6V lead acid battery

[flippityflop]

i've got a 6V lead acid battery that i ripped off a portable CFL. i tried charging it with the old circuitry that it came with, but it won't get past the ~4V mark.

then i tried charging it from a wall wart of 12V  with a max current of 1A. it was exceeding 1.5A when charging the lead battery, so i used some a voltage divider from some spare high wattage resistor that i got to lower the charging to around 7V @ 800-900mA. it got the lead acid battery to 4.83V. but after that it plateaus, no matter if i leave it charging for extended periods of time.

i've never heard of lead acid batteries having memory effect, so i'm assuming that the battery is not in perfect condition. i have a welder that can supply 25VAC (@ 50A max, neither a constant voltage or current sourcew) which i can rectify and filter with a large capacitor... do you think using this 25VDC would bring it up to 6V that it originally were??

i'm not gonna leave it for too long at that though, i'll keep a eye on it.

[audioguru]

A lead-acid battery remembers that it has been killed by an over-discharge.
It should NEVER be discharged to less than about 5.5V for a 6V battery but some "deep discharge" batteries might survive a lower voltage.

Your battery might have its lead plates "sulphated" or the plates might have collapsed and are shorting one or two of its cells.