[volt-nuts] fluke 731b battery pack

[David]

On Wed, 3 Aug 2016 16:34:31 -0400, you wrote:

>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).

That is my experience as well however I do like the better low
discharge NiMH cells.  Some are better than other though.

>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.

Low leakage and high temperature NiCd cells seem to last a lot longer
than other types and you will not find any consumer versions of these
but the price is high.

>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

I looked at the schematic and is seems feasible without excessive
effort.  Either the existing simple series preregulator can be
modified or replaced allowing it to both charge the battery (through a
blocking diode) and power the instrument or a completely separate
power charging circuit can be added in parallel.

The difficulty of maintaining charge in a backup application using
NiMH cells would lead me to consider LiFePO4 cells instead.  The only
serious difficulty would be preventing excessive discharge which will
ruin a lithium (or PbSO4) based rechargeable battery in short order.