Updated: Feb 14, 2022
This is some more detailed information on lithium batteries that we have been able to glean in our journey to buying new batteries. We are definitely not experts. If you notice something that isn't correct here please comment and let us know.
Lithium batteries, as with all batteries, have several operational limitations. These are essentially temperature and charge voltage.
LiFePO4 batteries will be damaged if charged at or below 0C. Also charging batteries in hot conditions, over 45C, will reduce the life of the battery. These batteries will also be damaged if charged at a voltage over 29.2V.
To overcome these issues manufacturers incorporate a Battery Management System (BMS) which will stop the battery from operating in an environment where they will be damaged.
Lithium batteries should last longer than lead acid batteries as well. Our flooded lead acid batteries started to show problems after 7.5 years and according to our supplier may last up to ten years. When you start to research this you will encounter the term 'cycles' when checking battery specification. All this means is a charge and discharge cycle and with off grid batteries a cycle is usually a day. So a battery with 2,000 cycles in a life time should run for about 5.5 years. Of course it isn't that simple as it will depend upon the situation, including the 'depth of discharge', which is how much the battery is depleted each cycle. Lead acid batteries will have a short life if discharged over 20%. Although lithium batteries can be completely discharged they last longer if only discharged to 80%.
Battery Management System
Lithium batteries like all batteries are affected by temperature. However extreme temperatures or high voltages can have a fatal result for lithium batteries. Low temperatures, below 0C, or high temperatures above 45C, can cause major problems to the battery so these batteries are usually fitted with a battery management system (BMS). This is usually built into the battery to stop damage being done to the battery in these extremes. Each manufacturer will have their own version of this. These handle the extremes so the battery isn't damaged but they don't manage the usual charge and discharge which is managed through a solar controller or regulator.
Calculating storage capacity
Battery capacity defines how much power you have available when the sun isn't shining or the wind isn't blowing. Usually for off grid you need to have a few days of power stored for those cloudy days.
Calculating the battery capacity is essential to decide the size of the battery storage. Don't get lost in all the graphs that you will find as these will take into consideration the depth of discharge of the battery, the less the depth of discharge the longer the battery lasts (in theory).
Batteries are rated according to their capacity measured in amp hours or watt hours. Amp hours indicate the number of amps (current) able to be delivered in an hour (under certain conditions), so a 1000Ahr battery can deliver 1000 Amps for an hour or 500 Amps for two hours or 250 Amps for four hours at the particular voltage of the battery.
A watts is the unit of power (P). Power is the voltage (V) x current (I). Watt hours measure the amount of stored energy.
You can calculate the watt hours by multiplying your battery voltage by the amp hours. Our lead acid battery is a 1000 Amp hour for 24V battery.
1000 x 24 = 24000 watts hours or 24kilowatts hours.
This may seem a large amount but with lead acid batteries you can only use 20% of the total or you will reduce the life span of the battery so really the useful power of this battery is actually only 24000 x 80% = 4800 watts hours.
A lithium battery can use 80% of the battery without degrading the lifetime too much so to deliver 4800 watts you need a battery capacity of only 6000 watts.
Battery Recharge Cycle
All batteries charge in a cycle. With lead acid batteries the cycle goes through
Boost - full charging, using all available energy (within battery limits) to charge to almost full
Absorption - final stage of charging, trickling charge in to fill the last 1-2% to get to full
Float - fully charged battery with small top ups as these batteries will loose energy doing nothing
In addition lead acid batteries require Equalisation usually monthly. This is where each of the component cells in the battery bank are 'evened out'. This is done by overcharging the cells for an hour or two.
Lithium batteries behave differently to lead acid batteries. Unlike lead acid batteries they don't mind being heavily discharged and they don't mind sitting with less than a full charge, in fact this is recommended.
Also although lithium batteries are also composed of a number of 'cells', like lead acid batteries, these cells are managed by the BMS in lithium batteries so they don't need to be 'equalised'.
Lithium batteries really only need two steps to the cycle.
Constant charge - all available charge is applied, within battery limits, is used to charge the battery
Constant voltage - keeps the voltage at a constant level, topping up only when battery is being drawn on