Deep Cycle Batteries for Solar Applications
About Deep Cycle Batteries
Despite what you may read elsewhere there is very little currently available to replace the deep cycle lead acid battery as the heart of a stand alone power system. Crude, heavy and finite life aside, a lead acid battery is surprisingly efficient as an electrical storage device and about as affordable as anything that rivals it on the market today.
Available as "flooded", "sealed", "gelled", "AGM", and a few other marketing names that basically refer to these types anyway, the lead acid battery will no doubt be the first or final choice of most stand alone power system owners. While technology promises new things, they are yet to appear in economically viable forms to us end users.
Let's look at a the lead acid battery
A lead acid battery is a 2 volt device, end of story. It doesn't matter how big you build it or what type it is, the combination of lead and acid makes a 2 volt cell. What you have here is two types of lead, sandwiched together in (usually) flat sheets, immersed in a bath of acid. The acid bath is often referred to as the electrolyte. When you add charge an chemical reaction occurs and the state of the plates change. When you use power from the battery the chemical reaction reverses.
Your typical "12" volt car battery is actually a box containing 6 x 2 volt cells. If you could drill a hole down the length of a car battery so as the electrolyte could freely mix you would create a 2 volt battery. But there is nothing to try here, messing around with batteries can expose your good body to all sorts of dangers, so best take it easy here!
When you front up to the battery shop to get our storage device you will find two listings regarding battery performance, the first being "Cold Cranking Amps" (CCA), the next being the "C" rating.
Cold Cranking Amps (CCA) and the difference between starting and deep cycle batteries
Not really applicable to deep cycle batteries, CCA refers to the ability of a battery to deliver a high current for a period of 30 seconds. This is useful stuff if you need to start a big engine, or even a small one but it also demonstrates the difference between a starting battery and a deep cycle battery. A starting battery is designed to start an engine. To do this it must deliver a large amount of power over a short period. The construction tends towards lots of thin plates of lead in an acid bath. This type of design lends itself to high current over a short time but is not designed to be discharged to any great deal. Thin plates close together can warp and touch each other, resulting in a failed battery.
The amp/hour rating is much more meaningful to us consumers. What the battery manufacturer is telling us is how many amps of current their battery will deliver over a period of time.
The "C rating
The C rating determines how much current a battery can provide over a designated amount of time before the cell voltage reaches a certain point that marks the battery as "flat" and in dire need of a recharge. A deep cycle battery by the way has fewer plates than a starting battery. The plates are thicker and further apart. Distortion should not result in plates touching each other and some change in dimension and size over time is permissible.
You should get at least two C ratings with a deep cycle battery, the C10 and C100 figures which are the amount of power the battery can deliver over a 10 our period and the amount of power it can deliver over a 100 hour period. The 100 hour figure will always be higher because a lead acid battery can deliver more total power with a long slow discharge than it can with a high rate of discharge.
The C100 hour figure is more applicable to household power installations whereas the C10 figure might be used for some caravan installations.
Looking at the C rating
Here are some specs from an Exide Energystore deep cycle battery as may be used in a house or large motorhome installation. The battery in question is called the 6RP1080. Its a great big lump of lead that will hurt your back if not handled correctly, 143 odd kG to be precise. I have installed a lot of these batteries ...
Exide list the capacity as follows:
|BATTERY||C120 to 1.8 Vpc||C100 to 1.85 Vpc||C20 to 1.85 Vpc||C10 to 1.80 Vpc||C5 to 1.7 Vpc|
What Exide are telling us here about there battery is that it will provide a constant 9.83 amps for 120 hours (1180 ÷ 120) if we use the C120 figure. Typically I would be planning around the C100 figure so we could do the math here and divide the 1080 by 100 and get 10.8 amps, but both calculations are pretty meaningless. What they do is demonstrate that the capacity of a battery varies according to the way it is discharged.
Lets, say we are planning on using this battery for a large motorhome or house and we want about 4 days reserve power. 4 days is 96 hours so the C100 rating fits our bill nicely. Before we move on and calculate how much energy this battery will provide for us let's quickly look at another battery fact: Battery life is determined by (amongst other things) cycle life.
Battery Cycle Life
What a cycle is, is a discharge followed by a recharge. The number of cycles we will get is determined by the depth of discharge before we recharge back to full. In an ideal world, the discharge will immediately be followed by a recharge by the way, leaving a lead acid battery kicking around half flat will reduce its life quite quickly.
Exide list the cycle life of the 6RP1080 as follows:
1500 cycles to 80% DOD
2500 cycles to 50% DOD
3300 cycles to 30% DOD
5000 cycles to 10% DOD
If we were really optimistic or lived in a sunny dry climate we could plan 10% depth of discharge (DOD) and say that each night we would use 10% of the batteries capacity confident that this 10% would be replenished the following day while we were at work. If this perfect scenario happened we could expect the battery life would be 5000 ÷ 365 (days per year) = 13.69 years. And this may be true but there are a few other factors that affect a battery life ... but we don't want this page to resemble an epic novel so let's get back to how much energy this battery will provide for us.
The 6RP1080 is sold as a 6 volt battery. In fact it is three individual cells connected in series and packed into a robust plastic box. What we need to do is determine what voltage we want our battery bank to be. This in turn will determine how many of these lumps of lead we need to purchase. Take a peek back at our "Load Chart" page. First you determine how many watts of energy you are going to consume per day. Next you determine what depth of discharge you are going to plan on. I personally think 10% would optimistic so I am suggesting 30% Then you work out how many days you want to have for autonomy. Autonomy is the number of days you can go without recharging before you hit the planned depth of discharge.
Alternatively you look at this battery and figure it out backwards. The total amount of energy this battery will provide is determined by the voltage of the battery bank it is in. Obviously one battery is useless because 6 volt systems are not in vogue. Perhaps you remember early Morris or VW cars (showing my age here), but since the 60's things powered by 6 volts have vanished.
If we chose 12 volts we would need two of these batteries joined in series. We would then have a 12 volt 1080 amp/hour battery bank. This would give us 12 x 1080 = 12960 watts of energy (remember amps x volts from the "Basic Electrical Info" page) 30% of 12960 = 3888 watts or 3.8 kW to play with over 4 days. Not a lot really so we may consider getting 4 of these batteries and making a 24 volt system. using the same math we would conclude that we would get exactly double this to play with. 7776 watts should run a refrigerator and a few lights for 4 days.
I could keep going here, learning all about batteries is interesting. Not only that, a good knowledge of batteries will save you money and help to keep your valuable battery bank in top shape.
Lets summarise this page
- The batteries we want will be provided with discharge data ("C" rating)
- Battery life is, in part determined by cycle life
- The amount of energy a battery can provide us is determined by Battery voltage x Battery capacity (in amp-hours)