Makita 18v LXT Battery Dismantling & Testing

So I have four failed batteries and decided to find out what has gone wrong each of them.  First step is to get the cover off.  This is held on by four T10 Anti Tamper screws, on newer models there may be a white plug covering one of the screws.  I have found that the easiest way to remove it is to drive in a small wood screw and pull it out with a pair of pliers. I was surprised how much dust had made its way into the batteries.

Below is a description of the fault on each battery.

1. Battery One: Fails to charge with a over temperature alert.  Also the plastic at the side of the release button has bubbled. It is showing 7.99v across the whole battery which is way to low and 0.04v, 0v, 3.96v, 0.04v and 3.97v across each of the cell pairs respectively. Upon opening up the battery there is obvious damage to the second pair of cells.
2. Battery Two: Fails to charge with classic red / green flashing lights. It is 12.84v across the battery and 3.2v, 0v, 3.22v, 3.22v & 3.21v across each of the cell pairs respectively.
3. Battery Three: Fails to charge with classic red / green flashing lights. It is 15.54v across the battery and 0v, 3.89v, 3.89v, 3.89v & 3.89v across each of the cell pairs respectively.
4. Battery Four: Fails to charge with classic red / green flashing lights. It is 18.15v across the battery and 3.62v, 3.63v,3.6, 3.63 &3.62v across each of the cell pairs respectively.

By way of comparison a known good battery seems to measure around 20v when freshly charged.

So I think my best option to end up with an extra working battery is to use the board from Battery One and fit it to the cells and temp sensor from Battery Four.  Changing the board should be pretty simple it just requires de-soldering.  Fixing the other batteries is going to be a little more challenging. Up until recently replacement circuit board were not available so once you had seen the red / green flashing lights three times your battery was bricked.  However recently they have become available for a round £10 which is certainly much cheaper than a replacement battery.

That still leaves the failed cells and whist it is possible to solder cells together the heat required to solder them risks damaging the cells. Having investigated purchasing a spot welder I feel that is out of my budget but I may have a go at making a capacitor discharge one.