A Lithium battery is often the most expensive element in a solar installation, and as a battery is a long term investment, it is important to take into account all aspects of a battery before buying. There are a number of parameters that are important to consider when buying batteries, for example:
- Voltage and capacity
- charge / discharge rate (C-rate)
- Depth of Discharge (DoD%)
- Life span, warranty and (local) back-up support
Voltage and Capacity
The Voltage of a battery will have to match the Voltage of your inverter. These days, most home solar or backup installations are 48 Volts (V). The higher the voltage, the less energy losses take place inside the system.
The Capacity of a battery is the total amount of electricity that it can store, measured in kilowatt-hours (kWh) or Ampere-hour (Ah). If the battery capacity is measured in Ah, just multiply the Voltage of the battery with the Ah rating:
Wh = Ah x V
To calculate how long a battery will last, for your intended use, you can multiply the loads you want to connect, by the duration you want to run the loads for. For example, your load is 1 kW (1,000 Watts); a battery with a useable capacity of 4 kWh will run this load for 4 hours.
Time (hours) x kW = kWh
While capacity tells you how big your battery is, the C rate tells you how much power a battery can provide at a given moment, or how quickly the battery can be charged or discharged.
A battery with a high capacity and a low C rate would deliver a low amount of electricity (enough to run a few crucial appliances) for a long time. A battery with low capacity and a high C rate could run your entire home, but only for a few hours. The C-rate is relative to the capacity of the battery, e.g.:
In summary, 1C means that the battery is fully charged and discharged within one hour, 2C is 30 minutes, and 0.5 C means 2 hours.
Example: a Pylontech US3000C will have a C-rate of 0.5, while some other batteries have a 1 C-rate. To be sure that a 5 kVA / 4 kW inverter can run at its full potential, you would need at least 3 of these Pylontech batteries.
Depth of discharge (DoD)
The Depth of Discharge (DoD) of a battery refers to the amount of a battery’s capacity that has been used. Most battery manufacturers will specify a maximum DoD for optimal performance. For example, if a 10 kWh battery has a maximum DoD of 90 percent, you shouldn’t use more than 9 kWh of the battery before recharging it.
In theory, all Lithium batteries can be discharged at 100%, but this will shorten its cycle life and might affect warranties. The stated DoD in battery specifications should therefore always be considered together with the life span (see below).
Life span, warranty and back-up support
In general, most Lithium batteries will have an expected life-span of somewhere between 8 and 16 years. In most home solar systems, your battery will “cycle” (charge and discharge) daily. Over time, your battery will lose some of the charge.
This is similar to the battery in your phone – you are charging your phone each night, and as your phone gets older, the battery doesn’t last as long as it did when it was new. Extreme temperatures can affect the life span of a battery. The life-span of a battery is often stated in number of cycles, e.g. 4,000 cycles.
Your battery will have a warranty that guarantees a certain number of cycles and/or years of useful life, often for a certain maximum DoD. Warranty often states for example “10 years at a maximum (or average) DoD of 80%”, and that after 10 years “60% of the battery’s capacity” will remain. Always try to check for this statement in a battery’s specification, the DoD in combination with life span and remaining capacity.
Another aspect to consider is back-up support, and whether this support can be provided locally as well. For example, if a battery is assembled locally, and there is a specific battery cell that happens to be faulty, the manufacturer can come and simply swap out that cell. However, for many imported batteries, the unit has to be sent back to its origin for repairs, and you might have to wait for a long time to get your battery back once it is repaired.
Most batteries are designed to be “expandable,” which means that you can add multiple batteries to your solar system to get extra capacity. Check for this option to make sure you can expand your system in the future.
Some batteries can be wall-mounted, others have to be placed on the ground, and sometimes you need to buy specific cabinets. If, for example, you have limited space in your garage next to the DB board, the mounting system is something to consider as well.
Other aspects to consider are, whether you need extra specific battery cables, whether the battery is compatible with your brand of inverter and whether the battery is able to communicate with the inverter, and last but not least of course the what the total price is, compared to the quality of the battery. Price per kWh would be useful for comparison, taking into account, warranty, number of cycles etc; in short considering all the issues discussed above.