Hi there, K again. In this post, I want to continue talking about types of batteries. In the last post, I described starting batteries, marine batteries, and deep cycle batteries. For a solar system, you want deep cycle batteries, which will run your essentials over the course of the night. There’s nothing worse than waking up to melted ice cream because your batteries ran out of power. There are three popular types of deep cycle batteries used for photovoltaic systems: flooded cell batteries, absorbed glass mat batteries, and lithium batteries.
1) Flooded Cell Batteries
For years, the battery of choice was the deep cycle flooded cell battery, aka lead acid battery. These had thick lead plates immersed in a sulfuric acid liquid. People have reported having their battery bank for as long as ten years, but there was a lot of maintenance required and it was somewhat dangerous because the acid, if touched, was corrosive. Also, when you charge them in an enclosed area, the hydrogen gas from the batteries can explode.
How this works: Whenever you run an electric current through a liquid, molecules break apart, releasing electrons. The water that is in the sulfuric acid breaks down into hydrogen and oxygen and outgasses. You lose a little bit of water every time you go through a charge cycle, forcing you to refill your batteries on a consistent basis. The sulfur is also affected–over time it tends to break down and start sticking to the plates, causing problems with charging and discharging your batteries. Then you have to de-sulfate your batteries by using a unit you attach to your battery bank. This will take some of the power from your batteries and change the frequency causing enough disruption to knock the sulfur off the plates. Unfortunately, sulfur is heavier than water and will sink to the bottom of the battery; this will create a high acidity environment at the top of the battery, causing corrosion and distortion of the plates. As the plates thin, less power can be discharged. The way to fix this is what is called an equalization cycle. You need to bring your battery voltage up to 15 volts. This literally causes the sulfuric acid to boil and mixes the sulfur back into the acid.
Skip any of these maintenance steps for too long, and you end up with a pile of very heavy plastic boxes to deal with.
The last problem with these batteries is they’re temperature sensitive. Flooded cell batteries work most effectively at 70°F. As it gets colder, less energy is available for use. You need to either keep your battery bank in your house, or have an insulated battery box to keep them in. The size of the box will depend on how many batteries you have and the size of the batteries. Just as a point of reference, a typical flooded cell battery is 12” x 8” x 14”.
2) Absorbed Glass Mat Batteries
In the 1980s, a battery was developed to fix all the problems with the flooded cell battery. It was called the absorbed glass mat battery, or AGM for short. It was a lot like the flooded cell battery with lead plates and sulfuric acid, but the acid was absorbed into glass mats between the lead plates. The glass mat was essential in stopping the sulfur from adhering to the lead plates, meaning no need for de-sulfation or equalization. In the top of each cell of the battery are recombiners that recombine the hydrogen and oxygen back into water, meaning no need to constantly top off your water levels. This is much safer and more convenient.
The best part of these batteries is they’re not temperature sensitive. They will put out the same amount of power at 0°F as at 70°F.
The longest I’ve been able to keep a flooded cell battery bank was just short of 5 years. Eight years ago I decided to switch to AGMs. Then a year ago I decided to switch out the AGM battery bank to a larger AGM battery bank. I bought the same AGM brand and switched out my old batteries for the new ones.
I didn’t toss my old batteries. Instead, my smaller bank of old batteries was wired into Phoenix’s solar system to power her smaller setup replacing her flooded cell batteries, and it’s still running today.
3) Lithium batteries
Although there are other types of batteries out there such as the gel cell, I won’t discuss them since I never dealt with them. I would, however, like to discuss one of the newer batteries on the block, the lithium battery. Although I’ve never tried these in a solar system application, anyone who has a cell phone has experience with them. I have hesitated to try these in my solar battery bank for several reasons:
- Outrageously expensive. One of these batteries cost almost 5 times the cost of my AGMs at the same amp/hour rating.
- Short-lived. Every time I buy a new phone, I’ve noticed that the first time I charge it up, the charge seems to last close to 2 weeks (I don’t use my phone that much). Two years later, I charge up my phone, and 3 days later, I need to charge it again.
- Temperature sensitive. Lithium batteries will discharge at any temperature but need to be above 32°F to charge. If you live in a cold climate, you have to either keep your battery bank in your house, or divert some of your solar power to a heater to warm your batteries while being charged.
Where we live, there are no power lines so everyone who moves into the area will eventually put in solar. The biggest mistake everyone makes out here is after it’s installed, they leave their porch lights on all night. They think “the electricity is free, so why not?”. The truth is, the electricity is free but the batteries are not. Each one of these people will have their systems fail within a year. If you have a typical 12-volt system, your batteries will read 12.7 volts after sunset if fully charged. If you’re careful, and never bring your batteries below 12.6 volts, your batteries can last 10 years. If you constantly discharge your batteries to 12.2 volts, your batteries will die before the year is up. So… electricity is free, batteries are not!
Next, I’ll talk about different types of inverters. Stay tuned.
Next blog post: Part 6: Photovoltaic systems–types of inverters (by K)