[volt-nuts] fluke 731b battery pack

[Orin Eman]

The battery in the 731B is charged via a diode and 510 ohm resistor from
the rectified DC, before the smoothing capacitor.  The smoothing capacitor
and power supply is fed by another diode, so the battery is charged with a
pulse waveform.

If you have no battery, then this pulse charge waveform will get around the
power supply regulator (since the regulated supply and battery are diode
switched) and increase noise on the output.  I therefore don't recommend
running without a battery installed... (though you could remove CR5 or or
R30).

Even with the battery installed, if it's good, at full charge, you can
still get charging pulses bypassing the regulator.  I built a high-side
switch to switch between the battery and power supply on my 731B.

I got the local Batteries Plus to build a new battery pack.  They usually
have NiCd cells of the right size and will build a battery pack while you
wait.  With an NiCd pack so easily available, I saw no reason to change
battery type.

Orin.

On Wed, Aug 3, 2016 at 1:34 PM, Charles Steinmetz <csteinmetz at yandex.com>
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).
>
> 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
>
>
>
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