How did they test it was 100% charged? did they do a specific gravity test (if it was possible) on the electrolyte?
if you have a couple of cars and you can give a battery 2-3 more years life the charger can pay for its self within the life of both batteries
Even if it is possible to get at the electrolyte modern car batteries have a problem with stratification. You need to charge them in a way that stirs up the electrolyte. Trickle charging will never get to full charge.