Lithium ion battery monitor

I’m hoping somebody can point me to a resource that I’m having trouble finding. I want to build a contraption that will use my DeWalt 20v battery packs. I don’t want to over discharge the batteries. What I would like to do is use the same circuit that the tool uses to sense the battery is getting low to stop the contraption when that occurs. Is that all in the battery itself? Or is the tool monitoring the voltage?

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It’s not the only way by far, but a quick and easy way is with a module that does just that. And at <$15 for two of them, they’re a pretty cheap solution:


Thanks! Over-discharge! That was the term I couldn’t think of that would’ve helped tremendously in the search.

Some brands have the battery protection in the tool, and others have it in the battery pack. From what I can find DeWalt does it on the tool side.

Short version: The battery does not have a cutoff, so you’ll have to monitor the battery via your device. The circuitry on the battery only acts to balance charge (which is important) but does not protect the cells from being drained too much. (Which is common to any ‘protected’ cell.)

Depending on how you do it, you could use something like an older 5V chip like the AVR in an arduino to sense individudual cells voltage (up to 4.2V ish) directly via C1-C4 contacts. If you use newer microcontrollers (3.3V), or want the whole pack voltage, you’ll need a resistor divider to bring them within range (0-3.3V for most adcs, notably 0-1V on some esp8266s (like the esp-01 versions, if they don’t have a built in divider))

It will vary depending on brand. When I looked into it in the past. For the most part, they kinda suck. The only common brand (in the 18-20 range, though most are actually 5 li-ion cells, so depending on your marketing that may be 3.6V5 = 18V, or up to 4.25=21V, though things like Ryobi’s 18V, originally comes from NiCd batteries, at 1.2V*15=18, which was a higher number than others at the time.) that I know that even has a low voltage cutoff is Ryobi’s 18V, and even that will actually drop out only too low to prevent damage (<3V) in some cases. (Generally ~ 3V is the absolute minimum you should discharge Li-Ion batteries, often better to keep it in the range of ~ 3.3 - 4.1V even though the rated range is often 3.0-4.2, with a nominal 3.6 or 3.7V (slight differences) as it will greatly increase life.) Almost all rely on the tool to do it. Some of them (one of the more expensive brands) doesn’t even have the balancing that DeWalt does.

You can do it, but expect to do basic battery management yourself, even if it’s just a cutoff. Which you can do with a simple transistor (which may need to be beefy, even a relay in some cases) on the battery outputs. One of them Makita? even helpfully supplies an output which seems intended to hook directly into a transistor to do that. (So why not do it on the battery itself…?)

Frankly for Li-Ion batteries, most people will need to read carefully about them. Then be even more careful. I can personally attest that you can move very fast if a li-ion battery starts to go off in your pocket.

I’d just use a chip. Pick one out on TI tell them you want samples

You can get fancy stuff like this

Thanks for the input. I dunno why I bothered with contraption earlier. I guess because I have a few things that I think would be cool to use these on. Right now, I’m thinking of a work light. Yes I can buy a DeWalt work light. But $80 for a 2000 lumen light just seems excessive to me when for less than $22 I just ordered four 9-30 v flood lights meant for automotive use.

With the info and James’s post and the link he posted, I know what I want to incorporate in the light interface. Either a FET or a relay with a comparator to turn off the light if the voltage gets below some adjustable but likely 15.5 v. Oh, and a rocker switch. And maybe a magnet or five.

Having decent portable power is handy. I’ve got 3D printed adapters to allow my cordless batteries to act as a very portable power supply for a few of my breadboards. I don’t use them often – I’m usually working at my bench, but on the rare occasion I want to put something together somewhere other than my office/lab, it’s glorious!

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