HP, I would bet that you have at least one dead cell in your battery string. They will be worthless under load but act as a decent filter capacitor for the phase control amplifier to look at a smooth voltage. If your input dips, it might take a few cycles for the firing angle of the SCRs to skew to make up for it. That's my guess from the info that you have provided. Most of the newer chargers run SCRs these days. Joe
