Soon after installing the solar system on our RV we have realized that the solar works wonderfully, but on the days when there is no sun, charging the batteries from the generator simply does not work well at all.

Why would that be? We had a nice 60 amp Progressive Dynamics converter with Power Wizard, which is supposed to make charging a snap, we thought. Ha, not so fast, dear RVer. After putting a volt meter on the batteries and the converter, reading the manual for it and thinking about how the batteries charge, the reasons became apparent. They are as follows:

  • The rated amperes on the converter are the maximum it can put out during the bulk charge time. The bulk charge time is relatively short and replenishes only a portion of the discharged battery capacity, up to about 80%, even in a properly designed charger. This problem is even worse in our PD unit, which does not account for voltage drop in the wiring and therefore stops the bulk charge even sooner.
  • As soon as the bulk charge is over, the converter stops increasing the voltage, causing the amps delivered to the battery to drop even more. The converter holds the battery at lowered absorption charge voltage for a set period of time, then switches to the float charge.

I have watched the amps going to the battery on the Trimetric monitor during a blizzard once, when there was no solar charging at all. My batteries were at 65% charge from the overnight running of the furnace, and the PD never delivered more than 12 amps. Its voltage setpoint is so low that it could not deliver a proper bulk charge. I ran the generator for 3 hours and barely got the batteries to 85% charge.

RV converters are not designed to recharge the batteries quickly. The manual for the PD states that the converter requires 80 hours to bring the batteries to full charge. This, coincidentally, means that the amp rating on the converter does not matter at all: a 20 amp unit is just as useful as an 80 amp one. The converter almost never deliver more than 3-4 amps anyway.

But the marketing departments earn their keep: the more amps listed on the product page the better, right?! So the consumer pays more money for the amps that they will never see, and will never be able to use.