Mpfana Manu is a mechatronic engineer who's working in green energy electronics products.
It's pretty tempting right? A car battery looks very much like a deep-cycle battery and looks very capable of fitting perfectly in your solar power system. But do not fall for the temptation, car batteries cannot be used in a solar power system because of particular reasons you're going to learn in this article.
You're also going to learn the consequences of including a car battery in the battery bank of your solar system and how to maintain your battery bank so that it last longer.
Batteries are the heart of every solar electric system, especially when you're off the grid. If you can take good care of them, you can easily increase the lifespan of your solar system before it starts tripping.
And also considering the amount of money you cashed in to buy all the batteries in your battery bank when the system was installed, you ought to diligently protect that investment. Replacing a failing battery in your power bank with a car battery is not an intelligent move. In fact, it will throw the initial investment down the drain. Here's why.
Why a car battery can't fit in a battery bank of a solar system
Car batteries are designed to release lots of electrical power in an instant. They can discharge almost half of all their stored energy in a few seconds. This is good when they're installed on a car because a car only demands significant electrical energy when its engine is starting (except for electric cars of course).
But when it comes to a solar system, batteries are required to give a slow and steady electrical power output for a lengthy period of time. And also, batteries should have the ability to discharge up to 80% of the stored electrical power without significantly changing the rate of discharge.
Car batteries can't do that. Use a car battery until it's 70% discharged and you're most probably going to have a hard time recharging. Most of them need an electrical boost to start recharging normally when they've been discharged over their limit.
A totally discharged car battery may never recharge again especially when it spends a long time before getting recharged. This is how some car batteries die. When it's installed in a car, it quickly gets recharged by the car's alternator when the car is revved. But when it's installed in a solar system, it'll swiftly suffer a premature death and the solar system will start tripping.
There's another more horrible reason why a car battery is bad news for your solar system. You know a big solar system doesn't run on a single battery. If a single car battery is to be installed among deep-cycle batteries making up the whole battery bank, it'll ruin all of them as itself also gets ruined.
The characteristics of a deep-cycle and a car battery are very different. And as a result of that, when the batteries are connected in series/parallel one battery will discharge/recharge faster/slower. The overall effect will be that all the batteries will reach a compromise level of performance at which they work as one power bank.
That level of performance will be ridiculously low compared to the performance of a normal battery bank. The battery bank will deliver much less energy than it took when it recharged or it may never fully recharge or it may stop delivering power before getting even 50% discharged.
What exactly happens to the overall battery bank depends on the type of car battery and how it's connected (series/parallel) in the battery bank.
What is worse is that even when you remove the car battery and replace it with a proper deep-cycle battery, the battery bank will still behave as if the car battery is still in use. It's in the nature of batteries to adapt, so all your batteries would have been crippled by the car battery while it was still in use.
The only option you'll be left with is to replace all the batteries in your battery bank. That's hundreds, if not thousands, of dollars worth of an investment forfeited.
What is the best way to replace batteries of a solar system?
Now that you know how horrible it can get if you connect a car battery in your power bank, it's obviously necessary that you also know the right thing to do when a single deep-cycle battery needs to be replaced.
If your battery bank was initially set up by someone who had done his homework very well, all the batteries must have been of the same size and model. If one battery needs to be replaced, here's what you do.
- Check the model and size of the batteries making up your battery bank and write it down.
- Calculate the age of the battery bank from the time it was initially installed. This is the age of all your batteries. Note it as well.
- Recharge all the batteries to their maximum capacity before replacing.
- Buy/get a second-hand deep-cycle battery of the same model, size and age, and install it in the battery bank.
Battery manufacturers and DIY shops do sell second-hand batteries. If you intend to get it from a manufacturer, you can also sell the broken battery that you want to replace and then buy the second-hand battery. The model, size and age will make sure that you get the battery with the same characteristics as the rest of the batteries making up your power bank.
Someone might ask what happens if you buy a new battery of the same model and size. The answer is nothing happens to your power bank, it continues working as it used to. But the new battery would quickly adapt and start behaving as the rest of the old batteries.
So all of its improved and better properties as a new battery would be waisted after several cycles of charging and discharging. That is money waisted since brand new batteries cost more than second-hand batteries. The most cost-effective way is to just get a second-hand battery similar to those in your power bank.
If it happens that you cannot find the exact model of the battery that you need, I advice that you consult a battery manufacturer/engineer and ask which other model has almost the same characteristics as the model of the rest of the batteries in your power bank.
You can compromise on the age but it's safe to get a battery that is newer or of the same age as your other batteries. But do not compromise on the size. Get the exact size as the one you're replacing.
Tips to maintain your batteries so that they last longer
- If your batteries are flooded lead-acid batteries, check for the liquid level every month and add distilled water accordingly. If your battery bank recharges more frequent than once per day, check the liquid levels after every two weeks.
- If your batteries are connected in parallel, use a multimeter to check for the voltage across adjacent batteries. There should be no difference in the voltage readings. The maximum voltage difference that you can live with is 0.7V. You can balance the voltages using a battery charger. A voltage difference that is higher than 3V indicates that a battery is failing. Replace the battery before it affects the whole power bank. You can do these voltage tests every month but if the system recharges and discharges frequently, you should also do it frequently.
- After every month or two, you should shuffle the batteries. Those that were in the middle should be shifted to the ends. And those that were at the ends should be shifted to the middle.
- Check the inter-connecting cables and make sure all connections are tight and intact. Apply a layer of petroleum jelly on all connections to prevent water ingress.
- The battery bank should not be placed directly on the ground/floor. Make a wooden frame on which all the batteries can fit together. The frame should keep the batteries off the ground so that they don't get cooled by the ground/floor especially in winter.
- If your charge controller can set the limit to the extent of discharge of your batteries, keep that limit as low as the system will still strive. Batteries that recharge after discharging 50% of their stored energy last longer than those that recharge after discharging 80% of their stored energy.
Do you know what I did? I translated the complicated chemistry that happens when a car battery joins a solar battery bank into words that I hope can be understood by everyone, even folks who did no chemistry. If there's any part that's still hard to understand, don't hesitate to complain in the comments section and I'll be more than happy to help you out. If all is clear and understandable, then cheers.
BRENDA ARLEDGE from Washington Court House on March 24, 2020:
Your article is very informative.
You are a good writer.
Thanks for sharing.