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New EEVblog BM786 Multimeter

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Fungus:

--- Quote from: JDW on March 08, 2023, 09:07:05 am ---I don't know the best way to calculate battery life for the BM786.  If all the batteries are in series, then we would take the mAh rating of one, and I assume that to be about 850mAh for a single Alkaline AAA, which at an 8mA meter draw (I do DC work, not AC) would mean just over 100 hours:

--- End quote ---

Yep, that's how to calculate it....


--- Quote from: JDW on March 08, 2023, 09:07:05 am ---One could argue the same about the 121GW, yet it's manual says >500 hours, which is a nice guide to have.

--- End quote ---

The 121GW uses AA batteries with about 4x the capacity. From that we can conclude that the power consumption must be similar.  :)

J-R:
I think it's best to start by pulling up the datasheet of the batteries you are using.  For example, here are some Energizer AA and AAA alkaline datasheets:
https://data.energizer.com/pdfs/e91.pdf
https://data.energizer.com/pdfs/e92.pdf

The lowest published capacity and current draw data point for both is 25mA, so ~3,000mAh for AA and ~1,150mAh for AAA.  Even though both DMMs draw less than half that current, given the low-voltage cutoffs of the DMMs I think the numbers are probably reasonable as-is for this theoretical exercise.  Some of the batteries' capacity will be unusable.

The BM78x has 3xAAA batteries, and the 121GW has 4xAA.  The mA draw for each that is published is based on the operating voltage of the DMMs, and we know both are wired to run the batteries in series, so for runtime calculations you would simply use the mAh capacity of one battery.

One variable is that devices typically require a specific amount of power (V*A) to run, so as the voltage drops, the current will increase to compensate, and vice-versa.  For the 121GW it might be 30mW (4*1.5V*5mA) and for the BM78x it might be 36mW (3*1.5V*8mA).  We have to make some assumptions.

So for a true number, you would have to characterize the DMM with a power supply or do real-world tests with actual batteries.  Another variable is the point at which each DMM begins to become inaccurate, even though it is still "operating". 

However, for back of the envelope calculations, I think we can start with:
121GW - 3,000mAh/5mA=600 hours maximum
BM78x - 1,150mAh/8mA=143.75 hours maximum

The BM78x manual states the low battery indicator kicks in at 3.7V, which is 1.23V per cell.  That is leaving a lot on the table.  So I would lean towards ~100 hours as being more realistic.

The 121GW has a published life of 500 hours typical, and that seems like a reasonable number at face value.

Fungus:

--- Quote from: J-R on March 08, 2023, 07:24:15 pm ---The BM78x manual states the low battery indicator kicks in at 3.7V, which is 1.23V per cell.  That is leaving a lot on the table.

--- End quote ---

Yep, but I hope nobody is thinking of using ordinary Alkalines in one of these.

Get some Eneloop AAAs. Discharge curve here.

Image courtesy of forum member HKJ: https://lygte-info.dk/

J-R:
What, 99% of handheld DMMs ship with alkaline cells, then probably 99% of the time those are replaced with alkaline cells...  Leaks are rare, all things considered.

EEVblog:

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