You cannot, really, overcharge a lead-acid battery, certainly not with a trickle charger.

If you try to charge a lead-acid battery too far above it's full charge voltage, and assuming the charger has the POWER to do so, all that will happen is that you will BOIL the electrolyte away. Even should that happen you can refill the battery, recharge it over 3 or 4 days and it can be put back into service.

Trickle chargers simply do not have the output wattage available to raise the electrolyte temperature enough that it begins to outgas, boil off.

 

Toyota TSB PD091-06 is information for mainetance of HV and auxillary batteries. Someone on another site asked me about this and this is what I found for her. Goggle Toyota TSB PD091-06.