In article ,
Michael A. Terrell wrote:

"Dave Plowman (News)" wrote:

In article ,
Michael A. Terrell wrote:
So how much current do you think they're actually capable of
delivering--say with the little battery fairly fresh and fully
charged?

Too many variables. What is the amp hour rating of the battery?

Tiny in automotive terms.


Really? My jump packs have 17 Ah batteries.

I suppose it depends on your definition of tiny, but 17 amp.hr is a lot
smaller than any battery in any car I've owned or seen. Both my current
ones have 70 Ah types.

Product ID: BSL1117
Cold Cranking Amps: 240
Voltage: 12
Termination: NUT & BOLT
Chemistry: SLA OR VRLA VALVE REGULATED S
Weight: 13.3
Width: 2.99
Length: 7.13
Height: 6.57


My truck uses a size 27 battery with these specs:

Product ID: MTP-27
Amps: 1000
Cranking Amps: 1000
Cold Cranking Amps: 810
Voltage: 12
Termination: A
Pro-rata Warranty: 85
Weight: 47.4
Width: 6.81
Length: 12.06
Height: 8.88


So, it has 24% of the rating of the truck battery which isn't
'tiny'.

Err, you don't appear to have given the amp.hr capacity - the very thing
you mentioned.

It is intended to start a vehicle with a run down battery, not
one with mechanical problems.
The portable pack is designed with a
different type of battery, as well. The available current is determined
by the plate area and thickness.

I've seen someone use 12V alarm batteries to jump start a service
truck. They were rated at 7 Ah.

Batteries? Parallel them and they become like a larger one.


What is the internal resistance of the switch?

Dunno

The resistance of the
cables?

Mine has 16mm² cables. The resistance of the alligator clamps is more
likely the limiting factor.


No, all of it is important. If the resistance of the clamps is as
high as the leads, they will overheat since all the resistance is at the
same spot rather than distributed along the length of the cables.


The resistance of the starter motor? The temperature? How clean
are the battery clamps, and what is the resistance of the battery
cables? The resistance of the starter solenoid?

Non of which matters since the power pack makers claim a maximum
output.


BS. They all matter, if you aren't an ignorant troll.


What do you think the short circuit current of a fully charged 12V
car battery is?

Are you trolling? The maker of the jump start pack claims a maximum
current. Since they can't possibly know exactly what the starter motor etc
draw is, just how is it relevant?

My point is (with experience of several jump start packs including
expensive ones) is that they will not do lots of starts of a vehicle with
a flat battery without permanent damage to the SLA. Somewhere round a
dozen or so seems to be it.

--
*Corduroy pillows are making headlines.

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