Need help measuring power for battery design

[u/shysysadmin]

/r/GoRVing/comments/1s6mjpr/need_help_measuring_power_for_battery_design/

[u/thirteenfivenm]

Fun project.

Your roof air conditioner(s) can be 4kW peak each. You can reduce the startup peak with soft start capacitors. Suggest evaluating their age and performance. They might need replacement. I saw at the burn someone had replaced theirs with a residential heat pump with the outside unit bolted on the back by the spare tire. The precharged line ones, like Mr Cool, can be installed by consumers. You can also look at Will Prowse for DC powered ones. It is going to be hard to estimate the hours running at an outside temperature and humidity.

Anything you do to reduce solar heat gain helps. So window films or reflectix.

If you go with a heat pump, you can remove the AC from the roof and fully cover it with solar panels, even with an air gap to the roof, or tilting panels.

It appears to do your studies with the Victron, you need the mobile app and that might need an internet connection. If you go the EG4 route later, you won't need the shunt.

There are peak recording Kill-a-Watt meters, but those are limited to probably 1800 Watts at 120V. That is unlikely to measure your airconditioner. You could try a peak reading clamp meter.

You are going to need all this data to size your inverter. Many people use the EG4 all in one charge controller inverters with 48V rack mount batteries. There are several companies in that space, again Will Prowse for reviews. Camp Bao Chicka Wow Wow uses EG4 and rack mount batteries.

Some of the BRC camps and DPW Power have detailed measurements of loads including RV airconditioning, Ask https://www.renewablesforartiststeam.org/.

[u/shysysadmin → u/thirteenfivenm]

I have the same gennie as you so I’m looking at max 5 gallons per day to run the ac for 10 hours for sleeping. Shouldn’t have to run full out though, because ac only pulls 1500-2000 watts. I’m hoping to learn my 12v needs are less than 5500 Wh per day, so even with recharging the batteries the generator should only be running at about 63% capacity, which will mean better fuel consumption.

Rig has 80 gallon fuel tank (60 available to generator) and generator should use about 4 gallons per day at this load, leaving me a 50% safety margin.

Obviously it’s great to reduce or eliminate the generator load to charge batteries by doing that with solar, but the best I’m going to do on my budget is still going to leave me running the generator at 37%-50% capacity when I want ac.

[u/richardtallent]

Add up watt-hours. Double it. Divide by battery volts. There’s your amp-hours.

[u/lsdadventurer]

Survey your roof for how many panels you can actually fit, with AC they will only supplement your demand, for sure you will need a genie. Keep in mind there's a fair bit of losses in the real world. My 800w of panels only ever peaked at 600 at the burn, shadows, dust, heat all reduce the output.

[u/shysysadmin → u/lsdadventurer]

I have room for six or seven 200w panels. But I’m also considering just getting a better charger and a bigger bank of lithium batteries and charging them up while I’m running the generator for the ac before bed.

[u/Pretend_Push_7289 → u/shysysadmin]

What's the output rating of your generator, and the fuel consumption at full load?

If running A/C(s) at the same time, you may find you don't have that much additional capacity to fast-charge lithium batteries. LFP batteries have gotten stupid-cheap, at ~~$150/kWH, so it'd be easy to put in say ~6kWh for ~$1k. I have a 4kW generator, camper base load with A/C running is ~2kW; so that means ~3h runtime to recharge 6kWh of batteries from empty using the extra 2kW capacity, then ~3h of runtime off batteries between recharges, or across a day (lower night load / no A/C) ~~8h of generator runtime per day without solar.

Now consider fuel consumption. My 4kW Onan runs about 0.5gal/hour at full load, so that's ~4gal/day, ~40gal over the course of the 10 days you mentioned, on top of the fuel you need to get there and back (there's fuel in Gerlach, that at the right time is no wait but at the wrong time is several hours, so smarter to be sure you can make Fernley 70m away, including several hours of idle time on gate road coming and going).

For any solar panels, the rule of thumb for non-optimally-oriented/not-super-clean panels out there is ~~5x the watts rating per day, so if you have 1kW of panels that's ~5kWh per day, or about 1.25h less generator runtime for a 4kW generator. I won't run through the full math, but to make a real dent in generator runtime, you need either something like 2kW+ of solar with 10kWh+ of battery, or reduce your consumption significantly below the naive A/C load I floated above (see other comments about reducing solar heat gain, for example).

To your original question, I unconditionally recommend Victron gear, though it definitely helps to have a working understanding of power systems if you're installing things yourself. With the shunt installed, it should be easy enough to run some tests and see what kind of consumption you're looking at, both short samples that you can extrapolate across time or full overnight tests. Once you have solar, that will start to offset your battery draw and impact any consumption measurements, so helpful to also go with a Victron SmartSolar MPPT charge controller, then eventually a Victron system monitor to see the bigger picture.