Batteries for solar power storage

Lets face it, 'batteries' is not the most exciting topic to discuss, however there is actually quite a lot of difference between them and many considerations when choosing and using them with your solar setup. I will try and keep it simple and focus on the most useful and important info.

Batteries are all DC current, unlike your mains power which is AC.

You will need to purchase a good battery or batteries to store your solar energy, so it's worth getting it right and treating your batteries well, as they are fairly expensive and you want them to last as long as possible.

The best batteries for size against power are the type that you have in your mobile phone and small devices. However, these devices require little power and need to be small. There are basically four types of cell phone batteries: Lithium Polymer, Lithium Ion, Nickel Metal Hydride and Nickel Cadmium.
These batteries become very expensive when you ramp them up to useful household sizes and become unpractical as they are so expensive. For your solar power setup, you will be using lead acid batteries which are far more economical. These are similar to car batteries.

I will presume that you are using a 12v system for the purposes of this article.

Lead Acid Batteries

Lead-acid batteries are the most common in PV systems because their initial cost is lower and because they are readily available nearly everywhere in the world. There are many different sizes and designs of lead-acid batteries, but the most important designation is whether they are deep cycle batteries or shallow cycle batteries.

Car batteries are shallow cycle batteries, they are designed to give large amounts of power in small bursts ie to start your engine. If you use these for general purpose applications, and frequently use all the power, they will not last long. Clearly you want to be able to use all of the power stored in your battery without worrying, so you will need to buy deep cycle lead acid batteries which can be run down and recharged in 'deep cycles' ie used until flat and then recharged to full again etc..

There are two main type of deep cycle lead acid battery:

Sealed deep-cycle lead-acid batteries

Sealed Gel Cell (gelled-electrolyte) batteries

Either of these types will suit your needs and they are maintenance free. Providing you have a good charge controller and keep them always within 11.3 - 13.5 volts, you should get 500-1000 complete cycles from your battery depending on the model.

The gel type have the advantage of being extra sealed and are often called marine grade, meaning you can get them a bit wet and they will never leak into the water if dropped in. They are often used for jet bikes and boat engine starting. The only downside is that they are seriously effected by under/overcharging. One charge above 14v could ruin the battery. This shouldn't be a problem if you are using a charge controller and an inverter.

Battery Size

Lead acid batteries are measured in ah or amp hours. This is a rough guide to how much power they can output. In general, they range from 1ah to 300ah. Since we know we are running a 12v system and that watts = volts x amps, we can see that a 1ah battery would give a maximum of 12 watts output and a 300w battery would give 3600 watts output. These are rough guides, which will depend on the battery model and it's state of charge.

Batteries are often quoted with an amount of hours as well, usually 10 - 20 hours, which implies how long they can sustain that current. In the real world, it doesn't mean very much and quite simply the ah rating is the best guide to what you are likely to get from it.

If for example you need 300w of power, you would look for something like a 35ah battery to be safe. As a guide, you would need somewhere in the region of 150-200w of solar panels to charge this battery in a day.

You will want to buy enough battery storage to keep about 4-5 days charge, this really all depends on how much space and money you have for panels.

What size battery do I need?

An important guide to the size of battery required is as follows:

You should charge your batteries at up to 15% (max) their amp rating.

As an example, if you have 200w of solar panels working at the standard output voltage of 13-14v.
200w ÷ 13v = 15amps. Due to inefficiencies, in reality, you will only get 10 amps on a very good day. In this instance, you would need a battery rated at least 60ah.

State of Charge

Measuring your battery accurately is easier said than done, since they are sealed, you can't test the chemicals. With car batteries, you can test the 'specific gravity' of the battery liquid to find the state of charge.

You can only find the current of a battery when there is a load (power) running, which makes it awkward and complex cicuits are required.

The simplest way to find the charge of your battery is to measure the voltage. This is easy to do with a multimeter across the terminals, or you can buy a cheap voltmeter to attach to your battery. When the battery is empty (but still healthy) it will read about 11.3v. When it is full, it will read about 13.3v. The sliding scale between 11.3-13.3 will give you the state of charge. For example, a measurement of 12.4v is roughly half full.

You can also see from this how the power output of a battery will change depending on charge. A 20ah battery could output roughly 270w when full, but will drop to about 220w when empty.

Battery Safety

Be careful! Batteries can output very high currents and give a nasty shock if mistreated.

If you take care, you will have no problems, but some guides need to be adhered to:

  • Don't leave them where children hang about
  • Wear rubber gloves when wiring them or else only use one hand (sounds stupid, but a shock with one hand goes only through your arm, a 2 handed shock goes through your heart).
  • Don't let them get wet unless marine grade type
  • Don't short circuit, this can ruin them and cause a dangerous burst of current.
  • Think about what you are going to do carefully before doing it and do it with a steady hand.
  • Keep them away from heat/fire and everyday activities.

Battery Wiring

If one battery is enough, wiring is pretty self explanatory. Follow the polarity of the panels, through your charge controller, to your battery.

If you need multiple batteries for greater storage, or wish to change the voltage, there are some considerations.

Series Wiring
Series wiring refers to connecting batteries to increase volts, but not amps. If you have two 12 volt batteries rated at 35 amp hours, for example, by connecting the positive terminal of one battery to the negative terminal of the other, then you have series wired the two together. In this case, you now have a 24 volt battery and the rated 35 amps does not change. If you were to series wire four, you'd have 48 volts at 35 amps, and so on.

Parallel Wiring
Parallel wiring refers to connecting batteries to increase amps, but not volts. If you have two 12 volt batteries rated at 35 amp hours, for example, by connecting the positive terminal of one battery to the positive terminal of the other, and the same with the negative terminal, then you have parallel wired the two together. In this case, you now have a 12 volt battery and the rated 35 amps increases to 70 amp hours.

You can combine these principles to create battery banks with the amps and voltages that you need. For example a set of 4 12v batteries could be wired to another set of 4 12v batteries to create a 24v system with 4 times the amps (power).



Charging

Always use sets of batteries bought at the same time and of the same make and size.

When you get a new battery, charge it to full, then run it down and do this cycle a few times. This is important and will increase the power and life of the battery. In general operation after that, it is best not to deep cycle too much, but make a point of doing it occassionally. Try and keep the batteries within the 12v - 13.2 range where possible.

With all this in mind, you should be fine to store your solar power. There are many other things (believe it or not) that can be said about batteries and how they work, but none of it will help you greatly. The way that they charge is quite complex for example, but don't worry about it.



To use the power stored in your batteries for general AC appliances, you need an inverter.

Click here for information about inverters >>>
 
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