I think putting an diode on the positive pin should do the trick. Just make sure to put it correctly

kotel wrote on 2025-03-06, 10:17:

I think putting an diode on the positive pin should do the trick. Just make sure to put it correctly

A diode is also my first though .... but ....

Do we know what voltage the "barrel battery" is (I wasn't able to identify it in the linked photo)?

A diode will typically have about a 0.7v drop in the forward direction,
a CR2032 is nominally 3v which is enough for most RTC/CMOS backup
(many boards use a CR2932)

... 3v-0.7v = 2.3v - a CR2032 on it's own is considered effectively dead at this voltage.
Will the RTC/CMOS retain content reliably at 2.3?

In some cases where I've run into this with other equipment, I've used two
CR2032s (6v) and 4 diodes (~2.8v) for ~3.2v which has worked well!

Another option is an LIR2032 (3.6v) - even though this is a "rechargable" cell,
I'd still use a diode as the NiCad recharging circuit is unlikely to be suitable.
3.6-.7 = 2.9 (and you have the added advantage that when it does go flat
every few years, you can pull it out and recharge it!)

On a related note: I've got some Logitech "Solar keyboards" - they have solar cells at
the top which charges an internal 3V ML2032 - these eventually go dead and are
fairly hard to find and pricey - some replace them with CR2032s and tape over the
solar cells - which looks terrible - I did some testing with a SONY CR2032 and a 4.5
digit DVM with a 2ma scale - meaning I would see currents of 0.0001 ma
- never saw any "reverse current" with the CR2032 - even in bright sunlight.
So I *didn't* tape over the cells and have been using them for a few years, changing
the CR2032 as it needs (not very often) and have observed no problems.

(But don't use a CR2032 with out reverse protection in this case - the NiCad charge
circuit is unknown and likely to cause bad problems)

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Usually there's a 100...220ohm range resistor near the diodes around the area where the battery is. It is the trickle charge resistor for the NiMH cell, removing that resistor is usually enough to prevent charging from happening. Sometimes it is also possible to bypass a diode to reduce voltage drop from the battery, as mentionud by Dave, extra drop on a 3V cell isn't all that good to have although luckily that drop won't be 0.7V but something near 0.4V since the current through one is very low. But still the less drop the better. RTC itself may stop counting much sooner than the SRAM losing its content, you'll eventually see time going more and more wrong as the voltage drops but at least the settings remain intact.

Tiido wrote on 2025-03-06, 14:33:

... luckily that drop won't be 0.7V but something near 0.4V since the current through one is very low. ...

I agree that in many cases very low current = less effective voltage drop - but I've found
you can't rely on it -- varieties of diodes differ greatly! (and it can change with temp and
other environmental factors)

And yes - that the primary reason I show ~v . ~ meaning "somewhere around".
I generally don't do anything like this without taking a LOT of measurements to see
exactly what the result will be.

And yes, if you can ID the trickle charge resistor - pulling that is the best
way to avoid reverse charge- but I'm leery to suggest that to someone I
don't know when I'm not able to help - no idea what their rework capability
is (and I've seen the results of people trying to do "something
simple" who didn't have skills)

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

They do differ a lot, but the ones I have seen used for this task on these boards are pretty much always around 0.4V range. Sometimes I end up replacing the diodes for something modern with much lower voltage drop too but in the end the limiting factor is what is getting powered. MC146818 or a compatible will suck any battery dry in months regardless of what diode or battery one uses 🤣.

But yes, what I mentioned does require some knowledge and preferably tools which is why I didn't try to go into any specifics. If one understood the "clues" that one will find their way around the problem on their own.

Thanks for the suggestions so far.

I'm pretty sure the old battery was 3V.

I am comfortable working with a soldering iron and would have no issues with removing a resistor.

I need to find the optimal position for the CR2023 battery holder and event the legs to reach the the solder point for the old battery or alternative points where the connection from the battery will power the CMOS chip.

I'm open to all options. The easier, the better of course.

Here are some close-up pics of that section of the board. If any more pics are needed of the whole board or other sections of the board, I can take more.

I know those traces don't look nice but I've tested them and they have continuity. The board also seems to work fine on initial inspection by booting to the bios and testing the keyboard.

Here was my own thread about this:
Better understanding barrel battery charging circuit

If you have an EXT BATT connector and use it as designed, you shouldn't have to modify the board. However, 3V will likely not be enough for those pins; it was designed for one of those 4.5V Rayovac computer clock batteries, and you can substitute a 3xAA holder. Part number BH3AA-211-ND at DigiKey is a ready-made solution with the proper connector on it.

jakethompson1 wrote on 2025-03-06, 17:26:

Here was my own thread about this:
Better understanding barrel battery charging circuit

If you have an EXT BATT connector and use it as designed, you shouldn't have to modify the board. However, 3V will likely not be enough for those pins; it was designed for one of those 4.5V Rayovac computer clock batteries, and you can substitute a 3xAA holder. Part number BH3AA-211-ND at DigiKey is a ready-made solution with the proper connector on it.

Thanks for the link and the advice.
My board has markings for an external battery. It is those pins you can see in the images that I posted, next to the diodes.

As for resistors mentioned in an earlier comment, I can only see two in the surrounding area, labelled R29 and R32.

byte_76 wrote on 2025-03-06, 18:24:

Thanks for the link and the advice. My board has markings for an external battery. It is those pins you can see in the images th […]
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jakethompson1 wrote on 2025-03-06, 17:26:

Here was my own thread about this:
Better understanding barrel battery charging circuit

If you have an EXT BATT connector and use it as designed, you shouldn't have to modify the board. However, 3V will likely not be enough for those pins; it was designed for one of those 4.5V Rayovac computer clock batteries, and you can substitute a 3xAA holder. Part number BH3AA-211-ND at DigiKey is a ready-made solution with the proper connector on it.

Thanks for the link and the advice.
My board has markings for an external battery. It is those pins you can see in the images that I posted, next to the diodes.

As for resistors mentioned in an earlier comment, I can only see two in the surrounding area, labelled R29 and R32.

From my understanding of that particular circuit, R32 feeds directly into the positive terminal of the battery and that in turn is fed by D10 which is the only diode I can see that goes 5v (? guessing) -> D10 -> R32 -> RTC-NiMH+

See if there's 5v on the input of the D10 diode and if there is, snip that and the RTC battery positive terminal solder point should now be ~0v rather than ~4.5v. The nice aspect of this is that if you snip the leg of D10 and bend it out of the way, you can test whether it has the desired effect and if not you can easily put the diode leg back in place and easily resolder it.

Once the charging circuit is disabled, you can safely put a coin cell holder where the NiMH RTC battery originally soldered in.

Thermalwrong wrote on 2025-03-07, 23:31:

From my understanding of that particular circuit, R32 feeds directly into the positive terminal of the battery and that in turn […]
Show full quote

byte_76 wrote on 2025-03-06, 18:24:

Thanks for the link and the advice. My board has markings for an external battery. It is those pins you can see in the images th […]
Show full quote

jakethompson1 wrote on 2025-03-06, 17:26:

Here was my own thread about this:
Better understanding barrel battery charging circuit

If you have an EXT BATT connector and use it as designed, you shouldn't have to modify the board. However, 3V will likely not be enough for those pins; it was designed for one of those 4.5V Rayovac computer clock batteries, and you can substitute a 3xAA holder. Part number BH3AA-211-ND at DigiKey is a ready-made solution with the proper connector on it.

Thanks for the link and the advice.
My board has markings for an external battery. It is those pins you can see in the images that I posted, next to the diodes.

As for resistors mentioned in an earlier comment, I can only see two in the surrounding area, labelled R29 and R32.

From my understanding of that particular circuit, R32 feeds directly into the positive terminal of the battery and that in turn is fed by D10 which is the only diode I can see that goes 5v (? guessing) -> D10 -> R32 -> RTC-NiMH+

See if there's 5v on the input of the D10 diode and if there is, snip that and the RTC battery positive terminal solder point should now be ~0v rather than ~4.5v. The nice aspect of this is that if you snip the leg of D10 and bend it out of the way, you can test whether it has the desired effect and if not you can easily put the diode leg back in place and easily resolder it.

Once the charging circuit is disabled, you can safely put a coin cell holder where the NiMH RTC battery originally soldered in.

Thank you for this feedback, this is exactly what I needed to know.

You are absolutely correct, there is 4.727V measured on the side of R32 that goes into the via and there is direct continuity from there to the positive battery solder point.

I might just remove R32 and D10 for the cleanest result.

I'll post a pic again when it's done.

The job is done.
Tested and working. Thank you for help.

You can buy on sites like amiga store adapter for 2032 with diodes on it. It costs around 5€, and it work perfect!