Just needed a diode in series with my BMS board
Bathroom scale. And if the circuit can run off 4.2V happily enough instead of 3V, is there any chance that would mean 1kg != 1kg?
You could test it yourself by putting 1kg on it
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Any risk of frying it?
Hi @Kimmo,
There’s a risk that it could be fried by supplying a higher voltage than expected.
What is the scale currently powered by?
Is there any particular reason for wanting to be able to power it with a LiPo?
It runs off a 3V button cell. Rather than having to continually obtain disposable cells for it, I’d rather make it rechargeable now that it needs a new cell.
How about a voltage divider?
I suppose it’s a trade-off between current draw, accuracy of the reduction factor and available values, and it’s a dirty hack because because of the parasitic drain involved?
But if that drain can be low enough, then it’s just a trade-off between required charging frequency and the size of battery I need to put in it, right?
Why LiPo? NiMh would be easier and safer. See:
Hacking bathroom scales with an Arduino – the easy way – STEVEP.XYZ
Hi Kimmo
As the load is probably not constant a voltage divider will not be suitable
I think we have had discussions in the past about the suitability of voltage dividers for different jobs.
Cheers Bob
If you wanted to use a 3.7V LiPo you might get away with putting a !N400X diode in series with the 3.7V cell.
Cheers Bob
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The chip under the black glob might run on a higher voltage, it might not. Usually chips will be designed with a range of operating voltages. The design of these things was probably done before LiPo’s became readily available. At the time button cells were common because of use in watches.
Will it work with a LiPo ? One way to find out, try it.
But the manufacturer wrote 3V on the board for a reason and with ‘design for price’ a LiPo will probably kill it; eventually. You could use a LiPo and a 3V3 regulator. The linear one Core Electronics have is quite small and cheap.
The voltage to the device should not determine the accuracy of the weight measurement.
Regards
Jim
EDIT: The quiescent current of the linear regulator will drain the battery, maybe faster than you want.
Also the 1V drop out would be a problem at low battery levels.
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Hi Kimmo,
I think @Jeff105671 has the right idea using NiMH batteries, safety first after all. you’ll get 3.6V with 3 of those in series. Perhaps it will be a more manageable voltage for your circuitry but still be cautious.
@Robert93820’s recommendation of using a !N400X diode is also quite handy.
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Cheers, folks! Of course - a diode was the dirty hack I’m after.
I’m just using what I have on hand here, which includes a bunch of USB-C BMSes and LiPo cells of various sizes.
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According to the site I referenced that should be 2 in series, although I doubt that three would cause a problem.
I just used these as my reference
The main point is don’t overdo it
Seems to work.
Tested that little cell with one of those USB analysers, it’s around 37mAh. Should last a while; the scale draws 3mAh when working, and close as dammit to 0 in standby
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Hi Kimmo,
Glad it has worked. Hot swapping Batteries can be risky business but from your tests it looks like it should be fine. Well done!
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Hi Jeff
NiMH cells 1.2V
3 in series 3.6V
Cheers Bob
The project I referred to had 2 batteries in series. I doubt that 3 would fit. It is not easy to tell whether they are alkaline or NiMH, but that would either be 3V or 2.4V. Based on the writeup I believe it would work at 2.4V, or 2 x NiMh as I suggested. 3 x NiMh or 3.6V isn’t substantially different than LiPo and has the same risk - maybe OK, maybe not.
