When you equip your home with a photovoltaic system for self-consumption with storage, solar batteries represent a significant part of the total investment. It is therefore essential that the choice of the type of batteries takes into account the user’s objectives in terms of price and use. As the supply of batteries is plethoric, a competent installer will be able to offer a solution adapted to each project.
Below are some parameters that will affect the life of a solar battery:
- The charge and discharge current: expressed as a C-rate. For example, a charge at 1C means that the battery will be charged with a current equal to its capacity (100A for a 100Ah battery) therefore in one hour of time . Charging at 0.5C means that the battery is charged with a current equal to half its capacity (50A for a 100Ah battery), therefore in two hours. A significant number of manufacturers of lead batteries (gel and AGM) recommend charging their batteries with a current not exceeding 10% to 20% of the capacity of the solar battery, which would take from 5 to 10h (0.2 to 0.1C). For the lithium solar battery, most of the solutions available to the general public are recharged over a period of 2 hours to 5 hours (0.5 to 0.2C). Several manufacturers of lithium batteries offer solutions allowing very fast charges, at 3C or 4C (15 minutes), but these represent a limited interest for solar self-consumption installations because the price remains high and the charge, in this type of application, can usually be spread over a few hours. The bottom line is that, for the same battery, the faster the charge, the shorter the lifespan.
- The depth of discharge: also called DOD (for “depth of discharge”) and expressed as a percentage. For example, using a solar battery at 50% DOD means that only half of the energy stored in the battery will be used day by day. The lifetime of a battery is expressed in number of cycles (one cycle represents a charge and a discharge) as a function of a depth of discharge. To simplify, the deeper a battery is discharged, the fewer cycles it will perform (deep discharge = reduced lifespan). Below is the DOD VS Cycles curve of an LFP battery.
- The ambient temperature: the higher it is, the more it negatively impacts the lifespan of the solar battery. To give an example, the manufacturers of lead-acid batteries indicate the number of cycles that a battery can perform under ambient temperature of 20°C, each time the ambient temperature increases by 10°C, the number of cycles is halved. It is therefore essential to ensure that the room in which the solar battery is installed will remain in the correct temperature range throughout the year.
The other parameter which can be interesting to take into account when choosing a solar battery is the price per kWh. Many people take into account the total capacity of solar batteries and not the usable capacity when calculating it, which is not a good practice as it is the latter capacity that must be taken into account for the calculation to be consistent. To calculate the price per kWh, we must take into account the depth of discharge at which it will be possible to exploit the solar battery and therefore the energy that it will actually be able to supply. For example, a 10kWh battery usable at 50% DOD will have a usable capacity of 5kWh. These 5kWh should be taken into account when calculating the price per kWh which therefore equals battery price divided by 5.
IMEON ENERGY does not recommend to use lithium batteries (LFP, NMC, LMO …) for solar installations intended for the electrification of isolated sites. In order to be managed correctly, lithium batteries compatible with IMEON inverters must be able to send the inverter instructions concerning appropriate charging, which may not be feasible to execute if the electric grid is not there. For instance, when a battery requests a full charge, the IMEON inverter may need to use the public grid to comply with this instruction if the sunshine is not sufficient. For the electrification of isolated sites, one can therefore consider lead batteries (AGM, gel, OPzV, OPzS batteries).
IMEON ENERGY manufactures hybrid inverters for battery self-consumption which allow installers to work with different types of batteries and therefore choose the most appropriate storage system in function of the requirements. This flexibility allows, for each project, to fulfil the expectations of the user taking into account other objectives such as price and lifespan.