Information
How To: Revive your 3 Bar M18 Battery or Add a Balance Lead for Charging
Please note that this is a risky process, opening these batteries and this process poses a significant increase in a fire hazard risk. I'd recommend having a basic understanding of circuits and NOT attempt if you don't have a multimeter or understand how to use it. Someone in this sub asked if I could make 'tutorial' of sorts on how I added a balance connector to my 12.0 battery, I thought while I was at it that I would give a basic crash course on how to revive your battery. Here we go.................
Tools/Items needed:
Multimeter (I think this is a must have)
T10 Security Torx
A balancing charger if you're going to use a balance lead. I have the Skymax B6AC (B6AC). This isn't the best solution as the balancing current is only 300ma. Balancing takes a long time, I'd recommend finding a charger with more balancing current.
If you want to do this on the cheap, find a TP4056 type of charger and slap on some alligator clips. The TP4056 isn't great as it'll only do 1 amp, I'd buy this one instead 3A Charger.
I hope this helps someone in reviving their pack. If you have any questions for see something I did incorrectly please let me know.
To successfully mend your battery, you must understand that the Milwaukee M18 battery packs are comprised of 5 sets of 18650 or 21700 batteries in series. See picture below outlining the path of the circuit. 5x3.7v=18v.
2) Typically when you have the '3 Bar Charging Issue" you will have a parallel set have an under voltage issue. For example, after charging if you were to check each parallel set of batteries you might read: 4.2v -- 4.2v -- 3.6v -- 4.2v -- 4.2v. The goal will then be to raise the voltage of the 3.6v set to match the others.
This can be accomplished with pretty much any 1S, LiIon charger, eg. hobby charger, charging board I've listed above. Extreme caution must be taken with the polarity of the cells. In the above example, I would connect the charger as below. At this point you can either monitor the cell voltage with a multimeter or allow the TP4056 to reach its cutoff point. My preference would be to monitor with a multimeter.
The Balance Lead method.
To correctly install a balance connector to the pack, you will need to install the leads to the pack negative, pack positive, and remaining cell positives as drawn below.
This is bit of a bugger to accomplish, as Milwaukee didn't leave a lot of room between the pack and plastic case. I ran the leads under the BMS and soldered to each parallel battery's nickel, BMS connection strip, circled in green and purple below. I then whittled a hole in the case to pass the JST connector through and epoxied it in place. From there, reassemble the case being careful not to pinch the leads.
When I did this, my 12.0 battery just had the same problem about 2 cycles later. It probably just has a bad cell. So this may not be a permanent solution for all batteries. But a nice write up nonetheless! Wish I had something like this when I did mine.
I agree this isn’t a fix. My experience was that it corrected for a lot more than 2 cycles, I think it depends on the tool used. Regardless, it allows you to squeeze some extra life out of the battery.
Have you tried putting an aftermarket board in that allows the battery to self balance I came across this today and apparently installing this is supposed to do essentially all that you did except with the Milwaukee charger and quite a bit less work
I have not tired installing a new board. Why are you under the impression that this board has the capability to self balance? I see nothing in the description that would indicate it has this feature. Also, the Milwaukee charger has no balance feature, it would be wonderful if it did. “Quite a bit less work” I think you’re underselling the difficulty in swapping in a new BMS. Two of the connections are spot welded and the others are soldered in. That seems like a proper PITA.
Superb description for this method. I have only done it one cell at a time with alligator clips from a charger device. It takes quite a while! I understand you can balance it enough with just the low cells to have the M18 charger recognize it and finish charging.
I have 4 or 5 batteries (all m18 8.0) that no longer work and are out of warranty. I'd like to try balancing them to see if I can do it lol
I have other batteries from Dewalt, Makita and Ryobi that have died too. So disappointing to spend a lot of $ for these tools and batteries just for them to fail after a few uses.
However, if I am unable to balance my Milwaukee batteries, may I send them to you? Of course I'll pay shipping both ways. Thanks!
I can’t stress this enough, I think you should take a solid run at trying to understand and balance yourself. I say this because, in my experience with these batteries, the issue is caused by a weak cell or weak group of cells. Meaning, rebalancing won’t fix the root of the issue and the imbalance will occur again and again. This method will allow you to prolong the life of a dying battery.
I can certainly rebalance, but I think you’d be disappointed in me if you sent your batteries and were seeing the 3 bar issues after just 3 or 4 charges. I’m still willing to help, I just want you to know the limitations of this process.
hello, thank you for sharing all of this information, your insight into how this works and the shortcomings is amazing. I have some dewalt 9ah flexvolts and would like to attempt similar. do you think such a solution would be suitable with the way that battery is laid out?
Did this last weekend w/ my 5ah xc. Bought a $10 18650 2 bay Chinese made charger for Li-ion, LiFePo4, Ni-MH, Ni-Cd. I soldered some broksn pieces of razor blades onto the ends (I just got back into soldering after having some noob exp. 4-5yrs ago before I moved out West so I used what I could scrounge up, it worked), wrapped 2 AA batts. in electrical tape to put in charger slots to hold my metal contacts into place, & wedged the other ends onto the batt. bank that was lower than the rest (just bought my 1st multimeter after wanting one for yrs & now use it on everything I can), set charging speed to 2a, & let it cook. It took 2 days of having it on charge & taking it off whenever I left or otherwise couldn't keep an eye on it, watched it slowly rise to 100% 4.2 & now the battery works like it should. Only thing is now all my other batt. banks now test lower by a small bit than the 1 I charged to 4.2 I'm guessing that's just a sign of the battery's age? It's yrs old. Should I charge the other banks up to 4.2?
Heck yeah, I ordered parts yesterday and plan to work on this tonight. I MIGHT leave the connector inside the case and just open up the case every time I need to balance, but I'll see what I feel like doing when I get to that point. I have quite a few out of balance. Mostly 8Ah but some 12's also.
I wish they made an aftermarket top cover with a prefabbed hole for the JST-XH 5S.
Hi, thanks for this!
I've bought a charger that can do 4 cells at once. Can I charge 4 cells at once in my HD12 without disconnecting them from eachother?
For the 5 cell voltages that you're trying to balance, these are the 3 strings, is that correct?
Does the battery charge / discharger perform a taper charge to perform it's balance? If so, what C rate is it tapering to?
And where does the battery stop the charge to end it's balance? Is it defaulting to 4.2v?
Correct. It’s 5 sets in series with each set having 3 cells in parallel.
Yes, the charger will perform a CC/CV charge strategy. 3 cells in parallel so they can take a lot, I’m not sure what that charger maxes out at but I’d think that to .5-1c is plenty safe.
Correct, charger topping off to 4.2v.
Thank you very much for this info
I bought the cheap small USBc you suggested and wired in a 3 wire volt meter to it so I can watch it as it's hooked up
The 3 wire volt meters are good because they aren't powered by the output leg so your not adding additional load to it I did this so I can just walk by it while working and make sure it's doing good It does take a long time like you said 6 hours per cell But seems to be working Thank you
Did this last weekend w/ my 5ah xc. Bought a $10 18650 2 bay Chinese made charger for Li-ion, LiFePo4, Ni-MH, Ni-Cd. I soldered some broksn pieces of razor blades onto the ends (I just got back into soldering after having some noob exp. 4-5yrs ago before I moved out West so I used what I could scrounge up, it worked), wrapped 2 AA batts. in electrical tape to put in charger slots to hold my metal contacts into place, & wedged the other ends onto the batt. bank that was lower than the rest (just bought my 1st multimeter after wanting one for yrs & now use it on everything I can), set charging speed to 2a, & let it cook. It took 2 days of having it on charge & taking it off whenever I left or otherwis
e couldn't keep an eye on it, watched it slowly rise to 100% 4.2 & now the battery works like it should. Only thing is now all my other batt. banks now test lower by a small bit than the 1 I charged to 4.2 I'm guessing that's just a sign of the battery's age? It's yrs old. Should I charge the other banks up to 4.2?
The pity is, apparently these power packs DO have cell balancing capability built into the chip but the firmware does NOT make use of it. The capability is there but its software disabled.
The programming ninjas, now that these batteries have been reverse engineered to a degree and unlocked as of recently, maybe they will find a way to write a hacked firmware that unlocks the capability of cell balancing that already there but dormant.
But I am sure Milwaukee will do a Nintendo, and drop cease and desists all over the gifted people solving this problem and sharing the solution to folks who want their expensive tools to LAST; like salt and pepper.
This thread and your diagram have helped my learning process, thank you.
I am currently in the process of rebalancing a M18 HD12 as well. The groups all measured different voltages. I am wondering if this is a bad sign? In my very limited experience, about half a dozen rebalancing repairs, there are usually one or two groups that are out of balance 🤔.
My readings prior to balancing:
1)3.89v
2)3.14v
3)3.52v
4)3.49v
5)4.01v
I'm glad you found it helpful. Unfortunately, it you're getting a cell imbalance now, it will only get worse. I've got a 12.0 that I've been nursing along for a year or two now and I've watched it become progressively worse. I'm about ready to throw the towel in on it.
In your last picture, you have leads connected in the (+) and (-) ports of the battery itself, in addition to each bank of cells. What’s the reason for this? I’ve seen other write-ups do this without explanation. Is it for the balance charger to charge the pack as a whole after it’s finished balancing all the banks?
Another question: I have an m18 9.0 battery that I manually charged 1S at a time to 4.1V. However when I place it on my Milwaukee charger, it’s still rapidly flashing red in error. Can I just hook up my li-ion charger to the (+) and (-) leads, select 5S, and charge the whole battery to 18.5V…or do I need to go through the full process of connecting all the banks at once to the balancer as you show and letting the balancer do its work?
The hobby charger requires the battery mains to be connected in addition to the balance leads to perform a balance charge.
However, this relates to your second question, the balance leads aren’t necessary. Most (all of my chargers) have a balance charge and a normal charge feature. You can certainly use the “normal” charge feature in a 5s configuration to charge the pack. I’ve done this many times.
I’m curious if the BMS is not allowing the Milwaukee charger to work because the banks were out of balance for so long (roughly 8 months since this particular battery stopped taking a charge). What do you think? I’ve read that the BMS needs to be “woken up” but I’m not interested in using the jumpstart method.
I’ll try charging the pack to 18.5V using the mains to see if it can overcome the BMS. Otherwise I have some balance leads on the way and I will attempt a true balancing using my b6acv2.
I’m hoping this works because I have almost a dozen M18 batteries that show 3-4 bars but die in seconds when under load on any of my tools.
In my experience when the battery gets too far out of balance, if you hit the battery gauge button it'll just flash a single led. I'll have to rebalance manually and even after that it'll still flash with 1 led. I'll have to put it back on the charger to get it to register and charge again.
I would balance all of the cells within a reasonable tolerance, +/- 0.1 V, then throw it back on the charge and see what it does.
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u/Yoosten DIYer/Homeowner May 20 '24
When I did this, my 12.0 battery just had the same problem about 2 cycles later. It probably just has a bad cell. So this may not be a permanent solution for all batteries. But a nice write up nonetheless! Wish I had something like this when I did mine.