DIY: Repair Makita BL1830 Battery by Making One Good From Two Bad

This time I'll take two bad Makita BL1830 lithium batteries and will make one good operating battery. Plus I’ll show you two ways, how to solve protection circuit problem, when it was triggered and regular Makita charger isn’t charging battery any more.How I did it - you can check by looking DIY video or you can follow up instructions bellow.

For this project you will need:

Materials:

2 Makita bad batteries

Solder

18V Li-ion Makita Battery Protect Circuit Board: https://goo.gl/wfWfDB

lithium 5S (5 cells in series) balance wire with connector

Tools:

Screwdriver

Soldering iron

Knife

Multimeter

SkyRC iMax B6AC V2 (or other smart charger)

I have two Makita 18V 3Ah bad batteries. By saying bad - I mean, that they have low voltage and won't take a charge on regular Makita charger.
I took both batteries a part and found that they have a little bit different protection circuit boards. They are protecting cells from over discharge, overcharge, overheat and are counting the number of charging cycles. Plus one battery have balance wires to each pair of 5 groups cells connected in series.

Measured voltage of each cells group to identify bad cells. At each battery was dead one pair of 18650 cells. They was very badly corroded and of course won't hold or take a charge.

There is only way to solve this problem - by simply replacing dead cells.
I choose battery with balance circuit as a project base and another one - will be a donor. Using knife removed bad cells and tried to save battery tabs, because I’ll use them later. In same method removed a pair of good 18650 cells from donor battery.

Pretined and soldered 18650 cells in place.

Checked - what voltage we have. I took my Imax B6 charger and charged battery to raise voltage up to 18V. I did that, because I had a very small hope, that protection circuit wasn’t triggered and with normal battery voltage regular Makita charger will start charging. But that’s was a bust. So if I want to use regular Makita charger I have to replace triggered protection board. I found a replacement board on bangood.com for few bucks.

Desoldered old protection circuit. Because new protection board isn’t compatible with balance circuit so it was removed too.
Installed new board, soldered positive and negative terminals, plus red wire to first group of cells positive side.

In case you don’t want to buy replacement circuit and to be addicted to Makita regular charger, or maybe your charger is broken, there’s another charging option. All you need is to solder lithium batteries balance wire between each cells group and find a good place for connector at battery housing. This setup will allow you to charge and balance this battery with universal smart charger like Imax B6. Plus you will be able to see the state of each cell group connected in series, by looking at internal resistance and voltages.
I want to notice, that internal resistance is able to show only Imax B6 V2 model. I leave a link in description, where you could get one.

In my battery cells have from 23 to 31 milliohms of internal resistance. Because those cells are quite old and used - I could say, that this is pretty good result. If resistance will be 50 milli ohms or higher - I’ll won’t use them any more.

My battery is almost fully charged, each group of cells have nice and almost equal voltages. Of course, it is possible to charge this battery without balance wire. But in this way there’s a risk to overcharge some cells or not give a full charge to others.

Assembled battery back and put it on regular charger to check does replaced circuit board solved the problem. And yes, it did.
Battery fully charged and performs very well.