How To Choose the Right Batteries for Your Solar Home

Solar power batteries are 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. These batteries are designed to work with both grid-tied and off-grid solar systems.

By using a battery storage bank in your solar system, not only do you provide a constant level of power to your electrical loads, but you can also use your PV batteries to store energy during the day and use it at night, or in times of cloudy weather. Batteries make your solar power use much more practical and convenient.

Types of Batteries Used for Solar Power

The different kinds of batteries that can be used to store solar power 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 are 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
• 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 that starts your car. 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 solar battery is great for storing solar power since it uses less power over longer periods and you can use up to 80% of the battery capacity.

Using more than 80% capacity can harm your solar battery and shorten its 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 Power and 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 the specs on your batteries before buying them. By knowing what to look for and what each spec means, you can ensure your solar project's solar battery bank operates smoothly, efficiently, and free of costly mistakes. 

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 a 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 can be charged and discharged 3000 times before it dies – 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 harmful to breathe and can ignite and explode if exposed to a spark. On the other hand, discharging your solar battery too far shortens its 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 its 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 corrosion.

Do Not Mix Used Batteries With New Batteries

It's 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 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, wait until you have completely used up the life span of your batteries 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 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, 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, and thus just use the grid power as needed. 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 during power outages, power shortages, and solar system maintenance, you must use solar batteries.

How To Choose the Right Size Battery Bank for Your Solar System

What Is Solar Battery Bank Sizing?

Solar battery sizing, otherwise known as battery bank sizing, is one of the most important considerations when choosing the specifics of your solar electric system.

The main objective when sizing a battery bank is to get one that can handle the load coming from your PV panel array and provide enough stored power for your needs without having to regularly discharge to an unhealthy point.

By wiring multiple batteries together in different wiring arrangements, you can design a battery bank that's right for your solar power system and thus correctly perform solar battery sizing.

Factors Affecting Battery Bank Sizing

The number of batteries you use in your solar system depends on several factors.

The amount of money you have to spend on this solar project. Part of solar battery sizing is ensuring you can buy enough solar batteries to handle your power storage needs.

You must also take into account the number of days you want to be able to go before needing to recharge your batteries. If you need to be able to power certain appliances for a specific number of days at a time without interruption, you'll need more batteries to carry a bigger load. This is determined by the number of batteries you use and how you wire them to affect your battery bank's total amp hours (storage capacity).

Another factor that affects solar battery sizing is the amount of power you will be needing for all of your appliances. If your appliances require many watts (power), you'll need enough batteries to store the power so you can use those appliances.

The voltage your solar system produces is another important consideration. If your system produces 48 volts, then you're going to want to have enough batteries in your battery bank to store 48 volts. Actually, a little less is better – like a 36 volt system with a 24 volt battery bank, just to be sure your system can charge the battery bank even in the case of a sudden voltage drop. 

When sizing a battery bank, always size your solar panels bigger than your battery bank to be able to compensate for factors such as voltage drop, power fluctuations, and energy loss due to normal wear on the system.

To charge a battery, a generating device must apply a higher voltage than already exists within the battery. That's why most PV modules are made for 16-18V peak power point. A voltage drop greater than 5% will reduce this necessary voltage difference, and can reduce charge current to the battery by a much greater percentage. Our general recommendation here is to size for a 2-3% voltage drop. So for a 12 volt battery bank, a 16-18V solar panel should be used to allow for unexpected voltage drop.

Another important consideration when sizing a battery bank is the storage capacity you will need your battery bank to have. If your area gets fewer hours of sunlight in the day, you're going to want more batteries so you can store more “amp hours” of power in your reservoir and last out the long night's stretch. When sizing a battery bank, the more amp hours you have the longer your total power reserve would take to deplete.

When doing solar battery sizing, you must also take into account the rate of discharge you want to have. Remember, the slower your batteries can discharge the more hours you'll get out of them. You can find out a battery's rate of discharge by looking at it and finding the value marked: (C-?). If you see (C-10) then this means the battery takes 10 hours to discharge fully, if it's (C-5) then the battery takes 5 hours to discharge fully.

Lastly, when sizing a battery bank, you must consider the depth of discharge you want to go to before recharging. This is decided by your specific power needs and capacity,  which affects the battery's lifespan.

Solar Energy Batteries

Batteries are one of the most expensive components of a solar system, so it's important to understand what you're buying before you spend your money.

To get optimal performance from your system and maximum life out of your batteries, you must carefully choose the size, type, and number of batteries you use to properly maintain your battery bank.

As a general rule of thumb, the bigger your batteries are and the more batteries you have, the more convenient it is for you and the better it is for your batteries' health. With more batteries and storage capacity, you will have more power available. Plus, you will be discharging your battery bank in smaller (more shallow) cycles and thus increasing its overall lifespan.

As a general rule in solar battery sizing, it's always better to have more batteries in your battery bank and only discharge them 30-50% of the way down than to have fewer batteries and discharge them more. Use a battery bank sizer calculator that can help automate the process for you.