When to replace DM42 Battery?

Discussion around the SwissMicros DM42 calculator
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Jim_W
Posts: 33
Joined: Fri Mar 15, 2019 3:36 am

When to replace DM42 Battery?

Post by Jim_W »

My DM42 battery is at 2.76V with one bar missing. At what voltage should I change the battery? Thanks! - Jim_W P.S. I just uploaded two of my aviation programs. They are for flying VFR.
WillMarchant
Posts: 48
Joined: Sat Sep 16, 2017 8:38 pm

Re: When to replace DM42 Battery?

Post by WillMarchant »

Mine went berserk at about that Voltage level. Was fine with a new battery. Maybe there’s a threshold below about 2.8V? Now I’m curious if there is a dependable threshold or if it varies from unit to unit.
rprosperi
Posts: 1703
Joined: Mon Apr 24, 2017 7:48 pm
Location: New York

Re: When to replace DM42 Battery?

Post by rprosperi »

Yes, once you see it is a bar down, I suggest you install a new battery. The 'spec' limit is actually lower, but pragmatically this is the proper thing to do.

As there is no battery checking during program execution (to avoid slowing run time by thousands of checks) and sustained run-times require lots of current, these situations can lead to the battery failing to supply enough current, in-turn leading to a CPU reset from the voltage drop.
--bob p

DM42: β00071 & 00282, DM41X: β00071 & 00656, DM10L: 071/100
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deetee
Posts: 54
Joined: Mon Jul 22, 2019 8:01 am
Location: Vienna

Re: When to replace DM42 Battery?

Post by deetee »

Good question.

I made tests with the DMCP function read_power_voltage().

When there is power via USB this function delivers stable 3051 (mV)
My battery is quite drained but still works fine with 2064 (mV)

There is also a function get_lowbat_state() which signals a low battery, but I couldn't find a voltage value due this functions decides.

It seems there is no exact lower limit when the DM42 stops working confidentially. Mine (at 2064 mV) works/calculates perfect but has troubles when switching to auto power off (resets to DMCP instead of suspend mode).

Maybe there is some limit when the DM42 is not 100% reliable - seems to me around 2500 mV?

Regards
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deetee
Posts: 54
Joined: Mon Jul 22, 2019 8:01 am
Location: Vienna

Re: When to replace DM42 Battery?

Post by deetee »

I drained my battery as much as possible and ended at 1760 mV.

It still calculates, but with a little more "power stress" (ie entering the OFF/Suspend mode) the calculator remains in some undefined state (display flickers) and even needs the power grid to reset.

Regards
deetee
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