How to choose the right batteries for your solar power system.
Solar power batteries refers to the batteries used in the battery bank of a solar power system. Basically, this is your storage reservoir for any energy you create, but don't use. In other words, batteries collect DC energy from your solar panels and store it for use later.
By using a battery storage bank in your solar system not only do you provide a constant level of power to your electrical loads, you can also use your pv batteries to store energy in the day and use it at night or in times of rain or cloudy weather. Batteries make your solar power use much more practical and convenient.
However, batteries are one of the most expensive components of a solar system, so its important that you understand what you're buying before you spend your money.
In order to get optimal performance from your system and maximum life out of your batteries, you must choose the size, type and number of batteries you use carefully.
Below we talk about the different solar battery types along with how to choose, size, monitor and maintain your battery bank.
Types of Batteries Used For Solar Power
The different kinds of batteries that can be used to store solar power in are sealed lead acid batteries, liquid vented lead acid batteries, gel cell batteries, alkaline batteries for low temperatures, nickel cadmium and nickel iron batteries.
However, the most popular of all of these are sealed lead acid batteries because they provide the best performance for the lowest cost, with the least maintenance required.
Since sealed lead acid batteries are the most commonly used for PV systems and designed to be compatible with common photovoltaic components, we will concentrate on sealed lead acid batteries here.
Sealed Lead Acid Batteries
Sealed lead acid batteries typically come in 6 volt or 12 volt and are named according to their "depth of charge". When a battery is discharged and then charged back up again, this is called a cycle.
The two main types of lead acid batteries are Shallow Cycle Batteries and Deep Cycle Batteries.
Shallow cycle batteries are meant to be discharged only 20% of the way and then recharged again. This is the type of battery the starts your car and it's effective in doing so because it uses lots of power (like up to 20%) for a very short period of time. Then the alternator recharges the shallow cycle battery while you drive and you are good to use that top 20% to start your car again next time. Never use this type of battery with a solar energy system.
Deep cycle batteries (commonly used to power electric golf carts) can be discharged up to 80% of the way and then recharged again. This type of battery (solar battery) is great for storing solar power in because it uses less power over a longer period of time and you can use up to 80% of the battery capacity.
Using more than 80% capacity can harm your solar battery and shorten it's lifespan. Ideally, you should try to discharge your deep cycle sealed lead acid batteries to only 50% empty if you want them to last longer. Basically the less you discharge your batteries (shallower cycles) the longer they will last.
Determining a solar battery's approximate power / storage capacity.
Let's say you go out and buy a battery for your solar system that is 12 volts (push) and 105 amp hours (storage capacity).
You can find out approximately how much energy this solar battery will store (provide) by calculating the watt hours. To do this, just multiply the volts (V) x the amp hours (AH) and divide by 100.
Volts x Amp Hours / 100 = Watt Hours
12V x 105AH = 1260 / 100 = 12.6 Watt Hours
What this means is that you can power a 100 watt appliance for 12.6 hours on a fully charged battery.
Make sure you find out what the specs on your batteries are before buying them. By knowing what to look for and what each spec means, you can insure your solar project's solar battery bank operates smoothly, efficiently and free of costly mistakes. You can learn more about sizing a solar battery by clicking here to go to the Solar Battery Sizing section of our website.
Life Expectancy of Batteries
One thing you want to pay close attention to when buying solar batteries is how long they will last. The life expectancy of sealed lead acid batteries is rated using the number of cycles that battery can perform.
The "number of cycles" refers to the number of times the battery can be charged and discharged before it's dead.
So if your battery is a 3000 cycle battery, this means it will be able to charged and discharged 3000 times before it dies, that is providing it is consistently charged correctly and not discharged past acceptable levels. Batteries are considered to be at the end of their lifespan when 20% of their original capacity is gone.
The Importance of Using a Charge Controller
A charge controller (with built in Low Voltage Disconnect) is a vital component of any solar energy system that uses batteries. It controls the speed at which batteries are charged as well as protects them from overcharging and complete discharging.
This is very important because overcharging your solar battery bank can produce dangerous hydrogen gases that are both harmful to breath and can ignite and explode if exposed to spark. On the other hand, discharging your solar battery too far shortens it's lifespan.
By using a programmable charge controller you can set it to eliminate the chance of overcharging or over-discharging your battery bank and never have to worry about this issue. However as reliable as charge controllers are, you should still monitor your batteries (and charge controller) for any issues or malfunctions weekly just in case.
How Temperature Affects Batteries
Sealed lead acid batteries should be kept in relatively consistent and moderate temperatures.
A battery has less capacity when it is cold, and hot temperatures will shorten it's life. This is why it is important to monitor the environment your sealed lead acid batteries are in all year round and keep them within moderate temperatures.
Also make sure they have plenty of ventilation so they don't overheat and try to keep the other photovoltaic components at a reasonably safe distance from your battery bank, just in case of accidental exposure to corrossion.
Do Not Mix Used Batteries With New Batteries
It's also important to note that unlike with solar panels, it's not a good idea to add more batteries to a battery bank after the batteries in that bank have been used. In other words, all the batteries in your system must be equally used and all have the same remaining usage cycles in order for you to get the best results from your system.
Sure people break this rule all the time, but it is not the healthiest thing to do to your solar power system. If you're going to start with a smaller system and add to it as you go, I suggest you wait until you have completely used up your batteries' s life span with the smaller system and then buy all new batteries (or recondition them all) when you upgrade to the bigger system with more panels.
In this case, it may be in your favor to start off with batteries for solar power that have a shorter lifespan (cycle) - just so you don't have to wait as long to perform regular upgrades.
When it comes to deciding on the size of your battery bank, make sure you match your battery bank size to the solar panel array size. After determining your load requirements, build a battery bank that's big enough to store five days' worth of power plus another 30%. This extra 30% is to allow for changes and fluctuations in battery capacity due to temperature and other factors.
Using No Solar Batteries
Not all solar energy system types use batteries, some of them just send the power straight from the solar panels to the other photovoltaic elements and then directly to the household loads. In such cases (off grid solar systems - no solar batteries), you are subject to sudden fluctuations in power production and can even completely run out of power when there's no sun.
In cases where the solar system is hooked up through the AC breaker panel, systems with no solar batteries can still feed excess power back into the grid and get credited thus just use the grid power when they need it. This is sort of like using the electric company as a solar battery bank.
However, if you want to have power at all times including in power outages, power shortages and during solar system maintenance, you must use solar batteries.
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