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Creabest prides itself on making strong and reliable LiFePO4 batteries. It's no surprise that we get a lot of battery-related issues.
One of the most common questions is "I need more energy, can I parallel the batteries? How many LiFePO4 batteries can I parallel at most?"
Paralleling batteries in a LiFePO4 battery is one way to increase the battery's capacity (i.e. how long the battery can run on a single charge).
Of course LiFePO4 batteries in parallel doesn't mean having your battery power any device above its standard voltage output, but rather increasing the duration it can power the device. It should be noted that the increased ampere-hour capacity may require longer charging times when charging for LiFePO4 batteries in parallel.
Related knowledge of LiFePO4 batteries in parallel
First of all, we need to know that when two or more LiFePO4 batteries are connected in parallel, the current flowing through each battery cannot be exactly equal.
For example: Suppose you use two 12.8V 100Ah batteries in parallel. When the battery system is connected to a load of 50A, the load of each battery cannot be completely accurate to 25A. One battery might take a load of 21A, while the other battery takes a load of 29A. This difference in current becomes more pronounced when the load is larger.
Also, as the number of betteries in parallel increases, it becomes increasingly difficult to get power from each bettery and to push power evenly to each battery. To complicate things further, each battery has a unique internal resistance and therefore accepts/produces electricity at slightly different rates.
Reasons for Current Unbalance of LiFePO4 batteries After Paralleling
Current imbalance between batteries is caused by field installation variables such as cell and battery manufacturing process differences, battery connection resistance differences, and temperature differences between batteries.
Even if the batteries are produced in the same batch, their resistance and power supply capacity cannot be exactly the same. And the difference in current in each parallel branch causes the battery state of charge (SOC) to diverge. The battery with the best ability to deliver power will deplete faster than the rest. Then, the SOC of all batteries will converge again as the fastest depleting battery gets to a point where it can no longer deliver as much power to the system. At this point the stronger batteries will be those that initially delivered less power but are now at a higher SOC.
The conclusion is that there will be a difference in SOC between each LiFePO4 battery connected in parallel for most of the cycle life of the battery system. The difference in SOC between the batteries means that even when the load is disconnected, there will be eddy currents flowing between the batteries because they need to equalize the SOC state of the paralleled battery system. Sometimes eddy currents between batteries can be abnormally high and cause the cells to enter an unpredictable protection mode.
It is important to note that the discharge curve of a LiFePO4 battery is very flat so it can take many hours for the batteries to equalise SOC once a load is removed. It is possible that the batteries won’t equalise no matter how long you wait due to diffusion voltages, or what some call ‘surface charge’.
Of course, the battery management system (BMS) can handle the parallel branch current unbalance and uncontrolled eddy current, but it cannot solve this problem 100%, and can only achieve a relatively balanced state between the paralleled batteries. Once the current in the parallel branch is higher than expected, it may shorten the life of the battery.
LiFePO4 battery paralleling precautions
Before paralleling batteries, you may want to note the following:
1. Do not mix batteries of different brands and capacities
Make sure that the batteries connected in parallel have strict internal resistance pairing: absolutely the same capacity, same voltage, same brand and cannot be used with old and new batteries. If batteries of different capacities are used together, a current imbalance will occur between the two batteries, which will shorten the operating time of the battery system. If you mix batteries of different capacities for a long time, it will eventually lead to a short circuit in the batteries with smaller capacity. A shorted battery can cause excessive heat, further affecting the entire electrical system, or even damaging the battery.
2. Fully charge the battery first
Before connecting batteries in parallel, please make sure that all LiFePO4 batteries you want to run in parallel are fully charged individually with matching chargers. so that the battery is more or less balanced, maximizing your overall usage time.
3. Use the correct connection cable
Make sure your cables are the right size (diameter) to carry the required current - they must be the same size as your maximum requirements and usually match your inverter.
Therefore, the battery cable should be the same size or larger than the inverter cable. They also need to be of the right length for efficiency.
Ideally, make sure that your connecting cables are as short as possible and that the batteries are close together, and that the cables are of equal length between each battery.
4. Check the main system cable
The main system cable that powers the load should be connected to the entire battery pack to ensure that the batteries are charged and discharged evenly for optimum performance.
Also, make sure the main system cables and the cables connecting the batteries together are large enough to handle the total system current.
5. Connect the battery
Connect the positives to each other and do the same for the negatives. Then, connect the positive of the first battery and the negative of the last battery to the system.
Be careful not to reverse the positive and negative poles.
Why can't old and new batteries be used in parallel?
When two or more batteries in parallel are new and are of the same brand of capacity and voltage, the amount of power being pushed between the batteries is well balanced.
As the batteries age, their internal resistances gradually become mismatched.
Batteries are known to generate self-consumption. Even if the battery is left unused all the time (or only used once or twice), the circuits inside the battery etc. will consume a small amount of power. The cells inside the battery then slowly age over time.
For example, the LiFePO4 100Ah battery purchased a year ago (or half a year ago) and the 100Ah battery of the same brand just purchased will have slightly different states of the internal cells, and the internal resistance of the battery will also be different. You cannot use these two batteries in parallel.
Because the new battery has lower internal resistance compared to the old battery. Batteries with lower internal resistance are somewhat self-correcting and will charge and discharge at higher currents than others, ageing faster and catching up with their counterparts. Also, the internal resistance of the new battery is lower than the old battery, the current will now mainly flow through the new battery, affecting the life of the new battery.
Can Creabest LiFePO4 batteries be connected in parallel?
CREABEST batteries can be connected in parallel, but the following conditions must be met:
Only in this way can it be ensured that the internal characteristics of the batteries are closer and the parallel effect is better.
Obviously, if you plan to parallel batteries, buying 2 batteries at once is the best option. In addition, isolation fuses are used in parallel wiring to avoid mutual charging between the two batteries.
Reminder: Creabest's LiFePO4 battery does not support series use.
