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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.

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.

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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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.