Hi all, first let me say that I'm an electrical or maker newb, especially compared to all you veterans. But I'm committed to learning the craft because I think it's very fun.

Right now I'm trying to upgrade an old cordless ratchet from the 1980's, it's a Black and Decker 9050.

I have a good Samsung 21700 to replace the old ni-cad batteries in the ratchet. But I need a good protection circuit for it. Preferably a circular one that I can affix to the bottom of the battery. It needs to be able to flow 10 amps continuous, and cut off at 3.0v to protect the battery from the voltage going too low.

I guess it doesn't have to be circular, but it would be nice. Thanks!

 

What sort of circuit do you have in mind for this purpose?

 

Sounds like photonfanatic wants/needs a BMS (Battery Management System) for a single cell 21700 Lithium Ion battery. Something like this:
https://www.amazon.com/ACEIRMC-Charger-Protection-Lithium-Battery/dp/B08MPXHFJB/ref=sr_1_1
It states it's for a 18650 but the number is merely a size reference. The 21700 is slightly longer and larger in diameter. Aside from that the BMS shown is good for 10A.

 

Sounds like photonfanatic wants/needs a BMS (Battery Management System) for a single cell 21700 Lithium Ion battery. Something like this:
https://www.amazon.com/ACEIRMC-Charger-Protection-Lithium-Battery/dp/B08MPXHFJB/ref=sr_1_1
It states it's for a 18650 but the number is merely a size reference. The 21700 is slightly longer and larger in diameter. Aside from that the BMS shown is good for 10A.

Yes that is the type I'm asking about. However, if you look at the specs it says "Protection voltage range: 2.5V-4.2V."

I've read that these lithium batteries don't appreciate being made to go below 3.0v. And if you do that regularly, it shortens their lifespan considerably. My battery is Samsung 45-T 21700.

So I'm looking for one of those you linked, which cuts the power when the battery gets down to 3.0v. I've been searching through amazon but everything seems to cut off at 2.4 or 2.5v.

Unless maybe someone here knows that going down to 2.4v regularly is ok?

 

I've read that these lithium batteries don't appreciate being made to go below 3.0v. And if you do that regularly, it shortens their lifespan considerably. My battery is Samsung 45-T 21700.

Read this. Scroll down to specifications
https://18650battery.com/products/samsung-45t-4500mah-50a-21700-battery
Says "Discharge cutoff voltage 2.5V"

 

Read this. Scroll down to specifications
https://18650battery.com/products/samsung-45t-4500mah-50a-21700-battery
Says "Discharge cutoff voltage 2.5V"

Sorry about the newb question but, does that mean the battery itself will cut the power at 2.5v? Or are they saying "You'd better cut the power at 2.5v if you know what's good for you"?

After learning more about this subject (old thread I know) I have come to the conclusion that it's best to just pay the extra dollar or two, and get batteries that have the protection circuit built in.

Only time I can see that wouldn't be necessary, is when they're connected to some board that handles this for them.

 

A BMS is a Battery Management System. It controls how fast and to what voltage a cell is charged to. It also prevents the battery from being drained beyond the minimum voltage. Using the wrong charge profile can cause a battery to explode or erupt in flames. Over draining the charge can cause damage that later, when charging, can cause a battery to explode or erupt in flames.

 

Yes that is the type I'm asking about. However, if you look at the specs it says "Protection voltage range: 2.5V-4.2V."

I've read that these lithium batteries don't appreciate being made to go below 3.0v. And if you do that regularly, it shortens their lifespan considerably. My battery is Samsung 45-T 21700.

So I'm looking for one of those you linked, which cuts the power when the battery gets down to 3.0v. I've been searching through amazon but everything seems to cut off at 2.4 or 2.5v.

Unless maybe someone here knows that going down to 2.4v regularly is ok?

Samsung's datasheet for the cell specifies a 2.5V discharge cutoff.

 

Since the device does not use just a single cell battery, why not monitor the voltage of multiple cells in series?? I doubt very much that any "cordless ratchet" uses only a single cell.
If that "cordless rachet" does really only use a single cell, not a few in series, MY APOLOGIES for being confused!!

AND WHAT IS a ""cordless ratchet"???? I know what a ratchet wrench is, but I have never seen one that uses a battery!

 

AND WHAT IS a ""cordless ratchet"????

source: https://www.amazon.com/FULLYEA-Cord...Batteries/dp/B0F3WYQ26T/ref=asc_df_B0F3WYQ26T
Just one example. I have an air powered ratchet. Instead of a battery it depends on air pressure.
Here are many more:
https://www.amazon.com/s?k=dewalt+cordless+ratchet&i=tools&crid=1HSA5Q3MHEDEO&sprefix=deWalt+cordless+ra,tools,296&ref=nb_sb_ss_p13n-expert-pd-ops-ranker_ci_hl-bn-left_1_18

If that "cordless rachet" does really only use a single cell, not a few in series, MY APOLOGIES for being confused!!

16V and 20V. You're not confused.

 

OK, I do have an air powered wrench that does not look at all like the one shown. It is capable of quite a few footpounds of torque, able to loosen tight LUG NUTS, even. Quite different from the one shown.

What I see in the photo appears to use four cell battery packs, not single cells.

A very simple scheme to the track battery state of charge would be simply an LED with a series resistor connected across the load. The resistor would need to be selected to not let the LED light when the battery state of charge was low. Certainly even fully charged batteries have a fair amount of voltage drop at full load, so that should be an adequate scheme.