[RedDevil]

All the wonders a solid 8 volts feed will do for you cars electronic systems...
My Insight had a bad battery and it caused a whole flotilla of warnings and errors: VSC, hybrid battery, airbag, PS, you name it.

Turns out one of the 6 cells was slightly less charged than the others. So when the car charges the 12V batteries the other 5 cells hit full while the 6th isn't.
Due to the smartness of the engineers nowadays, the charging system won't keep pumping electrons into a fully charged battery; it would only waste power and shorten the lifespan. But it cannot detect that one of the cells is not really full yet.
The problem with lead acid is that self discharge increases when the cell voltage drops. Once out of line, and barred from pulling up by the other cells, it gets more and more out of line until it is nearly empty while the rest is full.

Then if you don't drive for a week or run a light for a while the bad cell gets fully discharged and may even invert while the other cells keep cranking out power. The 12V battery is now an 8 Volt battery...

I tried to recharge my battery with an Optimate 4 desulphating trickle charger. But it would only do 20% or so before cutting off. On discharge the 35 Ah battery could only sustain 400 Watt (33 Ampere) for 5 to 10 minutes before it died. When recharging it cut off again after taking only say 8 Ah.

Then I hooked the battery up with the 16V leads of a power adapter of one of my prematurely dead HP printers, together with a shunt as the adapter would shut itself down over 3 Ampere charging.
The voltage stayed at 14.something while charging for many hours while the battery was overcharging, producing minute amounts of HHO from its 5 good cells. then quickly shot up approaching 16 Volt while the current fell. The last cell was finally brimmed.

That was some 4 months ago. The battery is performing well since.