In the previous article in this series, we covered the essentials of selecting solar panels. Be sure to read that article, or go back to the first article in this series for a complete overview of how to design an off-grid energy system. In this article, we cover some of the important aspects of solar chargers to make the most of the available solar energy and battery capacity. Since this series is focused on off-grid applications, we are only interested in chargers appropriate for that task.
MPPT For Me, PWM For Him
Off-grid chargers come in two main flavors: MPPT or PWM. MPPT stands for Maximum Power Point Tracking, while PWM stands for Pulse Width Modulation. While MPPT chargers are significantly more expensive than PWM chargers, the difference in performance between the two types is well worth the cost. As the name suggests, MPPT chargers track the actual output of the solar panels and continuously select the optimum amount of power to extract from them. On the other hand, PWM chargers throw away a lot of power in exchange for simplicity and cheaper design. This is one case in which, inexpensive solar panels notwithstanding, throwing additional solar panels at the task is ill-advised.
Unfortunately, a lot of solar starter kits out there bundle PWM chargers, or low-capacity inexpensive MPPT tracking controllers, leading to a serious bottleneck in capacity. Just as quality batteries are worth the investment, so is the charger. Here is a comparison of a few MPPT chargers, and a representative PWM charger.
This chart shows just a sampling of the charge controller options found on the web. We've included some PWM options as a comparison for price and features. At the low end, there will be very little difference in price between PWM and MPPT options in the same output current class. At the higher end, note that the MPPT version will cost about twice as much as the PWM. However, don't be fooled by this. The 60A Morningstar PWM boasts that it can handle input arrays up to 4 kW, but it only charges with a max of 2.88 kW. Guess where that extra kilowatt of power is going? Into that gigantic heatsink attached to the charger, and not into your batteries. Larger PWM units are hard to find, primarily because the heat sinks get prohibitively large. You also won't see any specs on PWM charger efficiency, while most MPPT units shown boast efficiencies of 97% to 98%, or slightly higher.
But aren't we designing our input panel array to throw away peak power anyway? Yes, we are. However, without a long discussion of the technical details, the MPPT will be able to extract useful charging power off the edges and convert more of the available peak power into charging current as conditions change throughout the day. Just say no to PWM.
Select an MPPT Charger Based On Output Current
Focusing on the MPPT options only, notice that a defining characteristic is the output current, while multiple battery array voltages, such as 12 V or 24 V, are supported. This means that the output power of a MPPT charger increases as the battery array voltage increases, essentially for free. This is an important feature of multi-voltage MPPT chargers. This capability is a built-in upgrade path: increasing the battery array voltage effectively adds another charger.
Which charger should you choose? Note that the low-end 20A MPPT chargers are only capable of storing about 2.5 kWhr over a five-hour charging cycle, even with a 24 volt battery array. This means that it can supply a maximum of 100 W of continuous 24-hour service, no matter how many solar panels or batteries are thrown at the job. A better choice, if you can afford a few hundred dollars more, is to select an 80 A or 90 A charger, such as the last two options, and then feed them additional batteries and panels as your system expands. Even a 12 volt system running from the Outback Flexmax 80 can easily build to a 200 W continous load, or 400 W with a 24 volt battery array. We also like the Midnite Solar Classic Lite 150 for the 94 A current on a 24 volt array, allowing nearly 500 W of continuous load for only a few dollars more.
The Charger Determines the Available Daily Energy
Remember when we talked about only getting about five hours of useful solar energy per day? This limitation means that you will typically need to select a larger charger than you might expect at first. Consider the equation below:
Consider a simple example of providing a 250 W continuous load, using a 24 volt battery array. This leads to the following result:
In actual practice, because of two factors, the charging current would be a bit less than the simple equation above would indicate. The first of these factors is that with a high-voltage array, described in the next article, some additional tail energy can be extracted than would be possible with a lower-voltage array. The second factor is that while lead acid batteries are charging, the actual voltage delivered is larger than the nominal voltage by about 20%. So, a 24 volt battery array will charge around 28.8 volts, allowing more energy to be delivered from a given charging current. These gains are offset somewhat by charging losses and battery heating, so the simple equation above suffices for most purposes.
Now that we understand the charger options, the requirements placed on charging current, and how to make an informed buying decision, it is time to discuss solar panel arrays to feed the charger. This is the topic of our next article: Solar Panel Arrays ...
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Posted: 12 Nov 2015