[sdiy] Cheetah MS800 replacing NiCd Varta Mempac

Terry Bowman ka4hjh at gmail.com
Fri Jun 4 01:58:24 CEST 2021

> On Jun 3, 2021, at 11:25 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> On the Polysixes I’ve worked on, I always replaced the NiCd with a coincell battery holder. The diode that provides charging just gets removed.
> Then you can just stick a new watch battery in it every five/ten/whatever years.

This is by far the best way to do it (if possible). When rechargeable batteries are used to provide just enough current to keep a small amount of volatile RAM alive they're functioning like large capacitors. The device has to be connected to the mains often enough to keep them from running down. If left discharged for too long they will fail completely and have to be replaced. Coin cells don't have this problem as long as they aren't defective/half dead crap to begin with. This is why I only buy coin cells at the local Batteries & Bulbs store.

In the past I've seen this done with a battery holder and two or three AAA alkaline batteries but over the last decade I've had a LOT of name-brand alkalines puke. I've had this happen in as little as two or three months in low current devices such as a digital thermometer with a small LCD! The absence of mercury may have something to do with it.

I use lithium double/triple As whenever possible now. There is one serious caveat, though: new alkalines are typically 1.59V. Lithiums are 1.69V. Be careful with what you put them in because the more of them there are in series the greater the increase in initial voltage. For example 2X = 3.38V, 3X = 5.07V and 4X = 6.79V.

I haven't managed to fry anything yet but it is a concern so there are times when NiMH may be a better choice.

Speaking of capacitors, I have several old Uniden radio scanners from the early '90s that used .05F/5.1V supercaps, IIRC. Destroyable instantly by ESD or going a tenth of a volt over the rating. I repaired one by shoehorning a USED 2200µF @70V capacitor I had laying around into the housing. I left the scanner unplugged for five months and the memory held.

RECHARGEABLE lithium batteries do occasionally make front page news by spewing flames and smoke in the passenger cabin on a trans-oceanic commercial flight at 30,000 feet or exploding in the pocket of the proud owner of the latest smartphone.

As for NiCds, I've removed the backglass from more than one Late-Seventies Bally pinball. The corrosion on the logic board brought tears to my eyes.

Terry Bowman, KA4HJH
"The Mac Doctor"

"I've seen things you people wouldn't believe: attack ships on fire off the shoulder of Orion... beams...in the dark in the Tannhauser Gate. All those moments will be lost in time...like tears in the rain." — Roy Batty, Blade Runner

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