A lot of if not most off grid hams have only one battery in their system. Connecting a single battery is more or less self-explanatory. What should you do when you expand your system to include multiple batteries? Should they be in series? In parallel? Is one way “better”? Does it really matter? Let’s take a look at series and parallel battery connection techniques and what it might mean to your off grid power system.
Equality for all.
No matter what connection method you ultimately choose, start out with consistency in your batteries. All the batteries in your system should be the same voltage, amp-hour rating, and type (flooded, AGM, lithium, etc.). If you are buying batteries new, get the same brand and model.
When I’m looking to replace the batteries in my home system, I go one step further. I bring a battery analyzer with me to the dealer and identify a group of batteries that have the same or nearly the same internal resistance and the same manufacturing lot/batch number stamped on them.
It takes time and I have to check many batteries to find a matched set. I’ve found that if I’m polite and explain to the sales people what I want to do (and not interfere with other customers) they will understand and cooperate. You’re making a big investment. If you’re not inclined to be as quirky as me, at the very least check the date code and pick batteries that were made in the same month.
While we’re talking about date codes, choose the most recently-made batteries available. I’ve seen date codes on “new” batteries going back eight months! Batteries will degrade over time, even when not in use. Do you really want a battery that is already more than half a year old when you buy it? And it’s been sitting around all that time (probably) without a float charge?
If you are not buying in person (ie, on line), then you will just have to take your chances. Having a “matched set” is not mandatory, but if you have the opportunity to hand-pick your batteries, you should do so. series and parallel battery connection
Connection method #1: You can learn from my mistake (or, what not to do). series and parallel battery connection
Years ago when I was young and still learning all this stuff, I set my batteries up with connection method 1. I didn’t think it was a big deal at the time, but it was a horrible idea. Learning from your mistakes is good. Learning from someone else’s mistakes is even better! I urge you to learn from my mistake and not connect your batteries this way.
As shown in the diagram, the batteries are tapped at the positive and negative terminals of the last battery in the string (battery #3). What is wrong with this? The internal resistance of the batteries and connection wire, albeit very small, has an outsized effect on the current draw of each battery. This results in one battery producing significantly more current than the others. Remember, current will always favor the path of least resistance, even if the difference is very small.
Battery #3 will bear most of the load and drain the quickest. As this happens, Batteries 1 and 2 will increase output to make up the difference. No matter what, the batteries will never be balanced (in terms of sharing the load equally) and Battery 3 will have a shorter service life than the other two. As we already discussed, batteries should be installed and replaced as a complete set. So, you’ll be faced with replacing three batteries because one went bad. This method underuses two (or more) batteries while beating one to death.
Connection method #2 (waaayyy better!)
After learning my lesson the hard way, I reconfigured my system into Connection Method 2. The batteries are still interconnected the same way as before, except this time the tap for power is taken from each end of the string. series and parallel battery connection
Since the power is being drawn from the positive on the first battery and the negative on the last, it forces the current to travel through all three batteries to form a complete circuit. No one battery is stuck doing most of the work while the others just coast along. The inherent resistances of the batteries and the wire will still cause a more or less negligible imbalance. It’s a huge improvement over Method 1.
My home solar has been connected by this method for many years now. I’ve had no problems and each battery is pulling equal weight.
There is another connection method that is even better than this. However, to me it seems like something for those who are obsessively nit-picky. It’s too much effort for a small gain. There is a link at the end of this article for anyone who wants to look into it further.
The last connection method is to connect the batteries in series. It’s very straightforward: Connect positive to negative in a “daisy chain” pattern. The power is tapped at each end of the chain. This method is foolproof in that, since the batteries are in series, the current flows equally and the batteries will (mostly) balance the load between them. The principle is very similar to Connection Method 2, above.
There are a few things to consider. First and most obvious, the system voltage will be the sum of all the batteries. So, if you chain three 12 volt batteries together in series, your total voltage will be 36 volts. That’s fine as long as your charge controller and load are compatible with a 36 volt system voltage.
The other problem is that if one battery goes bad, you truly will lose the entire system. In the other two methods, if a battery goes bad, you can remove it and still run on the remaining batteries with normal voltage (although with less capacity). When batteries are in series, a failure could cause a total loss of power. You cannot simply remove the bad battery and continue at full voltage as with Methods 1 and 2.
System voltage will drop accordingly. In our example, a failed 12 volt battery in a 36 volt series means you’re now down to 24 volts. Voltage will dip below 24 volts if the defective battery itself becomes a load and drags down the others. Are your controller and load ok with that? Probably not.
So which method is “best”?
Whether to use a series or parallel configuration is largely a matter of personal preferences and the technical properties of your equipment. There is no “best” method. However, if you are going with a parallel system, wire your batteries as described in Method 2 above. The goal is to have each battery carry as equal a load as possible.
Here is a very informative, easy to read web article that goes into detail about how small resistances can have a huge effect on battery performance. It also describes other effective connection methods. This website is worth bookmarking.
ICYMI, check out last month’s Off Grid Ham article for more battery tips.