Off-Grid solar power is increasing in popularity in the regional areas of the Sunshine Coast and Brisbane as more and more people want to disconnect from the power grid.

There are a huge variety of systems on offer however if you are considering installing an off-grid solar power system then please read the following information and contact SunWorks Solar your off-grid solar specialist for more information

Can I use Car Batteries for off-grid solar power?

Off-Grid solar batteries - do not use car batteries

Start batteries are the type of batteries that are used in cars, boats and generators and are not suitable for frequent deep discharging due to the way they have been constructed. They have thin plates with a large surface area and are designed purely for short-term high discharge currents like engine starting. As a result, they cannot be used in Off-Grid solar power systems.

Can I use second-hand Batteries for off-grid solar power?

The idea might sound good at first, but do you know how old those batteries are? Do you know what their history is? Do you know how they were they treated by their previous owner? The answer is generally no and whilst it might be tempting to use cheap batteries you will pay for it in the long run.

Off-Grid Solar - do not mix batteries

Can I mix and match batteries?

You should never mix different batteries in particular do not mix these combinations:

  • Old and new batteries
  • Different battery sizes
  • Different battery types
  • Different brands
  • Different chemistry or technology

When you construct a battery bank with a mix of different batteries, the weakest battery will determine the performance of the whole battery bank. It does not matter if the other batteries are new and strong, if one bad battery is used the whole battery bank will be adversely affected.

What is battery cycle life?

Once you have made the choice of what type battery you are going to use. You will need to determine how long these batteries are going to last. What is their maximum lifetime?

To find an answer to this question you will have to look in the battery datasheet. On the battery datasheet you will find a graph or a set of numbers which shows the Battery cycle life at various depth of discharge levels.

Off-Grid Solar - Battery Depth of Discharge

The deeper the depth of discharge the fewer number of cycles the battery will last for. As shown in the graph above if you only discharge your batteries by 20% they will last for 8,500 cycles however if you discharge them to 80% then they will only last for 1,500 cycles.

What is a cycle?

One charge and one discharge event is called one cycle. Typically, you will go through one cycle every 24 hours. In the graph below you will find the state of charge of a battery in a solar system over a few days. You can easily see that in this system there is a cycle taking place each 24 hours.

The cycle starts when the battery is full, the batteries are then discharged and recharged. The cycle ends when the battery is full again.

What is depth of discharge?

Depth of discharge is an indicator of how empty a battery is.

If the battery is full it is 0% discharged. If the battery is discharged only a small amount it might have an 20% depth of discharge. If the battery is discharged a lot, it might have an 80% depth of discharge.

Off-Grid Solar - DoD

The abbreviation of “Depth of Discharge” is DoD.

What is state of charge?

Another way to indicate the charge status of a battery is “state of charge”. State of charge is an indicator of how full a battery is and this value is also indicated as a percentage.

“State of charge” is the opposite of the “depth of discharge”.

Off-Grid Solar - State of Charge

What happens if I deeply discharge a battery bank?

As discussed above the deeper you discharge a battery the shorter the cycle life will be and the faster the battery will age.
The reason for this aging is that during discharge batteries shed some of the active materials on their plates.

  • If you only discharge the battery a bit, a small amount of material will be shed.
  • If you discharge the battery deeper, more material will be shed.
  • If the battery is discharged beyond approximately 80% depth of discharge, the shedding process speeds up.

This is why the number of cycles a battery can deliver over its life time will be less at deep discharges than at shallow discharges.

What is the recommended Depth of Discharge?

To achieve maximum battery lifetime, you should not discharge your battery too deeply, ideally the maximum depth of discharge would be set at 30%. However this might not be desirable as a lot of the available battery power never gets used. As a compromise SunWorks Solar recommends a maximum of 50% depth of discharge. This way you will get to use a large part of your battery capacity whilst still getting reasonable battery life time.

For more information on Off-Grid Solar power contact Sunworks Solar today.