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.