How to balance Milwaukee M18 tool batteries

[HR19]

Hi there. Some lithium tool batteries have a tendency to go out of balance over long periods, such as Milwaukee's notorious HD 9.0 and 12.0 batteries (the ones I have). They don't have built in cell balancing (even though they have hardware for it), so I'm trying to think about ways to add it. I've seen some people add balance leads for an external balance charger, but I'd rather find a way to add internal balancing so I can keep using my Milwaukee chargers. However I haven't found any balancing boards that are quite small enough to fit inside the existing case. So first question is: does anyone know of any very small (even if very low current and passive) balance boards? Like 15mmx20mm or smaller, and thin. If not, my other idea is to design one using 4.1V zener diodes with 10ohm resistors to create a sort of pseudo-balance board so as they charge up, if they're out of balance, the highest ones will slowly drain to allow the lower ones to catch up. Should this theoretically work the way I am expecting, or am I missing something about circuit design? Thanks!

Also if there happens to be anyone with experience designing circuit boards who's willing to help me design one like what I just mentioned with the diodes, that would be super cool.

 

[rjenkinsgb]

Do you have any photos of the battery internals and the PCBs in them?

I've never used Milwaukee stuff. I just searched for those batteries & immediately found mention of problems, places banning them due to fires etc.

One comment was someone transplanting the protection board from a 5AH into a 9AH.

I also found a listing for them, stating they have individual cell monitoring. Sounds like some production site really messed up!

Milwaukee M18B9 M18 9.0Ah Red Lithium-Ion Battery

REPLACED BY :Milwaukee M18HB12

www.bigredpowertools.co.uk

 

[ahsrabrifat]

Hi there. Some lithium tool batteries have a tendency to go out of balance over long periods, such as Milwaukee's notorious HD 9.0 and 12.0 batteries (the ones I have). They don't have built in cell balancing (even though they have hardware for it), so I'm trying to think about ways to add it. I've seen some people add balance leads for an external balance charger, but I'd rather find a way to add internal balancing so I can keep using my Milwaukee chargers. However I haven't found any balancing boards that are quite small enough to fit inside the existing case. So first question is: does anyone know of any very small (even if very low current and passive) balance boards? Like 15mmx20mm or smaller, and thin. If not, my other idea is to design one using 4.1V zener diodes with 10ohm resistors to create a sort of pseudo-balance board so as they charge up, if they're out of balance, the highest ones will slowly drain to allow the lower ones to catch up. Should this theoretically work the way I am expecting, or am I missing something about circuit design? Thanks!

Also if there happens to be anyone with experience designing circuit boards who's willing to help me design one like what I just mentioned with the diodes, that would be super cool.

You can design such a circuit. Pick a compact SMD Zener diode (4.1V, like the BZT52C4V1 or MMSZ series).
Use 10Ω, 0.25–0.5W resistor.
Lay out a compact 5S or 15S board, depending on pack configuration.
Consider daisy-chaining the balancing modules in a thin PCB strip to fit alongside or between cells.

 

[HR19]

Do you have any photos of the battery internals and the PCBs in them?

I've never used Milwaukee stuff. I just searched for those batteries & immediately found mention of problems, places banning them due to fires etc.

One comment was someone transplanting the protection board from a 5AH into a 9AH.

I also found a listing for them, stating they have individual cell monitoring. Sounds like some production site really messed up!

Milwaukee M18B9 M18 9.0Ah Red Lithium-Ion Battery

REPLACED BY :Milwaukee M18HB12

www.bigredpowertools.co.uk

They do have cell monitoring but I want balancing so I don't have to open them up to re-balance if/when they get out of balance again.

 

[HR19]

You can design such a circuit. Pick a compact SMD Zener diode (4.1V, like the BZT52C4V1 or MMSZ series).
Use 10Ω, 0.25–0.5W resistor.
Lay out a compact 5S or 15S board, depending on pack configuration.
Consider daisy-chaining the balancing modules in a thin PCB strip to fit alongside or between cells.

I was wondering if that would work, thanks. I can't seem to find the specific BZT52C4V1 or a 4.1V in the MMSZ series. I did find some axial diodes that are 4.1V and have a relatively tight tolerance, listed as 4.0-4.2V. But it seems like 3.9 is the highest under 4 that comes in tight tolerance. Maybe a tight 3.6+ a 1N4001 or comparable to make a tight 4.1V is the best option. Either that or purchase a large bundle of Zeners and test their actual voltages before assembly.

 

[rjenkinsgb]

They do have cell monitoring but I want balancing so I don't have to open them up to re-balance if/when they get out of balance again.

Yes, understood - but apparently some versions in that series do have balancing, so the question is about the details of the internal circuit board - does it already have space for the extra components?

If not, a "drop-in" replacement board could be made - with adequate info of what is presently in the battery packs!

 

[Tony Stewart]

Zeners must be able to shunt all the power pumped into each cell, so I suspect your Zener solution will burn up.

Tell us the max power under CC at say 4V. I would use something more accurate like a TL431 and a power resistor on the PNP collector to dissipate the power outside and place on the bottom. This will a ribbon cable inside or magnet wires to drive the power resistors at 3V at max CC. Chose >>2x power rating to cut the temp rise to 125'C << 50%.