3.3 Parallel battery bank wiring
It matters how a battery bank is wired into the system. It is easy to make a mistake. One of the most common mistakes is to parallel all the batteries together and then connect one side of the parallel battery bank to the installation. As indicated in below image. What happens when a load is connected? The power coming from the bottom battery will only travel through the main connection leads. The power from the next battery has to travel through the main connection and through the 2 interconnecting leads to the next battery. The next battery up has to go through 4 sets of interconnecting leads. The top one has to go through 6 sets of interconnecting leads. The top battery will be providing much less current than the bottom battery. What happens if the battery bank is being charged? The bottom battery gets charged with a higher current than the top battery. The top battery gets charged with a lower voltage than the bottom battery. The result is that the bottom battery is worked harder, discharged harder, charged harder. The bottom battery will fail prematurely.
Why is cable resistance important when wiring battery banks? Remember that a cable is a resistor. The longer the cable, the higher the resistance. And cable lugs and battery connections also add to this resistance. To give an indication of this, the total resistance for a 20cm 35 m 2 cable together with its cable lugs is about 1,5 mΩ. You might say that 1.5 mΩ is not much. But the internal resistance of the actual battery is also low. Therefore, it does matter a lot! The internal resistance of a battery is typically between 10 to 3 mΩ. If you construct an electrical diagram it will look like this: Current always chooses the path of least resistance. Most of the current will travel through the bottom battery. And only a small current will travel through the top battery.
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