Here's the thing...

After fully charging the batteries the BDI shows full charge and the actual battery voltage is 26,19 V.
I have load tested the batteries and three of them got 4.2 and one got 5,0 Volts.

After 10-20 mins of work BDI suddenly goes form full charge to full empty. The voltage is then 25,4 V and the battery icon flashes. Then I charge the batteries for 1 minute just to raise the voltage above the 26 V. After the key on the BDI goes to full charge again.

I've checked the power on the terminal 2 and the difference is 0,55 V. Also I've checked all the contacts on that line and all seems fine.

The batteries are weaker but it seems that they are not done yet. If it weren't for the BDI the machine would work much more.

Should I replace the batteries just to satisfy the BDI?
Is there some other way to take the most from them??