which voltage is your truck? 72? or 80?
in either case if you read 72v using volt meter on battery it is low and it should be charged.
If you have already attempted to charge it and voltage does not increase you may have a bad cell not letting it charge up fully.

You need to know exactly which voltage your truck is, the dataplate should show you this. If it shows 72/80v and the tags on your motors show 72/80v then you need to know what voltage your battery is on the battery itself.
Count the cell caps on the top of the battery, if it has 36 filler/vent caps then the battery is 72v, if there are 40 filler/vent caps then of course it is 80v. Remember an old rule of thumb, to determine the voltage just count the caps and multiply by 2.

Then open logic card and check jumpers J1, J2 and J3 and make sure jumper is on correct pins.
J1 = 36/48v
J2 = 72v
J3 = 80v

Once this is verified you should determine if battery is problem or if BDI needs to be recalibrated. (in most cases it is a battery problem)
Most of the time these problems your having occur because of a defective battery or incorrect voltage battery installed and the truck jumper setting has not been set correctly.

If you are just complaining because the battery drains down after sitting for 40 days then your best course of action is to just unplug the battery from the truck when not in use. Something is draining it down if you are leaving it plugged in to the truck but not using it for such a long period of time. You could plug into the charger when not using the truck and run a refresh charge every week to insure it does not drain down.