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How does cordless tool battery protection work?

52 Ford

Stainless
Joined
May 20, 2021
Can someone explain to me how the battery protection works on something like a Kobalt 24V tool battery? You have 3 spades on the tool that interface with the battery. Obviously positive and negative, but that third pin is measuring something (temperature of the battery?).
 
Talking 3 wire lithium ion batteries in general, typically a thermistor for temperature monitoring. Sometimes a communication line if there is more going on in there.
 
generally there is a battery problem terminal (low battery, battery hot etc) that tells the tool to shut down. The thermistor is used to stop overheating during charge and is used by the charger. While tool battery adaptors can move the power, they generally don't move the protection signals because they are manufacturer specific. You can trash an expensive battery easily with battery adaptors.
 
There is an internal sensor for temperature connected to a charge/usage control PCB. The PCB will shut off all current to the tool if the battery voltage gets too low. It also will shut off if the battery temperature becomes too high either in use or charging.

Li batteries are prone to going into an exothermic reaction if they get too hot so control is critical. The battery manufacturers test the chemistry of new formulations in a pressure-tracking reaction calorimeter to determine the critical temperature so they can stay well below it.

I believe the third terminal is related to current monitoring rather than temperature. Excess current can damage the circuitry so the PCB will shut down if detected.

Also, best practice for battery life is to let the battery cool between use and charging or charging and use.
 
You have 3 spades on the tool that interface with the battery

Some manufacturers put the protection and monitoring in the battery. Some put it in the tool. The other part of the system is the charger.

So the location of the protection/monitoring electronics determines what happens.

With protection in the battery (which Makita use), the third terminal is a signal to the tool that it should shut off whatever draws high current from the battery. If you notice, when the battery is flat, the rotating bit stops but the LED light still works.

With the protection in the battery, the terminal also signals to the (relatively dumb) charger what it should do. If the protection is in the tool, the charger needs to be more sophisticated.
 
rabbit hole search terms this way > battery management system, fuse wire lithium ion
 
i have an M18 milwaukee entourage that use shoebox-size chargers . they are very sensitive and won't work until the battery is perfectly positioned to the base.

i also have a couple of $15 horror freight 12v lion drills that have a hole in the top of the
battery for the barrel plug from a simple wall-wart .

ironically , the cheese-o drills are great screwdrivers . milwaukee ...not so much.
too much torque . even on the lowest clutch setting it can break the head off a #10
deck screw or 1/4 inch lag screw . it can . however drill a 3/4 hole in steel plate with
no fuss. i don't know what FUEL does inside the tools , but the angle grinder will
drain a 3A battery in minutes .
 
I was really tired when I asked this question. I should've elaborated.

I'm making some corded adapters for some of my power tools. For testing/experimenting, mostly. Also some playing around... Anyway, in the case of the Kobalt stuff, I plan on sending right at 24VDC to the tool from a large power supply. It'll be pretty interesting to see how some of those tool perform with a consistent supply of power vs a battery where it can only output so much electricity at a time.
 
i think you'll find it won't work well . you may smoke the brains of your lion power
tools. while it would probably be fine with old school ni-cad tech, modern tools
communicate with the battery.

if i didn't need cordless... i'd just use my corded tools.
 
DeWalt at least sells a specific corded to cordless adapter. I wouldn’t be surprised if one is available for your tools as well.

Probably only useful if you already own several tools of theirs. I’d check the duty cycle ratings for your device as well.
 
if i was guessing
Id say the vast majority of the battery management system would be in the chargers
in the tool itself its probably just reading voltage.

not for tools but...
 

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I was really tired when I asked this question. I should've elaborated.

I'm making some corded adapters for some of my power tools. For testing/experimenting, mostly. Also some playing around... Anyway, in the case of the Kobalt stuff, I plan on sending right at 24VDC to the tool from a large power supply. It'll be pretty interesting to see how some of those tool perform with a consistent supply of power vs a battery where it can only output so much electricity at a time.
I think that unless the power supply can be current limited, inline fast blow fuses would be a wise precaution.
 








 
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