Because batteries appear to be simple boxes with two terminals, we often assume that they are simple devices. They are in fact, one of the most complicated and least understood parts of a solar or backup system. And yet almost every system must have one.

If you use poor quality batteries, or you don’t use enough batteries, your systems will always have problems – and your clients will blame you and the technology. Since batteries are usually the cheapest components in the system, it makes perfect sense to spend a little more and build a successful system by not cutting corners on the batteries.

In renewable energy systems, we commonly use lead acid batteries. This is the same technology that you find in your car battery, and in fact, automotive batteries will work in renewable energy systems for a limited time. The problem with automotive batteries is that they are shallow-cycle, meaning that the lead plates within the battery are thin, and are designed for supplying almost all of the battery’s energy in a very short time (~5 minutes). This is exactly what your car engine needs to start, but your renewable energy system needs to draw energy out of the battery over a much longer period of time (20 – 100 hours). If you use automotive (also called SLI or starting) batteries in your system, they will not hold as much capacity, the thin plates will soon give out, and your batteries will fail.

Deep cycle batteries have thicker lead plates, with less overall surface area as compared to an automotive battery. Because there is less surface area for chemical reaction, deep cycle batteries produce less instantaneous current than an automotive battery but can produce that amount of current for a much longer period of time.

In battery language, a cycle on your battery bank occurs when you discharge your battery and then charge it back up. A lead acid battery is designed to absorb and give up electricity by a reversible electrochemical reaction. How deep a battery is discharged is termed depth of discharge (DOD) while the state of charge (SOC) is 100% minus the DOD. This means that a 25% DOD equals a 75% SOC. A shallow cycle occurs when the top 20% or less of the battery’s energy is discharged and then recharged. We suggest you use deep cycle batteries whenever possible and cycle them as low as possible.

The most common type of deep-cycle battery is “flooded” meaning the electrolyte or battery acid is liquid. This battery’s vent caps can be opened allowing you to add water to extend their life. Since they are not sealed, they also discharge hydrogen and oxygen when charging, meaning that you should provide a way for the batteries to be vented to outdoors. Because these batteries are not sealed, they will need to have distilled water added to them every 2-3 months. Remember that we only add distilled water and not electrolyte (battery acid/sulfuric acid) since only the components of water have escaped (oxygen and hydrogen). These batteries will also need an equalization charge, which means a charge at a higher than normal voltage occasionally to re-mix the sulfates.

Flooded batteries will give the longest life of any lead acid battery and have the best price, but do require periodic (and correct) maintenance. Another advantage of flooded batteries is that they can be manufactured “dry”, meaning without electrolyte (acid) in them. This is important in Africa because batteries start to lose their charge as soon as they are charged. Flooded batteries have the highest self discharge rates. For example, within 6 months of manufacture they could be almost 100% discharged if they are left sitting without a charge. And once a battery becomes fully discharged it can suffer irreparable damage – in fact, even allowing it to sit too long at a partially discharged state can cause damage.

(You may also find sealed flooded lead acid batteries in your markets. They are simply flooded batteries that can’t be opened. These are “maintenance-free” meaning they can’t be maintained because you can’t add water. We don’t recommend them, because they will have a very short life in a renewable energy system.)

In a gel battery the electrolyte is fixed as a gel – they are maintenance-free, valve-regulated (VRLA) batteries. They are the best performers in hot environments, but have the slowest charge rate of any battery. They are slightly more expensive than AGM batteries but can give twice the cycle life. Historically, they have been our best performers in Africa and are popular because they are maintenance-free. Remember that a gel battery, like all sealed batteries, requires a lower charge voltage than a flooded battery.

In an AGM battery the electrolyte is in an “absorbed glass mat”. This battery type is widely used in UPS machines and other applications where rapid charge and discharge is important. Although there are some quality AGM batteries, on balance we prefer the gel technology for most of our applications in Africa.

Battery state of charge is determined by reading the static (i.e. not charging or discharging) battery voltage or the specific gravity of the electrolyte. The density or specific gravity of the sulfuric acid (H2SO4) electrolyte of a lead acid battery varies with the state of charge and temperature. The density is lower when the battery is discharged and higher as the cells are charged. This is because the sulfates are suspended in the electrolyte when the battery is charged. Specific gravity is read with a hydrometer, which will tell the exact state of charge. A hydrometer cannot be used with sealed batteries.

Voltage meters are used to approximate the battery’s state of charge. They are relatively inexpensive and easy to use. The main problem with relying on voltage reading alone is the high degree of battery voltage variation through the working day. Battery voltage reacts highly to charging and discharging. In a PV system at any one time, we are usually either charging or discharging the batteries, and sometimes we are doing both at the same time! As a battery is charged, the indicated voltage increases and as discharging occurs, the indicated voltage decreases.

These variations may seem hard to track, but in reality they are not. A good accurate digital meter with a tenth of a volt accuracy can be used with success. The pushing and pulling of voltage, once accounted for by experience, can also help indicate the amount of charging or discharging that is taking place. By comparing voltage readings to hydrometer readings, shutting off various charging sources or loads and watching the resulting voltage changes, the system owner can learn to use indicated voltage readings with good results.

It is worth repeating: the most important factor in your battery’s life is the depth of discharge. This means how much of your battery’s available energy you draw out in each cycle (before re-charging it). The deeper the discharges, the shorter the life (the less cycles) in a battery. We normally recommend that a battery system be sized for no less than 50% depth of discharge. Clients always use more power than they anticipate, so it is a good idea to add a cushion to your calculations. And with solar systems, a planned 20-30% depth of discharge leaves you a few days of safety (autonomy) in case of heavy cloud cover for a few days.

Battery sizing in power backup systems is typically more aggressive – 50% to 80% depth of discharge – in order to reduce cost. Be sure to inform your clients of the number of cycles they can expect at their chosen depth of discharge. Then let them chose whether to spend a little more now or a lot more later, depending on their cash flows. In Africa, the customer will almost always choose to discharge more deeply now and replace them more often.