VOGONS


First post, by timw4mail

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I have a motherboard with a barrel-shaped rechargable CMOS battery. The board has the four-pin external battery connector, and I'd like to use that and snip off the barrel battery. I'm wondering if there are pre-made CR2032 external battery holders, or if I would have to make one myself.

Reply 1 of 15, by myrsnipe

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I had a 486 DX2 pickup yesterday with battery damage. Ordered https://a.aliexpress.com/_mMwWmZo to use with the external connector so I don't have to mess with the charging circuit. Note this is a 6V version, check with your motherboard what voltage you need over the external connector. (Note this is a 5lot, but you should be able to find them single on Ali as well)

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Reply 3 of 15, by jakethompson1

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Before you do this, I would warn you that I made some myself, and the CR2032 is sufficient to run the clock on a PCI Pentium board I have, but on the 386 and 486 boards, it only maintains settings in CMOS overnight (so it's definitely connected right) and the clock doesn't advance.

Reply 6 of 15, by timw4mail

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jakethompson1 wrote on 2022-07-11, 15:19:

Before you do this, I would warn you that I made some myself, and the CR2032 is sufficient to run the clock on a PCI Pentium board I have, but on the 386 and 486 boards, it only maintains settings in CMOS overnight (so it's definitely connected right) and the clock doesn't advance.

The existing battery seems to only properly keep CMOS settings anyway. I did see a thread about a little PCB to adapt a CR2032, but have to get one printed, and sourcing the parts is not my idea of fun (although, the parts list is like 5 items, to be fair).

Reply 7 of 15, by jakethompson1

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timw4mail wrote on 2022-07-11, 17:07:
jakethompson1 wrote on 2022-07-11, 15:19:

Before you do this, I would warn you that I made some myself, and the CR2032 is sufficient to run the clock on a PCI Pentium board I have, but on the 386 and 486 boards, it only maintains settings in CMOS overnight (so it's definitely connected right) and the clock doesn't advance.

The existing battery seems to only properly keep CMOS settings anyway. I did see a thread about a little PCB to adapt a CR2032, but have to get one printed, and sourcing the parts is not my idea of fun (although, the parts list is like 5 items, to be fair).

I use 3xAA battery holders which work fine and run the clock on those boards.

Reply 8 of 15, by konc

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timw4mail wrote on 2022-07-11, 17:07:

I did see a thread about a little PCB to adapt a CR2032, but have to get one printed, and sourcing the parts is not my idea of fun (although, the parts list is like 5 items, to be fair).

There are also places that sell all the components together, in case you are considering this option. For example https://store.ribit.se/egna-produkter/varta-killer-v3-0.html

Reply 10 of 15, by Jo22

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myrsnipe wrote on 2022-07-12, 14:06:

The use for the resistor in the kit above is to drop voltage to expected output? Seems like it's meant for Amiga computers

Um, if it's in series, maybe it's intended a as a current limiting.

If it was installed against ground, it might be a voltage divider of some sort.
However, the amount of current wasted doing so wouldn't be trivial.
A button cell would quickly be depleted that way, I suppose.

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Reply 11 of 15, by TheMobRules

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myrsnipe wrote on 2022-07-12, 14:06:

The use for the resistor in the kit above is to drop voltage to expected output? Seems like it's meant for Amiga computers

A resistor in series with the diode is usually suggested in the coin cell datasheets to limit the current so that in case the diode fails short the current flowing to the battery does not go over acceptable limits. An alternative also mentioned in datasheets is to use 2 diodes in series instead of one, some motherboards do this for the external battery connector.

Reply 12 of 15, by keenerb

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https://www.amazon.com/Alinan-Button-Battery- … r/dp/B09KTVG1Y5

I bought some of these to replace old mobo batteries with. They work very well. You'll need to have a diode in-line if you replace the actual battery with it. You can use LIR2032 LION batteries if you absolutely need 3.6v, but 3v from CR2032 have been enough for almost all my systems.

Reply 13 of 15, by jakethompson1

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TheMobRules wrote on 2022-07-12, 17:48:
myrsnipe wrote on 2022-07-12, 14:06:

The use for the resistor in the kit above is to drop voltage to expected output? Seems like it's meant for Amiga computers

A resistor in series with the diode is usually suggested in the coin cell datasheets to limit the current so that in case the diode fails short the current flowing to the battery does not go over acceptable limits. An alternative also mentioned in datasheets is to use 2 diodes in series instead of one, some motherboards do this for the external battery connector.

Would the two diodes vs. one diode be a factor in which boards can use a CR2032 as external battery as-is and which can't?

Reply 14 of 15, by TheMobRules

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jakethompson1 wrote on 2022-07-12, 19:08:

Would the two diodes vs. one diode be a factor in which boards can use a CR2032 as external battery as-is and which can't?

It mostly depends on the chip that handles the RTC/CMOS settings, some of them work just fine with little over 2V, so the voltage drop from the diodes does not affect them. Others require at least 3V, so those usually do not keep time properly and need a higher voltage between the battery terminals (usually 4.5V, or even 6V). Also not all types of diodes used have the same voltage drop, depending on the current flowing through them.

The strangest case I found is a 386 motherboard (ECS 386A) which uses a CHIPS 82C206 for RTC/CMOS. The board has no internal battery (good thing, since it means no leakage) and when I got it there was a 3.7V non-rechargeable lithium battery pack plugged in to the battery connector. It was dead, so I replaced it with 3xAA (4.5V)... which worked fine for a couple of weeks until the batteries died. Same with the replacements I installed. I decided to check with a multimeter and the 82C206 was drawing several mA of current!! So for some reason with that higher voltage the chip was working as if it was powered from the PSU instead of entering low power mode, which only draws a few microamps!

I then tried 2xAA or a CR2032 and it did indeed switch to low power mode with 3V. But it seems that voltage is not high enough, so even though the settings were kept the RTC didn't tick when the machine was off.

The definitive solution was to use a 1/2AA 3.7V lihtium battery (same voltage/type as the original), so I connected it using a holder. That one seems to hit the voltage sweet spot to work in low power mode while having enough voltage to keep the clock working. But in my experience 3.7V non-rechargeable lithiums are somewhat harder to find.

Reply 15 of 15, by wiretap

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For motherboards with a 3V or 6V external battery rating: https://github.com/wiretap-retro/EXT_BATT-3-6V-Version
For motherboards with a 3.6V-4.5V external battery rating: https://github.com/wiretap-retro/EXT_BATT-4.5V-Version

Both use common button cell batteries, and have charge circuit protection for the rare occurence a motherboard has charging on the V+ pin of the EXT_BATT header.

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