Many people want to run multiple lithium batteries in their system to increase their battery storage. This makes sense, sometime one battery just doesn’t provide the runtime you need. To run multiple batteries in a single system, without increasing the system voltage, you will be looking at paralleling batteries.
What is paralleling?
When we talk about parallel batteries we are talking about what we see in image 1. Paralleling is a method of wiring batteries that increases capacity while keeping battery voltage constant.
What is in a lithium battery?
Another important thing to remember is that lithium batteries have BMSs (battery management system). The BMS protects the pack from fault conditions commonly, over-voltage, under-voltage, temperature, and sometime over-current. When a fault condition detected, the BMS switches of the battery.
Why is paralleling risky?
1. May void your warranty
Some lithium battery manufacturers specify that their batteries cannot be paralleled. If you decide to not follow their specifications, you will void your warranty. Some other manufacturers specify certain conditions on parallel use. These conditions may specify the maximum amount of batteries allowed in parallel, the maximum charge and discharge currents for parallel use, and more.
2. BMS damage, especially on high current
One of the reasons that paralleling is risky is because of the limitations of the BMSs inside the lithium batteries. An example will explain this concept best.
Example. Lets assume you have the following system:
– 2x 100Ah lithium batteries. 100A max discharge, 50A max charge.
– 12v 2000W inverter. We will assume that this is being used to its full capacity.
As can be seen in (image 2) when the inverter is running at full capacity, the batteries are both working together to supply 167A. Assuming the batteries both contribute half of this current, they will each supply approximately 83A (It is important to note that is unlikely that each battery will supply exactly half the required current). In the event that one of the batteries BMS trips (switches off), then suddenly one battery is left trying to supply the full load on its own. This means that one single 100A rated battery is trying to supply a load of 167A. This will very likely destroy the battery that is still on.
It is difficult to guarantee how much of the load or charge current each battery will contribute to. If the total load exceeds the ratings of a single battery, you are risking damaging your batteries.
If you were to parallel, how might you do it well?
1. Use batteries designed to be used in parallel
Using batteries such as the Pylontech RT12100G31 is an excellent choice if you want to parallel batteries correctly. These batteries have built in communications that allow batteries to “talk” to each other. This communication prevents damage caused by parallel use and is a very reliable method of paralleling lithium batteries. This method of paralleling using communication allows the battery current ratings to “add up” when paralleled.
2. Check your manufacturer warranty
If you plan on paralleling your lithium batteries check to make sure you aren’t voiding your warranty. Look through the specifications carefully. Some manufacturers limit the charge/discharge current when the batteries are used in parallel.
3. Balanced wiring
It is important to ensure that the load/charge is evenly shared between paralleled batteries. This means that the cable run to each battery should be equal. This can be done in a few ways as show in the image 4. The goal of the wiring is to ensure that voltage drop is shared evenly across batteries.
4. Low current (below single battery limit)
Running low current devices of parallel lithium batteries can be a safe option. Keeping the load/charge currents below the rating of a single battery minimises the risk that you will damage a battery due to over-current. Running a fridge, for example, off parallel lithium batteries can be a good way to increase runtime.
5. Charge batteries evenly first
Before connecting lithium batteries in parallel, charge them evenly. Putting each battery on a quality charger and allowing them to fully charge is good practice. Evenly charging the batteries reduces the risk of high current flowing between batteries due to different charge levels.
6. Fusing
Fusing each battery can be a good step to improve the safety of your battery setup. Fusing each battery ensures that no battery will exceed its maximum current rating. This can help to reduce some of the risks of parallel batteries. Note, however, that if a BMS trips and causes high current on a battery, the fuses of the batteries will blow. Fusing is not a recommended method of getting around current limits, simply a backup protection to improve battery safety.
