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I'm not having a good weekend - car wouldn't start, even when jump started using a conveniently placed 12v generator battery (almost identical to this question, I suspect dodgy cabling to the starter). The battery was fine at first, but various failed attempts to start the car and diagnose the problem and various other things have run it down.

I'm trying to recharge the battery to properly test my theory that it's a problem with the wiring. I don't have access to a car battery charger and won't have access for several days, at least. As mentioned jumpstart didn't work.

What I do have is a li-po battery pack designed for laptops, appliances etc that outputs at 12v, 16v and 18v (3.5 amps), which I can connect to crocodile clips and to the accessory socket. I double-checked with a multi-meter that the output was right and that the polarity was as expected before connecting.

This didn't go as expected:

  • Connecting to the battery while connected to the car, set to 12v, connecting positive to positive then negative to ground, the battery pack instantly switched off. This is the same behaviour as if it detects a short circuit. (same thing happened at 16V)
  • Trying again connecting straight to the terminals of the battery while disconnected from the car, the same thing happened.
  • Trying again via the accessory socket (with the battery reconnected to the car), the battery didn't react but instead I think the fuse blew out inside the plug I was using to connect to the accessory socket. Testing the accessory socket cable and plug I used with a multimeter, it now seems to be dead.
  • Testing the above with a low-power solar battery maintainer (2.4 watt), the accessory socket works fine (charges very very very slowly but observably while the panel is plugged in) and has no problems connected direct to the battery using the same crocodile clips that I used with the battery.

What could be going on here? Why might what seems like perfectly normal charging circuits for a 2.4 watt solar panel be treated like a short circuit that blows an accessory plug's fuse and trips a battery pack's electronic protection? What's special about a car battery and 12v / 3.5a from a lithium polymer battery that would make it behave this way?

All I can think is that a lead-acid battery probably draws current differently to an electronic appliance, but I can't think why it would result in this.

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This battery pack is rated for 4A at 12V. But a really empty car battery can easily draw much more that 5A when connected to 12V, which is definitely too much for your battery pack. The LiPo battery itself can deliver lots of, but not unlimited current. At high currents, it can get really hot and even start a fire. The battery pack contains a protection circuit, which trips if the current exceeds a threshold.

(The mains power cable of your house can deliver some 100A, but the cabeling in your house can not handle this - To prevent fires you have circuit breakers tripping at too high currents. The breaker doesn't care whether there's a short circuit or just too many devices connected!)

The simple solar panel doesn't have this problem. You can easily short-circuit it, and it will just deliver its maximum current.

It's a little confusing that the fuse of your accessory socket blows up. The original use of the socket is a cigarette lighter, and this is typically fused with 10, 15 or sometimes 20A. Either your LiPo pack is capable of delivering extreme currents for a while, or these sockets are not designed for cigarette lighters any more and are fused for lower current.
And again, your solar panel does not deliver more that 0.2A at 12V (0.2A*12V=2.4W), and the fuse doesn't blow.

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  • Thanks, just to be clear, it was the one accessory plug/cable/adaptor that stopped working, not the socket in the car itself. Maybe this one simply had a lower than usual fuse or cable.
    – user56reinstatemonica8
    Apr 10, 2016 at 20:15
  • The battery pack likely was able to provide a significant surge of current, enough to blow the fuse. The pack's overcurrent protection may not kick in as quickly as the fuse does.
    – David Schwartz
    Apr 10, 2016 at 22:42
  • Lipo battery packs used by radio controlled airplanes average around 40A and these are roughly the size of a block of butter. Car charger packs can be much bigger. (large RC planes can easily pull 200A or more)
    – slebetman
    Sep 27, 2017 at 3:59
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A lead acid battery, as used in motor vehicles has a very low internal resistance, this means that when directly connected across the terminals of the li-po battery pack, it does act very similarly to a dead short. Most li-po packs are not designed for very high current draws, and have short circuit protection built in. This is likely tripping.

Really, the only thing readily available for the task at hand is a proper car battery charger.

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Usually the car is charging the 12V battery with over 14V to get it up to something over 13V, so a charger with 12V will do no good.

It should work while the battery is lower, but it won't get you anywhere where you can use the battery to start a car.

The next problem might be the way the charger detects a short circuit, probably by measuring the flow. The battery is probably very large compared to what is charged usually, so there will be a huge flow and the charger will shut down.

You could try to put in resistors, but they need to be large enough to reduce the flow and strong enough to hold it, so I wouldn't play around if you can't check the numbers. Or try, but keep a fire extinguisher close! :)

Would recommend getting someone over with a car and jump start the battery. Use the recommended way to reduce risk for the other car.

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  • I forgot to say I'd also tried it at 16v with the same result
    – user56reinstatemonica8
    Apr 10, 2016 at 20:12

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