CMOS battery life

[ant3202]

Re: CMOS battery life

depends on the quality on the batteries, some bads one, ard 6 mths, shd be at least 3yrs

[Eagle2a]

Re: CMOS battery life

Thought about posting a new thread in the laptops forum but decided to post here since it deals with the same topic. Had to replace the motherboard in my Toshiba Satellite 1735 laptop recently and found that the new motherboard wouldn't retain the CMOS data. I was expecting to see a standard coin battery but it turns out that the CMOS/RTC battery is soldered to the motherboard, and web searches seem to show that it's a rechargeable lithium-ion battery. Apparently it's no longer holding a charge if that's the case. I initially thought about desoldering it and replacing it with a standard coin cell/holder but then found out about it probably being rechargeable. I don't think that's a good idea with the motherboard circuitry trying to charge the battery. Also it's been quite difficult locating a replacement battery. Thanks for any advice and suggestions. Attached is a photo of the battery.

[PCBONEZ]

Re: CMOS battery life

A battery part number or at least the voltage would help.
.
I think if you look you can find a rechargables that will fit in holders.
.
You can also solder wires or pins and stick a holder somewhere else in the chassis.
.

[PCBONEZ]

Re: CMOS battery life

Look here:
http://www.powerstream.com/licoin.htm

[Agent24]

Re: CMOS battery life

Is there a jumper anywhere to choose between standard and rechargeable?

I ask because an old Socket 3 (!) board I have used an onboard soldered in NiCd but it had failed (of course) - but there was provision on the board for a normal battery to connect via a pin header (those kind which are a 3v lithium AA-size battery inside a black plastic case with a lead and plug)

I installed the header, changed the jumper, new working battery and the board doesn't try to recharge it.

[PCBONEZ]

Re: CMOS battery life

My very first tinker toy PC was an old Zenith 286.
Had a dead leaking NiCd.
I got a holder for some standard AA's and added a diode in the wire so the charger couldn't charge them.
The AA's lasted a couple years until I modded the case to hold a mind-numbingly fast 486-DX 100.
.

[Scenic]

Re: CMOS battery life

@ eagle2a: let the laptop sit for a while with the charger plugged in.
took a couple hours for the CMOS battery (also rechargable) in one of my old toshi's to take a charge after it was left unused for ages..

it kinda seems to tickle-charge it if it "thinks" there's no battery or it's dead, so it'll take quite some time till it reaches the voltage threshold where the laptop actually starts charging it normally..

[PCBONEZ]

Re: CMOS battery life

That's true.
CMOS battery charge circuits are not fast chargers at all.
They are intended only to keep the battery topped off.
Restoring dead ones is not in their job description.
Eventually they will do it if the battery isn't shot but it could take a day or more before you've actually got a full charge.
.

[Eagle2a]

Re: CMOS battery life

Thanks for all the excellent advice! I'll try it and see how it goes.

@Agent24 and PCBONEZ:
Now that you mention it, I think my very first PC (a blazingly fast 80286 that I later overclocked from 6 to 8 mHz as I recall) had some sort of AA-size battery pack for the CMOS battery. I totally forgot about that overclocking kit. It had a round clicky knob that allowed you to dial up the clock speed in 2 mHz increments. I think I tried to get 10 or 12 mHz but the system totally crashed.

[Eagle2a]

Re: CMOS battery life

@Agent24:
No, there isn't a jumper on the board. Sorry I forgot to answer your question.

And it does appear that the CMOS/RTC battery is finally starting to hold a charge. I'm not sure how much and how long it'll last but I'm not having to reenter the CMOS data every bootup like before. I'll have to experiment with having the main battery in or out to see if it has any effect, as well as how long the CMOS/RTC battery is keeping its charge. Thanks again for the replies and advice!

[Agent24]

Re: CMOS battery life

Yeah in a laptop I guess it's unlikely they would design it to have that option, although jumpers are not always pins with a plastic cap, sometimes they are wire links soldered in or even just pads bridged with solder. (I guess you probably know that, but just thought I'd say it)

Anyway, a replacement should be easy, just find something that's the same voltage (I would guess 3v like all BIOS batteries I've seen) and around the same size which should give you around the same capacity.

[japlytic]

Re: CMOS battery life

Typically, coin cells last for around 3 to 5 years. Anything shorter, this could mean you have a serious problem, especially if you are using a quality battery.
I've seen one last for about 7 to 8 years.

[Eagle2a]

Re: CMOS battery life

Thanks again for all the feedback and replies. After over a week of observation, it appears that the CMOS/RTC battery is still not charging but the CMOS data is generally retained if the main laptop battery is present. The CMOS data is lost immediately if the main battery is removed (without external DC power) or if the external DC power is unplugged without the main battery present. I haven't decided whether it's worth all the time and work required to completely disassemble the system again to replace the CMOS/RTC battery. I was running the laptop on external DC power without the main battery present but I may just keep an old battery inserted now to retain the CMOS data.

[goodpsusearch]

Re: CMOS battery life

https://www.badcaps.net/forum/showpo...&postcount=135

~1997 board. ~14 years old. The battery reads 2.72V. Is it too low to use on another motherboard?

[brethin]

Re: CMOS battery life

https://www.badcaps.net/forum/showpo...&postcount=135

~1997 board. ~14 years old. The battery reads 2.72V. Is it too low to use on another motherboard?

Depends on the motherboard, some have issues posting when the battery is that low and some will post with no battery at all.

[Agent24]

Re: CMOS battery life

I haven't decided whether it's worth all the time and work required to completely disassemble the system again to replace the CMOS/RTC battery.

Personally I would replace it, even if just to prevent it from leaking.

[Longbow]

Re: CMOS battery life

Coin cells are not rechargeable. However, some backup circuits do use rechargeable cells. Obviously either one has a finite lifetime. Shorting battery terminals, especially on alkaline or lithium coin cells is an invitation for an explosion and should never be done. Often on MB, there is a separate terminal that allows the bios to be reset without removing (or shorting out) the battery. On coin cells, it is irrelevant what the voltage reading is when powered on. Both types must be measured with power removed and should hold their voltage. One interesting by-product of a failing backup battery or a failing supercap is erratic and faulty circuit operation. In other words, the correct backup voltage is usually necessary for correct circuit operation as well as for holding memory when the circuit is off.

[PCBONEZ]

Re: CMOS battery life

Coin cells are not rechargeable.

Not always true.
The the link in post #24.

[larrymoencurly]

Re: CMOS battery life

I've been averaging 3 years from nonrechargeable lithium cells, but I expect the one on my BioStar G41-M7 to wear out faster because it's right next to the north bridge chip and conducts a lot of heat through at least one of the contacts.

About a month ago, I checked a 486 mobo from the 1990s with a Dallas clock chip on it, and it was still keeping good time, after being adjusted a few years ago.