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Show HN: 32V TENS device from built from scratch under $100

littlemountainman.github.io

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Re: Show HN: 32V TENS device from built from scratch under $100

#3

Post by cenamus »

dmitrygr wrote: post #2

> down to ~3.0V (discharged). That is NOT how one uses lithium batteries one foe snot want to go boom. Consider 3.6V as empty. Discharging them down to 3.0 can cause them to go boom when recharged...

Of course you can go down there? That's literally the lower limit of design voltage?

Below 2,5V is usually when you don't wanna use them anymore

Re: Show HN: 32V TENS device from built from scratch under $100

#4

Post by dmitrygr »

cenamus wrote: post #3
dmitrygr wrote: post #2

> down to ~3.0V (discharged). That is NOT how one uses lithium batteries one foe snot want to go boom. Consider 3.6V as empty. Discharging them down to 3.0 can cause them to go boom when recharged...

Of course you can go down there? That's literally the lower limit of design voltage? Below 2,5V is usually when you don't wanna use them anymore

only if you want them to lose capacity fast and become spicy pillows in your lifetime

Re: Show HN: 32V TENS device from built from scratch under $100

#5

Post by clumsysmurf »

dmitrygr wrote: post #4
cenamus wrote: post #3

Earlier quoted context omitted.

Of course you can go down there? That's literally the lower limit of design voltage? Below 2,5V is usually when you don't wanna use them anymore

only if you want them to lose capacity fast and become spicy pillows in your lifetime

2.5V is the absolute minimum typically mentioned in spec sheets, 2.8V and above is perfectly fine.

Re: Show HN: 32V TENS device from built from scratch under $100

#6

Post by kees99 »

dmitrygr wrote: post #2

> down to ~3.0V (discharged). That is NOT how one uses lithium batteries one foe snot want to go boom. Consider 3.6V as empty. Discharging them down to 3.0 can cause them to go boom when recharged...

Most cell OEMs will specify safe discharge (low threshold) voltage in a datasheet. 2.75V is quite common [1].

That being said, system designer might choose higher cut-off point, since:

1) charge/discharge curve is S-shaped. There is very little energy in that last few millivolts;

2) battery (protection) circuit, and/or battery itself probably have some small leakage current. However minuscule, over months/years on a shelf, even some nano-amps of leakage will add up. If you want device to survive that, you have to factor this in, so that rest cell voltage still stays above safety threshold even after storage.

Also, "Li-ion" is quite a wide category. Don't use arbitrary voltage as a fast rule. Look up datasheet, or characterize actual cell you use. For some[2], disconnecting at 3.6V would mean leaving 50% of capacity unused. For other[3], that would be a reasonable, if somewhat conservative threshold.

[1] https://docs.rs-online.com/080b/A700000007848112.pdf

[2] https://www.murata.com/-/media/webrenewal/products/batteries...

[3] https://ntrs.nasa.gov/api/citations/20140005830/downloads/20... (page 4)

Re: Show HN: 32V TENS device from built from scratch under $100

#7

Post by Kirby64 »

dmitrygr wrote: post #2

> down to ~3.0V (discharged). That is NOT how one uses lithium batteries one foe snot want to go boom. Consider 3.6V as empty. Discharging them down to 3.0 can cause them to go boom when recharged...

3.6V is considered the nominal voltage, certainly not the low end cut off.

3.0V is considered basically the highest voltage. Most chemistries suggest even lower, 2.8, or even 2.5 in some situations assuming you can control the cutoff carefully. Perfectly safe to do so. You only start to have issues when you’re south of 2.5 without a load.

Most advanced battery usages let the cells drop even below that during heavy load.

Re: Show HN: 32V TENS device from built from scratch under $100

#8

Post by lambdaone »

Monophasic waveforms are generally considered less safe than biphasic waveforms. That's why many TENS units have an output stage based around a pulse transformer, so they can deliver two pulses, one in each direction, shortly after each other. Leaving this out seems to me to be a false economy when you've gone to all the effort of building the rest of the system.
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