8 amps or rather 4 amps each, into an 8d battery is next to nothing. Consider the amperage that the battery is capable of in comparison. However, the 8 amps in relation to what state the battery is in at that time is your concern. A 50% discharged battery getting only 8amps and something is wrong, conversly a fully charged battery with 8 amps continueing to be pushed into it is a problem to. Bring your batteries to what you think is fully charged with your onboard charger, then disconnect them at the battery posts and let them sit and rest for 24 hours to let the electrolyte equalize. Now take a hydrometer and note the spcific gravity of each cell. This will let you know the state of charge of each cell and the battery as a whole. If the battery is fully charged, great the charger did bring it up to full charge. Now hook up the charger and if its a smart charger, it will go through its entire charge profile but then settle down to off or 1 or 2 amps. If not there is something wrong with the charger. Buy a hydrometer and a good one at that, you'll use it frequently. Btw flooded batteries self discharge at a pretty good clip in warm temperatures.
