Featured image is of the smoke from the East Canyon forest fire. It started around 1pm and is about 10 miles from us. We are not in any immediate danger except the smoke may blow our direction and be annoying. Welcome to Colorado in a dry summer.
Yesterday I did the second part of a multiple part series about building an off grid solar system for our cabin. The first part was about the differences of a grid tie system vs off grid. The second part was about AC vs DC. In this part we are talking about batteries.
Batteries are the most important part of an off grid system. You can have solar panels, a solar charge controller, an inverter, but if you don’t have batteries nothing else happens. The biggest surprise is that lithium ion is not necessarily the best option.
Electric batteries have been around since 1800 but we want batteries that can be recharged. The earliest rechargeable batteries were lead-acid, invented in 1859. Today rechargeable batteries are mostly associated with Lithium-Ion type batteries.
When choosing a battery for powering a home there are a few factors to consider:
- Amp Hours
1) Cost is always the first thing to consider when buying batteries. On the cheap end you can get Marine Deep Charge batteries from your local auto store. On the expensive end you are looking at Lithium. In the middle you have flooded lead acid and absorbed glass mat(AGM).
You want to spend a little more on batteries than the cheapest so that they last longer. The lead acid batteries we bought for the trailer were a little more but have lasted for six years at the point.
2) Voltage is important because that determines how your DC system runs. Recall from the last part of this series that DC electricity is the only type of electricity that can be stored in batteries. So by default when you have batteries you have a DC system.
Most DC systems are 12 volts as are most rechargeable batteries. However the longer lasting lead-acid based batteries come in 6 volt packages. This means we have to connect our batteries in series bring the voltage up to 12.
3) When you are talking about a batteries capacity, amp hours give you a way of comparing batteries. On the set of batteries that we chose for the cabin the amp hours are 416ah @ 20 hour rate. This means that they battery will deliver 20 amps of power over the course of 20 hours.
We are going big with this system though and are going to double it. We will get 4 of those batteries and tie them into 2 pairs in serial for 12 volts. Then we will connect those 2 pair in parallel to double the amp hours to 832 amp hours. The main goal here will be to run an electric refrigerator 24/7 in the cabin.
4) Maintenance is an important aspect of having deep state rechargeable batteries. If you take care of your batteries then they will last longer.
With flooded lead acid batteries, like what we have on the trailer, you have to check the water levels of the batteries at least once a month. If the water level gets too low, due to evaporation cause by usage of the batteries, the lead plates will corrode and stop working. AGM batteries do not have this issue as they are sealed.
The other maintenance of a battery is making sure it doesn’t run out of juice. Not only is it annoying when the power outage causes the lights to go out, but it is also damaging to the batteries to let them get too low.
The depth of discharge is usually defined as a percentage of how much of the battery’s capacity has been used. For lead-acid type batteries the depth of discharge that you should never go below is 50%. Ideally you want to avoid going below 80% if you want them to last the longest.
That is correct, with lead acid batteries you are only getting half of the listed amp hours. Keep in mind that using them that much will make them last half as long as only using 20% of them.
Lithium Ion batteries have an almost 100% usable capacity with no ill effects for taking them down almost to 0%. You want to avoid completely draining them but even then it does not damage the battery.
This is an area where lithium ion batteries are on a different level of performance. The problem is that they are over twice as expensive and the technology is not nearly as proven.
For the cabin we chose AGM batteries for cost, and maintenance. We are getting more and spending less than with lithium.
We went on a beautiful ride around Mancos today. 22 miles in total.