Automotive electrical question

[bucolic_frolic]

For an occasional weekend amateur, with Youtube and Haynes, I do ok.

But this week ... I successfully repaired the main lights and circuits integrated card on a 90s Honda. Tinned up a few traces. Did an AMP draw test, lots of clicking there, and left it with open circuits overnight under the dash. Next morning ... battery voltage ... 7.59 volts. Lowest I've ever seen. That's not a slow leak. I've had slow leaks ... takes 10 days to draw down to 11.2 volts with a dome light on.

Charging battery while disconnected, prepping up fuse, amp draw, tests. But what could cause such a massive drawdown overnight? I'm thinking alternator/diode?

Thanks for a quick take.

[Clearly fogged in]

1. Sometimes things aren't what they seem

My first guess/suggestion would be the battery has an internal short - bring it up to float voltage, disconnect and hook up a small load like an instrument bulb. Recheck the voltage drop for a day. 11+ volts is one bad cell, less is more bad cells. The second would be a bad diode in the regulator (or alternator in the absence of an external regulator)

Too many times I've seen batteries load test good but fail in use.

[gibraltar72]

2. Check your draw as you pull fuses one at a time.

that should isolate which circuit is giving you issues.

[bucolic_frolic]

3. Passed parasitic draw test

3.2 milliamps. I have a very weak charger from a lawn mower, but the battery did get to 11.25V and rapidly fell almost a whole volt when hooked up for a few minutes. Would not crank at all. Will give it overnight to charge and see if any crank.

I'll do the fuse tests, and a diode test once I get it running, but so far no explanation. I don't see how I could have shorted something in the removal and replacement of the fuse box, or how all those disconnects dangling for 2 days would have drained the battery. Which is not that old by the way.

Not getting any crank. I'm still thinking starter or ignition short, but with no parasitic draw? That's my topic for research this evening.

[bucolic_frolic]

4. Just to close this off, thanks for the help

I'm still puzzled about the battery drawdown, but it's all back together, the battery holds a charge and starts, the ICU is repaired though I wonder if the thick solder will heat more than surrounding copper traces and burn the junctions, but hey you get what you get and do what's expedient.

I have no idea why disconnecting all those wire connectors into the ICU, and leaving them open and hanging out for 36 hours without disconnecting the battery led to dramatic voltage collapse. It's not like anyone on the videos I watched or the Haynes manual warning to disconnect the battery. Maybe it was in the disclaimed or general directions at the beginning of the chapter. The reason not to disconnect is that now I must reset the ECU - warm it up, pull the batter for 20 minutes, idled it for 10, turn it off and drive for a week or so.

Disconnect before you vivisect!