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" <[email protected]>
To: "Discussion of precise voltage measurement" <[email protected]>
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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