External battery

[FishFinger]

I just removed a leaking battery from a 286 motherboard, but I'm not sure if I should be connecting the replacement to the external battery pins, or to the pads where I removed the original.

Testing the pins on the external battery connector shows <0.2V and no charging current. So I'm assuming the external battery connection is meant for a non-rechargeable battery, like a lithium cell.

Testing the pads where the battery was removed shows 4.8V and 35mA charging current, which is pretty much spot on for recharging the 3.6V NiMh replacement battery I have.

 

[Dwight Elvey]

Hi
The external batteries were usually AA alkaline batteries for
the 286 aged processors. If you wanted to, you could run
wires out from the NiCad connector and put a 3 cell NiCad
pack external as well. The packs used for remote phones
work well and are available at many place ( RS and Office
Depot come to mind ). .
Dwight

 

[Chuck(G)]

I've seen two sorts of off-board setups for these--one is the rectangular NiCd packs usually made by Tadrian with a 4-position connector on the end; the other, commonly used on generic Far East boards, was 4 AA cells in a holder with leads terminated in the same sort of connector.

Be careful not to confuse one with the other. Alkaline cells do not take kindly to being charged. However, you can still use 3 AA alkaline cells with your setup if you include an inexpensive silicon diode in one of the battery leads to block charging current. The diode drops about 0.7 volts, so you'd have about 3.6-3.8 volts with 3 AA cells. The battery thus formed is good for its shelf life, as the draw from the RTC chip is minuscule.

 

[FishFinger]

Following the circuit traces there's already a diode on the board to drop the voltage, and prevent the battery from being charged, so it would seem that it's designed for an alkaline battery pack.

 

[antiquekid3]

Following the circuit traces there's already a diode on the board to drop the voltage, and prevent the battery from being charged, so it would seem that it's designed for an alkaline battery pack.

I'm not sure about the actual circuit you're referring to (I don't know a lot about 286s), but be careful that it's not a Zener diode. Those may look like ordinary diodes, but in reality, those are used to regulate voltage. However, if it's in series and not across the battery, I would presume it's a standard 1N4001 or something. Just a thought, though.

Kyle

 

[FishFinger]

It's just a regular diode. I'd automatically assumed that the external battery would be a rechargeable one just like the on-board battery I removed (a little 3.6V NiMH). So I was wondering why there was no current going to the external port.

It never occurred to me that it might take a normal non-rechargeable battery.