When looking for a Lithium-Ion Battery these are the main factors you should consider.
- Capacity
- Cycle Life
- Useable Battery % (DOD)
- Voltage
- Chemical Structure
- Battery Management System
- Battery Casing
- Warranty
Lithium-Ion Battery Capacity
The most important factor is the battery’s capacity. Battery Capacity is measured in kilowatt-hours commonly written as kWh. A kWh is a unit of energy. For example, if you use an appliance that needs one kilowatt of power to run and you run it for one hour then you will need one kWh of electricity. Similarly, if you run an air conditioner that continually draws 2.5kW of electricity then it will need 2.5kWh of energy to run for an hour. Therefore calculating what appliances you would like to power and for how long is essential.
Lithium-Ion Battery Cycle Life
This is the measure of how many times you can use the battery before it is no longer usable. A cycle means using or discharging the battery’s available power and then fully recharging it.
In the two datasheets below the first battery has 4000 Cycles while the second battery has a greater than 6000 cycles. The second battery has a 33% higher cycle life projected to last 16 years as opposed to 10 years making the second battery a better choice.
Battery Example 1
Battery Example 2
Lithium-Ion Battery Useable % DoD
The term DoD or depth of discharge is the amount or percentage of the battery that you can safely use without dramatically reducing the battery’s life or damaging it. In the two datasheets above the first battery has an 80% DoD meaning that you can safely use 80% of the battery for your daily cycles with the second battery having a 90% DoD meaning that 90% of the battery is useable.
This is very important as it directly affects the useable capacity of the battery that you are purchasing. For example, a 20kWh battery with an 80% DoD has 16kWh of useable power while the same battery with a 90% DoD will have 18kWh of useable power.
Lithium-Ion Battery Voltage
The voltage of your battery has a direct impact on the efficiency of your battery. The higher the voltage the more efficient.
Heat created due to resistance when stepping current down from 500-1000V from a solar panel string to 48V and then back up to 230V for single-phase power and 400V for 3 Phase power. The remaining power after this loss is referred to as round trip efficiency.
In the 48V example below you can see the battery’s internal temperature ranges between 50 to 55 degrees while in operation. This is as hot as the inside of your geyser and not only causes energy loss but ages your battery faster.
48V Battery Temperature
In the 459V battery example below the internal temperature of the battery operates very close to room temperature with the internal battery temperature between 25 and 30 degrees. This results in a much more efficient longer lasting battery.
459V Battery Example
To illustrate what to look for on a battery datasheet In the first example below with a 50V battery has a round trip efficiency of 90% meaning that 10% of the energy and usable capacity is lost to resistance. In the second example, the battery is 100.8 V which has less resistance with a higher round trip efficiency of 95%.
The higher the battery voltage the more efficient your solar system will be.
Battery Example 1
Battery Example 2
Lithium-Ion Battery Chemical Structure
The two most commonly available battery chemistries are NMC which is cobalt-based and LFP which is phosphate-based. Cobalt-based batteries when exposed to air oxidize, catch fire and have a thermal run away. Phosphate batteries are inert, do not burn when exposed to air, and are much kinder to the environment.
In other words, if you reverse your car into an NMC battery it will catch fire and not be able to be extinguished unless with specialized fire fighting equipment. In the same incident, an LFP will not catch fire.
Please see the video below for more.
Lithium-Ion Battery Management Systems
Every Lithium-Ion battery system needs a battery management system or BMS. It is the brain of your battery and plays a critical role in the advanced management, monitoring, safety protection, system performance and overall battery longevity.
Research needs to be done on who manufactures the inverter, the battery and BMS. These can come from different manufacturers and may not work optimally together. If there are problems customers can fall between the cracks with your inverter manufacture blaming the battery manufacture and vice versa.
A better option would be that the inverter, battery and BMS are all developed by the same manufacturer. This will provide the highest level of customer support, battery safety and maximum battery life.
Lithium-Ion Battery Casing
Battery casings come in plastic, flimsy metal, and strong metal. Strong metal providing the most protection.
A higher level of battery protection is provided by batteries that have an IP 63 or IP 65 rating which means they are water and dustproof. This will provide protection if a geyser leaks in the area or the battery is installed outdoors.
For an even higher level of protection batteries with a TÜV Rheinland certificate is proof of the safety, reliability, and excellent quality
Lithium-Ion Battery Warranty
Check what warranty is offered with the battery, for example the number of years, depth of discharge, battery cycles covered and other fine print. In some cases, if the battery is offline for an extended time and not connected to the internet the manufacturer will not honour the battery’s warranty.
Summary
The right battery for you is the one that has the capacity to meet your energy needs and taking all the above points into consideration.
If you would like to learn more please contact us for a Zoom Solar Workshop at power@msolarpower.co.za.
Looking forward to you being empowered by the sun!
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