[volt-nuts] fluke 731b battery pack

[Tom Miller]

I like the LiFe idea except for the restrictions on shipping by air or USPS.

That shouldn't be too much of a problem for most that have access to a local 
cal lab.




----- Original Message ----- 
From: "David" <davidwhess at gmail.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Wednesday, August 03, 2016 5:08 PM
Subject: Re: [volt-nuts] fluke 731b battery pack


> 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.
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