Maximising Battery Life with a MPPT Solar Controller

by Wayne Hunt

A comprehensive guide to MPPT Solar Controllers and a case study detailing their importance and power in aiding the efficient use of solar energy. Learn why these controllers are so important in terms of battery life, how they can keep batteries at an “idle” state and power all load from solar instead of the battery, as well as how to ensure you get the most out of your system by doing research and talking to professionals.

Solar Controller

I thought I would write a few words on why I consider the “solar controller” one of the most important components in any solar system.

Other than us, the people using the loads, the solar controller is the component that can make the biggest positive impact on your battery life.

If you are working to a budget, the solar controller is one area where I would buy the best you can afford.

PWM or MPPT SOLAR CONTROLLER?

Most of us know there are two types of controllers, PWM (pulse width modulation) and MPPT (multi-power point tracking).

PWM is the least efficient of the two technologies by a margin of 30 to 50%, which means you will get 30 to 50% more energy to your batteries just by having MPPT over PWM.

So MPPT is definitely the way to go.

Now it doesn’t end there!

For the purpose of this article, I’ll talk about a 30amp mppt solar controller, which is one of the most common sizes; you won’t have to look far to find a 30amp MPPT for anything from $150ish to $400ish dollars, now that’s a big range! So what are you getting for $400 that you are not getting for $150?

What if I told you you could be getting twice the life out of your batteries compared to exactly the same setup with the cheaper MPPT solar controller?

This is one of my pet topics, and I have spent a lot of time studying what gives us the longest possible life from our batteries, and the biggest benefit comes from the mppt solar controller.

So now we look at the how and the why

One of the common issues we see which affects all controllers is their location.

The best location for a solar controller is as close to the batteries as possible. This avoids “volt drop” between the battery and the controller.

This is important because of the potential to undercharge batteries. If your controller is five or six metres (cable length) from the batteries, the voltage measured at the solar controller is going to be less than the voltage measured at the batteries. The result undercharging on every cycle.

The solution is to either move your controller closer to the batteries or if you have a higher quality controller, some have the ability to communicate, usually by Bluetooth with a sensor or device reading the voltage and sometimes temp at the battery. This tells the controller to ignore the voltage of the cable and instead use the voltage that is being communicated from the battery.

The above is one of the most common issues in lost battery efficiency.

MPPT Solar Controller Speed

The next area is the “speed” of the controller; this refers to the frequency that it adjusts to get the optimum performance based on available light onto the panels.

Now we mentioned the big difference between PWM and MPPT; now we are talking about efficiency gain across the range of MPPT solar controllers which is around 15%, so the better quality of the controller, the more frequently it is optimising to the conditions giving you up to 15% more charge to the batteries.

Solar Controller Software/Firmware

Now the biggest saving of all is in the software/firmware of the controller.

Generally speaking, the better the quality, the more features they have that can impact battery life.

A safety measure is low temp charging of LiFeP04 (lithium-ion phosphate, which needs to be restricted once the battery temp is below zero deg C.

The better quality controllers allow us to do that with the same type of volt/temp sensor mentioned above.

Once the battery is full, we can programme when we want it to start charging again. This is important and again impacts the battery life, and only the better quality controllers can tune/adjust this.

Real-Life Data

I’m going to reference some real-life data from my system and from Allan Henderson’s system. We both have Victron Power Equipment.

Now we both have relatively large systems; however, in the case of my system, we run much bigger loads than Allan; we did this deliberately to demonstrate “what is possible”

  • My system consumes close to 150Ah overnight compared to Allan’s, which uses about 1/2 that amount.
  • We both have 2400W of solar and a 3000VA inverter.
  • Allan has 600Ah, and I have 900Ah, both Relion LiFeP04.
  • I need the bigger bank as our daytime loads are much higher than Allan’s.

We run

  • Heated towel rail
  • Electric hot water heating
  • Air Conditioner
  • Hair Dryer
  • Toaster
  • Induction Cooktop
  • Two fridges
  • Sonos stereo
  • 32′ smart tv

This is all off-grid; I have only listed our large appliances to give some scale to our consumption.

Now, this is where the real magic happens.

Most controllers, once the battery is full, go into a float, job done!

Then if it’s a nice sunny day, you might start to charge your e-bike batteries etc.

So you are now drawing from your batteries until they get to a preset re-bulk level and the controller starts to charge again, but for most controllers, you are still cycling the battery to a point, and that’s fine. It’s what the controller is designed to do.

However, with the better quality controller, they have the ability to keep the battery in an “idle” state and power all the load from solar with nothing coming from the batteries.

With the systems Allan and I use, we are able to measure and record this benefit to battery life.

Below are some screen grabs from our systems you can tell who’s is who’s as the name is at the top of the pic.

mppt solar controller

You will note in one of mine my battery is idle “8W”, and the solar is powering over 1000W of AC load via the inverter (it’s heating hot water), NOT using the battery.

So you can see my numbers are 60% from the battery and 40% from solar.

In Allan’s case, his numbers are massive, with less than 1/2 (46%) coming from battery!! And a massive 54% direct from solar!! 

Now I don’t mind saying we have been told it’s ridiculous to have this much solar, BUT as you can now see, our batteries will last 40 to 54% longer than if we had less solar and lower quality controllers.

I hope you find this a useful further insight into solar energy

I have to say, please do your research and talk to professionals before embarking on this type of project; when you have big solar, you have to be aware of your battery spec, inc max charge rates etc. My system can generate 340amps of charge to the batteries, which is too high, so through my controls, I limit my max charge to 200amps, but still have the capability to run big loads and charge simultaneously or just run big loads.