Affordable Charge Controller for AGM Battery

[bcroe]

Assume 100 watts and do the steps:

1. Voltage loss between Panels and Controller = 2 to 3%. 97 to 98 watts into controller.
2. Controller efficiency PWM at very bet is 67%. 97 in, 65 watts out.
3. Battery charge efficiency = 80% down to 52 watts.
4. Wire loss between batteries and load @ 1% around 51 watts. Good luck keeping losses to 1% for more than 5 feet to load.

50% IMO is to generous if you throw an inverter in the loop. Realistically something lower than 50%

There is no dispute about the losses of wires, batteries, inverters, etc. But lets
review charge controllers for a minute. I don't know much about PWM controllers,
since I would never use something with so many negatives.

My understanding is the PWM is really an on-off switch, varying the on time duty
cycle to limit average power as needed. So if the panel Vmp happened to match
the voltage required to match the battery requirement at that time, the efficiency
could approach 100%.

In fact battery charging voltage varies a lot, and panel Vmp varies quite a bit with
temperature, so they hardly ever exactly match. To the extent they mismatch,
efficiency is lost. There might also be a series diode loss to prevent night discharge,
which will be more damaging in the lower voltage circuit. The diode loss can be
avoided electronically at some cost.

MPPT controllers manage to always operate panels at the best possible point for
conditions, matching them efficiently to the battery. They have the additional
advantage of being able to operate at an uncritical, higher input voltage, reducing
wiring losses. Is this all accurate? Bruce Roe

[Amy@altE]

Looks like the King and the Queen are having a Royal discussion.

As long as I don't lose my head....

[J.P.M.]

Looks like the King and the Queen are having a Royal discussion.

[Sunking]

Oh sure, throw math and logic at me....

Reality sucks sometimes huh?

It really comes down to experience. All that stuff you learned in school is useless academic nonsense. It just gives you the tools to learn when you are out. That is why most teachers teach, they cannot cut it in the real application world. Takes 5 to 10 years of experience to learn the skills on top of education. It sucks, but real.

[Amy@altE]

Oh sure, throw math and logic at me....

It makes sense, the truth hurts.

[Sunking]

As I said, you guys have convinced me not to use .67 for PWM, I just haven't settled on a number yet, because it also takes into account dirt, voltage drop, etc. .6 is still too high, but I haven't been able to jump down to .5 yet (although that's probably where I will land).

Assume 100 watts and do the steps:

1. Voltage loss between Panels and Controller = 2 to 3%. 97 to 98 watts into controller.
2. Controller efficiency PWM at very bet is 67%. 97 in, 65 watts out.
3. Battery charge efficiency = 80% down to 52 watts.
4. Wire loss between batteries and load @ 1% around 51 watts. Good luck keeping losses to 1% for more than 5 feet to load.

50% IMO is to generous if you throw an inverter in the loop. Realistically something lower than 50%

[Amy@altE]

Stop and think about it Amy. If you use a very expensive 100 watt battery panel (you are forced to use battery panels with PWM) the Specs are:

Vmp = 17 volts
Imp = 5.8 amps

• Use a PWM controller and you have 5.8 amps of charge current right? Perhaps as much as 6 amps on a really dead battery pulling the panel down to Isc range.
• Use a MPPT controller on the same or a much less expensive GT panel and you get 95 watts (95% efficiency) / 12 volts = 7.9 amps right?

Which is Greater? 5.8 or 7.9?

As I said, you guys have convinced me not to use .67 for PWM, I just haven't settled on a number yet, because it also takes into account dirt, voltage drop, etc. .6 is still too high, but I haven't been able to jump down to .5 yet (although that's probably where I will land).

I was working on the assumption that he already has the 100W panel, and his original question was for a PWM charge controller under $50. If not, and he has a local source for a grid tied panel, and therefore doesn't have to pay $250 to ship it via truck, and is willing to spend $200 for an MPPT charge controller, then heck yah, a 250W 60 cell panel and a BlueSky SolarBoost SB2512i-HV or Morningstar SunSaver SS-MPPT-15L is the way to go.

[Sunking]

I was being generous just dropping to .6 for PWM, the folks here have recently convinced me I should go lower.

Stop and think about it Amy. If you use a very expensive 100 watt battery panel (you are forced to use battery panels with PWM) the Specs are:

Vmp = 17 volts
Imp = 5.8 amps

• Use a PWM controller and you have 5.8 amps of charge current right? Perhaps as much as 6 amps on a really dead battery pulling the panel down to Isc range.
• Use a MPPT controller on the same or a much less expensive GT panel and you get 95 watts (95% efficiency) / 12 volts = 7.9 amps right?

Which is Greater? 5.8 or 7.9?

[Amy@altE]

Thanks Amy, According to the NREL calculator I get 6.43 average sun hours a day. Is there any reason I should assume less here?

Is the 0.6 efficiency number based on a PWM based charger setup? I was hoping to squeeze a little more out with a MPPT charger.

.67 is used for MPPT, I was being generous just dropping to .6 for PWM, the folks here have recently convinced me I should go lower.

As SunKing said, you have to use worst case for sunhours, not average. I see 5.78 sun hours for Phoenix.

So, using more precise numbers, 100W x 5.78 x .67 = 387Wh in the winter. Still not enough with MPPT. Let's try average, 100W x 6.43 x .67 = 430Wh. Nope.

430Wh / 50W light = 8.6 hours you can run the light at full power 3 season out of the year.

[Sunking]

Is the 0.6 efficiency number based on a PWM based charger setup? I was hoping to squeeze a little more out with a MPPT charger.

Have you read this Stickie? It will answer all your questions. If you use a PWM controller and use 500 watt hours per day your panels must generate 1000 watt hours or a Fudge Factor of 2. If you use a MPPT controller and use 500 wh per day your panels must generate 750 wh per day or a Fudge Factor of 1.5.

Battery size is based on watt hours used in a day time 5 and divided by battery voltage. So assuming 500 watt hours per day and a 12 volt battery you get [500 wh x 5] / 12 volts = 208 AH.

[Sunking]

Thanks Amy, According to the NREL calculator I get 6.43 average sun hours a day. Is there any reason I should assume less here?

Yeah winter. For a battery system you use worse case month. Any month with less than 6.43 Sun Hours you go DARK without power.

Only Grid Tied systems use yearly average. You build credits in summer, and use them in winter. There is no credit in battery systems, you live day to day

[photolimo]

50W x 12 hours = 600watt hours (Wh) /12V battery = 50aH

100W panel x 5.5 sun hours in AZ x.6 efficiency = 330Wh of power generated. You need 600Wh. Unless that regulator brings the power use of the light way down, you have only half the solar you need.

Thanks Amy, According to the NREL calculator I get 6.43 average sun hours a day. Is there any reason I should assume less here?

Is the 0.6 efficiency number based on a PWM based charger setup? I was hoping to squeeze a little more out with a MPPT charger.

[Amy@altE]

I have a 50w DC light that I am looking to power for 12 hours a day. It has a voltage regulator on it to help manage the power consumption/brightness. I am hoping to fine tune the battery/panel power this way.

Thanks for the recommendation.

50W x 12 hours = 600watt hours (Wh) /12V battery = 50aH

100W panel x 5.5 sun hours in AZ x.6 efficiency = 330Wh of power generated. You need 600Wh. Unless that regulator brings the power use of the light way down, you have only half the solar you need.

[Sunking]

I have a 50w light that I am looking to power for 12 hours a day. Thanks for the recommendation.

You need more of everything as that adds up to 5 cents worth of electricity per day or 600 watt hours.

To do that you need 250 watt solar panel if using a 20 amp MPPT controller, or 360 watts of panel using a 20 amp PWM controller. Then a 12 volt 210 AH battery, about the size you have now except a new one.

[photolimo]

An important question hasn't been asked. How much power will you be using a day? All of the discussions have been assuming using 50% of the battery a day. If you are only using a small fraction of the battery capacity, you don't need to refill that much, just what you used.

I recommend a Morningstar SunSaver SS-10-12V. It does 3 stage charging, has a jumper to select flooded vs AGM, and costs under $50.

I have a 50w DC light that I am looking to power for 12 hours a day. It has a voltage regulator on it to help manage the power consumption/brightness. I am hoping to fine tune the battery/panel power this way.

Thanks for the recommendation.