Hi people!

So,a central warehouse has six MO20S and about a dozen batteries that are used every day.
One of MO20S has a problem that after 30 minutes of using truck battery indicator shows the battery is suddenly discharged to 20% and slows down to creep speed..
After several restarts (key on/off) the battery indicator is at 100% again and again after about 15-20 minutes of truck usage battery indicator again suddenly drops to 20% and creep mode is on...
Period between the "drops" are shorter as the battery discharges during use.
When battery is at about 50% of charge it's almost impossible to normally drive the truck,creep mode is always on...
This happens with any battery that is placed in the truck,all batteries are working normally in other trucks...
This problem is not mentioned in repair manual,i checked all connections,wires etc... everything is ok...

Any ideas what is the problem or where to look for it?

Tnx in advance