First, the tLDR version because this WILL get long...
TLDR - I basically check the NiMH batteries in my 486 notebooks for shorts, zap em' with a disused Skil cordless drill battery charger to break up shorts and put some power on the cells, then try to "excercise" them back to health by running it off of battery for increasing lengths of time.
Long Version
Well...the longest yet is a generic replacement for the NEC OP-570-4701 "Smart Battery" used by my NEC Versa M/75 and P/75. Longest runtime I got out of it was about 2 hours. It's a much newer battery pack I believe, it's the same one pictured on some ofthose shady replacement battery sites that look like they are stuck in 1996 (hence me toying with buying from them). I'm expecting this battery is no more than 15 years old.
All of the batteries I'm talking about regarding NEC Versa are Nickel Metal Hydride 7.2vDC 3000-4000mAH batteries. The oldest one is a OP-570-4001 from a Ultralite Versa that crumbled and would not POST, which is 3400mAH, and then there's the 4000mAH Smart Battery whopper I moentioned above that gets almost 2 hours of life when properly charged, and cared for.
However, I've also been having some luck in bringing some REALLY old ones back to life....
I got about 30 minutes out of a 28 year old OP-570-4401 in my NEC Versa 40EC playing Sim City 2000. Of course, the cells eventually exploded - not once, TWICE. The second time it was outside of the case and I carried it outside with a pair of tongs in a wash cloth. My V/50 has another 30 year old one that's starting to "grow" in battery life the more I use it.
How I've been achieving this is a process I'm developing to bring the original batteries back to life and "exercise" them to a reasonable runtime. I'm using information taken from scientific websites regarding how NiMH and NiCAD Batteries work and age on a chemical level.
NiMH is the eco and user-friendly chemistry. They don't explode or catch on fire, the are less as harmful to the environment, and it seems, I have a knack for reviving them building up.
The two things that kill NiMH batteries is letting them sit dormant, not running them down all the way periodically, or letting them get hot.
The two main things I read that kills battery life is what is called a "Stalagtite Short" which is when the compounds inside the cells conglomerate into a conductive solid resembling a stalagtie from a cave, and short out giving the battery some continuity, and not to mention munching up storage space for energy. The other issue is the "fine-ness" or how finely ground the compounds ar einside the battery. As the battery is used and as it ages, the compounds become more "coarse" and thusly can't hold as much energy. I mentally visualize this as the finer particles allow more energy to be physically stored in a smaller space as there are more gaps for electrons to fit around. Not 100% scientifically correct, but this gives you an idea.
First thing I do is I check the battery for both voltage, and continuity. The Voltage on those NEC batteries should never exceed 9vdc, and tend to stop powering the laptop below 5vdc. As long as there is at least 1 volt, there's hope for a charge (and less likely to be shorted out). If I don't see a charge, then I check for shorts - as if it has a charge, any shorts won't be visible anyway as it will overdrive/kick in some protection on my VOM. If it does have a short....then I'll try to break it by over-voltaging the short.
The NEC Versa Ultralite/E/V/M/P models all trickle charge the batteries at about 4vdc from the power control board. So usually what I do is "whack the cells over the head" with an over-voltage to break the short. That's how I revived the 15 year old battery in my Versa M/P machines.
One method I do this is using a 2A 21v cordless drill charger, which I wired wires into to press onto the contact pads. Sometimes I can hear a very mild boil and sometimes even hear the short break. For more extreme cases, I use my car battery charger on "Fast charge" mode - which only puts out 12VDC but with enough current to really provide a nice smack. You want to make sure NOT to touch the Sense pin, and if it's a smart battery, remove the control module if possible (the NEC batteries let you do this via a snap-off door next to the contacts - it attaches with a 4 wire miniature molex connector). I usually know when the shorts are broken because if the voltage is low I no longer have continuity, and usually I end u p with at least around 3-5vdc after just a few minutes "charging" with the over-voltage, or setting it in the laptop.
I tend to find if I keep using the laptop, charging the battery, pulling the cable, and doing this on repeat, eventually the battery will start to hold a charge and start to run for longer lengths of time. However, when I do this, sometimes I'll try not to let the battery get too hot, it should get a good bit warm, but too hot will damage the cells in some way that I still have yet to learn.
Out of doing this, I haave 2 smart batteries that work, one capable of almost 2 hours if I keep it in shape, and one dumb battery in my Versa V/50 that's decided it want's to become the Centurinarian of laptop batteries for some reason.
I plan soon to transplant the guts of that Skil charger into a wooden board that has special spots on it that let me just drop the battery on there and charge it/break shorts/ or even short the entire battery out.
Also, I've started trying this on my DFI MediaBook and already I'm getting a charge to store on the original NiMH 9vdc 1800maH batteries as well. So hopefully I i might be able to put this to use on a lot of different third/4th/5th hand 486 NiMH laptops and make them "mobile" again, evenif it is just to get plugged in on the patio for some sunshine arcade time.
