Batteries For Solar Power

Store extra energy produced by solar panels by using batteries for solar power after dark.
By Steven D. Gregersen
March 21, 2014
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Deep-cycle batteries are one of the most common batteries for solar power used by low-budget homesteaders. This is a deep-cycle battery purchased at Wal-Mart. It's rated at 125 amp-hours and comes with an 18-month free replacement warranty.
Photo by Steve Gregersen
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Creating the Low-Budget Homestead (Paladin Press, 2011), by Steven D. Gregersen, provides practical advice for building your dream homestead. Living an off-grid, independent lifestyle takes a lot of planning, but with Gregersen’s help, anyone can save time and money making their self-reliance dreams come true. In the following excerpt from “Power,” learn how to store electricity afterhours using batteries for solar power.

Purchase this book from the MOTHER EARTH NEWS store: Creating the Low-Budget Homestead.

Types of Batteries for Solar Power Storage

Batteries for solar power store the excess energy from your solar panels for use after the sun goes down or at times when your electrical loads are drawing out more than the panels are putting in.

You’ll have to choose between deep-cycle batteries, golf cart batteries, sealed (or not) batteries, lead-acid batteries, absorbed glass mat sealed lead acid (AGM) batteries, gelled electrolyte sealed lead acid (GEL) batteries, and a few others that aren’t so well known. Each one has strengths and weaknesses. For the most part you’ll get what you pay for, so make your choice wisely.

The two most common types of batteries for solar power, used by the low-budget homesteader, are the deep-cycle batteries sold to boat and RV owners, and similar (but larger and more durable) golf cart batteries. Golf cart batteries are usually 6 volts, so you’ll need multiples of two for a 12-volt off-grid system and multiples of four if your system is 24 volts. The big complaint about regular deep-cycle batteries is their short lifespan in off-grid applications. The big drawback to other types is the price. This is another area in which you’ll need to do your own research before you make a purchase.

You’ll hear different arguments about the lifespan of different types of batteries, and while these may be good for comparative purposes, there’s no way anyone can tell how long your batteries will last. The most important factors in battery life are how many charge/discharge cycles they go through, how deep these cycles are, and how well you maintain them.

When a battery is discharged and then recharged, it’s called one “cycle.” The problem is that not all cycles are equal. The deeper your battery is discharged, the more wear the battery sustains. Additionally, not bringing the battery up to full power before discharging it increases wear further. For this reason, it’s recommended that you have enough battery capacity to power your needs for five days without having to charge them. It may sound like a lot, but if your storage capacity is too low you’ll be discharging the batteries to a deeper level in each cycle, which shortens battery life considerably.

Battery life is extended significantly if you discharge them no more than 50 percent of their capacity before recharging. Batteries are rated by how much energy is available from the full charge level down to the level where the battery is completely discharged. In order to ensure that the batteries are never discharged below 50 percent, some manufacturers recommend that you determine how much storage capacity you need and then double that. For example, if you need 100 amp-hours (Ah) of storage capacity, it’s recommended that you purchase the number or size of batteries required to achieve 200 Ah of storage capacity.

If you get in the habit of running your batteries in a continually discharged condition, it will shorten their life. This often happens in the winter, when daylight hours are short and nights are long. In these circumstances, batteries may never reach full charge during the day, so you’re constantly using them in a partially discharged state. Batteries function best in temperatures between 60°F and 80°F. Temperatures over 100°F shorten battery life significantly. Colder temperatures require higher charging rates to bring the battery to full charge. If you store your batteries outside, it’s best to get a charge controller that compensates for battery temperature.

Battery Rating: Voltage and Storage Capacity

Voltage is the number of volts the battery is rated at. This might be 2, 6, 12, or 24, depending upon battery type. Most batteries used for home power systems will be either 6 or 12 volt. Most charge controllers come in 12-, 24-, and 48-volt configuration. In order to get the correct voltage, you put together different combinations. Two 6- volt batteries connected in series will equal 12 volts. Four 6-volt batteries connected in series will get 24 volts, as will two 12-volt batteries. Unless you really know what you’re doing, I wouldn’t recommend anything larger than a 24-volt system. (More on that later!)

There are two ratings you’ll want to take a look at. The first is the ampere-hour (Ah) rating. The amp-hour rating is the maximum sustained amperage that can be drawn from a fully charged battery over a specified time period (usually 20 hours) until the battery is dead. For example, our deep-cycle batteries have a 125 Ah rating. That means they can be discharged at a constant rate of 6.25 amps for 20 hours.

The second measurement is reserve capacity. Reserve capacity is the amount of minutes a battery can maintain a useful voltage under a constant 25-amp discharge. Our batteries have a 205 (minute) reserve capacity, meaning that they can power a 25-amp load for 205 minutes without falling below 10.5 volts.

It’s important to note that these figures are for comparison purposes only. A battery’s performance is impacted by its age, number of discharge/ recharge cycles, depths of discharge/charge, and temperature. Even the condition and size of the wiring and connectors affect the amount of power that’s available. Wiring that’s too small or connections that are loose (a fire hazard!) or corroded may significantly reduce storage capacity and output.

Remember the formula for converting volts and amps to watts? Multiply your volts by the amps to get the number of watts produced. A 12-volt battery with a 125 Ah rating will produce 1,500 watts over a 20-hour time period. If you take the 1,500 and divide it by 20 (total number of hours), you’ll see that this battery, under ideal conditions, will power one 75-watt lightbulb for 20 hours. In theory, it will power a laptop computer (50 watts) for approximately 30 hours. In reality, it won’t last that long!

If you figure the watts of power available by the amp-hour rating compared to the reserve capacity rating, you’ll get different numbers of watts available for use. The reason is that the rate of discharge impacts the amount of energy the battery can release. You’ll get more power from a battery if you discharge it at a lower rate. The higher the discharge rate, the quicker the battery runs out of power.

Charge Controllers

The charge controller keeps the voltage at levels that will bring your batteries to a full charge without harming them.

In normal operating mode, the charge controller varies the voltage output of the solar panels according to the charge level of your batteries. If your batteries are low, the controller increases the allowed voltage to bring the batteries up to full charge quickly (bulk charge setting), then maintains that voltage (absorption phase) for a specific time period to ensure that the batteries are fully charged. Once that occurs, it reduces the voltage (float or maintenance phase) to prevent overcharging the batteries.

Controllers also put your batteries through an equalization cycle every three to four weeks. In the equalization cycle, the inverter steps up the charge level in order to ensure that the battery is deeply charged. As a battery is discharged, the outside of the plates discharge faster than the inside does. Because the charge level on the surface is less than at the center, the center of the plates slowly transfer power to the outer surface so that the voltage is equal throughout the plate. When the battery is recharged, the opposite occurs and the surface of the plates charge faster than the center. If the battery is discharged again before the inside of the plates are fully recharged, you can end up with a situation where the inside is constantly undercharged and begins to deteriorate. The solution is to periodically overcharge the battery in order to “force” the electricity deeper into the battery’s plates. This is called equalization. When done properly, it will extend the life of your batteries significantly.

Charge controllers are rated in amps and your solar panels are rated in watts. So once again, you’ll have to do a little math to choose the right one.

Most charge controllers require that you wire your solar panels in parallel circuits. In other words, you may have six panels with a maximum output of 20 volts each hooked into a 12-volt system. That means the maximum voltage pushing the electricity to the charge controller is limited to 20 volts. Remember, voltage is electrical pressure. I said earlier that some solar panel manufacturers recommend that you go to a 24-volt system if the panels are over 50 feet from your batteries and inverter because you need the extra voltage to overcome the resistance in the wiring. You could go to larger wire, but you may end up spending more for the wire than you did for the panels!

There is a charge controller manufactured for off-grid use that alleviates the problem. It’s manufactured by Outback and allows you to wire your panels in series, which increases the voltage going to your charge controller. The increased voltage from your panels to the inverter means you lose less power in transmission and you can get by with smaller size wire. It’s a much more efficient system than conventional charge controllers, sometimes increasing output by 30 percent, which means you may need fewer solar panels. The downside (there’s always a downside!) is that they cost considerably more than other charge controllers. My advice? Do the math to decide if the increased cost is worth it.

Learn more about low-budget homesteading: Go Green with Low-Budget Housing


This excerpt has been reprinted with permission from Creating the Low-Budget Homestead by Steven D. Gregersen and published by Paladin Press, 2012. Purchase this book from our store: Creating the Low-Budget Homestead.


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