Sustainable Living: A Primer on Batteries for Use with Solar Storage

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by Jeremiah Johnson, Ready Nutrition:

Ready Nutrition guys and gals, this article is being posted to address questions posed by one of the readers, as well as any others who might want to know about batteries for use with solar panels. The main thing is to understand this is an overview and will give you the basics to make your own choice when it comes to choosing the right batteries for the job. Let’s get into it!

First of all, when you charge up a battery and then use it all up (until it is drained), this greatly reduces the battery’s longevity. Depth of Discharge (abbreviated DoD) is the capacity of a battery that has been used…and there is an optimal level for best performance specified by individual firms. The higher the DoD (usually expressed as a percentage), the more you can utilize before it uses its charge.

A 15 kWh (kilowatt hour) battery with an 80% DoD means that you can safely use up to 12kWh of power before you have to recharge it again. Seems simple enough, but in times of need people allow it to go beyond the DoD and then this decreases the life of their battery as well as its effectiveness. That power rating for your battery is the kWh (kilowatt hour) figure here. This power rating refers to the amount of electricity the battery can deliver one time/all at once.

The reason this is important with solar panels? A battery with a low power rating, yet a high capacity can deliver power over a protracted period of time for some tools or appliances that you need. You have to figure out the cycle…and this is an amount that is estimated numerically in your warranty. A cycle is just that: the cycle of when you charge up a battery and then drain it out with use. Then we need to take into consideration “round-trip” efficiency...the amount of power you will be able to use, as opposed to what you “feed” into your battery for storage.

An example would be a battery with a 70% round-trip efficiency may have 10 kWh worth of juice pumped into it, but you can only use 7 kWh of that electricity. There are a lot of factors that contribute to this “round-trip” efficiency: the quality of your battery, the number of times you drained it down to nothing (and went beyond DoD), the weather and temperature, and many other factors. This figure tells how your battery is performing, and also gives a good idea of how you treat it. Akin to anything else, you should “baby” the batteries and take pains to extend their lives. It’s your money, and your power, after all.

That manufacturer will also have a warranty to cover the battery. The batteries for your solar array usually have a lifespan of 7 to 15 years. Much of this (again) will be determined by how you take care of them, as well as the power consumption and load you place upon them. You need to figure out how much of an array you will need (such as 2, or 4 for a “bare bones” minimum just working off of your solar panels without any other way of recharging them.

You’ll also have to do your homework on how much power you consume, as well as the times you have to augment those solar panels with something else. In my latitude in Montana, we have periods of time for weeks at a stretch without any sun at all and snow on top of that. Are you in an area completely off-grid, or is this solar array your backup supply in the event that day should arrive that everyone loses their power?

I’m a big believer in “gennys” or generators, as a good backup. I’m not going to get into how to build a wood gasifier for an engine to charge up an array. It’s beyond the scope of this article. Your battery array needs to be doubled when you figure out what the minimum is that you need: one to do the job while the other is being charged in some way. If you want my recommendation for a type, I would go with a good Lithium-ion battery, and next a Nickel-cadmium one. The latter has been around a long time and is relatively easy to maintain.

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