Problems with 3 AA batteries CMOS battery replacement.

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Problems with 3 AA batteries CMOS battery replacement.

Postby Rob8 » 2019-5-15 @ 11:41

Has any one replaced the rechargeable battery on a 486 motherboard with the triple AA battery holder?

I have, and run into the following problem.

After about 2 months, on off usage of the computer I get a CMOS battery error on boot.
Testing with my multimeter showed 0 volts across the pack. Removed the batteries and tested individually.
1.xx volts on 1
0.77 volts on the next,
and
-1.xx volts on the last. It had reverse polarity.

My only guess is that perhaps the charging current is messing with the pack or something. So I have bought some Diodes to stop the reverse charge current.

Anyone had similar problems?

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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby HanJammer » 2019-5-15 @ 12:39

Never connect triple AA battery pack into anything else than 'external battery' connector. Ideally use reachargable batteries if you are not sure if the external connector is not charging batteries when the power is on (you can also verify it with multimeter). I've seen some motherboards that had this connector routed through the jumper you would have to manually set depending on the type of battery used (internal/external), and some without jumper - then again, there could be a diode preventing the charging hooked up to this connector, but I wouldn't bet my life on it (rechargable external battery packs were used back than as well as regular non-rechargable batteries).
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby jesolo » 2019-5-15 @ 13:38

I'm not aware of any 486 motherboard where the external battery connector (header) is re-chargeable.
They are usually always non-chargeable connectors which means that you need to use standard AA (non-chargeable) batteries. You can use re-chargeable batteries but, even when not in use, these will usually discharge much faster than standard batteries.
As HanJammer mentioned, there is usually a diode connected on the positive line, which will also slightly decrease the voltage (hence, why you usually require slightly more voltage than what the original on-board battery was, when connecting an external battery pack).

I do, however, have a 386SX motherboard that contains both a chargeable and non-chargeable external battery connector.
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby appiah4 » 2019-5-15 @ 14:06

2 months on a recharge circuit? You should consider yourself lucky it didn't catch fire at some point..
A500:Rev6|+512K|ACA500+|C1084S
i386:Am386SX25|4M|GD5402|ES688|MuntPi3
i486:U5S33|8M|GD5428|YMF719|DB-S2
i586:P133|32M|T64+/MX2|V1|CT3980/32M
i686:K6-2/400|128M|V2/SLI|CT4520/32M
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby HanJammer » 2019-5-15 @ 14:51

appiah4 wrote:2 months on a recharge circuit? You should consider yourself lucky it didn't catch fire at some point..


Naah... it will dammage the battery (and possibly the charging circuit), eventually cause bad leak but it won't catch fire - charging current is very low.
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby jesolo » 2019-5-15 @ 17:52

HanJammer wrote:
appiah4 wrote:2 months on a recharge circuit? You should consider yourself lucky it didn't catch fire at some point..


Naah... it will dammage the battery (and possibly the charging circuit), eventually cause bad leak but it won't catch fire - charging current is very low.

I accidentally connected my battery pack to the previously mentioned 386SX motherboard's chargeable circuit and after a couple of hours, those batteries became hot, very hot.
I'm just glad I caught it in time.
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby Rob8 » 2019-5-16 @ 12:09

Thanks guys for the advice, will make sure it has a diode to prevent charging.


appiah4 wrote:2 months on a recharge circuit? You should consider yourself lucky it didn't catch fire at some point..


It was turned off for most of the 2 months. Only on when I was playing with old games or software, and that really wasn't much.


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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby Baoran » 2019-5-16 @ 15:14

jesolo wrote:I'm not aware of any 486 motherboard where the external battery connector (header) is re-chargeable.
They are usually always non-chargeable connectors which means that you need to use standard AA (non-chargeable) batteries. You can use re-chargeable batteries but, even when not in use, these will usually discharge much faster than standard batteries.
As HanJammer mentioned, there is usually a diode connected on the positive line, which will also slightly decrease the voltage (hence, why you usually require slightly more voltage than what the original on-board battery was, when connecting an external battery pack).

I do, however, have a 386SX motherboard that contains both a chargeable and non-chargeable external battery connector.


I have come across multiple 486 motherboards that charge anything connected to external battery connector with 5 volts.
One that I have doesn't have any battery on the motherboard and just external battery header that has 5 volts when checked using multimeter.
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby HanJammer » 2019-5-16 @ 16:00

Exactly. These connectors never were standardized. I've seen some that charge connected battery pack, some that don't. Some were just hardwired to 'internal' battery circuit, some had separate circuit that had to be switched by hand.
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Re: Problems with 3 AA batteries CMOS battery replacement.

Postby retardware » 2019-5-16 @ 22:18

Yes, there is no "standard".
In my experience usually it works best (or at all) when you take the same kind of battery that is mounted on mobo.

In case of these three NiCd cells blue barrel, use container with three cheapo AA or AAA NiMH cells.
It's also a good way to dispose of AA/AAA rechargeable cells that are beginning to get weak. Their remaining capacity will be sufficient for maybe one more decade of operation.
(Btw, in my experience, customers do not notice and happily pay the new cells they are being charged for ;) )

In case of 3V Lithium button battery, use container with two cheap AA alkaline batteries.
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