Batteries in Solar Systems

Some fairly useless information ...

A century or two ago archeologists working in Egypt discovered curious clay jars sealed with tar and containing a carbon rod. They were baffled as to what these ancient relics could be. Modern science now tells us these items were in fact batteries. It is highly probable that they were used in electroplating, a metallurgical pursuit that goes back to ancient times. This of course throws into doubt who were actually the pioneers of the electrical age! Without pondering this further let's progress onto what we are really about, and that is storing the power from solar panels.

In order for your solar system to be useful you need storage ...

If you want to collect solar energy you are going to need somewhere to store it. This is where batteries come in! Here we are dealing with lead acid batteries and although you could conceivably use ni-cad batteries (at somewhat of an expense) it is the lead acid battery that we will deal with in this section. Modern lead-acid batteries come in flooded, gelled and absorbed glass mat types just to name a few. For the purpose of this lesson let's assume that they are all the same.

Voltage:

A lead acid cell, regardless of size is a two volt device. Regardless of how large you build a lead acid battery the nominal voltage will always be 2 volts. If you put 6 of these batteries in one box and join them in series you will get a 12 volt device as is used to start your car.

Battery Capacity:

Battery capacity is measured in amp-hours (A/H or amp/hour or AH). This measurement is the amount of amps (energy) the battery will provide for one hour. Battery capacity is also relative to speed of discharge. A slow discharge over 50 hours will produce a higher total energy from the same battery than a rapid discharge. Temperature also affects capacity. A battery bank at 30 degrees Celsius will have a significantly larger capacity than if it were cooled to 0 degrees Celsius.

Battery capacity is stated as discharge over time. The time depends on what the battery manufacturer had in mind when designing the battery. Forklift and electric vehicle batteries usually have a stated capacity over 5 - 10 hours @ 30 degrees Celsius, batteries specifically for solar installations have the capacity stated at 100 or 120 hours @ 25 degrees Celsius.

Summary so far: Battery capacity is measured in amp/hours and provided to us by battery suppliers as capacity over time.

Battery Life:

If you get a brand new battery and store it with some means of maintaining its full of charge status (like a trickle charger) and put it on a shelf in the shed its life will be determined by the length of time it takes for the acid to degrade the bits the acid is soaked in (like all the internal bits). Typically this could be 10 - 20 years.

You are not going to buy a battery and store it on a shelf are you?!
The next thing that determines battery life is cycling (using then recharging) it. A discharge followed by a recharge is a cycle. The depth you discharge a battery to before recharging it is the depth of cycle. A small cycle could be a battery discharged a little, say 10%. A deeper cycle could be more like say 30 - 50% and a really heavy cycle could be 80%. If you discharge a deep cycle battery by more than 80% on a regular basis you will quickly ruin it.

Battery manufacturers will state the cycle life for batteries designed to store power. Batteries designed to start things are not storage batteries. They are "cranking" batteries and there capacity is not stated, rather there usefulness as a starting battery is stated in "cold cranking amps". We are not interested cranking batteries here but you should know the difference.

Typical spec for a deep cycle battery for solar system use:

4000 cycles to 10%
3300 cycles to 30%
2500 cycles to 50%
1500 cycles to 80%

From the above information we can determine a life expectancy: A solar system discharged on average 10% per day could be expected to have a battery life of around 4000 days or 10.9 years.
If you were a little harder on your battery and discharged it to 80% down every day before recharging it fully you could expect a battery life of around 4.1 years.
Given that the worst thing you can do to a battery is to leave it standing around "flat" and a daily discharge of 80% is enormous and in reality not likely your battery life is realistically about 8 - 15 years. Quite commonly I see good quality deep cycle batteries still in service after 20 or more years so the above figures are conservative ...

The manufacturer of this battery also states capacity as follows:

Capacity C120: 375, C100: 340, C20: 213, C5: 171. These listings are not until the battery reaches zero volts but are typically the capacity you will get until the voltage reaches 1.8 volts per cell or for a 12 volt bank until it reaches 6 x 1.8 = 10.8 volts.

Summary so far ... Battery capacity is measured in amp/hours and provided to us by battery suppliers as capacity over time. Battery life is determined by the number of cycles. A battery has a greater total capacity if it is discharged slowly.

A final rule: (made to be broken of course). Plan to be able to use about 50% of a manufactures stated battery capacity at the C20 rate. More than this constitutes a pretty heavy discharge.

Working out what you need:

It is a relatively simple matter to determine what size battery bank you require. You have already read and retained the prior knowledge imparted to you by way of our "load sheet" page (haven't you)?
Another big word we solar designers use in calculating what to install where is "Autonomy"
Autonomy is simply the number of days you would like to have power when things are inclement ... like no sunshine! A good figure would be about 4 days.

Get your total electrical load in watts per day. Multiply this by four days. Double this because you only want to discharge to 50%
Divide the result by the battery voltage you have chosen and you will have your required amp/hour capacity at the C100 rate.

OF course you may well choose a higher or lower autonomy than 4 days.

Choosing a battery bank voltage:

No matter how much I stare at my thumb there seem to be no rules written on it! Here is a guide though:

Up to 1000 watts: 12 volt battery bank
1000 - 3000 watts: 24 volt battery bank
Above 3000 watts: 48 volt battery bank or larger. It is not uncommon for battery voltages to be as high as 120 volts or more in large household power systems.

Measuring battery level:

This is covered in the "system monitoring" page of this information section.

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