Our RV's Limiting Factors: Batteries

This is the second part of a four part series that talks about what prevents our RV from being perfect for us, along with potential solutions for the problems. This specific post will talk about batteries. The posts in the series are:

  1. Water
  2. Batteries (this post)
  3. Size
  4. Heating

BATTERIES

We had to give up some creature comforts when we moved into the RV, both due to space and because of our goal to be propane free, aka fossil fuel free. Being fossil fuel free means that we are set on generating all of our electricity through our solar panels and don’t supplement by using a gas or diesel generator. But this means that we can’t always run our appliances when we want to or for as long as we want to, mostly on cloudy days. This restriction is mainly due to our battery capacity. Our electricity usage isn’t as restrictive on sunny days if we are conscious about when we are using the electricity. We can go full out on our usage but we have to make sure the solar panels are getting full sun so they are charging at max capacity, our batteries should be more than 50% charged (ideally 100% so we have enough battery life to use electricity the next day), and we don’t use more wattage than what is being generated by the solar panels.

 
The day our solar panels arrived during our rebuild.

The day our solar panels arrived during our rebuild.

 

For a long time I was confused by the relationship between batteries and solar panels. For some reason I just couldn’t wrap my head around it. I finally figured it out and the best way I can explain it is to pretend that electricity flows the same way as water: a battery is like a bottle while a solar panel is like a funnel. The sun “rains” energy down and it’s caught by the funnel. The more solar panels you have the bigger your funnel is and the more batteries you have the larger your bottle is. A bigger funnel means you can catch more rain and fill your bottle faster; the more batteries you have, the larger your bottle is and the more water you can hold.

Plenty of sun, funnel catches enough to fill the bottle quickly.

Plenty of sun, funnel catches enough to fill the bottle quickly.

Less sun, same funnel can't catch enough to fill the bottle.

Less sun, same funnel can't catch enough to fill the bottle.

Less sun, but a bigger funnel that can still catch enough to fill bottle (but not as fast as the first diagram).

Less sun, but a bigger funnel that can still catch enough to fill bottle (but not as fast as the first diagram).

 Our “funnel” is currently very large for an RV; we have 1,220 watts of solar panels. Our “bottle” is also extra big, at 928Ah. But only half of the 928Ah bank is usable. That’s 464Ah or 5568Wh of storage capacity. Without factoring in efficiency losses (there are several), our 1,220W array would need 4 hours and 34 minutes of perfect sunshine (midday, right angle, no haze) to bring the batteries up to 100% from full discharge (50%). It’s impossible to get that so what we’re really talking about is anywhere from 6 - 8 hours of decent sun - basically a full day of clear skies. With fully charged batteries, we have about 3 - 4 days of “autonomy”, meaning we have enough juice to last 3 - 4 days without getting any additional sun. Obviously this doesn’t mean we can run every electrical item in the RV.

So the problem isn’t straightforward and it’s not totally fair to point the fingers solely at our batteries. This is a complicated topic (and we’re still learning) and the simple and short explanation is: components in a solar system have to be sized to balance each other. If we wanted more days of autonomy – a bigger “bottle” – we could add more batteries to the system. BUT, then it would take longer to charge them up with our solar panels. This wouldn’t be an issue during the summer because the sun stays out for so long. But it would be an issue during the other times of year and it would mean that our batteries (not current setup of batteries but if we added more batteries) would be only partially charged batteries most of the time. Batteries don’t like to be kept partly full for long and this would end up shortening the life of our battery bank.

Bringing our newly purchased batteries home.

Bringing our newly purchased batteries home.

We think (like I said, we’re still learning and figuring it out) that if we were to add more batteries to our system, the right thing to do would be to also add more panels as well, so we can still recharge them regularly and fully. But we physically can’t add more batteries or panels, so basically this means that the system we have is as big as we can practically use.

This is our battery compartment maxed out with 8 batteries.

This is our battery compartment maxed out with 8 batteries.

Potential Solution: A solution to this problem is lithium-ion batteries. The benefits that lithium-ion batteries have over lead-acid batteries (what our golf cart batteries are) are:

  • It is lighter weight but has a higher power density
  • Has higher usable capacity
  • They're not as sensitive as lead-acid batteries are to being kept partially charged.

Below is a chart that shows that the stats on our batteries and the stats on lithium-ion batteries for us to have equivalent battery power:

Lead AcidLithium Ion
ModelInterstate GC2-XHDSmartBattery SB300
Battery Voltage612
Capacity, each [Ah]232300
Bank Capacity [Ah]928600
Max Depth of Discharge50%80%
Usable Bank Capacity [Ah]464480
Bank weight [lbs]520186
Quantity Required8 (4 parallel strings of 2)2 (parallel)
Bank price$880$7,000

So based on numbers from a year ago, it would cost us $7,000 to have an equivalent amount of lithium-ion batteries as we have now.

Even though the amp hours are less in the example above, they'd actually be about the same usable electricity because lithium-ion batteries can be discharged to 20% versus the 50% on lead-acid batteries.

If we had lithium-ion batteries then we would have the space and weight capacity to have more batteries to store more electricity from our solar panels, especially during long sunny days and we wouldn’t have to worry about them being partially charged during the winter months. So why haven’t we just gone ahead and switched out our current batteries for lithium-ion? As you can see above, because they are super $$$!

We’d love to hear from you in the comment section below: have you taken the plunge to outfit your RV with solar panels? If so, what batteries do you use, lead-acid or lithium-ion? 


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