[volt-nuts] fluke 731b battery pack

[Charles Steinmetz]

Eric wrote:

> NiMH cells seem to work in the 731b so I'd prefer to use them over NiCd
> because of the higher capacity and lower toxicity of potential leaks. Is
> there a downside to doing this?

In my experience, good NiCDs are preferable to NiMHs.  Good NiCds have 
substantially lower self-leakage than NiMHs (this is true even of the 
"NEW!! Low-discharge!!" NiMHs).  NiCds also don't degrade nearly as fast 
if they are left too long on trickle charge.   Both of these features 
translate directly into increased life for the NiCds.  NiCds are also 
quieter, and capable of larger current drains, due to their lower 
internal (series) resistance (high current is not really an issue in 
your application).

Consumer-type NiCds may suffer from the problem PHK noted (poor quality 
due to low production volumes), but there are still many industrial and 
military applications that specify NiCDs for some or all of the reasons 
given above.  Excellent NiCds are readily available -- just look for 
aerospace-grade parts rather than consumer batteries.

Many built-in charging circuits are crude and leave the batteries on a 
trickle current that is really too high, particularly given the 
temperatures inside electronic instruments.  So whichever batteries you 
choose, plan on redesigning the charging circuit.

That brings up the possibility of using either LiFePO4 or SLA (sealed 
lead-acid) batteries -- if you have to redesign the charging circuit 
anyway, you can just as easily design it for LiFePO4 or SLA.

I have not evaluated the 731B power supply in particular, but LiFePO4 
would be my presumptive choice unless I encountered an insurmountable 
obstacle.

Best regards,

Charles