How do I know what battery bank I need?

Much like you use batteries to power electronic equipment, batteries are a fundamental part of an off-grid or battery back-up system. This is where the power generated by the solar panels is stored. If you want to run things like your TV, laptop and wi-fi router during Load Shedding, you will need a suitable battery bank to cater for your requirements. The more power you need, the more batteries you will need. If there is more than one battery connected in a system, we refer to it as a battery bank. This is generally the most expensive component of a Power System. It is important that you get this right the first time to avoid unnecessary expense.

What type of battery should I get?

These days most people opt for sealed batteries as they are maintenance free. The following is based on the most popular sealed, maintenance free battery options.

  • Lead Acid Batteries
Otherwise known as flooded batteries or wet batteries, they are the most cost-effective battery of all, but often only come with a 12-month warranty. This battery has a lifespan of 1 to 3 years.
  • AGM Batteries

These are also lead acid batteries but the plates inside have improved technology known as Absorption Glass Mat (AGM) which makes these batteries more efficient than their counterparts. They charge very easily and hold charge well. Although they also generally only come with a 12-month warranty they tend to last much longer than standard lead acid batteries (4 to 10 years) provided they are cared for properly.

  • Gel Batteries
Gel batteries are called gel because they use silica (or sand) to turn the acid inside the battery into a thick liquid. They have a similar efficiency to AGM batteries but tend to be expensive and much more sensitive to external factors. One mistake (like overcharging or charging too quickly) and they can be irrevocably damaged. On the positive side they tend to have better depth of discharge than AGM batteries.
  • AGM + Gel Batteries
These batteries use the positive attributes of both gel and AGM batteries to create a more efficient and longer lasting battery. They are suitable for renewable energy systems and with a longer design life (8 to 12 years). They still come with the standard 12-month warranty.
  • Lithium Batteries

Lithium is an exceptionally light metal which gives these batteries the highest energy density of any other battery cell. They can store more energy than other batteries. While other batteries can only be used at 50% capacity (otherwise known as depth of discharge) before they need to be recharged, lithium batteries can be used at 80% depth of discharge. They are also able to handle extreme temperatures, both hot and cold. These batteries usually come with a much longer warranty (from 3, 5 and 10 years) and an even longer design life. While they are more costly from the outset, they end up being cheaper in the long run.

How do I determine what I need?

Calculating what you need is always best left to the experts as there are several factors that need to be considered when designing a battery bank. Our technical consultants are more than happy to assist you with this free of charge. However, for now, these are the basic calculations to use when sizing a battery bank suitable for your requirements.

First decide what voltage system you require. Most Solar Power or Back-Up Power Systems come in 12V, 24V or 48V. Again, this depends on several factors. For the purpose of this exercise, and to simplify things, the more power you need (or the more you want to expand) the higher the voltage should be to allow for this. A 12V system is generally suitable for smaller applications up to 2kWh, whereas 24V and 48V are better suited to larger systems from 2kWh and up. If you are unsure what kWh is referring to, please read our blog “How much does a Solar Power System Cost?” for more clarity.

How do I calculate the AH rating?

I am going to use the same load (equipment that needs to be powered) as an example of how to size a suitable battery bank:

10 x 10W LED Lights = 100W
1 x LED SMART TV = 100W
2 x 30W Smart Phones = 60W
1 x Router = 20W
1 x Laptop = 65W

Total = 345W

Load Shedding can sometimes last up to 5hrs per day, so 345W x 5hrs = 1725Wh or 1.725kWh. As this is below 2kWh, I could get away with a 12V system, but prefer a 24V system to allow room for expansion.

We then use the basic Ohms Law to calculate the battery bank required:

  • Watts/ Volts = Amps
  • Watts/ Amps = Volts
  • Volts x Amps = Watts

Batteries are measured in Amp Hours so you will need to do the following calculation:

Watt hours/ Volts = Amp hours

Therefore 1725Wh/ 24V = 71.88Ah

As you will recall, most batteries are only meant to be used at 50% capacity, so you’d need to double the amp hours of the above i.e., 71.88 x 2 = 143.76Ah.

Therefore, a 150Ah 24V battery bank will be suitable for the equipment you need to power.

As most of our Lead Acid, AGM and AGM+Gel batteries are available in 12V cells, you will need 2 x 150Ah batteries connected in series to get 24V.

If you connect batteries in series, it increases the voltage, if you connect them in parallel it increases the amperage.

Again, please keep in mind that these are the very basics of system design. There are many other factors that need to be considered, so please always consult a professional.

How do I effectively take care of my battery bank?

  • Batteries should always be elevated off the ground. Battery Boxes/ Cabinets are great for this.
  • Install them in a dry, well-ventilated area.
  • There should be at least 5cm of space between each battery.
  • Inverter cables should be connected to the positive and negative terminals on either side of the bank to ensure even discharge. If it is connected to one battery, that battery will take strain and reduce the lifespan of the rest of the bank.
  • The above goes for the charge controller and battery charger cables too, for even distribution of current.
  • If you have a 24V bank, you will require a 24V battery charger. Try to avoid charging the batteries separately with a 12V battery charger, this will create inefficiencies between the batteries. Always use the same voltage battery charger as the battery bank. If they are different, you will irrevocably damage your batteries.
  • Do not put additional equipment onto a battery bank because you think there is enough storage to handle it. Battery banks are sized according to the load as per the above. If you add additional equipment without proper knowledge of what you are doing, and the battery is discharged very low, you will damage your battery and greatly reduce its lifespan.
  • Investing in a Battery Monitor is a good way to actively monitor what is happening with your battery bank. It allows you to monitor the state of discharge and quickly identify any issues that may occur.
  • Do not store batteries for more than 3 months. Batteries need to be charged so they don’t go flat. If they are being stored, take them out and recharge them once a month to ensure longevity.

What else should I remember when investing in batteries?

  • *Please always check the C-Rating of a battery so you can compare apples with apples. This is the rate of time it takes for a battery to charge or discharge. They are generally rated at C10 (10 hour) or C20 (20 hour), but these differ from battery to battery. For example, a 240Ah battery rated at C20 is the same as a 200Ah Battery rated at C10.
  • Check how many cycles the battery has. A cycle refers to how many times you can discharge and recharge a battery.
  • You pay for what you get. Batteries are the most expensive part of a power system and if you are going to go cheap, don’t expect them to last very long.
  • For your own safety, please do your research on a company before opting for their service. There are a lot of start-ups that do not have the technical knowledge to effectively assist you with what you need, and batteries can be a hazard if not suitable for your requirements or not installed properly. The internet does not count as a reputable source of information.

We hope that this clarifies things a bit when it comes to battery banks and gives you a better idea of what your needs are. If you are looking for something to tide you over during Load Shedding or if you want to go completely off-grid, please contact us for assistance. We have almost 20 years of experience designing battery banks for our clients, free of charge!

Look out for the next instalment of our blog series “Can I expand my Solar Power System at a later date?"

Check out Part 1 of this blog series " How much does a Solar Power System cost?"