Sometimes, but not always, I find my car battery completely flat, with zero volts after letting the car parked for 2-4 weeks.
The battery seems totally OK after recharging, but since this happened more than a few times now (randomly) I would like to log the current and try to find out why and when it is happening. The battery is 80Ah/12V, and I suspect it to go totally flat in 2 weeks, maybe faster, so a resolution of 10-100mA will be needed. Sometimes the battery is just fine and can start the car after 1-3 months, sometimes is zero volts flat after 2 weeks. I can provide another Pb battery (from a UPS) for the logging device.
Any ideas how to probe and log such a small current (10-100mA) without disconnecting/cutting any wires, please?
Hmm, what kind of car starts at 10-100mA? Or am I misunderstanding what you are saying? The cold cranking amps of most motors are in the 100s of amps, depending on the engine size. Also, if you are logging the current while it sits idle over weeks or months, how do you know the leakage is 10-100 milli-amps?
If this is a lead/acid battery, you may have a cell (or all cells) starting to build up sulfates on the plates, which shorts out the cell and drains the charge, especially true if the battery sits unused for long spans. Lead/acid batteries can be very unpredictable because when you charge at 30 amps, which is reasonable for an alternator to do, this action can break the built up sulfates and when you stop, the battery doesn't discharge itself until the sulfates build up again. After some time, however the battery dies because of the level of sulfates deposited build up to a level that the charging will not break them.
Or, like maybe you are probably thinking, you have some type of short in your system that drains the battery over time... who knows? In the US, most automotive parts store, like O'Reilly's, or even Walmart will check your battery with a sophisticated testing system and be able to tell if you have bad cells, for free.
If you are thinking something else other than what I mentioned, never mind... Hope you find your gremlin.