NiMH or Lithium?

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Joined: 31/12/2012
Location: New Zealand
Posts: 2137
Posted: 05:18am 22 Mar 2023      

if a power outage is only half an hour long, and during it there is a 30 second long transmission, then the current draw (at 100mA) averages out to the equivalent of less than 2mA. this pales by comparison to the draw of your MX170 (5mA), or pico (25mA at prompt, connected via USB).

with a pico, i could see any extended outage putting a dent in 9v lithium battery (1200mAh). for instance over a weekend (midday Friday to midday Monday) 20mA x 72 = 1440mAh, which would see the battery drained. even just a 1 day outage would drain 1/3 of the battery's capacity.

i'd be looking at sticking with the NiMh battery, but having the MX170/pico take note of when it was changed and sound a 10-year alarm to remind that a new rechargeable battery should be fitted. i assume your units can get the time from somewhere (perhaps transmitted over the base HC-12)?

rob   :-)

addendum: you could look at using a LiFePO4 cell and a small boost converter, these can have a quite long service life if treated well, but the extra complexity would largely nullify any cost savings. even regular 18650 Li-Ion cells can have a remarkably long service life these days.
Edited 2023-03-22 20:41 by robert.rozee