When my Kobalt 40V battery flashed red on the charger, I thought that it was done. The light simply stayed red on the LED and did not change to the normal green charging light. I tried to remove and put back the battery a few times even left it on the charger a bit longer, but the red only kept blinking.
None of those efforts helped.
I chose to research the problem before simply tossing what should be a good battery. It is a model of 2.5 Ah that I think still has a lot of life left, because I did not use it too often. When I pressed the button to check the charge directly on the battery itself, not all four green lights up.
That simply did not match the situation. According to the battery itself, everything was fine, but the charger still refused to acecpt it.
How to Jumpstart a Kobalt 40V Battery That Flashes Red
I found a way to revive such batteries that do not charge normally. The process requires a second Kobalt 40V battery in good shape, together with two wires with clips on the ends. First I found which terminal of the battery is the positive and which is negative.
A little plus symbol is pressed in the plastic body at the positive contact. It's easy to miss if one does not look carefully.
I laid both batteries on my work surface, with terminals upward and easily reached. The dying battery that did not want to charge was on one side, while the known good one was on the other. Them side by side helped a lot for matching the terminals.
I checked everything twice before connecting.
With one wire I connected the positive end of the good battery to that of the dead. The negative ends were connected with the second wire. The wires must touch flat on the metal terminals of both batteries.
It does not need a perfect link, only stable enough to bridge between them.
The batteries stayed connected like this for around fifteen seconds. In the video I warned clearly: disconnect right away if the wires start heating or smoking, to avoid a short circuit. I spent the whole time watching, to make sure that nothing warms or goes bad.
The wires did not get heated, and nothing strange happened during those seconds. It went well.
After that, I pulled both wires away from the batteries. The idea is that the good battery gives only enough voltage to wake the safety circuit in the dead one. Whether that exactly happened or not, I checked if the charger now accepts the battery.
I took the one that before flashed red and put it back in the Kobalt charger. As soon as it sat in the charging spot, I looked at the LED light. I really expected to see the same red glow, which would mean that we are at the start again.
It worked finally. The charger accepted it and fed energy without problems.
I left the battery on the charger and watched as the green lights up while it charges fully. The red warning did not come back. It did not need special gear or hard repair, only a quick voltage push from another battery from the same series.
The whole revival cost maybe two minutes of real work, after I had the second battery and the wires ready.
