Reviving a dead power tool battery

Has this happened to you? You go to use one of your cordless power tools and find that the battery is dead. So you remove the battery from the tool and put it in the charger. That’s when you notice that its not charging! Why won’t it charge?

The voltage on your battery pack may be too low for your tool’s charger to begin charging. This may have been caused by leaving the pack discharged for a long period of time, to the point where the individual cell voltages have dropped below 2.5V. To confirm this, take a multi-meter and measure the voltage on the pack. If, for example, your tool uses an 18V battery pack , most likely the tool’s charger won’t charge if the pack voltage is lower than 12.5V (ie: 5 cells at 2.5V each minimum).

18V battery pack should measure at least 12V

Don’t throw out that battery pack just yet. You may still be able to revive it!

The first step will be to open the battery pack and have a look inside at the cells and the BMS board. Hopefully your battery pack is not glued together, but has screws holding it together that can be removed. If it is glued, then you will have to carefully pry it apart. Even though your battery pack does have screws, they may be the “tamper proof” type that require special driver bits to remove them. Fortunately a “security bit set” is inexpensive and can be purchased on Amazon.

Tamper proof screw and the security bit needed to remove it
Set of Security-Bits available on Amazon

Once you have the battery pack open examine the cells and the BMS board. Check to make sure that none of the individual cells or the board have any damage. Look for any deformed cells or burn marks on the BMS board.

For more information on how a BMS board works check out this article:

BMS (Battery Management System) board

Disconnect the wires from the BMS and measure individual cell voltages with your multi-meter. If they are below 2.5V that may likely be your problem. Shown in the photo below is a cell being checked which is showing only 0.03V. Cells discharged this low may still be able to be revived, but will likely have a diminished capacity.

Individual cell voltages are way too low!

First step in the revival process is to try and charge each cell individually. Use a battery charger that can charge multiple types of batteries. I use a SkyRC charger to charge the individual cells in this battery pack. This charger can charge Lithium batteries and NiCd / NiMH as well. Since these are Lithium cells I set the charger to Lithium mode. However, because the lithium cell voltage is so low the charger complains!

SkyRC charger in Lithium battery mode showing cell voltage is too low

In Lithium mode, this charger is behaving the same way as the power tool charger. You will need to raise the voltage of the cell to be able to charge it. One way to do this is to push some current into the cell while monitoring the cell voltage. This can be done on this SkyRC charger by setting it to NiCD /NiMH mode and using a low current setting such as 0.1A

Boosting the voltage of a lithium cell

Start charging and watch for the voltage on the cell start to rise. If the voltage doesn’t start to rise after a few seconds or the cell starts to get warm (touch it to check) then immediately stop charging that cell! You have a bad cell that must be replaced! Replacement cell can be purchased on Amazon and range from $2 to $10 depending on their capacity and quality.

Cell voltage starts to rise quicly

Once the cell voltage gets close to 3V you can stop charging in NiCd mode and switch back to Lithium mode. You may still get a low voltage warning and this is because the cell voltage has quickly dropped again below the threshold. No problem, just go back to NiCd mode and charge up to 3V again. Once the cell has had enough current it will keep up its voltage longer. Do not exceed 3.6V on the cell while in NiCd mode!

Cell has sufficient voltage for Lithium mode charging

Put the charger back into Lithium mode and try to start charging. If the cell has a sufficient voltage, the charger will begin charging the cell in Lithium mode. The charging characteristics in Lithium mode are quite different from than in NiCd mode. In NiCd mode, the charger will attempt to drive current into the cell regardless of the voltage across the cell. This is known as “constant current” charging where the charger behaves like a current source. This is fine for a NiCd cell as long as the charge current is 1/10 the cell capacity. When we try to boost the voltage of a Lithium cell using the current source characteristic of the charger’s NiCd mode, we are basically trying to force some current into the Lithium cell. This can be a safe process as long as the current is low (ie: 0.1A or 100mA) and you keep and eye on the Lithium cell voltage and temperature. NEVER try to boost a lithium cell’s voltage by wiring it in parallel with a good cell. The good cell will try and dump as much current as it can into the cell with low voltage. This can damage the low voltage cell or worse yet, could cause it to explode! You have been warned!

Charger in Lithium mode and has begun charge cycle

The charge cycle in Lithium mode begins with a constant current phase, where the charger will attempt to “push” a constant amount of current into the cell while the cell voltage rises. The photo above shows the fist cell being charged at 200mA (ie: 0.2A) and the cell voltage has risen to 3.08V. This constant current phase will continue until the cell voltage reaches 4.2V. It is at this this point that the charger will switch to a “constant voltage” cycle. During this constant voltage phase, the charger will maintain the cell voltage at 4.2V while the current going into the cell slowly decreases. The cell is considered fully charged once the current decreases to zero. At this point the charger will sound an alert beep to signal you that its done!

Charger has finished charging in Lithium mode

Now that you have successfully charged one cell, you can repeat the process individually with the other cells in the pack. When attaching the charger to each cell be careful of the cell polarity orientation! Make sure that you attached the positive lead of the charger to the positive end of the cell. Once all the cells have been charged, you can again take a voltage measurement with your Multi-meter across the whole pack. The meter should read 4.2V times the number of cells. In the photo below there are 5 cells wired in series and the meter reads 21V. This is the correct voltage for a fully charged 18V battery pack. Note: Lithium batteries have a nominal voltage of 3.6V. Honest manufacturers will market their 5 cell packs as 18V 🙂

All cells are now fully charged and the pack shows 21V

Now you can put the wires back on the BMS board and this time check the voltage on the terminals of the BMS board. The photo below shows that the voltage on the BMS board terminal is now 21V. We are now ready to do the final discharge test to determine this pack’s remaining capacity.

Voltage on battery BMS board terminals is now 21V and fully charged

Connect the pack to your charger and do a discharge test. The photo below shows that the test completed using my SkyRC charger and that the capacity of this pack is about 1A (ie: 1090mA). This is still pretty good considering that the original capacity of this pack was 1.5A. This pack should still have a useful life.

Discharge test complete, capacity still ~1A

At this point, go ahead and recharge the pack via the BMS terminals and take note of how much current was used to charge the pack. A slightly higher value than that what was found in the discharge test is a good thing and indicates that the pack is recovering still further with each charge/discharge cycle.

Battery pack has been recharged with 1211mA

Go ahead and resemble the pack back into its case. As a final check measure the voltage at on the terminals of this battery pack. As shown in the photo below, this pack is ready to go back into service!

This pack is ready to go back into service

Here is a list of the tools and equipment used to restore this battery pack. Please use the links provided should you wish to purchase your own charger or security bits.

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