the only way to be certain about the BDI is to hook up to the machine with PC Service Tool and check all the settings related to tbe BDI.
The default spec on the batteries is 110AH rating. If those new batteries are not that then the settings in the BDI/Controller may be wrong causing this issue.
In most cases a default reset on the system will fix any irregular issues but you still may want to verify all the settings are correct before spending that amount of money on a new BDI when it may not be the issue.
Note: doing these suggestions are only recommended after the batteries, battery charger, and all the cables and connections are verified to be good.

If the batteries have water and are showing charged with your DMM, then the batteries are likely not the issue and the BDI may be faulty.

No such luck. Only have a multimeter.

Do you have a Curtis handset? Check what voltage the controller is reading.