Makita Battery Repair

This began as a project to repair several "dead" Makita LXT batteries, although once I realised most of the internal components were not salvageable, it became a case of converting the working parts into a single comically oversized battery.

While it is definitely inconvenient for smaller drills and impact drivers, on larger SDS drills it's much less noticeable, and tools with higher power draw like angle grinders also see a benefit in the higher available current and seem to stall less easily than with a regular battery.

• Micro T10H screwdriver
• High power soldering iron (Or spot welder)
• Metal tabs (Can be cut from brass sheet)
• 3mm Acrylic sheet (40cm x 4cm)
• Heat gun
• 2-Part rapid epoxy

As with many types of cell chemistry, once the voltage of lithium cells falls below a certain point they are likely to become irreversibly damaged. For lithium cells the quoted figure for the minimum voltage cells should be discharged to is usually around 3.3v, however, cells will always self discharge even when not in use; a process which will occur faster in damp environments. Many of the cells I recovered from the batteries were below 1v for this reason, and the worst offenders usually showed some signs of corrosion.

Makita batteries add an extra level of complexity to the repair process. Once a certain number of attempts to charge the battery have failed (Due to the charger not accepting batteries of such low voltage), the charging board within the battery will be locked out, preventing it being used even if all the cells were swapped out for new ones. If this is the case, 3rd party replacements can be bought for under £10, although I did not need to source a new one for this project.

Of the 3 batteries I had, only one had a working PCB, although the cells themselves were old and rapidly lost their charge, so I began the process of dismantling all 30 cells (10 per battery) to determine how many were still in a usable condition.

Makita batteries are usually held together with a set of four T10H screws. Due to the narrower recess that the screws are in, you'll need either a dedicated T10H screwdriver or a micro screwdriver bit set. Standard 1/4" bits will be too wide for the hole. One of the holes may have a resin cap which will need to be drilled out first.

It's important to avoid shorting any of the terminals once the case is off, so it's best to ensure the battery is discharged first, although a "working" battery will still be around 15v when discharged. If the battery pack doesn't hold any charge, or the voltage is significantly below 15v, some of the cells are likely to be damaged. At this point, you can begin separating them to test them individually; I found that a pair of small metal snips was the easiest option, and began prying the Nickel strips up carefully just enough to get one side of the snips under. This can be risky on the positive terminals because without due care, one side of the snips may end up piercing the plastic wrap of the cell and creating a short circuit between the +ve and -ve terminals (since the -ve terminal forms the entire outer shell of the lithium cell.

If you have access to a battery spot welder, it is recommended to remove the Nickel tabs, although I opted to leave the tabs in place as detailed in the next step.

To test the cells, I fully charged them in my 18650 cell charger at a low current, although some cells were below the threshold to be detected as lithium cells, and I had to manually charge them to around 3v from a DC source before allowing the 18650 charger to charge them all the way. I left all the cells for around 2 weeks to see which retained their voltage.

Of the 30 cells I recovered, only 18 stayed above 4v, although I retained another 5 cells that had dropped to around 3.8 - 3.9v since they would be useful for "throwaway" projects. The remaining cells had to be recycled at a supermarket battery collection point since they had dropped far too low to be useful for any purpose (most of which had lost their charge after just a few hours, let alone weeks).

After re-wrapping any cells with slight amounts of surface corrosion from other cells that had leaked onto them, I arranged the stack of cells in a 5x4 arrangement and secured them temporarily with duct tape. Typically, Nickel strips are used for battery tabs due to spot welding relying on a high resistance (Relative to Copper) to heat the metal to a point where it welds itself to the battery. Brass on the other hand has a higher conductivity and was readily available to me at the time of this project.

The main risk of soldering tabs is the heat effect on the cells. Soldering irons can easily exceed 400°C; Well above the safe limit of lithium cells, although I mitigated the risk by pre-tinning the Brass tabs and those left on the cells and using the highest heat setting for the shortest amount of time. The short soldering duration, combined with soldering to the cell tabs rather than the cells themselves meant that each solder joint only required 2-3 seconds of heat and transferred much less heat into the cells themselves due to the lower surface contact area of the existing spot welds. The hasty soldering did result in some messy solder joints, but they were all still electrically sufficient.

One issue I had not accounted for was corrosion forming at the solder joints after a couple of months, this may be due to insufficient cleaning away of the flux, so I gave the battery pack a thorough cleaning with Isopropyl alcohol spray and using an old toothbrush to remove any remaining corrosion. It may also be possible that the contact between the brass and nickel strips was causing some sort of galvanic corrosion, although it seems less likely. Regardless of the cause, this inspired me to re-build the battery case extension so that any new corrosion would be visible from the outside.

3D printing is one option available for creating spacers to extend a battery's case by the height of one or two rows of cells, however, using a strip of 3mm acrylic was far quicker, stronger, and has much better transparency than 3D prints.

I used a circular saw to cut a 40cm strip down to a width of 39mm although it is possible to score acrylic using a steel ruler as a guide, and snap it along the score line. Beginning with one end at the centre of the case's release mechanism, I began the process of heating and bending the acrylic strip around the corners of the battery (Removing the acrylic from the battery pack each time to avoid heating the cells). Once I had reached the start point, I marked it, cut off the excess and joined the two halves using a square offcut of acrylic and some superglue. A strip heater would have been a better option as it's easier to get better bends without causing the acrylic to bubble.

It is possible to achieve bubble-free bends with a heat gun, but the acrylic I used produced strange crystalline artifacting even before softening, so I accepted the acrylic would only be clear on the flat sides and carried on without much worry for the bubbles.

The final steps to reassembling the case were to cut a notch for the battery release latch to travel into, and to secure the acrylic to the base using a 2-part epoxy. After this I used some 60mm screws (Salvaged from an old Xbox 360 heatsink) to clamp the top of the case together again.

As a finishing touch, to hide the residual epoxy and theme the new extension into the old battery case I masked off the centre of the acrylic and cut a jagged pattern on each side to give the impression the two halved had been "torn apart" as if by the sudden arrival of 10 new cells.

While this project used almost everything from the old and broken batteries, I still have several empty cases and dead charging boards which I plan to use for future projects such as a smaller 1x5 row Makita battery, as well as experimenting with a USB-C socket and 5v boost converter PCB to allow the broken charging boards to be used again (Even if only for their 18v terminals).

I also plan to add a battery life indicator to the front of this battery, though with less granularity than the standard 4-bar gauge on most power tool batteries. Currently I'm looking at retrofitting a laptop battery charge indicator which, depending on the brand, may give anywhere from 6 to 10 individual lights to show the current charge level.

Despite the case being that of a 3Ah battery, the cells all originated from higher capacity batteries, putting the total capacity around 8Ah (Accounting for the age of the cells), although I have no easy way to verify that!