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Old Today, 12:48 AM
tjcooper is offline
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Sunnyvale Ca
Joined Jun 2002
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3S Lipo to act as car jumper for battery not working.

There are several discussions of people using large 3S LiPo to jump start a car battery that has gone down because the lights were left on, etc.

I took the old SEL Pb acid out of my jumper kit and replaced the battery with 5 sets of 3S LiPoly in parallel. I have about 35 AmpHour in the set at 12.6 voltDC.

I used my Harbor Frieght battery tester as a load (it pulls 45 amps out of a new car battery during its 10 second test time). When I hooked it to the LiPoly setup, the voltage went instantaneously from 12.6 vDC to zero volts when I threw the load switch. Recovers to 12.6 vDC when I release the switch. Is the internal impedance of my collection of 3S LiPo's so low that it cannot support a resistive load of 0.3 ohms that my battery tester gives out when switched on?

Anybody got any ideas of what I am doing wrong?

I see all these car jumper kits on Amazon with LiPo packs that cannot be bigger than 3S at 15AMPhour. They all claim to start at least 4 times over a dead car battery with the kit. How do those systems do it?
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Old Today, 03:28 AM
DjZorlag is online now
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"An occasional user"
Joined Jan 2016
73 Posts
You have something wrong in the voltage measurement. The IR of those packs surely isn't that low. If the packs are in a decent condition, it will be close to 1mohm. Quite a bit smaller than 300mohm of the tester load. Are all the cells in packs at 4.2v? Did you measure the lipo voltage from the balance lead?

Even one, say 2Ah 3s pack should be able to recover a car battery (just limit the charge amps with some series resistance, it might otherwise damage the Li-Pos)
Electroboom in YT revived a car battery with a couple of alkalines so much energy isn't needed at all to give the car battery a few percent of charge to be able to start the car.

E: I googled that battery tester. It seems to have a voltage meter. But check your connections and measure the voltage from the balance lead with a DMM while doing the test with the automotive meter.
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Last edited by DjZorlag; Today at 03:38 AM.
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Old Today, 09:55 AM
rampman is online now
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USA, AZ, Gilbert
Joined Jan 2006
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Test the 3S batteries seperately.
If OK, reinstall and test for the poor connection. You most likely have a connection break under a load.
A digital voltage meter is your best friend though if you know your way around a 12v test light that would work just fine.

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