In our last post, we went over our process for selecting the Victron Quattro inverter. We discussed a few of the benefits offered by this inverter as well as gave an overview of how we wired everything. Now we finally have enough of our system hooked up to run a few tests and verify that things are working as designed.
The first order of business was to charge the batteries up. Our Nissan Leaf batteries have been amazing at holding their charge. We took readings on the cells when we first took the Leaf battery apart and got readings of 3.99 Volts. The batteries have been sitting idle for sixteen months since that initial testing, and they were still reading 3.97 Volts per cell.
After all this time, we were finally ready to charge the batteries up. We hooked up our 50 Amp shore power cable months ago, but didn't have everything hooked up to test it. Now we do. The battery charger in our Victron 48/5000/50 100-100 120V is fully programmable and capable of delivering up to 70 Amps of power into the batteries. Like most modern high-end chargers, the Victron charges the batteries in stages. These stages are bulk, absorption, and float. The stages exist because of limitations in the way lead-acid batteries are made and how they accept a charge. Research has shown that in order to get the most out of a lead-acid battery system, these stages must be followed.
But our batteries are lithium - do the same charging considerations apply? The simple answer is "no". Lithium batteries like the ones we have powering our RV are much more simple to charge. Luckily for us, the Victron charger is fully programmable. Victron even sells a couple lines of lithium batteries that are fully compatible with their system. So we set our charger up as if it was a Victron lithium system and tweaked the settings for our battery. What this consisted of was a linear charging curve that terminates at 57.4 Volts (4.1 Volts per cell) at 52 Amps. Our absorption time was set for an hour and the float voltage was set at 54 Volts.
Finally, we plugged everything in and crossed our fingers. To our relief, everything was very well behaved and charging started just as expected. We kept an eye on the BMS, our temperature probes, and the output of our Victron Inverter on the laptop. Charging was complete in about three and half hours.
We were itching to see this battery power just about anything. We have our 120V wired up and roughed in but only have a few things actually fully hooked up. Fortunately, one of the things we do have hooked up is our mini split air conditioning system and our small air conditioning system dedicated to cooling the battery and inverter.
Running air conditioning on battery and solar power is kind of the "holy grail" of off-grid RV systems. So we decided to run our A/C system and see how things went. The first thing to note is that it was an absolutely beautiful day in Arizona - 74 degrees and slightly breezy. Seriously, it would have been difficult to ask for a nicer day. Great for us, but this means that the A/C was not going to generate much of a load. We have the inside of the bus completely stripped and not much in the way of insulation. Even though it was 74 degrees and breezy outside - inside the RV it was an even 80 degrees. We set the thermostat for 70 degrees and commenced with the test.
The total amp draw while running both air conditioning systems (the small one in the battery bay and our "house" mini-split) was between 10 and 12 Amps out of the 57V battery for about 35 minutes to cool down the bus. About 40 minutes after we started the test, the inside of the bus was a cool 70 degrees and we still had 99% of our battery capacity left. At that point, we were only pulling about 1.5 Amps out of our battery (we had enough battery to last about 300 hours at that load). The one huge plus of this is that we intend to chase weather as close to this as we can. We won't mind a single bit if our A/C system doesn't have to work very hard or if we can get by on just opening some windows and running a few vent fans. :)
All that being said, this was not a very good load test at all. The most Amps ever drawn out of the battery was only about 12 Amps and the temperature of the battery didn't move at all.
One week later it was 92 degrees outside. In the bus, we saw 97 degrees. This would be a much more formidable challenge for our system.
We charged the batteries up to 57.4 Volts. Again we set the air conditioner to 70 degrees. Now instead of cooling down 10 degrees, it would need to cool down 27 degrees. The air conditioners would have to work much harder. This was immediately obvious by the number of Amps that were being drawn out of the battery. On the 74 degrees day, both our A/C systems pulled a max of about 12 Amps total, but now we were seeing loads in the 30 Amp range (@57V). After about 30 minutes our load had dropped to about 20 Amps and the inside temperature had dropped from 97 to 84 degrees. We let the air conditioners run for another hour (about an hour and a half total), and the inside cooled to about 74 degrees. At this point, we were pulling a steady 16 Amps from our batteries and it was starting to decrease as seen in the test before. Our BMS indicated that we had 91% battery left.
While we hope to chase that perfect weather, we know that isn't always possible. Especially if we come back to Phoenix anytime between March and November - ha! It's good to know that our system will hold up to the hot days.
What We Learned
These tests were rudimentary, as we are still busy building out the bus and fixing things. They mostly served to give us confidence that the system we designed is working how we expected it to work (this is the first time we have designed an RV power system after all :)). Even at the higher load we never saw our battery temperature raise even half a degree. And the system never behaved in a manner that would cause us to question if something was wrong. We are optimistic that we can move forward as planned and that we can expect good results from our batteries and inverter once we hit the road.
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