I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar

On Monday, April 1, 2013 8:08:55 PM UTC+1, Phil L wrote:

I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long
I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?
All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.
Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar

Reducing output to 9v will achieve nothing.


NT

wrote:
On Monday, April 1, 2013 8:08:55 PM UTC+1, Phil L wrote:

I have a 12v solar panel for charging batteries, the batteries I'm
trying to charge are 7.2v ni-mh, 4500mah - sunshine is not a
problem as it will be used abroad, in summer, and I've got all day
long
I've tested the panel and it puts out 21 - 24 volts, although the
wattage is very low, about 3w.
I've charged the batteries with it before, but have been told this
is not reccomended, although the plug in charger for these things
puts out about 11v, so without spending a fortune, how can I drop
the voltage to say, 9v? All the converters I've looked at online are
weird, the input is 12-24v but they only put out 5v.
Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery,
but it's got to be plugged in in the morning and collected at night,
but the mains power is a 30 minute journey, hence my need for solar

Reducing output to 9v will achieve nothing.



Why not? - is putting 23v through a 7.2v battery no worse than putting 9v? -
what about 230v?

On 01/04/2013 20:08, Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar


I wouldn't want to charge a 7.2 battery pack with a solar panel directly.

It is difficult to make meaningful suggestions without knowing what yuo
are using the 7.2 pack for, or the discharge rate/reharge interval.

Would one these chargers
http://www.ebay.co.uk/itm/321098456040

Connected to one of these solar charge controllers
http://www.ebay.co.uk/itm/121087310293

that is also connected to a 12v gel battery work for you?

--
Toby...
Remove pants to reply

On Monday, April 1, 2013 8:28:14 PM UTC+1, Phil L wrote:
wrote:
On Monday, April 1, 2013 8:08:55 PM UTC+1, Phil L wrote:

I have a 12v solar panel for charging batteries, the batteries I'm
trying to charge are 7.2v ni-mh, 4500mah - sunshine is not a
problem as it will be used abroad, in summer, and I've got all day
long
I've tested the panel and it puts out 21 - 24 volts, although the
wattage is very low, about 3w.
I've charged the batteries with it before, but have been told this
is not reccomended, although the plug in charger for these things
puts out about 11v, so without spending a fortune, how can I drop
the voltage to say, 9v? All the converters I've looked at online are
weird, the input is 12-24v but they only put out 5v.
Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery,
but it's got to be plugged in in the morning and collected at night,
but the mains power is a 30 minute journey, hence my need for solar

Reducing output to 9v will achieve nothing.


Why not? - is putting 23v through a 7.2v battery no worse than putting 9v? -
what about 230v?

1000 21v panels in series wouild charge at the same current. The battery pack see its own terminal voltage regardless


NT

Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar


The panel voltage is completely irrelevant. Once the battery is
connected the terminal voltage of the panel will be pulled down to that
of the battery. All that matters is the amount of current going through
the battery. The only problem with a high panel voltage is that charging
will continue even when the battery is fully charged.

However the current limit in your case will be set by the panel. I
suggest that you connect the panel directly to the battery via an
ammeter* of some sort, as a test. *Or buy a five quid multimeter from
Maplin etc. In the UK use a halogen lamp to represent the sun! I think
you'll find that the max charge rate will be about 0.15A,which is fine,
and when the battery is charged it will decrease somewhat. It will never
decrease to almost nothing though, so you won't be able to leave the
battery on charge indefinitely. If the panel manages to give a faster
charge rate than you'd like use a resistor as a dropper, or put
something over part of the panel to screen it.

My own (crude) practice is to charge 6V nicads from a 12V vehicle using
a dropper resistor. Find the resistor value that gives a safe charge
rate from your solar panel (working at max output) by trial and error.
If you know the battery is flat and you will be there to disconnect
after a while you can obviously use a resistor that has a higher charge
rate. For continuous charge you have to use a high value resistor that
gives a trickle charge. This is an electrically inefficient method but
that doesn't matter when the power is free. The resistor needs to be a
decent wirewound type rated at 5W or thereabouts.

I've charged my 6V lanterns from the van like this for 30 years with no
problems. I also occasionally (when the proper charger is otherwise in
use) charge 12V nicads from a 30V power supply using a big rheostat as a
variable resistor to control the charge rate.

Bill

On 01/04/2013 20:08, Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar



I don't know much (anything?) about the voltage profile of solar panels,
but if a 24v panel only produces 3w, that suggests that it delivers a
current of only about 1/8 amps (125mA). Presumably you only measured its
open circuit voltage, and if you were to impose a load which tried to
draw more than 125mA, its voltage would fall? Whether that would allow
the current to rise, I don't know.

Anyhow, the only thing that will damage the battery is if you try to
charge it at too high a rate. The mains charger charges it at 500mA
(*not* 500mAh - that's a capacity not a charge rate), which it
apparently doesn't mind. I can't see that the solar panel would have a
cat in hell's chance of delivering more than 500mA - so there shouldn't
be a problem. the most likely problem is that it may take about 3 days
to charge!

If I'm wrong, and there's a danger of too high a charge rate, what you
need is a *current* limiter rather than a voltage limiter - but I don't
think that's going to be necessary.
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

"Bill Wright" wrote in message
...
Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm trying
to charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will
be used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage
is very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say,
9v?

All the converters I've looked at online are weird, the input is 12-24v
but they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but
it's got to be plugged in in the morning and collected at night, but the
mains power is a 30 minute journey, hence my need for solar
The panel voltage is completely irrelevant. Once the battery is connected
the terminal voltage of the panel will be pulled down to that of the
battery. All that matters is the amount of current going through the
battery. The only problem with a high panel voltage is that charging will
continue even when the battery is fully charged.

However the current limit in your case will be set by the panel. I suggest
that you connect the panel directly to the battery via an ammeter* of some
sort, as a test. *Or buy a five quid multimeter from Maplin etc. In the UK
use a halogen lamp to represent the sun! I think you'll find that the max
charge rate will be about 0.15A,which is fine, and when the battery is
charged it will decrease somewhat. It will never decrease to almost
nothing though, so you won't be able to leave the battery on charge
indefinitely. If the panel manages to give a faster charge rate than you'd
like use a resistor as a dropper, or put something over part of the panel
to screen it.

My own (crude) practice is to charge 6V nicads from a 12V vehicle using a
dropper resistor. Find the resistor value that gives a safe charge rate
from your solar panel (working at max output) by trial and error. If you
know the battery is flat and you will be there to disconnect after a while
you can obviously use a resistor that has a higher charge rate. For
continuous charge you have to use a high value resistor that gives a
trickle charge. This is an electrically inefficient method but that
doesn't matter when the power is free. The resistor needs to be a decent
wirewound type rated at 5W or thereabouts.

I've charged my 6V lanterns from the van like this for 30 years with no
problems. I also occasionally (when the proper charger is otherwise in
use) charge 12V nicads from a 30V power supply using a big rheostat as a
variable resistor to control the charge rate.

Bill



Perfect description Bill, and I agree with everything you've said. Simple
current-limited charging for NiCds and NiMH batteries may not be the
kindest, or lead to the longest life, but is perfectly adequate. Most
cordless phones for the last 20 years or more, have used the simple 'series
resistor' charging scheme without issue, and if the panel is only rated at a
few watts anyway, then it is already a fairly high impedance voltage source,
so will be self limiting current-wise, in any case.

Arfa

I think the point is that you need to use a constant current source and
monitor the cell voltages so when its the stable max voltage you just sample
charge or do nothing every few seconds. The problem is if you use a simple
charger you will run them at too high a current during the charge cycle and
overcharge them if you leave it on.
Brian

--
From the Sofa of Brian Gaff Reply address is active
"Phil L" wrote in message
...
I have a 12v solar panel for charging batteries, the batteries I'm trying
to charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will
be used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage
is very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v
but they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar

In the main the suggested way to charge these is as separate cells, not in
series. The issue of reverse charging occurs in the battery pack charged as
they come, as one or more batteries go flat before the others and if you
continue to use them this way, assuming the equipment will still run, the
empty cell is charged in reverse.
So the regime that pays dividends I have found with packs is not actually
to use them till the equipment stops, but know how long they will last and
take them off just before they run out and charge them with a constant
current charger.
In many ways they can be seen as supecharged nicads, but the newer ones
retain their charge better.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Phil L" wrote in message
...
wrote:
On Monday, April 1, 2013 8:08:55 PM UTC+1, Phil L wrote:

I have a 12v solar panel for charging batteries, the batteries I'm
trying to charge are 7.2v ni-mh, 4500mah - sunshine is not a
problem as it will be used abroad, in summer, and I've got all day
long
I've tested the panel and it puts out 21 - 24 volts, although the
wattage is very low, about 3w.
I've charged the batteries with it before, but have been told this
is not reccomended, although the plug in charger for these things
puts out about 11v, so without spending a fortune, how can I drop
the voltage to say, 9v? All the converters I've looked at online are
weird, the input is 12-24v but they only put out 5v.
Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery,
but it's got to be plugged in in the morning and collected at night,
but the mains power is a 30 minute journey, hence my need for solar

Reducing output to 9v will achieve nothing.



Why not? - is putting 23v through a 7.2v battery no worse than putting
9v? - what about 230v?

Yes well what I tended to find happened using resistors was that one cell or
more died fairly fast. I'm not sure of the reason, but I'd imagine the
reverse charging issue when running the gear did something and the crude
charger nevetr quite got all the cells up to the same charge in the first
place.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Bill Wright" wrote in message
...
Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm trying
to charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will
be used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage
is very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say,
9v?

All the converters I've looked at online are weird, the input is 12-24v
but they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but
it's got to be plugged in in the morning and collected at night, but the
mains power is a 30 minute journey, hence my need for solar
The panel voltage is completely irrelevant. Once the battery is connected
the terminal voltage of the panel will be pulled down to that of the
battery. All that matters is the amount of current going through the
battery. The only problem with a high panel voltage is that charging will
continue even when the battery is fully charged.

However the current limit in your case will be set by the panel. I suggest
that you connect the panel directly to the battery via an ammeter* of some
sort, as a test. *Or buy a five quid multimeter from Maplin etc. In the UK
use a halogen lamp to represent the sun! I think you'll find that the max
charge rate will be about 0.15A,which is fine, and when the battery is
charged it will decrease somewhat. It will never decrease to almost
nothing though, so you won't be able to leave the battery on charge
indefinitely. If the panel manages to give a faster charge rate than you'd
like use a resistor as a dropper, or put something over part of the panel
to screen it.

My own (crude) practice is to charge 6V nicads from a 12V vehicle using a
dropper resistor. Find the resistor value that gives a safe charge rate
from your solar panel (working at max output) by trial and error. If you
know the battery is flat and you will be there to disconnect after a while
you can obviously use a resistor that has a higher charge rate. For
continuous charge you have to use a high value resistor that gives a
trickle charge. This is an electrically inefficient method but that
doesn't matter when the power is free. The resistor needs to be a decent
wirewound type rated at 5W or thereabouts.

I've charged my 6V lanterns from the van like this for 30 years with no
problems. I also occasionally (when the proper charger is otherwise in
use) charge 12V nicads from a 30V power supply using a big rheostat as a
variable resistor to control the charge rate.

Bill

On Apr 1, 10:38*pm, Roger Mills wrote:
On 01/04/2013 20:08, Phil L wrote:









I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - *sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.

Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar

I don't know much (anything?) about the voltage profile of solar panels,
but if a 24v panel only produces 3w, that suggests that it delivers a
current of only about 1/8 amps (125mA). Presumably you only measured its
open circuit voltage, and if you were to impose a load which tried to
draw more than 125mA, its voltage would fall? Whether that would allow
the current to rise, I don't know.

Anyhow, the only thing that will damage the battery is if you try to
charge it at too high a rate. The mains charger charges it at 500mA
(*not* 500mAh - that's a capacity not a charge rate), which it
apparently doesn't mind. I can't see that the solar panel would have a
cat in hell's chance of delivering more than 500mA - so there shouldn't
be a problem. the most likely problem is that it may take about 3 days
to charge!

If I'm wrong, and there's a danger of too high a charge rate, what you
need is a *current* limiter rather than a voltage limiter - but I don't
think that's going to be necessary.
--
Cheers,
Roger


You can buy a control box to do the above automatically.
The sellers of the panel usually have them.

On 01/04/2013 22:02, Bill Wright wrote:
Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm
trying to charge are 7.2v ni-mh, 4500mah - sunshine is not a problem
as it will be used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the
wattage is very low, about 3w.
I've charged the batteries with it before, but have been told this is
not reccomended, although the plug in charger for these things puts
out about 11v, so without spending a fortune, how can I drop the
voltage to say, 9v?

All the converters I've looked at online are weird, the input is
12-24v but they only put out 5v.


Will the 23v damage the batteries if I leave it as it is?

Eventually yes. Batteries need to be protected from both being
discharged past their point of no return and from overcharging.
However at the maximum current your solar panel can supply it will take
a long while before problems occur. You are more likely to encounter the
opposite problem of the panel leakage stealing charge from the batteries
when it is dark if the cheap panel is lacking a diode in series (as many
of the cheaper solar power units are).

The plug in charger takes 9 hours @ 500mah to charge one battery, but
it's got to be plugged in in the morning and collected at night, but
the mains power is a 30 minute journey, hence my need for solar

The panel voltage is completely irrelevant. Once the battery is
connected the terminal voltage of the panel will be pulled down to that
of the battery. All that matters is the amount of current going through
the battery. The only problem with a high panel voltage is that charging
will continue even when the battery is fully charged.

Until the pressure in the cell becomes too great and it explodes. This
is hardly recommended charging procedure with an excessive voltage.

The panel can only output about 3W at nominal 15v or so which is 200mA
it might manage a bit more into a dead short but probably not 400mA.
That is still below C/10 charge rate so apart from the overvoltage
problem your cells could survive on this almost indefinitely.

A simple circuit to regulate the voltage for charging the battery will
do what you want. The very simplest would be a resistor to drop the
excess voltage when 200mA flows (about 47R and 2W ought to do it).

If you want to be a little bit more elegant you probably should have a
diode in series to stop the solar panel discharging your battery
overnight (good ones should already have a reverse blocking diode).

The final refinement would be to put a 2W 8v2 or 9v1 Zener diode in
parallel with the battery to make a crude voltage regulator. Strictly
also a small resistor from the regulated node to the battery as well.

http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm

The circuit is incredibly simple and the bits can be bought from Rapid.

The 5W Zeners are probably better value at 6p each.

http://www.rapidonline.com/Electroni...r-diodes-80342

The power resistor still costs slightly more at 7p!

http://www.rapidonline.com/Electroni...esistors-80872

(expect to pay much more for them at Maplins)

However the current limit in your case will be set by the panel. I
suggest that you connect the panel directly to the battery via an
ammeter* of some sort, as a test. *Or buy a five quid multimeter from
Maplin etc. In the UK use a halogen lamp to represent the sun! I think
you'll find that the max charge rate will be about 0.15A,which is fine,
and when the battery is charged it will decrease somewhat. It will never
decrease to almost nothing though, so you won't be able to leave the
battery on charge indefinitely. If the panel manages to give a faster
charge rate than you'd like use a resistor as a dropper, or put
something over part of the panel to screen it.

My own (crude) practice is to charge 6V nicads from a 12V vehicle using
a dropper resistor. Find the resistor value that gives a safe charge
rate from your solar panel (working at max output) by trial and error.
If you know the battery is flat and you will be there to disconnect
after a while you can obviously use a resistor that has a higher charge
rate. For continuous charge you have to use a high value resistor that
gives a trickle charge. This is an electrically inefficient method but
that doesn't matter when the power is free. The resistor needs to be a
decent wirewound type rated at 5W or thereabouts.

I've charged my 6V lanterns from the van like this for 30 years with no
problems. I also occasionally (when the proper charger is otherwise in
use) charge 12V nicads from a 30V power supply using a big rheostat as a
variable resistor to control the charge rate.

It is only a problem when you forget and leave one on continuous charge
from a too high voltage source for weeks at a time. At a C/20 charge
rate you will get away with it for quite a long time (especially in this
case as they will get a daily rest when the sun goes down).

It depends a bit how you intend to use this device. If you only put
batteries in it when they need charging and take them out again at the
end of charging (about 24 hours at 200mA so ~ 3 days good sun) then no
problem. If you forget about them for weeks on end with a too high
supply voltage then expect to kill the weakest cell in the stack stone
dead. Eventually it will leak and damage whatever you use it in.

--
Regards,
Martin Brown

"Brian Gaff" wrote in message
...
I think the point is that you need to use a constant current source and
monitor the cell voltages so when its the stable max voltage you just
sample charge or do nothing every few seconds. The problem is if you use a
simple charger you will run them at too high a current during the charge
cycle and overcharge them if you leave it on.
Brian



Potentially with a 'big' charging source, but not really in this case where
the charging source has the power of a mouse due to its high impedance, and
will get 'switched off' for at least 8 hours a day whilst the sun has gone
into hiding round the other side of the planet. As I said earlier, cordless
handsets have always been charged by a simple high impedance source, created
by a resistor from the base station's main raw power rail, and I have always
been amazed at just how long the batteries last ( I used to repair loads of
cordless phones for a local shop) given that the handsets just sit on the
base with charge continuously applied, for most of their life.

Arfa

On 02/04/2013 08:41, harry wrote:
On Apr 1, 10:38 pm, Roger Mills wrote:
On 01/04/2013 20:08, Phil L wrote:

I have a 12v solar panel for charging batteries, the batteries I'm trying to
charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be
used abroad, in summer, and I've got all day long

I've tested the panel and it puts out 21 - 24 volts, although the wattage is
very low, about 3w.
I've charged the batteries with it before, but have been told this is not
reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?

All the converters I've looked at online are weird, the input is 12-24v but
they only put out 5v.

Will the 23v damage the batteries if I leave it as it is?
The plug in charger takes 9 hours @ 500mah to charge one battery, but it's
got to be plugged in in the morning and collected at night, but the mains
power is a 30 minute journey, hence my need for solar

I don't know much (anything?) about the voltage profile of solar panels,
but if a 24v panel only produces 3w, that suggests that it delivers a
current of only about 1/8 amps (125mA). Presumably you only measured its
open circuit voltage, and if you were to impose a load which tried to
draw more than 125mA, its voltage would fall? Whether that would allow
the current to rise, I don't know.

Anyhow, the only thing that will damage the battery is if you try to
charge it at too high a rate. The mains charger charges it at 500mA
(*not* 500mAh - that's a capacity not a charge rate), which it
apparently doesn't mind. I can't see that the solar panel would have a
cat in hell's chance of delivering more than 500mA - so there shouldn't
be a problem. the most likely problem is that it may take about 3 days
to charge!

If I'm wrong, and there's a danger of too high a charge rate, what you
need is a *current* limiter rather than a voltage limiter - but I don't
think that's going to be necessary.

You can buy a control box to do the above automatically.
The sellers of the panel usually have them.

And it will only take you a thousand years or so to recoup the
extortionate price they charge for it to solar suckers like Harry.

--
Regards,
Martin Brown

On Tue, 02 Apr 2013 08:59:58 +0100, Martin Brown wrote:

You are more likely to encounter the opposite problem of the panel
leakage stealing charge from the batteries when it is dark if the cheap
panel is lacking a diode in series (as many of the cheaper solar power
units are).

+1 found that out the hard way. B-(

Small solar panel to keep the battery on the genset topped up (in
theory). After 6 months connected to panel it was due for a test, battery
flat... not amused. Pull starting is possible but hard work (diesel
engine). After the test, check for leakage etc in the set none, charge
battery, takes charge but one cell might be iffy. Put back into store not
connected to panel. This was several months ago, needed it in anger the
other day. Started fine, even with it being -3 C...

--
Cheers
Dave.

Phil L wrote:
I have a 12v solar panel for charging batteries, the batteries I'm
trying to charge are 7.2v ni-mh, 4500mah - sunshine is not a problem
as it will be used abroad, in summer, and I've got all day long

Thanks to all who replied, given that I have 3 battery packs, all quite old,
I'm inclined to leave things as they are, they're only about £13 to replace,
if and when one or more need replacing so I'm not overly bothered if one
should get damaged / destroyed.

There's no chance of losing charge back into the panel as I'm on the bank
(lakeside fishing) all day long and have little else to do other than eat,
drink, charge batteries and hopefully catch fish, so they are removed from
charge well before dusk, as well as being checked regularly throughout the
day.
As I stated in my OP, it's worked well and charges the battery as quick as
mains voltage, but with the bonus of not having to stop fishing for 30 mins
morning and night, coupled with a long walk in the hot sun, so I'm not going
to do anything, but maybe on hotter days I'll partially shade the panel as a
precaution.

On a related note, don't test a solar panel's output at twilight by using
your tongue on the terminal pins, it hurts - christ knows what it would be
like at midday.

On 06/04/2013 15:09, Phil L wrote:


There's no chance of losing charge back into the panel

A diode, costing 10 to 20p, in series will stop this. The panel you are
considering may already have this feature.

http://www.solar-facts.com/panels/panel-diodes.php

http://www.rapidonline.com/Electroni...es-P600-200040


x5 to x10 more expensive, similar can be obtained from Maplin
http://www.maplin.co.uk who may have a store near you thus avaioding
minimum order or postage charges.


--
mailto:news{at}admac(dot}myzen{dot}co{dot}uk

On Saturday, April 13, 2013 9:03:33 AM UTC+1, alan wrote:
On 06/04/2013 15:09, Phil L wrote:

There's no chance of losing charge back into the panel

A diode, costing 10 to 20p, in series will stop this. The panel you are

minimum order or postage charges.

Diodes are free & plentiful in scrap electronics. For 20v 350mA you don't need to worry about specs, just avoid any glass point contact type.


NT