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. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
