Batteries 101 - Part 1: Depth of Discharge

In the next two articles I will be focusing on the use of a forklift battery in off-grid renewable energy systems. However, much of the information also applies to other deep cycle batteries that are commonly used off the grid. For more information about using a forklift battery, see our blog post "The best kept secret in renewable energy."

Many new off-grid users make mistakes that result in less than desirable results.  But once they learn their mistake and fix the problem, the issue clears up.  For the sake of comparison, I will use the analogy of a car. Think of this kind of mistake as being equivalent to running out of gas in your car.  Once you add more gas, the problem clears right up.  No damage done.

Fatal Mistakes

However, there are a few mistakes that I will call "fatal mistakes". Not because they result in someone's death, but because they result in premature death to your equipment.  Make one of these mistakes, and you could cost yourself a significant pile of money.  This would be equivalent to running an older car for many miles without checking the oil.   Once you run the car for a short while without oil, you will likely either burn up the engine or greatly shortened it's life.

On the bright side, you can avoid these mistakes. All it takes is a little education and getting a routine in place.  For example, when was the last time you completely ran the oil out of your car and caused major damage?  It's likely this has never happened to you and never will.  Does that mean you are a certified auto mechanic?  Of course not.  But you are an informed user who has taken a little time to learn the basics of what it takes to safely operate your car.  Do the same with your renewable energy system and you will be well on the way to long life and happiness (for your system, that is).

Battery Cycles

Let's start out with a common mistake. This mistake is a major problem, but isn't going to have fatal implications right away.

The life of a renewable energy battery consists of cycles.  A cycle starts when the battery is fully charged and ends once the battery has been discharged and then recharged to 100%.  Solar and wind systems often require the battery bank to go a longer period of time between cycles due to the length of time between sunny or windy days.  The battery bank must sit in a discharged condition during those periods of time, waiting for the next sunny or windy day to be recharged.

Our Solar System...

For us, operating with a solar system in the winter time, a cycle could easily last 2-3 weeks.  The problem is that lead-acid batteries don't like to sit in a discharged condition for long periods of time.  Sulfation builds up on the lead plates and reduces the potential for electrical actions to take place.  It is recommended that you not let your battery bank cycles last longer than 7-10 days.

For us, in the winter that means running the generator to charge the batteries every 7-10 days even though they may still have plenty of electricity left in them.  This is an important part of maintenance until we get to the sunnier time of year. During the summer the solar panels will keep up with our power usage and fully charge the batteries.  And while I try to keep this in mind, I use a handy reminder as a backup.  The Trimetric system meter that is mounted in our Kitchen has a reminder that flashes to notify me when the batteries have not been fully charged for 10 days.  It is that easy!

How Low Can You Go?

When using lead-acid batteries, one of the most common fatal mistakes is to discharge one's battery bank too deeply. This causes permanent damage that impacts performance and reduces the life of the batteries.  The non-negotiable lowest depth of charge you should EVER let your batteries drop to is 20%. This means that you have consumed 80% of the battery's capacity and only 20% is left.  However, when using deep cycle batteries with a renewable energy system, your batteries will last longer and yield more total power over their lifespan if cycled to a lesser percentage (such as 50%).

The Electric Forklift

Batteries made for these units are ideal for off grid useFolks in the forklift industry consistently say to discharge one's battery bank all the way down to 20%.  In the application they are familiar with (forklifts), this is probably good advice.  They are using their batteries very hard.  In one shift, they will run the forklift down to 20%, and then recharge it overnight.  With that sort of use, a battery is typically going to die from wearing out the lead plates rather than from sulfation.


Sulfation on lead plates

But for our application, we are using our batteries very lightly in comparison. And as a result will be more likely to lose our batteries due to sulfation.  And while forklift applications are fully recharging their batteries every night, we are only recharging ours once every week or more.  This means that the batteries are in a discharged condition for a much longer period of time.  That is exactly when sulfation builds up on the lead plates.  So if you set your "recharge point" at 50% rather than 20%, you will lessen the amount of sulfation that builds up on the plates. That in turn increases the number of cycles the battery will be able to run through over it's life.

Virtually all the experts (including battery manufacturers) in the renewable energy industry go with the shallower discharge model. For example, discharging to only 50% is better than discharging all the way down to 20%.  While a deep cycle battery certainly can go down to 20% and will be able to go for longer periods of time between sunny days (or running the generator), your batteries will not last as long in the long run.  In fact, with the information given by the manufacturer of my battery, I should theoretically get 19% more total electricity out of it if I cycle to 50% rather than 20%.


That is the theory for how deeply to cycle your batteries.  Sometimes circumstances dictate that practice differ from theory. For instance, I am okay with going a little lower than 50% for 3 or 4 months in the winter. The reason is our batteries are very lightly cycled for months (never less than 90%) during the summer. I figure it will balance out in the end.  So I sometimes let the batteries drop to 40% or even 30% during those darkest winter months.  Another situation in which I might be inclined to bend my 50% rule is when the only source of generation is a fuel powered generator.

For someone to be at that stage, it tells me that the budget is tight (which I totally understand and have been there myself).  This means the battery bank is probably undersized.  This also means that if the 50% rule is strictly followed, the you will run the fuel powered generator a lot.  And with today's fuel prices, that means a lot of money!  Therefore, in this particular situation, I would consider running the batteries down to 30-40%.  While the batteries probably will not last as long, the savings in fuel will most likely be substantial enough to more than offset the loss of battery life.  Making a decision like this is much easier if the battery bank is not brand new.


Nick Meissner

Nick Meissner’s adventure with homesteading and off-grid living began in the late '90s with a less-than-bare-bones budget. Over the past 12 years, Nick has taught thousands of people about renewable energy, homesteading, water systems, and independence in general. He's deeply in love with his beautiful wife Lisa and thoroughly enjoys their two children.