If you want to store the energy produced by your solar panels, it’s not a good idea to use car batteries. Even though car batteries are relatively inexpensive and easy to find, they won’t be a useful battery for your solar power system.
Because of their short burst discharge cycle, car batteries won’t store or deliver enough power in an off grid situation. The type of battery you need is a deep cycle battery, designed for a high number of charge/discharge cycles in a solar power battery bank.
When you’re new to all things solar power, it’s important to understand some basic information if you expect your system to work well and power your devices.
In this article, I’ll go over those basics for you, so you don’t make costly mistakes.
What’s The Difference Between Car Batteries And Deep Cycle Batteries?
Car batteries give a short-lived, high burst of current to start a car. Once the car starts, its alternator rapidly recharges the battery, so it’s ready to start the car again. Whereas, deep cycle batteries charge and discharge over a longer period of time, so they can provide continuous power instead of one quick burst.
You can think of the two batteries like runners. A sprinter and a marathon runner are both runners, but they don’t run the same kind of races.
As well as providing battery storage for solar power systems, deep cycle batteries power equipment like forklift trucks, golf carts, wheelchairs, and industrial floor cleaners. All things, which need sustained running power rather than a starting burst.
Car batteries are starter batteries, that’s their job and they do it well, reliably providing the power you need to start your car thousands of times.
But because of their shallow discharge depth, they don’t have the ability to run anything for very long. And if you discharge a battery below its rated discharge depth, you’ll ruin it pretty quickly.
The capacity of a car battery is rated in amps. The number of amps being the level of current it can provide for a very short jolt of power.
With deep cycle batteries, the capacity is rated in amp hours. Amp hours being the number of amps the battery can deliver over a period of time.
Amps and amp hours are not the same thing.
What Does Depth Of Discharge Mean?
Depth of discharge (DOD) is simply a measure of how much of the battery’s capacity you can use before you need to recharge it.
For a car battery, the depth of discharge is about 10%. If the battery is a 760 amp model for example, you can draw 76 amps without damaging it and reducing its lifespan.
With deep cycle batteries, you can draw much more of the battery’s capacity before you need to charge it back up.
How much you can draw depends on the type of battery.
Depth Of Discharge For Different Types Of Deep Cycle Battery
What Is The Depth of Discharge – DOD – For A Lead-Acid Battery?
The cheapest kind of deep cycle batteries are lead acid batteries. These have a depth of discharge of 50%. If you had a battery (or battery bank) with a 225 amp hour capacity, you could use 112 amp hours without damaging the battery.
Now, quickly revisit the car battery figure above. That battery has an 760 amp capacity.
If you didn’t know that amps and amp hours are 2 different measurements for a battery’s capacity, you might make the mistake of thinking that the car battery would give you more power, because 760 is more than 225.
But the 760 amp battery only has an amp hour capacity of 70 amp hours. AND you could only use 10% of that without causing damage. So that’s 7 amp hours, which isn’t much power. You could run a fridge for around one hour.
What Is The Depth of Discharge – DOD – For A Gel Battery?
Gel batteries are a different type of lead acid battery. The battery acid is mixed with silica to form a gel, which then sets. These batteries have a 75% depth of discharge.
What Is The Depth Of Discharge For An AGM Battery?
AGM stands for absorbent (or absorbed) glass mat. This is yet another lead acid battery technology. In this design, the battery acid is held in a fiberglass mat. You can achieve an 80% depth of discharge with these batteries.
What Is The Depth Of Discharge For Lithium Ion Batteries?
Lithium Ion batteries are the new kids on the block when it comes to deep cycle batteries for home solar battery banks. They have a depth of discharge of 80% – 100% depending on the brand.
They’re more costly than traditional lead acid batteries, but you get to use most of their rated capacity, and they last for many more charge/discharge cycles (as long as they don’t catch on fire!)
Understanding The Charging Rate – The C-Rate
The C-rate tells you how fast a battery charges and discharges relative to its maximum capacity. Most batteries state their capacity based on the 20 hour C-rate.
When you divide the capacity of the battery in amp hours by the C-rate you will get the number of amps you can draw over 20 hours to empty the battery. In the case of a 225 amp hours battery this would be 225/20 = 11.25 amps.
Why is this information important?
By knowing how many amps you can draw from your battery at the C-20 rate, you can make sure the battery you’re buying is the right size for the load you plan to put on it.
If the total of all of your appliances and devices is 11.25 amps or less, then your battery will operate at the rated capacity of 225 amp hours.
But if you pull a heavier load from your battery, it doesn’t just run down faster, which you would expect, it will actually hold less power overall. So, if you’re drawing say 20 amps, then you don’t have a 225 amp hour battery, you really only have a 207 amp hour battery.
Factor in the depth of discharge – 50% on a lead acid battery – and you will have 103 amp hours available instead of 112.
That might not seem like much difference, but as someone who has lived with off grid solar for 12 years, let me assure you that an extra 8 amp hours in your battery in the depths of winter is a huge amount.
Why Do Some People Use Car Batteries With Their Solar Panels?
Some people think they can save money by using a car batteries. Some only want to run a low power LED light and charge a phone, which they can do with a car battery, at least for a short while.
Others, in third world countries often can’t afford to buy deep cycle batteries. And in some countries, for those living in remote areas, deep cycle batteries are simply not available.
If a car battery was the only kind of battery you could get your hands on, it would be better than nothing. But for most people looking to store power from their solar panels, it makes zero sense to use a car battery.
For the small amount of power you would get out of it compared to the cost of the battery, you would actually pay a higher cost per amp hour of storage capacity, than if you had opted for a proper deep cycle battery.
Wrapping Up
Because of the small depth of discharge that you get with car batteries, they’re not up to the job of storing the power your solar panels will generate. You need deep cycle batteries for a dependable battery bank that will see you through 5 years or more of daily use.
Before you buy any batteries, work out your expected load in amps, and make sure you choose a battery that can handle that load, at the normal 20 hour C-rate.
