If you are using solar panels, charge controllers are critical to protect your battery system. There are many choices and a big difference in cost which can make choosing the right controller very confusing.
Why use a charge controller?
There's a fundamental mismatch that the controllers solve. The characteristics of a PV solar panel are very different than what works best for charging batteries. If you look at a power curve for a typical solar panel, you can see that the solar power is a constant constant current device, where the current established is based on the amount of sunshine.
The first problem you would have is a lot of wasted power! The panels want to provide maximum power 30 volts and your 24 volt battery might be at 24v. This 6 volt difference represents a 20% loss of power available for charging. The second is problem is that when the batteries are fully charged there is nothing to disconnect the panels from the batteries. There's also no way to evaluate when the batteries are finished charging.
Charging Up Batteries
A deep cycle battery used for storing energy requires a varying amount of current to charge properly. Overcharging a battery shortens its life, and can even destroy a battery completely. Batteries are expensive and they need to last for a long time.
Battery charging changes with different battery types and temperatures. Getting the most out of these essential pieces of technology requires a well engineered charging system that can take everything into consideration and provide an optimal charge. Your batteries will last longer and that saves you money!
Modern charge controllers often have a three stage charging system. First is 'bulk' mode which provides a fast charge, the second is 'absorption' which provides a finishing charge, and finally 'float' mode which keeps the batteries at a small charge designed to keep the batteries maintained properly.
Lower cost controllers are often have two stages first 'bulk' and then switching to 'float'. If you have a small system and a limited budget this might be your choice. You should probably use a generator to do an equalization charge once a month which is a good idea anyway but more important in your case.
An equalization charge is designed to circulate acid in the batteries. Water tends to settle in the bottom of your batteries and an equalization charge (overcharging or gassing) causes charge bubble that help the water and acid to circulate in the battery, improving the distribution of acid. Three stage charging reduces the need for an equalization charge, but should still be done periodically. Check with you battery supplier for more information on suggested length and period.
Extracting maximum power from your panels requires that you take power from the panel at its maximum power point and convert the that power to a voltage and current that is best for the battery system. Devices that use this idea are known as a PPT (Power Point Tracking) controllers. They cost more but can make a very big difference in how much power you collect from your panels, the longevity of your batteries, and reduce the number of panels you need to meet your power requirements.
A common term you will see when you look at charge controllers is PWM. This stands for Pulse Width Modulation and is a means to provide charge regulation without wasting power. A high quality PPT controller will use PWM as part of it's design, but it does provide an important step up from the least expensive regulators in use today.
When selecting a controller be careful to purchase a regulator that has a current and voltage rating that matches your panels. You need a current rating that matches the current your array produces. If you are collecting 40 amps of current, you will want a 60 amp PPT regulator. Your array will produce more power than without the regulator so you need to allow for increased output.
PPT controller allow you to raise the voltage of an array by wiring panels in series. This reduces wire size and current and allows for a little bit quicker system startup in the morning and a slightly longer charging day. If you want to do this be sure that your panels, in the series configuration you want to use will be within the regulator's specifications.
For very large systems, OutBack and others make charge controllers that can be teamed, using a small computerized device to coordinate charging for your system.
I think it might be good to explain another kind of regulator called a shunt regulator. A shunt regulator is often used with large wind power systems, but can also work very well for solar panels. This type of regulator is great because when the energy from your collection system is not needed the power is 'shunted' to another load. This can be a space heater or a water heater. In my system I use both a PPT regulator and a shunt regulator. The PPT regulator gets me the maximum power from the panels, and the shunt regulator provides a little bit of heat in my home office in the winter months.
Normally power not needed to charge your batteries is unused. From a system efficiency point of view shunt regulation is a superior choice.
Whether your system is small or large picking the right charge controller is important if you want to have a worry-free system that maximizes your solar investment. Take your time and make the best choices you can. The equipment will last for many years and the return on your patience is considerable!
Sample power curves for a high quality solar panel.
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