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View Full Version : A Tip for replacing a leaking nicad on a 286/386



David_M
December 19th, 2016, 01:55 AM
Some months ago I replaced a leaking nicad battery on a 286 motherboard with a cr2032 battery and holder (the battery holder fits the old holes perfectly).
I cut the positive power track and inserted a small signal diode to prevent charging the non rechargeable battery. It worked for a time but the voltage was borderline for the CMOS/RTC chip due to the .6v drop across the diode.
I Also found the battery very quickly goes under voltage so the fix leave a lot to be desired.

Just recently picked up some schottky fast recovery diodes in a pack on ebay for fixing power supplys. The FR107's in the pack can't handle more than an amp so are no good for power supplies.... aside from fast switching they have another very desirable attribute, they have a very low forward voltage drop.
So today I thought I'd replace the signal diode I used on the 286 board with one of the FR107's. Measuring the forward voltage drop it is now only .09 of a volt... far better than the .6v of the general purpose silicone diode I had been using.

So my tip is if you are going to replace a nicad with a CR2032 battery on an old board use a FR107 and not a general purpose diode.

alan8086
December 21st, 2016, 11:57 AM
That's a good tip - thanks! I'm about to do the same on a Compaq portable III and was wondering the same thing.

David_M
December 21st, 2016, 03:16 PM
I just noticed yesterday that replacement nicad and NiMH batteries have been showing up cheap on ebay again. When I did the original replacement I didn't find any cheap replacement options.
At the time rechargeable LR2032 batteries were not overly expensive but I elected not to use one because if I ever sold the board somebody might replace it with a normal battery.

I also recently replaced one of those Dallas RTC chips that have the built in lithium battery, I desoldered it and put a socket in the board and bought a new chip off ebay. I was expecting new old stock, but I was pleasantly surprised to see the new DS12887 has a 2016 date code.

http://www.ebay.com.au/itm/1-x-Ni-MH-80mAh-3-6V-button-Rechargeable-Battery-w-tab-/180675592054?hash=item2a111abf76:m:mDO_xX-8PzdI0U-mBt7vH3g

RuudB
January 3rd, 2017, 05:54 AM
So my tip is if you are going to replace a nicad with a CR2032 battery on an old board use a FR107 and not a general purpose diode.
I had to replace some nicads as well but I didn't solder them directly on the motherboard. I placed the nicads on an empty space in the case using some double sided tape and connected them to the board using two wires. Same functionality but not the problem in case this nicad starts to leak as well.