Batteries

Batteries can store electrical energy. This makes them an essential part of off-grid systems, where solar energy is stored in batteries for use at night or on overcast days.

• Batteries store electrical energy in chemical form. They have a limited lifespan, usually expressed as cycle life, which describes how many times the battery can be discharged and then fully recharged.
• When the battery is full, it is said to have a state of charge (in: state of charge or SOC) of 100%.
• All batteries are sensitive to low temperatures: they lose a large part of their capacity in case of freezing, or shut down completely to avoid damage.

Lead acid batteries are a simple and well understood technology. They do not use internal electronics for protection, so they can be easily damaged, for example when deeply discharged. It is recommended to use a battery protection device.

They have some disadvantages compared to more modern lithium batteries:

• Weight (4 times heavier than lithium batteries, for the same usable capacity)
• Volume (twice as bulky as lithium batteries, for the same usable capacity).

Car batteries or starter batteries are 12V batteries that are used in cars to start the engine. These batteries are designed for a short period of time to start the car and then to be recharged.

Using this type of battery for energy storage in an off-grid system will break them very quickly.

These batteries are designed for energy storage, and are the cheapest option to buy for off-grid use at the moment. They are filled with liquid acid. When they are charged, some of this liquid evaporates, and they must be refilled with distilled water every two weeks. They should also be kept in a ventilated area. If properly treated, they will last 5-7 years, sometimes longer, depending on use.

This type of lead-acid battery is suitable for energy storage and does not require maintenance, but is a bit more expensive than flooded batteries.

Lithium-ion batteries are more energy dense and have a higher power output than lead-acid batteries. For this reason, they are used in many mobile applications, such as laptops, cell phones, cordless power tools, electric bikes and electric vehicles. The technology is more recent.

Lithium batteries contain a battery management system (BMS). The BMS can shut down the battery to protect it from misuse (high and low voltage protection, high and low temperature protection, etc.). They are less likely to break if misused.

Lithium batteries cannot be charged at temperatures below freezing, which should be considered for unheated camps. They are also more complex and may not be as mature as lead acid batteries. Some LiFePo4 batteries have a built-in heater, which will warm the battery with energy from the solar panels before it starts charging in low temperatures.

Lithium iron phosphate batteries, LiFePo4, or LFP, have a much longer life than conventional lithium ion batteries, but they are somewhat heavier but still much lighter than lead acid batteries. Today, they often replace lead-acid batteries in camps, cabins, recreational vehicles, or as backup power in homes.

Today, mainly 3 types of batteries are used in off-grid systems:

• Deep cycle lead acid batteries (the cheapest to buy)
• AGM lead acid batteries (the cheapest to buy without maintenance)
• LiFePo4 batteries (better performance and longer life than lead-acid)

Voltage and state of charge for different types of batteries:

A battery bank consists of several batteries connected together to act as one. When installing a new battery bank, make sure all batteries are:

• The same type and manufacturer
• Of the same age (ideally new)
• The same state of charge

If you use different batteries, they will degrade much faster than normal.

Batteries can be connected in series, parallel or a mixture of both. Connecting in series increases the voltage, but the overall energy capacity of the bank is the same no matter how they are connected.

The capacity of a battery is often expressed in ampere-hours (Ah). To calculate the more useful energy capacity in watt-hours (Wh), multiply the amp-hours (Ah) by the battery voltage (V).

Lead acid batteries, despite the toxicity of their materials, are one of the most recycled products in the world, thanks to a well-developed industry over the years. For lithium batteries, being a new technology, this recycling industry is not yet developed. Considering that all batteries are basically a block of precious materials, it is likely that this industry will develop in the coming years.