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As a followup to Little/no charging but alternator and battery test good, the battery is only charging when the charger has a significant load in parallel with it (e.g. turning the key to the on position). When charged this way, the battery reaches around 13V and works for several days of starts before dropping too low to start the vehicle. The alternator does not succeed in charging it, and without a load, the dedicated charger's control logic immediately switches to "float" mode (and the battery immediately reaches 12.5V, but drops to 10V as soon as any load is applied).

What kind of state could the battery be in that causes this? And is it recoverable (e.g. with a suitable charging procedure) or does it just need to be replaced?

  • For the alternator to charge, the engine has to be running, so how do you not have the key in the "on" position? – Solar Mike Apr 29 at 20:15
  • @SolarMike: This is with a dedicated, AC powered battery charger, not the alternator. The alternator is not succeeding in charging it at all. The AC charger does, but only with a load to dump some current through. – R.. Apr 29 at 20:28
  • Try testing the complete system with a known good battery, both the external charger and the alternator - I don't believe the battery test equipment have given good results. – Solar Mike Apr 29 at 20:33
  • I'm about to the point of just saying the battery is bad and taking whatever pro-rated value I can still get out of it. With the AC charger refusing to charge it without a parallel load despite it clearly not being charged (but being chargable), there's definitely something wrong at the battery, and it's just a matter of whether it could be reconditioned to work without replacing it. – R.. Apr 29 at 20:50
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    Get what they will give you for the battery as scrap value and buy a new one. – Solar Mike Apr 29 at 20:54
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Given how complicated this seems from both posts, I would suggest that you fit a known good battery and run the complete set of tests on the charging system.

I would include the tests for voltage drop across all the cables just to make sure. These tests are usually given in the workshop manual, but basically involve having all heavy loads on (lights, heater, rear window heater) and using quality voltmeter (I used an AVO meter for this type of test - moving coil - not digital).

The general result should be less than 0.3V for a length of cable less than 0.5m roughly 2 feet. A higher result can indicate a cable with a problem. A quick test of the problem can be to make a "new" cable and put it in place or in parallel with the old one and see the difference.

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The battery was bad.

It continued to exhibit weird behavior I've never seen from a battery before. After getting it to charge once, it consistently charged to proper voltage using the 15 amp dedicated AC charger, and provided good current for cranking, but seemed to have really low capacity. After a couple hours with minimal draw, it would be back down to 10.5 volts or lower.

Today I took it back to Napa where I bought it last year, and after seemingly successful charging for 15 minutes, their charger/test rig suddenly deemed it in need of replacement. Everything looks good with the new battery.

My best guess at the mechanism of failure is a partially or fully shorted cell. That would explain the 10.5V figure - 5 cells fully charged - and the charger's reluctance to charge it. Successfully charging it to 12V then would be severe over-charge of each cell, and would, as I observed, rapidly drop to the nominal cell voltages as soon as any load was applied.

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