This topic contains 8 replies, has 2 voices, and was last updated by Roger Fell 4 days, 1 hour ago.
-
Posts
-
I’ve been researching solar systems recently. Charge controllers are a confusing topic as the technical jargon is pretty intense for the average layperson to grasp. I thought I’d start a thread about them. Maybe some folks who have installed solar can give some advice here on what they picked and why.
Seems to be two popular choices MPPT and PWM type. I just read an article that basically says the MPPT controllers will provide more power to the batteries but the PWM type will make the batteries last longer.
http://www.ecodirect.com/Charge-Controller-Comparison-s/251.htm
The alternate power source and storage batteries are the two principle cost items of an alternate energy system. The battery charge controller based on MPPT or PWM can do one of two things. The charge controller circuit of a MPPT controller (Maximum Power Point Tracking) unit is designed convert the voltage from your solar or wind power sources to the voltage optimum for maximum transfer of power to the battery or grid via a grid tie system. A PWM charge controller does not operate to optimize power transfer to the batteries; it is optimized to extend the life of the batteries. By applying higher voltage in short bursts a PWM controllers essentially clean off the lead plates that are a central part of the battery storage system. How to charge controller selection depends on if it makes sense to optimize power transfer, with an MPPT controller, or to optimize battery life, with the PWM charge controllers.
I guess it boils down to how much extra battery life is a PWM going to give you, if it is only say 20% well who really cares, I’d rather buy batteries a little more often and have max charge on my panels.
What are you folks seeing out there in the real world?
Ray - Love Your RV!I’ve been following the same confusing issue for the last year, there is a plethora of opinion, and some pretty muddy water when it comes to what approach is best, or even some basic electrical knowledge, most particularly, how voltage drops actually behave.
There seems to be a lot of different energy consumption styles, some just run a few lights, while others have huge inverters and run multiple electric kitchen appliances during the brightest part of the day, and watch big screen TV with surround sound for hours in the evening. I suppose the basic idea is how much energy will it take to keep you comfortable in the style you’re accustomed to. So, first step would be an energy audit.
As for battery life, the main issues are not to discharge the battery to deeply or quickly, and to actually bring it back to full charge.
Handy Bob has a good page on the more technical side of solar,
http://handybobsolar.wordpress.com/
His writing style would best described as cranky, but he does have a lot of good information.
Lots of reading there for sure. He seems to be saying for a smaller system under 600 watts with wet cell batteries like Trojans the the way to go is with a PWM not a MPPT controller. Interesting because I’ve had tons of folks tell me the PVM controllers are cheap and won’t work as well.
Seems there are so many variables between batteries, panels, controllers and usage patterns there is no real definitive right answer but many right answers depending on your unique setup.
Ray - Love Your RV!I believe his best piece of advice is follow what the battery manufacturer recommends for charging.
We use electric lift trucks at work, the dedicated chargers for these use a steady flow of high amperage dc to recharge the batteries, which are the same technology we use on our RV batteries. These units get deeply discharged every day and are recharged overnight, about once a month we top off the battery water. I’ve been working at this job for over 6 years, we’ve not changed a battery pack yet, and most of the units were old when I started.The new smart chargers are very good at recovering a completely dead battery, we use them in the shop. The pulse charge method does help break down the sulphation, but it’s a long slow process that can take two days on even a #24 battery, mostly it prevents the heat buildup that a deeply discharged battery has when the charge is pushed in quickly. The majority of the time we don’t use them, instead we use a standard charger to quickly charge a cranking battery back to a usable level of charge, and then use the smart charger overnight to finish the job.
Found this PDF explaining the some differences.
http://www.morningstarcorp.com/wp-content/uploads/2014/02/Trad-PWM-vs-TrakStar-MPPT-April-2014.pdf
This is what I was wondering as well?
PWM Over MPPT:
The preceding discussion of PWM vs. MPPT may cause some to wonder why a PWM controller would
ever be chosen in favor of an MPPT controller. There are indeed instances where a PWM controller can
be a better choice than MPPT and there are factors which will reduce or negate the advantages the
MPPT may provide. The most obvious consideration is cost. MPPT controllers tend to cost more than
their PWM counterparts. When deciding on a controller, the extra cost of MPPT should be analyzed
with respect to the following factors:
1. Low power (specifically low current) charging applications may have equal or better energy harvest
with a PWM controller. PWM controllers will operate at a relatively constant harvesting efficiency
regardless of the size of the system (all things being equal, efficiency will be the same whether using
a 30W array or a 300W array). MPPT regulators commonly have noticeably reduced harvesting
efficiencies (relative to their peak efficiency) when used in low power applications. Efficiency
curves for every Morningstar MPPT controller are printed in their corresponding manuals and
should be reviewed when making a regulator decision. (Manuals are available for download on the
Morningstar website).
2. As explained in the Environmental Considerations section, the greatest benefit of an MPPT regulator
will be observed in colder climates (Vmp is higher). Conversely, in hotter climates Vmp is reduced.
A decrease in Vmp will reduce MPPT harvest relative to PWM. Average ambient temperature at the
installation site may be high enough to negate any charging advantages the MPPT has over the PWM. It would not be economical to use MPPT in such a situation. Average temperature at the site
should be a factor considered when making a regulator choice (See Appendix).
3. Systems in which array power output is significantly larger than the power draw of the system loads
would indicate that the batteries will spend most of their time at full or near full charge. Such a
system may not benefit from the increased harvesting capability of an MPPT regulator. When the
system batteries are full, excess solar energy goes unused. The harvesting advantage of MPPT may
be unnecessary in this situation especially if autonomy is not a factorRay - Love Your RV!This is the system that I was thinking of installing on our trailer.
http://solarwholesaler.ca/shop/250-watt-rvboat-solar-kit-with-mppt-charge-controller-starting-at-546-89/
The maximum charge rate I’ve seen on our battery was 26.7 amps, and that only lasted for about 10 minutes and then dropped to about 16 – 17 amps. There didn’t seem to be much point in installing a larger system if the batteries can’t absorb the extra amperage.
Looks like a great deal!
I guess the only scenario would be if the batteries are run down overnight, having more panel capacity should help bring them up faster in the morning when the sun is not full strength yet.
Ray - Love Your RV!I intend to get the new batteries first, run them down to about 50%, then recharge and see how many amps they will draw in the process. Having extra capacity to charge doesn’t mean the batteries can accept the charge that fast. It will also avoid heating the batteries which is very destructive.
The 60 AH battery I’m using currently took 3.5 hours to reach full charge after being discharged to 46%. Most of that time, the amp draw was in the single digits.
I’d hate to get to small of a system! So battery tests first, then decide whether a 250 watt or a 500 watt System. I’m installing the inverter and outlets this weekend, as I’d like the inverter draw to be included in the battery test. Gotta get back to work!
The system uses a MPPT controller hooked to a panel where the Vmp voltage is 30.3 volts with 8.27 amps at that level. Even under low sun angles this panel should have no problem attaining 14 to 15 volts to start the charging process first thing in the morning. Granted, there wouldn’t be a lot of amps, but that would build as the day progressed. It would be interesting to see how it performed in overcast weather, one of the scenarios that the PWM system would hardly charge at all.
You must be logged in to reply to this topic.
