The charger won’t recognize deeply discharged cells.
Required Tools and Preparation
A dead 18V battery pack can be revived with healthy donor unit that boosts its voltage. This allows you to restore function without buying anything new and keep you from wasting a perfectly good battery.
To complete this repair, you’ll want to have a multimeter, some jumper wires, and a healthy (compatible) battery for donation.
On the battery side, first determine whether the battery is actualy dead by checking the voltage read out using a multimeter. If the battery’s discharged down to the point where it can’t hold a charge, the voltage readout will be very low and thus cause the charging circuitry to be confused.
It doesn’t know what to do with such a low voltage so it just shows an error light rather than trying to charge. And it does this abruptly, effectively shutting off your tool until you remedy the problem.
Now get yourself a healthy battery pack of the same or similar voltage rating (within the voltage range). Any compatible 18V to 20V battery will do. I like to use a Bosch pack (or similar donor pack) since the voltage difference create the energy transfer.
Connect both batteries together using jumper wires, positive-to-positive and negative-to-negative. Now the current flows from the good battery into the dead battery and increase its voltage so the charger can read it again.
When connecting the leads take your time. Watch the wires for any sign of smoking or overheating when transferring power. In general, nothing go wrong as long as things remain cool, but if you see any heat, quickly pull the leads apart before you cook them out.
Leave both batteries plugged in for about 15 seconds; typically that’s all it takes. It will wake up the cells on one pack and raise the voltage a little bit. This will clear the codes and reset inside circuits so they won’t shut down again.
After boosting, remove both batteries, and put one of them (the dead one) on your charger. Both Hitachi and Hikoki chargers uses the same tech. They will work fine with each other. You wouldn’t of need any extra purchases or special equipment.
It will indicate that it sees your battery by lighting up solid red instead of flashing; it’s charging properly now. That tells you it has enough power to charge, and the voltage is above the minimum operating level.
Leave it on the charger while the light is solid red, which means the charger recognizes the battery and charging is proceeding normaly while the cells balance and equalize their voltage levels. During this time, the light will not be flashing. There will be no error lights. It won’t fluctuate while it is accepting the full charge cycles. It’ll just be solid red.
When it’s fully charged, the light goes green and lets you know it’s done. After taking it off the charger and testing it in a drill, you’ll notice good consistent power output with no sag under load based off the test.
