[sdiy] Cheetah MS800 replacing NiCd Varta Mempac

Richie Burnett rburnett at richieburnett.co.uk
Fri Jun 4 18:24:41 CEST 2021

Hi Brian,

The new one is a 55615603940 3/V150H 150mA 3.6V NiMH varta Mempac.  The 
manufacturer's datasheet says very little about charging other than 
"Excellent overcharge capability due to patented GCE-Electrode" and 
including some charging curves for charging currents of 0.03*C, 0.1*C and 
0.3*C over a period of a few hours.  There's also a statement that says 
"Also permanent charging exceeding the limits of specified temperature 
ranges may reduce the battery life. The maximum life is achieved, when 
charging at an average temperature of +20 to +30˚C."  I can't see that 1mA 
of trickle charge at 3.6V (only 3.6 milliwatts!) is going to significantly 
raise the temperature of the battery which is a substantial chunk of metal. 
So that is down to the ambient temperature inside the enclosure and 
ultimately the climate of the country where it's being used!


-----Original Message----- 
From: Brian Willoughby
Sent: Friday, June 4, 2021 4:26 AM
To: synth-diy mailing list
Subject: Re: [sdiy] Cheetah MS800 replacing NiCd Varta Mempac

Speaking of the spec, do you have a spec for the Varta Mempac? The data 
sheet might have such details as the maximum recommended trickle-charge 
current. If you can find specs, you can work backwards from those to see 
whether it's compatible with the Cheetach MS800 circuit.

As for replacement with a coin cell holder, I've had bad experience with 
that. Every used synth that I've purchased needs to have the coin cell 
replaced very frequently. In contrast, the same CR3032 soldered onto the 
board performs a lot better than a CR2032 in a holder.

In contrast, I have no experience replacing a NiCd with a CR2032. Given the 
risk of battery acid damage, I suppose there might be a good tradeoff to be 
made. I've never had a soldered CR2032 leak, so I always replace them with a 
soldered model after they expire. The rechargeable are admittedly a 
different story.


On Jun 3, 2021, at 08:51, Joel B wrote:
> Not sure if still possible but I was able to find a company selling 
> replacement nicads that looked like they matched the spec of my cheetah 
> MS6 back in 2016… didn’t want to remove the charging system:
> This is from my receipt:
> 3.6V 110MAH NICD
> Item# NIC0010
> https://interstatebatteries.com/
> Worked for me.
> Joel
> On Jun 3, 2021, at 8:10 AM, Mike Bryant wrote:
>> I've always dropped in NiMH into NiCad slots okay.  But that's no 
>> guarantee.  As you say not getting rid of the old one is a far worse 
>> scenario.
>> -----Original Message-----
>> From: Richie Burnett
>> Sent: 03 June 2021 15:30
>> Hi all,
>> I'm currently servicing a Cheetah MS800 digital wavetable synth and want 
>> to change the old NiCd Varta Mempac to a brand new NiMH Varta Mempac 
>> before putting it up for sale.
>> The old blue NiCd battery hasn't started to leak yet, and is still 
>> holding it's charge but I know that old NiCd Mempacs are regarded as a 
>> ticking time-bomb in equipment of this age (>.<)  I wouldn't feel 
>> comfortable selling it without addressing this issue first.
>> The existing 100mA 3.6V NiCd trickle charges from the 5V supply rail 
>> through a 1k resistor when the unit is powered up.  Should I be looking 
>> at changing this trickle charge current when changing the battery type to 
>> the 150mA 3.6V NiMH replacement?
>> I did a search online and found very little solid information about this.
>> Everything from "The NiMH Mempacs are a drop in replacement" to "you need 
>> to decrease the trickle charge current because NiMH is less tolerant of 
>> being overcharged"  The trickle charge current already seems low to me.
>> I would be interested to hear your thoughts!
>> -Richie,

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