Battery wiring

Lately, I've become interested in the wiring methods/connections that I can use at the cabin. Obviously, we have your regular household 110 volt wiring in place. Now we add the off-grid power to that, the inverter and battery bank. Add to that the charging system for the battery bank, the generator and the solar panels and we start to get complicated. In the good old days of last summer, I simply used the charging system on my vehicle to charge the bank which generally took an hour or so and I was good to go. Now the system will be left in place so I have to be able to keep the system charged while I'm not there during the week.

This article follows my train of thought for MY system. Your system may be different; a different inverter for instance that you can change the input voltage of.

In a system of this type you have two basic choices for your battery bank, series or parallel wiring.

ah=amphours (capacity - volume, if you want - of the battery)

Briefly, in a series circuit, you're taking batteries of a lower voltage and connecting the positive to negative throughout the bank to get the voltage that you want.

Example: 6volt/20ah + 6volt/20ah = 12volt/20ah

Your flashlight is an example of a series circuit. 

What you are doing is increasing the voltage. This can be good because your longer life batteries (robust design, long life) are usually a lower voltage. This is because large batteries (both 2, 6 and 12 volt) are made up of 2 volt cells. Obviously, if the battery case is the same size, a 6 volt battery is going to have much larger 2 volt cells than a 12 volt battery of the same size so it would follow that the capacity (volume) is higher.

In a parallel circuit, you're taking batteries of a higher voltage but lower capacity (volume) and connecting them positive to positive and negative to negative to get more capacity.

Example: 12volt/20ah + 12volt/20ah = 12volt/40ah

While not directly relating to this, your home wiring is an example of parallel wiring.

There are benefits and drawbacks to each system.

Some prefer series connections because you can use high capacity golf cart batteries to get a wicked large capacity bank. This is a definite advantage. However, if you lose a battery you lose voltage and have to find a golf cart battery and possibly be down for days unless you happen to have a supplier nearby.

In a parallel connection, you use higher voltage but less robust (usually) batteries to create a wicked large capacity. However, the batteries don't last as long. If you lose one of the batteries you lose capacity (volume) not voltage and go to Wal-Mart and buy another battery. 

Parallel connections of  can cause all kinds of problems in large battery banks. The batteries in the strings with the main connections (in my case the inverter connection) take all the current load while the middle of the string contribute voltage and a little bit of current to try to balance out the bank capacity.

Update: In my effort to save you the reader time and money, I experiment with different setups. My current idea is to use 100 amp hour batteries in the CENTER of the battery bank to see how that effects the number of reserve hours. I am going to test the charging two ways:

Connect the charging to the end batteries like I always have and then try charging from the center batteries. Will it work? Will this experiment die a cold and lonely death? You'll have to check back to see.

What I decided to do.

Well, I've reached a decision (it's tentative at this point and open to discussion by email) is to use an example I found on the internet, with modifications of course.

My system relies on 12 volt batteries that I can replace in less than an hour. To keep the bank reliable, I've decided to do this. I'm going to do a "capacity test" here at the main house and see how it goes. I already have a baseline test from the current bank.

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