Posted: 7/7/2015 10:01:13 PM EDT
|
Let me preface this by saying I am a computer engineer with good understandings of electrical engineering.
I am looking for a solar charge controller to charge a 24v battery bank. My load, as far I can know right now, is going to be 20w continuous. This will need to operate day and night, rain, shine, or snow. Most commercial systems I see doing this are using a 240w panel and either a 50ah or 100ah battery bank. Low voltage disconnect built into the charge controller would be nice but could be accomplished via other means. I have looked at the Genasun controllers that can boost low panel voltages to charge a higher voltage battery. But once the panel voltage exceeds battery voltage does the excess voltage just get bled off as heat or does the charge controller perform the same DC-DC conversion to down-convert voltage to charge at a faster rate? Conversely, it looks like this Morning Star charge controller is able to perform a DC-DC down conversion for faster charging but no up conversion. I'm just wondering what arf's opinion is on such controllers and if I am over thinking this? |
|
MPPT type controllers are more expensive, but work as a DC-DC converter and use all the panel's power, minus efficiency losses. PWM controllers are cheaper and less efficient because excess power is wasted. Most MPPT controllers are a Buck converter only. What you seek would be a Buck-Boost DC-DC converter and in my experience is not that common.
Both have their place for off grid systems. I use an Outback MPPT controller on my small house solar generator. This allowed me to wire the panels in series with a nominal output of 109VDC, thus allowing me to use smaller, cheaper transmission wire to handle full wattage. My camper uses 17VDC panels wired in parallel. The panels were used and cheap, so I always have an excess of power available. Since the camper system is lower wattage (220W) I could get away with a PWM controller and accept it's 70% efficiency vs the MPPT'S 90%+ efficiency. 20 watts continuous is 20W x 24hr = 480 watt-hrs per day. If your system and it's longevity is important, I shoot for 20% depth of discharge for the battery bank or 480W/.20 = 2400 watt hours @ 24v = 100Ah. Say you have 4 sun hrs per day you would need at least 1 20 watts of panels ignoring efficiencies, so a 240 watt panel should work. Given a 240 watt panel, I would be inclined to use a PWM charge controller. |
|
Nice comparison. Great info. Thanks. |
|
Am I better off going with a controller that boosts voltage to charge? Say using 12v panels to charge a 24v battery bank over a system that bucks a 24v or 36v panel downwards?
It seems like the boost controller might have the advantage of being able to use power coming from the panel when it is not fully lit and above the battery voltage like in a buck system. |
|
Quoted:
Am I better off going with a controller that boosts voltage to charge? Say using 12v panels to charge a 24v battery bank over a system that bucks a 24v or 36v panel downwards? It seems like the boost controller might have the advantage of being able to use power coming from the panel when it is not fully lit and above the battery voltage like in a buck system. When the panel isn't fully lit it isn't producing much power anyways. It would be great if you could find a buck/boost controller that can do it both ways but as mentioned above they aren't common. So what is best? Taking advantage of the little bit of available energy when the panel is only partially lit or taking advantage of the vast amounts of energy when the panel is well lit during mid-day? There is 1 way to "take advantage" of both with just a buck controller but that's with multiple panels. This is one of the 2 major reasons the market it continually shifting toward higher and higher array voltages with MPPT controllers. For example, I have a 6 panel array, the panels are around 48 Volts OC, I run them in a 3-series, 2 parallel setup. I have 2 strings in parallel and each string consists of 3 panels in series. Those 3 panels in series give me a 144 Volts OC. When charging a 24 V bank that voltage can drop down to where the panels aren't really putting out any usable power and the array voltage is still higher than bank voltage. Obviously you can't do that with a single panel so your options are more limited. In the case of a single panel, unless you need to keep your setup compact and mobile, it may be cheaper to just buy an extra panel and use a simple PWM charge controller instead of buying and MPPT (buck) controller to maximize the use of your single panel. |
|
Sounds like you have a very small project.
I have one of the little Morningstar 12a controllers on a 100 watt panel that runs to two T105's that power an on demand pressure pump to take water from a 500 gallon tank to one leg of our water system. The input to the tank is a Sunpumps submersible. That little Morningstar controller has worked great a couple years now. I was going to get another C40 for commonality but for so small of a system it was kinda overkill. Our system that feeds the houses utilizes a couple of C40's and a couple of Outback MPPTs, hence the reason I figured another C40 for the water pump setup would be a good choice- interchangeable if need be. |
|
Thanks for the comments. I have a better understanding now.
It is a small project. I am building a wireless relay station that will sit atop of a small mountain. The challenges are keeping it compact (trying to stay within 25 sqft) to keep the land owner happy as it is family owned land they offer guided hunting tours on. The panels won't be high off the ground (probably 1-2ft) so its possible they could get covered with snow for a few days. I'm shooting for 5-7 days battery runtime. Hopefully being on the crest of the mountain will help with wind to keep the snow down but I'm not counting on it. I'm going overkill with solar to try and maintain charge when there is sunlight. My research has yeilded much the same that has been mentioned here. There are no charge controllers I can find that both buck and boost. But you have answered my specific questions. I appeciate the help. If there are any specific panel and battery brands that are recommended I am open to suggestions as well. |
| During charging, the panels act more like a constant current source rather than a constant voltage source. They get "loaded down" do whatever voltage the battery is in a PWM setup and they get loaded down for max watts in an MPPT situation. Effectively your voltage through the controller will be the same voltage as the battery. With MPPT controllers, the voltage of the panel has no influence on the voltage of your power source/battery. |
|
Quoted:
Am I better off going with a controller that boosts voltage to charge? Say using 12v panels to charge a 24v battery bank over a system that bucks a 24v or 36v panel downwards? It seems like the boost controller might have the advantage of being able to use power coming from the panel when it is not fully lit and above the battery voltage like in a buck system. Higher voltage is better. My observation of my system is that once the sun starts falling and voltage is dropping off, it does so quickly and there is little to gain from a boost converter. If you're worried about low output due to snow cover, you are better off with more panel power. |