since I am not familiar with how Hyster works, usually the AC drive motor controller will send a CAN or serial message to the BDI of what it calculates the battery percentage to be, which it calculates depending on voltage and voltage sag during loading, etc. The BDI only displays what information it receives. The BDI is not normally making the decision and is not normally connected to the battery to measure the voltage. I believe it would be best to contact Hyster for advice. if you have a handset or software to communicate to the motor controller, you should be able to see in its diagnostic screen what it is measuring the voltage to be or calculating the percent battery charge to be.

Battery open circuit voltage is 37.51 V batteries charged at 70% but battery indicator is showing 20% need to reset battery indicator

what do you mean by "reset"? perhaps it would be best to contact the manufacturer for support.