In Part I of this series, we provided an overview of the solar power system we deployed on the ground to keep our freezer and refrigerators running after Hurricane Matthew. In Part II, we discussed the combiner, an essential element that is often overlooked. Along the way, we discussed the principles of solar panels as a separate article. In this article, we will take a look at chargers, and the specific charger we used in this case. An overview photo of the inside components, taken from above, is given below (click to magnify):
The batteries shown are two 12 volt Duracell 29HM deep-cycle marine batteries connected in series to form a 24 volt array. We’ll discuss these, and other battery options and operating principles, in a future article. To the right of the batteries is a 1000 watt Microsolar pure sine inverter. This element also deserves its own article. To the far right is a Midnite Solar Classic 150 SL MPPT charger, and between them is an automotive 150 amp DC breaker that isolates the charger from the battery array. The breaker, and the wiring, which is all 6 AWG THHN except for the two leads to the inverter, will be covered in a special article about cabling. We placed all these items on a roller cart so that we could move them around in the building easily.
You may notice that the charger is upside down in this picture. This is because the wall is toward the top of the photo, and all of the cabling runs between the cart and the wall. This prevents tripping on the cabling, particularly in the dark, and causing a catastrophe. Normally, the charger, inverter and breaker would be mounted on a wall, or on a piece of plywood itself mounted on the wall. The display on the charger is simple enough that it being upside down wasn’t that big of a problem compared to the safety benefit of using it this way. In use, the extension cords also ran to the outlets on the far side of the inverter for the same reason.
When selecting a charger, the most important decision is whether to choose a Pulse Width Modulation (PWM) or Maximum Power Point Tracking (MPPT) charger. PWM chargers are simple and inexpensive, but waste hideous amounts of energy. MPPT chargers efficiently use as much of the available energy as possible, and are a must for anything but trivial solar energy collection.
PWM chargers have their place, and that is for tasks such as keeping a starting battery topped off in a piece of equipment. We have a very inexpensive 10 amp Outback Solar SmartHarvest PWM charger on one of our equipment shelters for exactly this purpose. It uses a small 10 watt panel, and every few days we move the charger leads to another battery. The fact that it throws most of the energy away when the battery is charged is inconsequential. We’ll talk about that kind of application, and the risks of some inexpensive PWM chargers, in another article also.
For an energy plant needed to keep freezers and refrigerators running, however, invest in a quality MPPT charger, there is really no other rational choice. Once this decision has been made, other factors come into play, which include the battery array voltage, maximum input voltage, the maximum charging current, and the charging algorithm.
Most quality MPPT chargers available today will support more than one battery array voltage. A less expensive charger such as the 20 amp MPPT version of the Outback SmartHarvest, supports 12 and 24 volt arrays, while the Midnite Solar Classic 150 SL we chose supports 12 volt (96 charging amps), 24 volt (94 charging amps) and 48 volt (84 charging amps) arrays. One of the main advantages to designing a higher-voltage battery array is that with the same charger, the amount of energy you can jam into the array increases accordingly. For example, with a 12 volt array, the 20 amp MPPT SmartHarvest charges with 240 watts, while the same charger provides 480 watts with a 24 volt array. For the Midnite Solar model, the figures are 1152 watts, 2256 watts, and 4032 watts, respectively, for 12, 24 and 48 volt arrays. The upgrade path in each case is an important feature; you can reconfigure a growing battery array later, and poof, the charger becomes that much more effective. Where practical, then, choose a charger that supports multiple array voltages.
Another important charger feature is the allowable input voltage. You will recall from previous articles that our array included three solar panels in series in each string. This leads to a maximum string voltage of around 113 volts. So, a 150 volt charger, such as the Midnite Solar we chose, would be a good choice. In this case, the SmartHarvest would not be a good choice, as it only supports a 100 volt input; with the panels we are using, you would only be able to use 2-panel strings with it. Remember, the higher the array voltage, the lower the array current, and the less power you will lose in the cables which bring that power inside to your charger. If you have a specific, lower power application, though, a less expensive, low current charger, with a higher battery array voltage to make the most of that current, and fewer panels, may be the correct design.
We’ve already seen the charging current pop up, and how that affects the maximum energy you can jam into your batteries during the limited solar day (see the SoftBaugh solar panel calculator for experimenting with options). As we saw in the last article, you can increase the solar day by throwing more panels at the problem, increasing the tail power in the morning and the evening. If you choose a charger with too low of a charging current, then most of the midday power will never make it to the batteries.
The final important charger feature is the charging algorithm. This is not the same as the PWM/MPPT decision. The charging algorithm determines how well the charger manages and protects the battery array. A naive charging algorithm will, like a car alternator, just output a fixed voltage, which may either leave the batteries not fully charged, or overload and damage them. Better, as is done by both the Midnite Solar and the SmartHarvest, is to change the operation of the charger as the batteries charge using different modes. Bulk mode, for example, jams as much energy as practical into the batteries when they are low. Absorb mode eases back as they become more fully charged while still applying the maximum practical charging energy. Float mode keeps them topped off when fully charged, and equalize mode is a rarely used maintenance feature. Look for chargers that incorporate at least the first three, most quality chargers will have all four.
This article discussed a lot of information about one single component, the charger. We’ll return to some of these concepts in future articles as we round out our complete system.
As usual, we provide Amazon links for typical items described in this article. In this case, there is only one category, and that is simply chargers.
Outback, SmartHarvest MPPT Charge Controller, 10A, 100VDC, with Load Control, SCCM10-100: This is a small MPPT charger, but still of interest simply as a price comparison. You can probably get them cheaper elsewhere. SoftBaugh will get some quotes on these and post an update if they can supply them for less than Amazon.
Outback, SmartHarvest MPPT Charge Controller, 20A, 100VDC, with Load Control, SCCM20-100: This is a larger version of the above, and is the higher-end of the low-range MPPT chargers.
MidNite Solar Classic 150 SL MPPT Solar Charge Controller: Midnite Solar chargers are our favorite brand for mid-range chargers (we go back to the Outback brand chargers for solar energy systems in the 4000 watt to 80,000 watt range). This model is the lowest-cost charger and works only in solar mode and has a simplified display, but still provides a very respectable charging voltage and output power, as we saw above.
MidNite Solar Classic Lite MPPT Charge Controller-150: The Classic Lite model is usually the next step up in price, and adds hydro and wind input options. However, the pricing on Amazon as of this writing is inverted. With the Lite, the programming is done with dip switch settings, but some network monitoring support is added. SoftBaugh will also get quotes on this one and see if it can be offered for less.
Midnite Solar Classic 150 Charge Controller 150VDC Input MPPT: This is the highest end of the Classic series, but as mentioned above, the pricing is currently inverted. This model adds a graphical display, like the SL, to the Lite, and also adds arc fault protection.
Note that each of the Midnite Solar models we list here come in 200 volt and 250 volt options also, but we recommend that most people stick with the 150 volt version, simply because the supporting equipment in the 150 volt range is less expensive. Worse, the output power actually goes down with the higher voltage models; these only charge 24 volt arrays at 78 and 62 amps respectively, and for the 250 volt version, the 48 volt performance drops to a disappointing 55 amps. The advantage of the higher voltage models is that they add a 72 volt battery array option, which we think is impractically large for most off-grid applications. Even so, the 72 volt, 43 amp mode only produces 3096 watts of charging power, while the 150 volt options beat this already at 48 volts with 4032 watts, as we saw earlier. One advantage may be that the higher voltage solar panel arrays have less in the way of transmission losses, but if a system is burning a kilowatt in the transmission line then it needs a serious rethink anyway.
Update 7 Nov 2016: SoftBaugh has some special offers on some of these items which are better than Amazon prices.