On some HP, Dell and Toshiba laptops the 7.2v are the main batteries AFAIK.

I am NOT talking about the main big battery that powers the entire system. I'm talking about the other battery connected to the motherboard through wires that looks similar to CMOS battery but it isn't (or is it?)

This is an interesting topic. I have Siemens Nixdorf PCD-2N and it also contains infamous Dallas chip, main batteries and additional acu, if I remember, 4,6V. I was also wondering what this acu is for and still haven't figure out the secret.

BlackVega wrote on 2023-05-27, 07:38:

What is that inner 7.2V battery supposed to do in most laptops?

Isn't that for powering RAM in sleep mode? I think the PSU in these early laptops was not efficient enough at low power draw to use the main battery and not drain it too quickly. But that's my educated guess, I'm not really into old laptops and every time I have one in my hands the battery packs are all dead (and usually leaking too) so I can't really test that theory.

Deunan wrote on 2023-05-27, 20:02:

BlackVega wrote on 2023-05-27, 07:38:

What is that inner 7.2V battery supposed to do in most laptops?

Isn't that for powering RAM in sleep mode? I think the PSU in these early laptops was not efficient enough at low power draw to use the main battery and not drain it too quickly. But that's my educated guess, I'm not really into old laptops and every time I have one in my hands the battery packs are all dead (and usually leaking too) so I can't really test that theory.

That's correct, at least for my old Toshiba Satellites. (In two of them all batteries including the main one still work surprisingly)

For some Dells, at least the XPi CD (Pentium [MMX]), CPiA (Pentium II), and C500/600 (Pentium III), it is the reserve battery used to hold the contents of NVRAM and power the RTC. It's supposed to last for 40 days from a full charge. They do not have a separate 3V "coin cell" battery like desktop motherboards.

Could a missing or bad 7.2v battery case a notebook to not function correctly? I have a toshiba pentium 1 with a bad (removed) 7.2v and main that refuses to POST after a restart.

Socket3 wrote on 2023-05-28, 00:46:

Could a missing or bad 7.2v battery case a notebook to not function correctly? I have a toshiba pentium 1 with a bad (removed) 7.2v and main that refuses to POST after a restart.

Depends on the notebook, I guess. I have a Dell XPi CD that works fine without it (it leaked and destroyed traces such that the battery connector on the board just came off). The BIOS does warn after POST that the settings have been reset but it boots and works fine after that.

Socket3 wrote on 2023-05-28, 00:46:

Could a missing or bad 7.2v battery case a notebook to not function correctly? I have a toshiba pentium 1 with a bad (removed) 7.2v and main that refuses to POST after a restart.

I removed both intenal batteries from one of my Toshiba Satellite 320 CDT and it has no problems with that

Deunan wrote on 2023-05-27, 20:02:

BlackVega wrote on 2023-05-27, 07:38:

What is that inner 7.2V battery supposed to do in most laptops?

Isn't that for powering RAM in sleep mode? I think the PSU in these early laptops was not efficient enough at low power draw to use the main battery and not drain it too quickly

Not sure if that's a thing and covering it up is a Nice Side Effect, but according to both Apple and IBM I've always seen it described as giving you 1/3 to 1 minute to change to another battery in standby mode without going through hibernation 😀

...How much were laptop batteries back then? Less than the late 2000s/today in absolute price and much less relative to the whole computer, I guess?

in the nec ultralite/versa series there was a bridge battery that i think allowed swapping batteries while running without ac power. i am the original owner of a versa p and opened it up (and a couple other versas i used to own) decades ago. i'm pretty sure i removed the cmos battery, but i wish i remembered if i did anything about the bridge battery. i'm worried that if i open it up again the case will completely disintegrate as just about everything snapped off the first time around and the plastic is 3 times older now.