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First post, by Luke4838P

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Hello.
As the title says, i've got my hands on a old 486DX4-100 Mhz PC that is marked "EXP 8049-REV 1.1A"
Fortunately the barrel battery hasn't damaged anything (even the tracks around the battery are fine) despite leaking.
The battery had 0.5v when measured but still kept track of settings and time.
I cleaned it the very little amount of green (mostly around the screw and the screw seal seems to have stopped that filth) and removed that cursed barrel battery, i wanted to replace that battery with a CR-2032 cell.
I have no idea what i need to cut as i've got no schematics of the board.
I have only found the EXP 8049 but clearly isn't mine as the one online says that it has a PS\2 mouse but my motherboard has no ps\2 mouse predisposition (at least not to solder it).

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Reply 2 of 3, by shamino

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Those pins above the battery provide a connection for a battery, but I don't know if the board would charge the battery through that connector. There might be a standard convention for that, I just don't know.
You could try probing with a meter on those pins to see if voltage is present when power is on but no battery present. If there's voltage present then it's trying to charge so you'd need to disable the charging before using a CR2032.

To disable charging, I think that diode D4 is the key but I haven't done any of these mods and you might need to probe around to make sure what the diode is connected to.
I *think* (don't blindly trust me) that removing D4 would both prevent the system from charging the battery and also prevent the system from powering the NVSRAM - so with that diode removed the board probably won't boot without a good battery installed.
If you defeat the charging behavior then you could also solder the battery/socket to the old pads that the rechargeable barrel battery used.

Reply 3 of 3, by Luke4838P

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shamino wrote on 2022-04-08, 21:16:
Those pins above the battery provide a connection for a battery, but I don't know if the board would charge the battery through […]
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Those pins above the battery provide a connection for a battery, but I don't know if the board would charge the battery through that connector. There might be a standard convention for that, I just don't know.
You could try probing with a meter on those pins to see if voltage is present when power is on but no battery present. If there's voltage present then it's trying to charge so you'd need to disable the charging before using a CR2032.

To disable charging, I think that diode D4 is the key but I haven't done any of these mods and you might need to probe around to make sure what the diode is connected to.
I *think* (don't blindly trust me) that removing D4 would both prevent the system from charging the battery and also prevent the system from powering the NVSRAM - so with that diode removed the board probably won't boot without a good battery installed.
If you defeat the charging behavior then you could also solder the battery/socket to the old pads that the rechargeable barrel battery used.

After looking around thanks to davidrg ,i found out on another schematic (DATAEXPERT EXP 8049 but i think it should be the same as it shows the EXT_BAT connector) that the pins above to where the barrel battery used to be (removed, good riddance) , that it is indeed supposed to be a connector for a non-rechargeable, external battery.
The pins that are supposed to be used for the external battery are 1 and 4 as 1-2 are for CMOS clear, measuring with a meter confirmed that 1 is ground and 4 is power.
Temporarly i could hook up a cr 2032 holder and two headers to the battery pins to save the BIOS settings/clock.
Thing is, i wanted to make a more elegant, long term solution by removing the charging circuit and making the on-board battery like my Pentium 133 that had a nice CR-2032 slot on the motherboard and it's still beatiful.

You say that i need to remove D4 to disable the charging circuit?
I've watched a video that shows how to remove a barrel battery but it says that depends on how the charging circuit is like that you need to either cut the trace, place a diode or remove the diode.
If i had the electric diagram i could have someone that is more capable tell me what to remove but i've not found the EXP 8049 electrical schematics.
I've found online an image of this board (I can see clearly the "Best performance with AM486 Micro processors" and "EXP 8049 REV 1.1A") with a CR-2032 coin battery on it.
I can't see if D4 is missing but if it is, it might be the key for this mystery.
EDIT: I found that another user on VGONS has placed the CR 2032 "mod".
If he can kindly tell me how he did it it will be much appreciated.

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