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. So why didn't it? We had to find out. 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:

  • 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, therefore the bulk charge doesn't deliver as much charge to the batteries as it could. We measured 14.42V at the charger during bulk charge, but the voltage at the battery was only 14.29V. This is a significant difference, believe it or not! Further, the converter stops bulk charging in 4 hours, even though its own manual says that 8 hours are needed to return battery to a 90% charge level. Therefore, running the generator for 4 hours would in practice recharge the batteries to about 85%.
  • As soon as the bulk charge is over, the converter drops the voltage to 13.6V at the converter, causing the amps delivered to the battery to drop even more. Progressive Dynamics calls this “normal mode", designed to “safely" finish the charging. They are afraid to cause gassing in the battery, and hold the voltage at a substantially suboptimal setpoint. The converter holds the battery at this lowered “normal" charge voltage for a set period of time (for 28 hours actually! – so it will never be reached on a generator), then switches to the float charge of 13.2V. The manual states that 80 hours on grid power are needed to restore batteries to 100% charge! 80 hours. This is 3.5 days plugged in continuously.

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 started out 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, and the voltage drop over the 6 gauge wire is so large that it could not deliver a proper bulk charge nowhere near the rated 60 amps. I ran the generator for 3 hours and barely got the batteries to 85% charge. I think that in order to get 60 amps out of this unit, the battery would have to be at 10 volts, meaning completely discharged.

So the main message here is that RV converters are not designed to recharge the batteries quickly. As we saw, the manual for the Progressive Dynamics converterĀ 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 80 amp one will simply reach the 85% charge point a little sooner, and will then hover at 13.6V longer. Ultimately the remaining 20% of the charge will still take several days, during which the controller will not deliver more than 3-4 amps anyway. It is because it delivers about 30% of the total charge in the very slow “normal" and float modes.

What it means in practice is that, unless you are plugged into the grid all the time, you will be operating your batteries between 65 and 85% of full charge all the time. This will cause premature battery failure, and will cause you anxiety of running out of power during prolonged cold weather camping trips.

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.

Next: The charging profile that you really want.