Connecting batteries together to generate high capacity banks at higher voltages than the individual batteries can supply can be a bit more complex than it seems on the surface. When power draws and charging currents are low, the arrangement of connections is not critical, but large banks such as those we are discussing are created because they need to supply a lot of power, and that power typically is replaced into the battery by charging currents as high as we can generate. When we are dealing with currents of 50 to 100 amps or more, sometimes a lot more, even very small resistances become very important in the way electricity flows around the circuit.
Things start very simple…
Getting More Volts
If we have two 100 amp-hour, 12 Volt batteries and want to make a single 24 Volt, 100 amp hour battery bank we really have no options. There is only one simple geometry that works:
Simple, Yes? I said so, right? Well, I lied… a little!
Here is the problem: There is nothing that intrinsically forces the two batteries to be at exactly the same voltage while charging or discharging.
It is true if the batteries are EXACTLY the same in all ways then they will distribute charging current and voltage evenly between themselves. But that is not very likely. One battery will have a very slightly higher internal resistance than its partner. That one will see a slightly higher voltage during charging, and reach full charge before the other. One battery will be very slightly overcharged, and the other very slightly undercharged, at the end of the charging cycle.
In the short run this is not noticed and is not important, but over many charge cycles the consistent difference causes the batteries to age differently, and the differences become larger and larger. Neither battery is being charged optimally, and very likely both will die earlier than they should.
If you have Flooded Lead Acid batteries that you regularly bring up to 100% state of charge, and equalize at higher voltage at the appropriate interval, this downward spiral can be arrested and reversed. What equalization does is force all of the cells of the batteries to be as close to 100% charged as they can be. This, at least to a first approximation, resets them all to exactly the same state. If this is done often enough, we can keep the batteries essentially identical and aging together. However, most AGM and GEL type batteries can not be equalized, and in some mobile applications getting them consistently to a full 100% state of charge is a challenge, so another approach is needed.
The solution to this problem is the installation of a “battery balancer”. The job of the balancer is to monitor the battery voltages individually, and bleed small amounts of current (less than 1 amp) between them to force the voltages to be identical during the charge cycle. For batteries that are in good condition, the amount of current needed to keep them in balance is very small. Most battery balancers have some type of alarm function to let you know if the voltage difference exceeds a preset limit (typically 100 or 200 milliVolts) so you know a serious problem is developing.
Getting more Amps
Connecting batteries in parallel to get higher capacity at the same voltage results in different issues. We have a couple different ways we can accomplish this. At a first glance, they might appear functionally identical, but it turns out that is not the case.
Connection geometry “A” is in most cases the simplest to assemble. Fewer, shorter cables are needed. Unfortunately, is is the worst arrangement for the batteries. The very small, but non-zero resistances of each of the cables, connection points and the batteries’ internal resistances matter when charging currents, or discharging currents, are high. Calculating the result is not simple, but it can be fairly easily accomplished with a circuit simulator and under high loads the battery closest to the load can easily work twice as hard as the one furthest “downstream.” It will cycle deeper and work harder, and die earlier.
Geometry “B” is an improvement. For a bank consisting of only two batteries, this is as good as it gets, but as we add more batteries to the bank, the differences between batteries in the middle of the bank and those at the “ends” grow larger and larger and the same issues resurface.
By now you have probably guessed that Geometry “C” is where we want to be. You would be correct. Each battery is not directly connected to any of its neighbors, but rather to a central connection point. Each cable from a battery terminal to the connection point is as close to the same length and construction as possible. As a result, each battery has exactly the same path to the load and charging sources as all of the others. Each sees exactly the same voltage during charging. Everybody is happy. Should one battery develop a problem, it can be easily removed from the system without extensive rewiring.
There is a more detailed explanation of this issue by the inventor of the original SmartGauge battery monitor here: http://www.smartgauge.co.uk/batt_con.html
He presents some more detailed rational and options. Coastal Climate Control went the trouble of actually confirming the issue experimentally. You can see his results here: https://www.coastalclimatecontrol.com/index.php/blog/272-current-affairs-making-the-right-connections.html
These data were measured on discharge, but exactly the same problems would show when charging with high currents.
More Volts! More Amps! More! More!
Sometimes we need a battery bank that is higher voltage AND higher capacity than the individual batteries available. Now things get more complex. Combining the best we have seen about connecting batteries in Series and in Parallel, we end up with a system that looks like this:
The cable connections in the “middle” of the bank do not need to match the others in length, but it is best if they match each other and are as short as practical. We now should add a battery balancer across EACH of the series pairs of batteries to make sure that each member of series pair each stay at the same voltage. This is the best and most robust system.
A small compromise is to make some additional connections, and now we can use a single battery balancer:
By adding the connections across the “middle” of the bank, we can ensure that the voltages all across those terminals are identical at all times. A single battery balancer can now ensure that the 24 Volt charging current is evenly distributed between the “top” and “bottom” of the bank.
One last modification can be made in a couple variations for very large battery banks is an intermediate between the last two options presented. This is how my system is wired. I use eight, 12 Volt, 116 amp-hour, Group 31, Firefly batteries to make a single 24 Volt, 464 amp-hour, battery bank. It is functionally divided in half, with two Victron BMV-702 battery balancers keeping things charged evenly. The connections look like this:
Keeping things as simple as possible, but no more so.
One of the reasons we used the Firefly batteries was to avoid the need to routinely bring the bank up to full charge. This means that fully complete absorption cycles are relatively infrequent, and the balancers become all the more important to maintain all the individual batteries at the same state of charge.