Yes, you can mix different capacity lithium batteries, whether a normal 12V 100Ah battery or a Lithium server rack battery.
There are a few points you need to consider when wiring in parallel.
- The batteries must have the same voltage
- The batteries must each have their own BMS
- There should be a fuse installed for every battery
- Have them at the same state of charge before connecting them
Let’s explore these three points. At the end of the article, you will find a diagram on how to wire these.
First some basic knowledge:
Connecting batteries in parallel increases the total capacity Ah of the battery, while connecting batteries in series adds up the battery’s voltage.
1. Batteries must have the same voltage
The total battery bank must be at the same voltage. You must create a separate system for different voltages if you have different voltage batteries.
Your total battery bank, which can have multiple different capacities (Ah), all need to be the same voltage, whether 12V, 24V, or 48V. You need to choose one of these three voltages.
The batteries must have the same chemistry as well. It is not possible to combine lithium-ion with LiFePO4. This is because lithium-ion has a slightly higher voltage than LiFePO4.
The batteries get charged all at the same time. A smaller battery will be at the same voltage as a larger battery. Therefore, the state of charge will be equal in all batteries. Let me explain with a diagram where all the batteries are charged to 70%.
2. Batteries must have their own BMS
The BMS is responsible for managing the charge and discharge process, keeping each cell within safe operating limits, preventing overcharging and over-discharging, balancing the cells, and providing essential data about the battery. Therefore, having a dedicated BMS for each battery can help to ensure that each one operates safely and efficiently, even when the batteries have different capacities.
Some batteries will have different internal resistance. Therefore one battery may be charged quicker than another. Once a battery is fully charged, the BMS will cut off the power to that particular battery to protect it from being overcharged.
3. Batteries must have their own fuse
A fuse for each battery can prevent excessive current from damaging the battery or creating a safety hazard.
The overcurrent protection for the BMS is not enough. You need a physical fuse that can blow to create a separation from the other batteries.
You can use a MIDI fuse if you have a 12- or 24V battery bank at 100Ah. If you have a 48V battery or a total capacity higher than 100Ah, you should use a Class-T or NH00 fuse.
This is because the fault current will exceed ten times the capacity. This means that a 100Ah battery can have a short circuit of 1000A. A MIDI fuse can only break 2000-3000A in a 12V circuit. Therefore if you increase the voltage and the battery’s capacity, a Class-T or NH00 fuse is recommended.
If one battery short circuits, all the other batteries will dump as much current as possible into the shorted one.
As you can see in the example, if Fuse 2 or 3 fails, then Fuse 1 will have to break over 3000A. Therefore I recommend having at least a class-T fuse or NH00 when your total battery capacity is over 100Ah. Read more about fusing here.
4. Have the same state of charge before connecting them
When you connect your batteries in parallel, they must have the same state of charge before connecting them. Because the voltage level of a LiFePO4 battery is flat in the middle, I recommend fully charging or discharging them before connecting.
If you do not connect the batteries when they have the same state of charge (voltage level), then the inrush current can blow your fuses and damage the BMS of the other batteries.
Schematic for multiple lithium batteries in parallel
Here is a diagram for multiple lithium batteries in parallel. You can add individual battery switches after the fuses. From the main busbar, it can go to your inverter, charge controller, or generator.
The negative cables can go to a busbar, then a shunt, then another busbar. If you have 3 batteries or less, you can connect them to the shunt without needing an additional busbar. This is because you can only have a maximum of three lugs on one terminal.
Conclusion
There you have it, connecting multiple lithium batteries with a different capacity. I hope you found this article with the schematics helpful. If you have questions, please leave a comment.
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