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Barrel battery alternative ?

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Reply 20 of 32, by appiah4

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HanJammer wrote:
Intel486dx33 wrote:

Well the connector is like that for a NON-rechargeable battery

Are you sure?

Many motherboards will charge the battery hooked up to external battery connector. It was never standarized.

You can usually verify this by looking at the EXT BAT connector area, if it has a load of diodes/resistors most probably it does not recharge the EXT BAT.

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Reply 21 of 32, by brostenen

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gdjacobs wrote:
Worst case, exothermic reaction and fire. https://www.aiche.org/sites/default/files/doc … onBatteries.pdf https://www.researchga […]
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brostenen wrote:

What exactly are the risks? Through the years, I have used a rechargeable lithium on some of my boards. They work as such, and they never get warm. That said, I never turn on a vintage computer, more than some 2 hours.

Worst case, exothermic reaction and fire.
https://www.aiche.org/sites/default/files/doc … onBatteries.pdf
https://www.researchgate.net/publication/3219 … er_Overcharging

And if the battery does not even get hot after 2 hours of power on? I did a test on the first board I installed such a battery on. 2 hours, and still as cold as when I turned on the machine. That said, it was on a testbench and not inside a case.

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Reply 22 of 32, by SirNickity

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appiah4 wrote:

You can usually verify this by looking at the EXT BAT connector area, if it has a load of diodes/resistors most probably it does not recharge the EXT BAT.

That is not a safe assumption at all. There are two diodes on my 486 boards. They prevent back flow from the battery to the +5V rail, and ensure nothing goes backwards from the RTC circuit to the battery / +5V rail. It IS designed to charge the battery through a current-limiting resistor. You can't just look for components, you have to trace the circuit and see what they do.

brostenen wrote:

And if the battery does not even get hot after 2 hours of power on? I did a test on the first board I installed such a battery on. 2 hours, and still as cold as when I turned on the machine.

Motherboard charging circuits just prevent wild inrush from +5V to the battery through a resistor. They don't do any end-of-charge monitoring, don't have thermal fuses, etc. They are wholly unacceptable as Lithium cell chargers. I'm not as familiar with coin cell chemistry as Li-Ion cells, but with the latter, there's very little (basically none) self-discharge, so other than the tiny drain from the RTC, they will accumulate charge from every time you power on the system. Eventually they will rise above their nominally-charged voltage and keep going up. Again, with Li-Ion, that means breakdown of the insulation, an internal short, and then fire. With a coin cell, I'm not totally sure, but I wouldn't do it.

Reply 23 of 32, by retardware

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SirNickity wrote:

They are wholly unacceptable...

I agree.

Just do this equation:
3 alkaline cells in series minus diode drop = ideal CMOS chip voltage (like with fully charged NiCd cells)

It is cheap, safe and lasts about a decade or more. After that, easy refill.

Reply 24 of 32, by appiah4

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What is the simplest way to understand whether your BAT_EXT header tries to charge your batteries or not? Run the computer without batteries and check them with a voltmeter?

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Reply 25 of 32, by retardware

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appiah4 wrote:

What is the simplest way to understand whether your BAT_EXT header tries to charge your batteries or not? Run the computer without batteries and check them with a voltmeter?

I think so, too. But I won't care to put rechargeables into a computer that only runs little of the time.
Alkaline+diode=best solution.

Reply 26 of 32, by HanJammer

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appiah4 wrote:

What is the simplest way to understand whether your BAT_EXT header tries to charge your batteries or not? Run the computer without batteries and check them with a voltmeter?

Just measure voltage on the battery pins (without battery installed of course).

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Reply 27 of 32, by SirNickity

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The only really fool-proof way is to trace the circuit. It's usually not that hard.

Find the '-' side of the battery, make sure that's tied to Gnd by using a multi-meter and checking for continuity between the battery '-' pin and the PSU connector or a shield or the ring around one of the mounting holes. This should always be the case, but it doesn't hurt to make sure. It only takes a second.

Next, put your '+' probe on the +5V pin on the PSU connector. Put the '-' probe on the battery '+' connector. It may have a small resistance (30-200 ohms) or it may be a diode drop -- which your meter might not be able to read. If you don't get a reading that makes sense, follow the trace on the top and/or bottom of the PCB and see where it goes.

If you have to, just put your meter in continuity mode and brush one probe over the pins of components in the area until you get a beep. This is useful for small SMD parts or when traces go under other things. It shouldn't take long to map out all the parts that connect to each other, since there's not much to these circuits.

Make a drawing as you go, and sketch out what connects to what. When you get to the trace that leads off into the weeds, it probably goes to the crystal oscillator circuit and/or the chipset. You should be able to verify that it heads off toward something that looks like a watch crystal or the main guts of the motherboard. That ought to be enough to assume that you've found the CMOS / RTC power line.

Here's an example from mine:

file.php?id=64525

(The gray part is original, the red is my modification to make it safe for an alkaline coin cell.)

Reply 28 of 32, by adalbert

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Couldn't you just use ML2032 rechargeable lithium battery and a circuit like this? It should limit charging voltage to 3.1V with zener diode and limit the current with a resistor.
With desktop motherboards obviously it is easier just to install a large battery pack and replace it once in a while, but I'm thinking about a solution for laptops, which often use Ni-Mh batteries too. But they shouldn't be disassembled too often in order not to stress plastic latches. So having a rechargeable, but not prone to leakage battery would be nice.

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Reply 29 of 32, by Caluser2000

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I just install fully charged NiHi AA or AAA batteries to the EXT_BAT connector making a holder out stuff laying about. If it charges well and good. If it doesn't still all well and good. No need to over think these things at all...

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Reply 30 of 32, by adalbert

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So please tell me how to fit AA battery here 😁

It had very small NiMH cells which already had leaked. I don't want to install new NiMH cells because they are hard to replace, it requires disassembling entire computer (and they may fail and leak again after few years). So I'm looking for a more permanent fix here. I have few Siemens Nixdorf PCD-4ND laptops which all use ML2032 rechargable lithium cells by default, and some of them are still working after 25 years. And none had leaked so far.

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Reply 31 of 32, by Caluser2000

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adalbert wrote on 2020-12-18, 17:10:

So please tell me how to fit AA battery here 😁

It's easy enough to make or buy a small enclosure to fit to the rear of the system.

Last edited by Stiletto on 2020-12-23, 17:58. Edited 1 time in total.

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Apparently 32-bit is dead and nobody likes P4s.
Of course, as always, I'm open to correction...😉

Reply 32 of 32, by RandomStranger

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There are 1/2AA battery holders. I'm planning on using them. They should fit right in the place of the original battery and it makes removing/replacing them just as easy as with coins.

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