Batteries: The Heart of a System

For the next few articles I'm going to talk more in depth about the individual components of a typical renewable energy system. I'll cover PV panels, wiring, safety components, inverters and generators. I'll start off with a discussion of batteries.

Batteries are the "pressure tank" of a renewable energy system. They are where the energy produced by a charging source is stored. It is important that they be reliable, well maintained and reasonably long lasting.

The type of battery used in a renewable energy system is called a deep-cycle battery. Unlike a standard car battery, which is designed to release a lot of power quickly, deep-cycle batteries are designed to release more of their power over a longer period of time. They can stand the constant long discharge and recharge cycles of a renewable energy system where a car battery would quickly fail.

Deep cycle batteries are rated in amphours. This indicates the amount of power that can be drawn from the battery over a specified period of time before the voltage drops below a certain point.

There are many different sizes and capacities of deep-cycle batteries. The two most common are the golf-cart battery, generally rated around 220 amphours, and the Trojan L-16 (or equivalent) which is rated at 360 amphours of capacity. Each of these is a 6-volt battery, a size which allows relatively easy handling and installation while still providing a reasonable capacity.

Now, you're probably wondering why use 6-volt batteries in a 24 volt system? A 24-volt battery that had the same capacity (i.e. 360 amphours) would weigh almost 500 pounds. By taking four of the 6-volt batteries and connecting them in series you can make the equivalent of a 24-volt battery with the same capacity and the individual batteries are easier to handle.

A series connected string of batteries has cable running from the positive terminal of one battery to the negative terminal of the next. This is the same way the individual cells in a battery are connected internally. All batteries are constructed of individual 2-volt cells, for example a 12-volt battery has six 2-volt cells inside.

Connecting individual 6-volt batteries simulates a 24-volt battery. When such a connection is made the resulting string of batteries will have one free positive and one free negative terminal. A series connection always increases the voltage and leaves the capacity (amphours) the same. When connecting batteries together in a series string it is important to have all the cables running between the terminals the same length.

To increase the capacity it is necessary to use a parallel connection. In a parallel connection the positive terminals of a group of batteries (or string of series connected batteries) are connected together, as are the negative terminals. This increases the capacity while keeping the voltage the same.

As an example, in our system we have 24 12-volt Dynasty sealed lead acid batteries (125 amphours each) connected in a series/parallel configuration. In our case we use a buss bar to make the parallel connections. We take the series pairs of 12 volt batteries and run cables from the remaining positive and negative terminals to a positive and negative "buss bar", which are large copper bars that act as main terminals. Using 12-volt batteries this way we created a storage bank that has a 1500-amphour capacity at 24 volts.

Here's how the math works. By connecting two 12-volt 125-amphour batteries together in series we increase the voltage (2 batteries x 12 volts) to 24 and keep the same 125-amphour rating. By connecting the resulting 12 pairs of batteries (each pair rated at 125-amphours at 24 volts) in parallel at the buss bar we increase the capacity (12 x 125-amphours) to 1500-amphours while keeping the voltage at 24.

All Contents © 1998
Wagonmaker Press
Thomas W. Elliot


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