Amp Hour (AH) Usage and Budgeting

In the previous posts we looked at the capacity or rating of the various chargers and components in the camper‘s electrical system and concluded that possibly the system has too little charging capacity given the size of the battery pack.

How does the system work in practice, given actual usage?

Use the generator for high draw appliances

The very first lesson we learned once the FLEXNet was installed was that the microwave oven (particularly in convection mode), the toaster and the coffee maker apply high current loads to the batteries through the inverter. For short duration use the net AH consumption is small but the convection oven in particular can easily consume over 100AH in a single use if the cook is baking something elaborate. The lesson - whenever possible run the generator to power the oven, toaster and coffee maker.

Solar only makes a modest contribution

The second lesson was that the solar panels only make a modest contribution. During the summer we were driving across Canada in generally sunny weather. Though Canada does not have what you would call direct overhead sun. We rarely saw more than 100AH per day from the solar panels and charger, and the very best day was 130 AH. Days of cloudy or rainy weather dropped that back to 30AH. Later in the year (Oct, Nov, Dec) we were in the South West (Arizona, New Mexico) with lots of sun but, less hours of daylight and at a lower angle. During this period the solar system delivered only around 80 AH per day.

The refrigerator is a substantial consumer

The refrigerator draws 5-6 amps when the compressor is running. During hot weather it can run for up to 20 hours per day and hence consume as much as 120AH per day. Cooler weather reduces this consumption but not by as much as one might expect as the interior of the camper easily maintains a 70+°F temperature.

Usage: 100-150AH a day

By using the generator for high draw appliances we have come to expect a daily usage of 100-150AH. That is up to 20% of the battery bank capacity.

Replenishing the batteries

While on the move

We have been able to replace that daily usage when we have a good sunny day combined with 4-5 hours of driving. The contribution from solar, a contribution from the alternator (without the diode isolator getting in the way) and a modest (up to 30 minutes) use of the generator will typically bring the batteries back to full charge.

While NOT on the move

On a sunny day without driving we can typically regain 50-60AH bringing the daily deficit into the range of 70-100AH. Which gives us an endurance of around 3-4 days without lengthy periods of generator use. This is based on a maximum discharge of 330AH (50% of the battery pack capacity)

Recovery from deep discharge

To date we have not at any time drawn the batteries down by 330AH or to 50%. Hence we have no experience of recharging from such a discharge. However we estimate that recovery from such a state of discharge would require the Outback Inverter/Charger to run for 8+ hours. This effectively means we would need a night in a campground with 110v/60Hz power or we would need to run the generator for 8 hours. I am are skeptical that solar, the alternator, nor the ChargeMaster (world charger) could, in any combination, recover the batteries (and continue to run the camper) from such a deep discharge in any reasonable time frame.

Shallow discharge, charge capacity

To date we have used our camper in a manner that recovers the batteries from a typical 100-150AH (20% of capacity) deficit each day. As a result of this practice the various chargers we have on board the vehicle do not spend much time in the bulk phase of the charging cycle, where their maximum current output can be utilized, but almost immediately go into the absorption phase. Consequently we have not felt the effect of any apparent lack of capacity in the Outback Inverter/Charger.

Because all travel to date has been in the US/Canada we have little experience with the ChargeMaster world charger.

The "last" 100 AH

Putting that last 100 AH back into the batteries (from 85% charged to 100% charged) is slow because at this point in the recharge cycle the chargers are in their absorption phase and charging current levels are down to 20-30 AMPs. This phase can easily take 4 hours. It is in this phase that the solar system and the alternator (via the Sterling Battery-Battery Charger) come to the party - those devices can complete the "top up" of the batteries while driving on a sunny day. But without the combination of sun and driving, the top up would require 4 hours of generator or shore power.

Inefficient use of the generator

It has taken me a long time to come to the following realization.

Our 3000 watt generator is much bigger than is required for recharging the house batteries. Even when the Outback charger is putting our 100 amps of charging current it is consuming less than 2000 watts of AC power. Thus if battery recharging was the only mission for our generator we could easily get by with one of those little 2000 watt Honda models. And certainly a smaller quieter generator would be very convenient for replacing those last 100 AH because during that phase less than 1000 watts of AC power is required.

I will have more to say about generator choice in a future post, but for the moment let us complete this discussion by noting that the generator has been sized to meet the power requirements of the camper A/C unit NOT to meet peak battery recharging needs.