The red light is flashing fast because it didn’t recognize your battery pack. Instead, rapid flashing red light indicates something wrong. So you try other charging spots, no luck.
The system detect a problem and won’t accept a charge. That’s voltage drop in the pack. DeWalt chargers don’t see cells below some minimum, they just won’t even try. And when voltage become too low, the electronics lock up. This is common with packs sitting around doing nothing for long periods.
How do you get the internal circuitry going again and not damage anything? Most folks bridge it using another working battery. Take a healthy, full charge pack that matches yours (same number volts). For example, use 36V or 40V pack to share charge from the good pack with the bad one.
Before doing anything though, you need to figure out your terminals, get those wrong and you will seriously damage something straight away! First find positive side by checking voltage with a digital multimeter and then probe around the contacts to confirm which is which. This is a small task that only take seconds but can make you feel much safer.
Executing The Battery Jump Start
Once you have found the terminals you are ready to connect some jump leads from positive to positive and negative to negative. This provides solid connection, but it is only meant to be an easy and temporary one through a length of stripped back wire. Ensure there is good contact but it should of be short lived. No one wants these connected permanently… Just long enough to fool the electrics into believing everything is fine and dandy.
Then you simply connect the leads for just fifteen seconds. The current will flow from the full pack into the empty one. As you watch the multimeter, you’ll notice a very small increase in voltage on the dead pack.
Fifteen seconds appear to be the magic number. If you go longer than that, you risk stressing the batteries too much so you patiently count down the seconds until you reach fifteen. At that point as soon as you remove the leads, the jump start is complete. There is no heat, no smoke, no warning lights, or sparks. There is also no sense of instability while the two are connected; it feels solid. After removing the packs, you leave them alone to settle down for a bit.
To test if the trick realy did work, you reinsert that previously dead battery into the DeWalt charger. The LED comes right on; instead of flashing quickly and frantically, it slows down and begins to blink steadily. Slow red blinking indicates that the pack isn’t being ignored anymore and charging has started again in a normal way.
You wait a few more minutes just to make sure, watching the indicator steadily blink, and it doesn’t stop, it’s still on slow. Bingo! The internal protection circuitry had reset successfuly, and your first attempt at this fix pay off here. The battery will accept charge as good as new once more. You can check the label on the charger to confirm that the status codes match what you’re seeing.
Everything goes smoothly from here with the charging process, and the cells gets their juice like they’re supposed to. There are no more annoying cold/hot delays because the system accepts the pack as a good, usable one. It will eventually go to solid when full, but in the meantime seeing that slow blink makes this victory enough.
Later that day, the battery charges up and keeps your tools going just like it’s supposed to… Just like it should’ve done straight from the box. There aren’t any oddball glitches or dips in power, nothing to throw wrenches in your project work. You can continue without stopping.
