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Apr 14, 2021 | Batteries, Installation

How to Correctly Size a Lithium Battery Bank

Written by Nigel Engelbrecht

We sat with Sam Smit from Green Power Projects and discussed how to size a battery bank. There are a few misconceptions surrounding batteries and one of them is sizing. In this article, we focus on lithium batteries as these are rapidly growing in popularity for residential installations.

“Here’s a mistake I see far too many installers making, especially the newer ones, incorrectly sizing a battery bank, and doing this using the cheaper 15 cell batteries (or 0.5c rated specifically).” Here Sam refers to lithium batteries which are now widely used and requested for many residential PV installations.

Sam Smit is the owner of Green Power Projects, a renewable energy company focused on residential solar and backup solutions. With over 10 years in the business, Sam gives us a basic rundown of HOW to choose your lithium battery to avoid disappointment and/or costly mistakes.

Sam goes on to explain that batteries have a limited amount of ‘power’ or ‘capacity’ they can deliver at any one time. Referred to as discharge i.e the amount of power that is discharged from the battery for your needs. “These batteries (all batteries for that matter) have a limited discharge current. In other words, the battery can only deliver a certain amount of power at any given point. This limited discharge current can also decrease based on conditions like SOC (most drop their max discharge current when the SOC reaches a certain point) and even temperature (if a battery gets warmer or colder that discharge current can vary as well)”.

SOC refers to the State of Charge – The state of charge is defined as the ratio of the available capacity Q(t) and the maximum possible charge that can be stored in a battery, i.e., the nominal capacity Qn. (6.1) A fully charged battery has SOC 1 or 100% while a fully discharged battery has a SOC of 0 or 0%.
Credit: Science Direct

We sat with Sam Smit from Green Power Projects and discussed how to size a battery bank. There are a few misconceptions surrounding batteries and one of them is sizing. In this article, we focus on lithium batteries as these are rapidly growing in popularity for residential installations.

Now, in a hybrid install, this is not all that critical as the inverter can limit the amount of current it draws from the battery and supplement the shortfall from grid power. Where you will have issues is in the event of a grid failure and the inverter switches to off-grid mode and is fully reliant on the battery.

When reference is made to ‘grid’ this refers to state electricity supply such as Eskom.

Hybrid systems are intelligent as they can alternate between off grid and grid connected with our without the use of batteries. This makes them very attractive option.

“In off-grid systems, however, and I’m not just referring to systems installed where grid power is unavailable, I’m talking about off-grid inverters (specifically the Axpert type). These are unable to supplement from the grid and are fully reliant on the max discharge current of the battery”. Sam talks of off-grid systems which means you are purposely disconnected from the main grid or there is no main grid available in your area.

Off-grid systems require large amounts of storage and in many cases a Generator as backup to compensate the battery and panel shortfalls that can and do happen on cloudy overcast days or during rainy weeks.

Example: To simplify how battery storage works with a 0.5C rated lithium battery: A 5kW inverter with a 2.4kWh battery at 0.5c has a max discharge current of 25A (this is equivalent to only 1.2kW). The moment the load exceeds 1.2kW the battery simply cannot deliver the power the inverter requires. If you don’t trip the battery’s BMS you’re going to experience such a voltage drop on the battery that the inverter will immediately drop the battery.

BMS refers to Battery Management System – A BMS is defined as an electronic system that manages a rechargeable battery (single-cell battery pack) by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and/or balancing it.
Credit: Science Direct

In the above example, the same equation applies to a lithium battery with a 1C rated performance. Take 5kw of battery at 1c has a max discharge of no less than 80-85A which is the equivalent to 4,8kw if we factor in cut off voltage etc … Essentially where the same 5kw battery at 0.5c will give you 2,5kw, you will need two 0.5c batteries to equal a 5kw 1c battery.

“So a good rule of thumb, especially when using the 0.5c batteries is to use at least 2 batteries equally rated at the inverter size. So a 5kW inverter would need 2 x 4.8kWh batteries”. Sam explains best practice here with 0.5c lithium batteries to 1 inverter. Again, a 1c Lithium battery will do the same as two .5c lithium’s.

There are a wide array of factors to be considered here. If you ever are unsure of something please contact your supplier and ensure you are providing the correct solution to your client.

If you would like to reach out to Sam regarding your energy needs please reach him via email at sam@greenpowerprojects.co.za
Facebook: Green Power Projects SA
Website: www.greenpowerprojects.co.za

Thank you for reading this article. If you feel we have left out any important information or would like to contribute to this site and content, please get in touch with us by leaving a comment or emailing us.

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