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Replacing CMOS battery with a different size in a laptop?

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pcm2a

Experienced Member
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Jun 24, 2013
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190
CMOS batteries are dead in every vintage laptop I have...obviously. Today I pulled the battery from an IBM and it had a wire going to the positive and negative side. The battery was a rechargable VL2020, 3V 20mAh. I have a big pack of CR2025 3V 160mAh batteries (not rechargable). I slapped one in it's place but it doesn't seem to have worked. Since both are 3V should it matter that one is a 2020 and one is a 2025? Maybe the IBM knows this battery is not rechargable and wont work with it?

Just read online that it's a horrible idea to have replaced a rechargable with a non-rechargable unless you can disable the recharge feature. I'll check the bios then take it back out i guess.
 
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Update:
It seemed impossible to me that this battery wouldn't work, even if it isn't recommended. I opened up a new CR2025, I have a pack of 30 from amazon for 3D tv glasses. I put this one in place and it seems to be working! I set the date, unplugged for a minute, plugged in and the date is set. I have it unplugged for an hour long test. If it works the question is what are the negative side effects of the laptop trying to charge that battery when I am using it? It's not like I use a vintage laptop for long periods of time.
 
Trying to recharge a lithium primary (non-rechargeable) can be very dangerous. You can use a diode or MOSFET in series with the battery to block the recharge current. If you use a diode, get a Schottky rectifier; the voltage drop is much lower than a conventional silicon diode. A MOSFET is better yet, as its voltage drop is extremely low.
 
It would be easier to get a $10 VL2020 than to try to fit a big blocking diode into the tiny space in the laptop. The diode might even be about the same price.

Should I pull the resume NiMH batteries while I'm in there? I have two laptops open and both have a shrink wrapped set of NiMH batteries that I believe are for resume features. I'm sure they are long since dead. Pulling them out might be safer than waiting for them to leak?
 
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Your poison, but if you want to go small with a diode, consider SMT. Doesn't get too much smaller than that. But they also come in the usual 1N914 tiny-glass leaded for very little. You only need one good for a few milliamps.
 
Looks like only $6-7 for a VL2020 with free shipping, I think I can hold out a few days and slap that in there :)

While I was in there I unplugged the NiMH battery pack so maybe it wont leave all over one day. I also did that in a Nec 4050C today while replacing the CR2430 with a CR2025.

I'm not sure where it is on my Compaq LTE Elites to replace.
 
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