Amiga 500 battery replacement

[banedon]

Hi guys

I've been having a great time watching a few Dave teardown tuesday videos and was especially interested in the one for the Amiga 500.
Given that I have an Amiga 500 lurking around, I've decided to crack it open and have a look. It seems to be a later model (circa 1989/1990) board model 6A and has a 1MB - for a total of 1.5MB - (do I hear an "ooooooooohhh!" from the audience?) RAM upgrade board in it with a battery backed clock.

The battery for the clock is understandably shot (23/24 years can do that to a battery ) so I was thinking of replacing it. However, I cannot find a supplier in the UK that I trust who carries a 3.6v 50mAh NiCd battery, so I've hit on a plan to replace it with a near-equivalent one:
http://uk.farnell.com/varta/55608303059/battery-pcb-mount-3-6v/dp/279341

From what I understand (and this is what I need to check with you guys) the following is true:

1) The mAh is capacity of the battery and as long as it exceeds the older battery's 50mAh then it should be ok (99% sure of this)
2) The battery will need to be 3.6v (I'm 100% sure of this one!)
3) The charging should be ok, although I should watch out for signs of over charging (getting excessively hot).

Are my above assumptions true and is there anything else I should be aware of? I don't watch to a) damage the Amiga, b) cause a fire and make myself homeless, c) have a battery explosion which somehow maims me(!).

Cheers for any advice!

[cybermaus]

Battery should be somewhat near the 50mAh, to  big could put too much stress on the charge circuit. To small and it will not be able to carry the clock for weeks. But it does not need to be too precise. Anywhere between 25 and 80 will probably do. Maybe even more. Just don't go adding a modern 1400mAh AAA triplet to it.

[SeanB]

That one will work, will just take a little longer to charge but as the charge circuit is just a resistor that is no problem. Pop it in and leave on overnight and it will be charged. If that cell gets hot in that computer you put it in wrong, the charge current is under 5mA.

[banedon]

Thanks for responding, guys.  I'll order a battery shortly and give it a go. Although i think I'll go for one with long "fly lead" legs as opposed to the plate-like PCB connections.

Cheers again!


[EDIT]

One last thing: would it be ok to put a slightly lower capacity mAh battery in?  The following is a 40mAh, where-as the original is a 50.
http://uk.farnell.com/varta/3-v40h-km-slf/battery-nimh-pcb-mount-3-6v/dp/854610

[SeanB]

Prety much anything that fits will work. The smaller one will have a slight overcharge and a shorter life, but will be fine if you are going to only use it occasionally. So long as you turn it on for a few hours every few months it will be fine. The discharge current from it into the RTC is about the same as the self discharge of the cell.

[banedon]

Thanks Sean - much appreciated

[westfw]

I'd be really tempted to put in a non-rechargeable battery; say 3x alkaline or a 3.6V lithium and a reverse current protection diode.  Cheaper, easier to replace, and probably as long an in-service lifetime as a tiny NiCd (NiCds tend to suck...)

[banedon]

That's not a bad idea actually. A 1N4007 should do the trick for the diode. Just go to get or series of batteries which add up to 3.6V

[AmmoJammo]

6A motherboard shouldn't have 1 meg on board, they still only had 512k, plus 512k in the trapdoor.

[SeanB]

Do not use a 1N400x with a lithium battery, the leakage current through the diode will be enough to charge the battery and damage it. Use a 1N4148 instead. I often used 4 AA alkaline cells in a 4 cell holder with a series string of 3 diodes ( here the 1N400x will be fine) to drop the voltage enough to what the RTC will take. They often would last a decade or more, just place them where leakage from the cells ( they all will leak in the end) is either contained in a box or in a platic sleeve.

[banedon]

6A motherboard shouldn't have 1 meg on board, they still only had 512k, plus 512k in the trapdoor.

Hi AmmoJammo.  I probably wasn't too clear when I posted above. What I have is 512KB in the Motherboard (4x TMS44C256-12N) and the card seems to hold 1MB (4x OKI M514256A-80R).
Interestingly, the motherboard has 8 IC positions with 4 used and 4 sets of IC solder points. Do you reckon I could get away with soldering in some DIP sockets and adding in some of the same RAM types for a total of 1MB onboard?  My concern here would be that the memory decoding might not pick up the extra RAM.  The ICs positions are populated every other socket.

Do not use a 1N400x with a lithium battery, the leakage current through the diode will be enough to charge the battery and damage it. Use a 1N4148 instead. I often used 4 AA alkaline cells in a 4 cell holder with a series string of 3 diodes ( here the 1N400x will be fine) to drop the voltage enough to what the RTC will take. They often would last a decade or more, just place them where leakage from the cells ( they all will leak in the end) is either contained in a box or in a platic sleeve.

Thanks for the warning, Sean: It's very welcome. Would you mind casting your eyes over a diagram I'm putting together? Because of my lack of knowledge (which I'm trying to improve) I'm quite cautious when dealing with any power sources of any type - even low end batteries.

[ShawnD]

On an A3000 I replaced a leaky battery with a cordless phone battery and added a long wire so it would fit on top of the floppy drive.  That way it was easier to get at if it failed and if it leaked it would only rust the top cover of the drive. 

This was probably done 10 years ago and was still working when I sold the machine last year.

[banedon]

Can some confirm what happens when a battery is charged as opposed to discharged?  I've looked around on the net, but cannot find out for sure what happens.

Do you apply charge to both ends of the battery, reverse charge it (i.e. apply power to the cathode)... I don't think the latter is true and I suspect would damage the battery, but I'm not sure.

Many thanks!

[AmmoJammo]

 

When a battery is being charged, you are applying a higher voltage across the battery than its nominal voltage...

But, what does this matter?

[banedon]

I'm trying to learn/understand how the process works...