Current flow based on voltage difference
Hard to give a definitive answer to this without seeing a schematic. If as you say there are diodes between the battery and the 28V supply / load (as shown below), and IF the 28V supply voltage is indeed greater than the battery voltage, the diodes will be back-biased and no current will be drawn from the battery, and you will not accidentally charge the battery.







simulate this circuit &ndash; Schematic created using CircuitLab

If the 28V droops though, perhaps due to excessive load, the battery will begin to start supplying current when the voltage falls to a level where the diodes can conduct.

That should not be an issue though. It should self balance and the load will draw split current from wherever can supply it.

However: If you do not already have so, I would add another diode from the supply to the load ( in position D2 of the schematic above) to ensure that when the supply droops or turns off.. you do not end up driving whatever it is from your battery.