How to Safely Revive Power Tool Batteries

In this Instructable, I'll demonstrate how to revive/recover a Greenworks 40-volt lawn mower battery. This process only takes a few minutes, and you don't have to open the battery or replace any parts. The same process works on most lithium power tool batteries with different voltages.

All you need is a voltmeter to test your non-working power tool batteries to see if they can be quickly and safely restored.

The Problem:

Power tools are designed to stop running before their battery voltage gets too low. However, with older batteries, the voltage will drop too low. When this occurs, your battery charger will indicate the battery as DEFECTIVE and will not allow you to charge it. Refer to the video for an example of a charger indicating a bad battery.

In reality, there may be nothing wrong with the battery; its voltage just dropped a little too low. With a battery in this condition, we can simply balance it with another battery.

Power Tool Battery Balancing offers a quick fix for low battery levels by using one battery to recharge another. It takes about 30 seconds before the non-working battery's voltage returns to an acceptable level and can be placed back into the battery charger for a full charge.

There is a chance that the non-working battery has a bad internal cell, in which case it cannot be recovered using this Instructable's balancing method.

Let's determine if your battery can be safely recovered or not in Step 1.

• Voltmeter.
• Calculator (to find 85% of the batteries rated voltage).
• The non-functioning battery you need to revive.
• Good battery(s) to fix the bad battery.
• Wire (14 gauge or larger is fine).

First, measure the voltage of the battery you are trying to revive. Second, calculate what 85% of the battery's rated voltage is. Example: 85% of that Greenworks 40-volt battery = 34 volts.

If you need help locating which battery terminal is positive and negative, enter your battery model in Google and you can find the information.

• If the measured voltage is equal or less than 85% of the listed battery voltage (example: you have a 40-volt lawn mower battery but its reading 34 volts or less) then the battery is probably defective with a bad internal cell. Balancing probably will not work and should not be attempted.
• If the measured voltage is greater than 85% of the listed battery voltage (example: you have a 40-volt mower battery and its reading higher than 34 volts) then the battery is probably good, and you can attempt to balance the battery. Go ahead and move on to Step #2.

As far as the 40-volt Greenworks battery goes, you can see it's voltage is above 85% and is just a little under-charged. It passes the test and we can try to revive it starting with Step 2.

Since the Greenworks 40-volt battery passed the 85% test, we now need an identical known-good, 40-volt charged battery to balance it with. If you don't have an identical charged battery, you can use multiple batteries. Ensure that their combined voltage does not exceed 2 volts above the rating of your soon-to-be-revived battery.

Example: I did not have a second 40-volt lawn mower battery, but I did have two 21-volt chain saw batteries. After connecting the two 21-volt batteries in series (using a piece of wire and left-over volt-meter banana plugs), I measured 40.4 volts. This was a perfect voltage level needed to revive my Greenworks 40-volt battery. Please watch the main video for visual instructions on using multiple batteries in series with this battery balancing method.

Note: Don't worry about the AH rating of the batteries during this procedure. The balancing is either going to work or not after about 30 seconds of being connected in parallel. Of course, it is best to use IDENTICAL batteries with the same voltage and AH rating.

Parallel batteries will naturally balance since they are directly connected to each other.

CAUTION: ONLY HOOK THE NEGATIVE TO NEGATIVE and POSITIVE TO POSITIVE when connecting the battery needing fixed with the good, charged battery(s). See the above diagram.

Any wire 14 gauge or larger should work fine to connect the batteries in parallel. Having volt-meter banana plug ends will help, but they are not needed. It's possible to just use wire pushed into the battery's terminal area. If you have an extra set of volt-meter leads, those will work too.

Once the connection is made, it only takes about 30 seconds for the low battery to regain sufficient voltage to be usable.

Follow the connection method used in the main video. You can instantly see the voltage level of the setup when connecting the batteries in parallel.

After 30 seconds, disconnect the batteries and place the revived battery in its normal battery charger.

After charging the fixed battery, you can test its voltage to confirm if it's fully charged.

I've been doing this trick for a few months now with that Greenworks battery. Maybe I can make it last another year?!

Thanks for reading.