i m

What sort of battery will you use? Will there be a back up charger for
mains for the winter?
Brian

--
----- --
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Jon Parker" wrote in message
...
I'm considering building a basic 12v power system for my garage comprising a
car battery, a solar panel on the outside to keep it charged, and a 12v
outlet on the exterior wall and probably one on the inisde also. Going to be
used for running the air pump mainly when pumping up bike and car tyres,
paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the
battery positive to the solar charger, or over to the 12v outlets, and to
prevent the outlet from receiving power when the battery is connected to the
charger. I'll need some sort of weatherproof box for exterior wall outlet of
course. I expect I'llneed to put a suitable fuse in the positive line,
around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

Maybe he has a garage where the car is actually the last thing that goes
inside it.
Brian

--
----- --
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Andy Burns" wrote in message
...
Jon Parker wrote:

Does that all make sense?
Even assuming you've got a 'free' 12V battery, what you spend on 3-way
switch, 2x sockets and a weatherproof enclosure will probably cost more
than occasionally running the car and plugging the air pump into the cigar
lighter ...

On 22/04/2020 17:20, Martin Brown wrote:

It is a problem with the UK's high latitude and low sun in winter so
probably applies to the entire country. I expect the signs work fine at
lower latitudes that get some decent amount of winter sunshine.

We used to have a "private" speed sign on the approach road to work. It
would have been well designed and maintained, and it worked fine all the
year round. It did have quite a big panel with an unobstructed view of a
lot of sky.

On Thu, 23 Apr 2020 09:11:21 +0100, newshound
wrote:

On 22/04/2020 17:20, Martin Brown wrote:

It is a problem with the UK's high latitude and low sun in winter so
probably applies to the entire country. I expect the signs work fine at
lower latitudes that get some decent amount of winter sunshine.

We used to have a "private" speed sign on the approach road to work. It
would have been well designed and maintained, and it worked fine all the
year round. It did have quite a big panel with an unobstructed view of a
lot of sky.

And that's the thing isn't it, it would have to be designed (both it
and it's location) to work with the energy available at that location
and in 'worst case' scenarios (little sun for a time, lots of usage
for a time, battery / panel efficiency reduction over time etc).

Anything less, causing the unit to fail down to lack of energy is
simply down to poor design limitation considerations.

So, if you have the OP's garage 12V supply situation. If you know how
much power the air pump takes (easy to read off the plate or measure
with a DMM), and the longest time you might run it for in a particular
period, you know what the battery will need to supply that energy and
therefore what size panel you might need to be assured you can cover
both that usage and any parasitic drain created by any charge
controller.

Or, that with a big enough battery and a matched panel (that wouldn't
do the battery any harm by overcharging in the 'best' conditions),
just a straight panel battery config (that the OP is not going to
never going to discharge the battery beyond say 50%) then he should be
good to go. ;-)

Cheers, T i m

On Wednesday, April 22, 2020 at 2:26:48 PM UTC+1, wrote:
There is no obvious need to switch the power between charge and the
sockets?

I know very little about solar PV. I considered that an appliance drawing more power than the panel could supply might harm the panel, so I thought it best to switch the panel out of the circuit to ensure power was only ever drawn form the battery.

If I were considering doing this, I would be installing a mains powered
12v (13.8v) SMPSU - do you not have any mains power in the garage?

I do have mains power in the garage. I will look into this solution, seems to be neather than my proposed solution!

Thanks

On 22/4/20 10:39 pm, Jon Parker wrote:
I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged, and a 12v outlet on the exterior wall and probably one on the inisde also. Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger. I'll need some sort of weatherproof box for exterior wall outlet of course. I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

Much better to invest in an air compressor which has many more uses like
many useful air tools and spray painting etc have many die
grinders{much better than dremels} sanders, multi tools,drills,staplers,
nail guns,sanding machines etc and much more

In article ,
Jon Parker wrote:
On Wednesday, April 22, 2020 at 2:26:48 PM UTC+1, wrote:
There is no obvious need to switch the power between charge and the
sockets?

I know very little about solar PV. I considered that an appliance
drawing more power than the panel could supply might harm the panel, so
I thought it best to switch the panel out of the circuit to ensure power
was only ever drawn form the battery.

If I were considering doing this, I would be installing a mains
powered 12v (13.8v) SMPSU - do you not have any mains power in the
garage?

I do have mains power in the garage. I will look into this solution,
seems to be neather than my proposed solution!

If you have mains power available, seems a very expensive way of getting
12v?

--
*When everything's coming your way, you're in the wrong lane *

Dave Plowman London SW
To e-mail, change noise into sound.

Jon Parker presented the following explanation :
I do have mains power in the garage. I will look into this solution, seems to
be neather than my proposed solution!

No need for a battery then, just a 13.8v SMPSU and only if you must
have the 12v. Why not look at a small 240v compressor?

Jon Parker wrote:
On Wednesday, April 22, 2020 at 2:26:48 PM UTC+1, wrote:
There is no obvious need to switch the power between charge and the
sockets?

I know very little about solar PV. I considered that an appliance drawing more power than the panel could supply might harm the panel, so I thought it best to switch the panel out of the circuit to ensure power was only ever drawn form the battery.

If I were considering doing this, I would be installing a mains powered
12v (13.8v) SMPSU - do you not have any mains power in the garage?

I do have mains power in the garage. I will look into this solution, seems to be neather than my proposed solution!

Thanks


You need to study the "load" carefully, as the SMPS pricing
will rapidly shoot skyward, if you attempt to get too much capacity.

The thing is, ATX power supplies come perilously close to
sufficiency and ampere-capacity. There are supplies with
ratings up around a kilowatt or 1200W or so. (Think 12V @ 80A
and the PCI Express cables)

ATX computer supplies put out 12.0V, while a
battery eliminator might put out 13.8V. When you look at this,
you might say "well, surely a small adjustment will give the 13.8V",
but it isn't always that easy. (Some equipment has overvoltage
protection, which is integrated somewhere and shuts off the
equipment if an "abnormal" value of voltage appears. The 13.8V
adjustment might cause that protection to trip. A control in
one part of the circuit, does not necessarily control every
subsystem in the box.)

Supplies come with "expectations" about the kind of load expected.

capacitive ["5000uf max" or a similar statement, phase margin]
resistive (ham radio???)
inductive (motors... 10X loading from motor when rotor stalled)

Motors are particularly tricky, because of their load current
behavior. When the rotor is stalled, some motors draw 10X the
nameplate current. On an SMPS with electronic overcurrent
protection, the time constant on overcurrent can be
quite short. Which means the SMPS turns off, as soon as you flick
the switch on the air pump.

On capacitive loads, the message here is about the "hobbyist"
approach to electronics. To make projects constructed at
home more robust, a hobbyist adds capacitors. A capacitor is
a short term energy store. And you would think, "well, perfect,
I slap those into my circuit, the motor 10X current comes from
the capacitor, honor is satisfied". But the SMPS has an internal
feedback circuit path, and if around 5000uF is added to
the output of the circuit (a small amount compared to what
clever hobbyists buy), this drives the control loop crazy.

As a general rule, easy public specs for SMPS do not provide
a phase margin statement, and what the limit is for adding
capacitors to the output. You must contact the supplier
and beg for the information (like we had to do at work
when buying these).

We can't "fix" the ATX supply particularly, by adding capacitors.

Ham radio operators, look for items similar to the following. Radios
intended for in-car operation, need a bench supply when the
radio is run in the shack at home. Devices like this attempt
to solve that problem. This one happens to be
adjustable, and you could, say, adjust it when a longish
cable runs to your linear. Key up the linear, adjust
the supply so that the linear sees the desired value
on its end (13.8V or whatever).

https://www.amazon.ca/MegaWatt-S-350.../dp/B00JZBE97U

OK, so that would probably work with a 30 amp resistive
load (say, four incandescent car headlights on hi beam).

It's harder to say, what would make a good DC power source
for an inductive load.

I tried looking in the transformer catalog, and couldn't
find just the right 18V transformer, to build a classic
supply. This would be the "linear supply approach", different
than SMPS, wasteful and inefficient if used all day long,
as some parts of the regulation circuitry kick off
tons of heat. Even the diode bridge rectifier can get
scalding hot when used at high load, such that the
diodes have to be mounted on a heatsink, as well as
the regulating circuitry. The heatsinks are measured in
"square inches of active area", and you might end up
needing a "cubic foot of fins for cooling" if the load
is high enough.

Like the SMPS, the price of implementation goes up
with the ratings. The linear might withstand more
abuses (such as the motor start transient), as the plus
for that approach. But it will be inefficient and
kick off heat, whereas the SMPS can be "80+ efficient"
and "active PFC", properties the power supplying company
likes. At these kinds of loads, some SMPS are
efficient enough, they barely need a cooling fan
to keep the smaller heatsink inside, cool.

Each approach has issues:

SMPS - unknown tolerance to motor startup current
- not typically rated for inductive loads, won't
say "sure, go ahead, put an X ampere motor on here".
- a solution that (nominally) lasts forever, perhaps
an inductive spike could damage it (I did that once,
that's how I know).
- a specialized SMPS could likely deal with any load
type, but at a guess, the designer would charge a fortune
for this ("because they could"). It's the average SMPS
which is not ready for all load types.

Linear - potentially more tolerant of loading types
- might support an inductive load
- something you build yourself, pretty simple hookup.
- a solution that (nominally) lasts forever
- inefficient, at these power levels "many chunks of iron".
Maybe 80 pounds weight.

Solar/battery - battery as reservoir, is very tolerant of load types.
Wonderful. Already I've gone to heaven. Don't
forget the fuses! Batteries take no prisoners.
A friend of mine got burned once, by not following
the rules ("do not wear jewelery near auto batteries").
His metal watch strap, the link pattern was burned
into his wrist! Cover the battery terminals with
insulators over top.

- can hook together using purchasable subassemblies.
I was even able to buy clamp style terminals to fit
around the round battery terminals, to cable up a
battery.
- can be charged by a wall powered charger if needed,
and this could even be used as a substitute for the
solar panel. You could hook up a "smart charger", if
you could get one that starts the charge cycle again
at 11.8V. (So the battery cannot be run flat and damaged
by neglect over long periods of non-usage.)
- big minus, replacement of battery at regular intervals,
as a function of how well the charging method makes
the best use of battery chemistry (sulphation).
- for automotive batteries, try not to use more than
25% of the amp-hour rating - the batteries last longer
if the discharge is shallow. To pump up a tire, you
might not have too much trouble with this. To pump up
all the tires at an auto rally, maybe not. Do the
maths, once you've measured the pump and fully
understand the loading it presents. More expensive deep
discharge batteries are also available.

For automotive batteries, you can find info on this site,
to understand some of the requirements, and then compare
what it says here, to what a smart charger is doing.

https://batteryuniversity.com/learn/...d_acid_battery

The battery plus smart charger looks like the winner to me,
mainly because (barring accidents involving car batteries),
it's a pretty simple setup. It can't provide power forever,
like the other two solutions could. If you keep a multimeter
handy to it, you can use that for determining current state
of charge (assumes maintenance-free battery where you can't
access the fluid with a hydrometer).

At work, each company parking lot had a battery "on wheels",
used for jump-starting cars with dead batteries. The device
was kept at the security desk, and could be signed out. And
that seemed to work pretty well. I didn't hear too many stories
about "hmmph, went to use it and someone forgot to charge
the battery". That way, we didn't need towing trucks
prowling the lot, looking for customers :-)

Paul

On Thursday, 23 April 2020 15:25:38 UTC+1, Jon Parker wrote:
On Wednesday, April 22, 2020 at 2:26:48 PM UTC+1, wrote:
There is no obvious need to switch the power between charge and the
sockets?

I know very little about solar PV. I considered that an appliance drawing more power than the panel could supply might harm the panel, so I thought it best to switch the panel out of the circuit to ensure power was only ever drawn form the battery.

PV panels can be shorted indefinitely without harm

If I were considering doing this, I would be installing a mains powered
12v (13.8v) SMPSU - do you not have any mains power in the garage?

I do have mains power in the garage. I will look into this solution, seems to be neather than my proposed solution!

Thanks

Then your proposed panel is pointless.


NT