Ed
I put 2 4D AGM batteries on my 420 in June '02.Haven't had to do ANY maintainance to them since. As far as I can tell, they hold charge about the same as the 2 (regular) 4Ds that they replaced, and charge up about as quickly. I've had no overcharging problems; but I use a Link 2000R to manage and check on charging - discharging. The major advantage is... no maintainance. Disadvantage ... cost.