Lithium Battery Replacement Help

[ckillerh3]

Hi -

I want to replace the old batteries in my A4000 and A2000s before they rot away and destroy my motherboards.  I've ordered the actual holders.  Is there anything special I have to solder in-place as well (diodes / resistors)?  I've seen some pics on the site here that show some electronics wired in-line with the holders.

Thanks for any help ! Or web links of course.

[billchase]

Yes, A diode needs to be soldered in place to prevent the
motherboard from charging the battery.  Check this link
for more details.

C Snyder

[boing]

I'd like to be able to put in a rechargeable battery so the diode schematic is a nogo for me.

[patrik]

@boing:

Just put in a NiMH battery in the same barrel-shape as the original then. They won't ruin your motherboard when they get old.


/Patrik

[Castellen]

Actually the NiMH batteries can still leak after a while, but they do tend to last a bit longer than the original NiCd batteries.
Varta rate their 3/V80H NiMH having a service life of approx 6 years @20°C.
From experience, the NiMH begin leaking after about 10 years, but some people report them leaking a lot sooner.

As for the lithium battery modification, the guide on Aminet is OK, but I'd suggest using a schottkey diode, or one with a low fowrard voltage drop (0.2-0.3V).
The lithium batteries are rated at 3.6V, and with a standard silicon diode's fwd voltage drop of 0.6-0.7V, that's right on the edge of the real time clock's operating voltage.

If you want to do the standard battery replacement, there's a guide I've written here:
http://amiga.serveftp.net/battery.html


Hmm, I really must take some better photos one of these days, those ones suck!
Doesn't help that my digital camera has a resoloution measured in kilopixels :-)

[TjLaZer]

[patrik]

@Castellen:

Is there a specific diode you would recommend. If so, what is its type-number?

I was under the impression that NiMH batteries as you say leaks with age, but that their leakage isn't especially corrosive - ie it doesn't destroy your motherboard. Is this wrong?


/Patrik

[ckillerh3]

The instructions on Aminet specify a radio shack diode and a 3 volt battery (not a 3.6 volt battery).  Also, Radio Shack does not carry the battery clips anymore.  I ripped some off old PC motherboards.

What is the deal from the previous posting suggesting a different diode?  It hinted it was "at the edge" of the RTC chip.  Does that mean it is at risk of melting it down?

[AmigaEd]

Does that mean it is at risk of melting it down?

Hi Stephen,
No, nothing is going to melt down.

What this statement means is that if you place the diode between the 3.6 Volt battery and the RTC that only about 3 Volts will be seen at the RTC. The diode will drop about 0.6 to 0.7 volts across it.

Sounds like you are making progress with the A4000.

Regards,
AmigaEd

[Azryl]

Remember to break off the bottom positive (+) pin!
depending on the type of socket you have of course :-)

Az

[Castellen]

For diode part numbers, I can't quite recall all the individual specs from various catalogues :-P
Usually I find it easier to use whatever is closest to hand.  Go to your local electronics shop and choose the first low loss diode you some accross.  Most schottky diodes will do the job perfectly.  It only has to conduct a few µA, so something with a fwd current rating of 100mA is more than adequate.
I've got a bunch of 1N6650 diodes laying about here I've been using in various random projects.  That would fit the bill nicely.

From the RP5C01 real time clock datasheet, the minimum data retention voltage is 2.2V.  So if you had a 3.0V battery, with a 0.7 fwd voltage drop diode in series, that's 2.3V at the RTC, 0.1V above the minimum required.
Obviously it will work, but with 0.2/0.3V diode, it'll work for longer as the battery's terminal voltage has to drop about 0.5V until the clock's time is lost.

As for the contents of NiMH batteries, I don't know much about chemistry, but I do know I've spent quite a few hours repairing equipment (not just computer) which has had PCB damage due to leaking NiMH batteries.
While I'd suggest they're a much better alternative to NiCd batteries, don't be under the impression they'll never need replacing.

[ckillerh3]

Hi -

Thanks for all the tips and suggestions.  The A4000 project is coming along nicely.  I also purchased a plug converter gizmo that converts a regular ATX supply to work nicely in an A4000 as well.  It is going to be a rainy weekend here in the North Eastern US . . .perfect to work on the old computer !