# So I'm a Newb. Picking a Charge Controller.......

patrickroy33
Registered Users Posts:

**5**✭
Real quick background. I'm newb with no one to defer to. I'm planning my first system for an off the grid location, with two honda 2000's for back up. This forum has been an asset to say the least. I've researched for weeks now and feel I have a good, but still basic, understanding.

In planning my system and I'm stuck on a charge controller. I really like the mppt style but price is an issue. I can afford one good one but if I need two I simply can't swing it. I'm hung up a little on the math.

I'm looking at 4 or possibly 6 sharp panels on a certain auction website. They usually come up pretty cheap and since I'm on a budget I don't think I can do much better for the $$ (most seem to be near .80 per watt). There are different ones, I've notice 235's, 240's, 250w's, etc. I will use the specs from the 250w, not sure which I'll end up with but they are all close enough.

•Maximum Power: (Pmax)* 250 W

•Tolerance of Pmax: -5%/+10%

•Type of Cell Polycrystalline silicon

•Cell Configuration 60 in series

•Open Circuit Voltage (Voc) 38.3 V

•Maximum Power Voltage (Vpm) 29.8 V

•Short Circuit Current (Isc) 8.9 A

•Maximum Power Current (Ipm) 8.4 A

And...not that it's of consequence but I've figured approx 16-20 6v 220amp batteries.

So the mppt charge controller have restriction for the most part on the max current, system voltage, and the max Panel Wattage. I know you can run in parallel and series with the panels to change figures like the amps, volts, etc.

here are the specs of the controller I'm lookin at

•Designed to accommodate project size from 12V ~ 48V ( minimum input voltage 40V)

•Max PV/DC input up to 150V

•Allows high voltage panels ( 24/48V) to charge low voltage batteries ( 12/24/36/48V)

•Parallel up to 254 units (IP address, MODBUS) forr large projects.

•High charging current up to 60A (maximum 1500w) output reduces overall charging time.

So here's where I need some confirmation that I'm doing the right thing in my planning. I need to make sure I don't exceed the MAX PV INPUT, Wire Higher than 48v on the panels and or battery bank, AND not exceed 60A

So, am I doing this correct?

If I have 4 panels. I would do one string of panels in series. Giving 12v, MAX PV 29.8 and MAX Amp34 And this would work.

If I have 6 panels. I would also do one string in series. Giving 12v, Max PV 29.8 and MAX amp 52, with this working as well.

Where I think I might be wrong is I'm not sure if the MAX PV of 29.8 multiplies with each panel when wiring in series.

Thanks so much guys!!!

In planning my system and I'm stuck on a charge controller. I really like the mppt style but price is an issue. I can afford one good one but if I need two I simply can't swing it. I'm hung up a little on the math.

I'm looking at 4 or possibly 6 sharp panels on a certain auction website. They usually come up pretty cheap and since I'm on a budget I don't think I can do much better for the $$ (most seem to be near .80 per watt). There are different ones, I've notice 235's, 240's, 250w's, etc. I will use the specs from the 250w, not sure which I'll end up with but they are all close enough.

•Maximum Power: (Pmax)* 250 W

•Tolerance of Pmax: -5%/+10%

•Type of Cell Polycrystalline silicon

•Cell Configuration 60 in series

•Open Circuit Voltage (Voc) 38.3 V

•Maximum Power Voltage (Vpm) 29.8 V

•Short Circuit Current (Isc) 8.9 A

•Maximum Power Current (Ipm) 8.4 A

And...not that it's of consequence but I've figured approx 16-20 6v 220amp batteries.

So the mppt charge controller have restriction for the most part on the max current, system voltage, and the max Panel Wattage. I know you can run in parallel and series with the panels to change figures like the amps, volts, etc.

here are the specs of the controller I'm lookin at

•Designed to accommodate project size from 12V ~ 48V ( minimum input voltage 40V)

•Max PV/DC input up to 150V

•Allows high voltage panels ( 24/48V) to charge low voltage batteries ( 12/24/36/48V)

•Parallel up to 254 units (IP address, MODBUS) forr large projects.

•High charging current up to 60A (maximum 1500w) output reduces overall charging time.

So here's where I need some confirmation that I'm doing the right thing in my planning. I need to make sure I don't exceed the MAX PV INPUT, Wire Higher than 48v on the panels and or battery bank, AND not exceed 60A

So, am I doing this correct?

If I have 4 panels. I would do one string of panels in series. Giving 12v, MAX PV 29.8 and MAX Amp34 And this would work.

If I have 6 panels. I would also do one string in series. Giving 12v, Max PV 29.8 and MAX amp 52, with this working as well.

Where I think I might be wrong is I'm not sure if the MAX PV of 29.8 multiplies with each panel when wiring in series.

Thanks so much guys!!!

#### Categories

- 26.8K All Categories
- 215 Forum & Website
- 123 Solar Forum News and Announcements
- 1.3K Solar News, Reviews, & Product Announcements
- 166 Solar Information links & sources, event announcements
- 878 Solar Product Reviews & Opinions
- 251 Solar Skeptics, Hype, & Scams Corner
- 21.4K Solar Electric Power, Wind Power & Balance of System
- 3.4K General Solar Power Topics
- 6.5K Solar Beginners Corner
- 975 PV Installers Forum - NEC, Wiring, Installation
- 1.9K Advanced Solar Electric Technical Forum
- 5.2K Off Grid Solar & Battery Systems
- 394 Caravan, Recreational Vehicle, and Marine Power Systems
- 1K Grid Tie and Grid Interactive Systems
- 625 Solar Water Pumping
- 796 Wind Power Generation
- 616 Energy Use & Conservation
- 513 Discussion Forums/Café
- 238 In the Weeds--Member's Choice
- 84 Construction
- 99 New Battery Technologies
- 92 Old Battery Tech Discussions
- 3.8K Solar News - Automatic Feed
- 3.8K Solar Energy News RSS Feed

## Comments

17,615✭✭Stop. You're going backwards. Or perhaps sideways.

Sixteen to twenty 6 Volt 220 Amp hour batteries based on ???

Load requirements come first. Always. Once you have that figured out then you size the inverter and battery bank. The panels and controller are there to recharge the batteries.

Picking panels first and then guessing at charge controller, batteries, et cetera is what's giving you that confusion headache you're feeling now.

If you've already got the power requirements please tell us what they are; there's no way to configure the rest of the system without it unless you like disappointment.

5✭Hey....thanks for answering back. I guess I'll explain a little more. I don't have an exact computation yet on the daily requirments, hence the range. I am getting a property and starting from scratch. It is presently off the grid (rural), in which the cabin and yurt are wired for power (genny) but it is not present. There are no appliances and such so I'm in the rough stages of planning. There is propane at the property with lines for things like a fridge but I'm thinking a DIY solar system will end up costing much less in the long run.

So these numbers are somewhat arbitrary, though I believe them to be close given the things I will need to power. So I will basically give you the math I employed to arrive at my numbers. (I used a very nice calculator on a website to help figure this out, I realize this may be rough but this is how I've gotten to the point I'm at)

I'll start with a round number. 3.5watts per day consumption. Energy independace for 3 days until battery bank is at 50%

3.5wattsx3=9.5kw x2 = 19,000kw. So my battery bank, needs to be 12v=1583ah, 24v=791ah, etc.

(I can source 6v golf cart batteries for ...220ah... for $80ea, SAMS)

so, at 6v I'll need 3166ah's. About 15 Batteries

Next to determine my panels I'll use the sharp 250's I referenced in my first post. My area gets 5 hours of sun but I want to be conservative and state 4, just to be safe.

so 3.5kw divided by (250w panel x 4 hours, 1000kw) Which gives a need for 3.5 panels (basically 4)

So on the low end of my range that's what I have come up with. 4 panels+15 6v batts. Inverter was easy to figure out, and of course I still have questions I'm searching for like how should I wire the battery bank, what voltage should it be at, how quick can my 2 honda genny's fully recharge the batts if I get to 50% power in the bank, etc etc. But the next step is charge controller. I'm really hung up on the math. Not sure if the way I'm looking at it above is correct.

PS....I thank you for taking time to help out newbs like myself. Its prob never easy to see the same qustions a million times. I always try to figure it out myself to save from the exact same things coming up on a forum but I really can't wrap my head around this one.

5,433✭✭✭✭There are several things you need to realize.

First, the difference between watts and watt/hours. (WH) WH is how you measure consumption, watts is how you measure a momentary load.

Next y ou have to realize a number of facts about how batteries a able to deliver power. It is not just a matter of the number of amp/hours (ah) of capacity.

Next, you need to realize that wattage numbers of PV is a number that is not directly translatable to useable power. Here is my rule of thumb (that has been been born out of years of experince). Take the name plate rating of the PV, divide that number in half to account for all cumulative system loses (PV, CC, wiring, battery charging eff, more wiring, inverter eff. etc) Then take that number and multiply that by 4 to account for the AVERAGE hours of good sun, you can expect per day on an annual basis. You can fudge that number up or down according to local conditions and seasonal use, but you fudge those numbers at your peril.

So, for example, a 1000 watt PV system might look like this. 1000/2=500*4=2000 watt/hours/day.

As a real life example, we live off grid. We have 400 watts if PV, into 459 ah of battery. On a daily basis we use 5-800 WH of power, and this sytem is pretty well balanced, and we seldom have to run a genny, unless there is no sun for several days. Newbies most often over estimate thier PV potential while at the same time underestimate their loads, leading to a poorly designed/functioning system. The other rule, is that loads ALWAYS grow with time!

So to run the broken record yet once again, do a REALISTIC daily load calc, along with you peak watt loads. All design considerations come from those calcs. Any other method is,, " ready, fire, aim"! You will also have to realize that conservation is your best friend. If you are talking about loads in the 19 kwh daily, you are going to need a huge system. Reduce those loads, and you dice the size and expense of the system. (edit, see tht you are talking about 3.5 KWH/day. That is going to require ~7 kw of PV, and a battery properly sized. There are lots of issues with series/parallel battery banks, see links below. For a bank that size consider 24 or 48 volts.

Tony

Ps I encourage you to read, and UNDERSTAND the following links:

http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

17,615✭✭Okay, let's start again using the right terminology.

What you're after is Watt

hoursper day. It's too easy to blend Watts with Watt hours.So you don't use 3.5 Watts per day. You probably don't mean 3.5 Watt hours either. You might mean 3.5 kW hours per day (3500 Watt hours). That's pretty big for off-grid. My cabin uses about 2.5 kW hours per day, including powering an electric 'frige and satellite Internet modem, comps, water pump, etc.

But let's use the 3.5 kW hours per day number. You'd want to use a 24 Volt system for that (see this thread: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power) which comes to approximately (3500 / 24) 146 Amp hours.

Take that as a 25% DOD figure and you get 584 Amp hours @ 24 Volts for your battery bank. That number you will have to adjust a bit up or down to fit available batteries. Up is better, but if you can control loads down a bit can work too. The 25% DOD automatically gives you two days, as you can take the batteries down to 50% if needed. On the third day of no sun you start the generator.

Using the inexpensive GC2 batteries at 220 Amp hours the closest you can get to this is two parallel strings giving 440 @ 24 Volts. That's roughly 2640 Watt hours. You really don't want to go with more than four parallel battery strings. At that it would be risking imbalance. You might go for three at 660.

What you would need to recharge 440 @ 24 Volts:

44 Amps @ 24 Volts = 1056 Watts. On an MPPT controller this works out to about 1360 Watt array. Round up if possible. With a PWM controller you need to be sure the Imp of the array meets your charge current target.

Can you see how this is going?

5✭Can I see how it's going? At a glance no, but I'll check this out more when in a few hours and post back up. I get it in general but want to back out your numbers and see exactly what's goin on

10,300✭✭✭✭i'm going to comment on the 3 days backup thing here. batteries are quite expensive and you will sometimes need more in pv to keep them charged properly. odds are there will be times you need to run the genny to charge up the bank as 4 days of cloud cover is more common than most would like to believe. i'm thinking design for 1.5-2 days instead and just remember to drag out the genny. a battery monitor may help some here too.

i also might suggest going for higher battery voltages which means a higher voltage inverter.

17,615✭✭I agree with Niel. The old "3 days without sun" isn't very practical because you need a much larger battery bank (batteries are expensive) and a much larger array to recharge it. If you're in an inherently sunny location it's way cheaper to run the generator, and if your in an inherently not sunny location solar is going to be extremely expensive no matter what. You can in those circumstances maximize your array size (panels are cheaper than batteries, so to speak) to get better output on less-than-perfect days.

Bigger battery banks bring bigger bothers.

5✭Ok....so.... I think I have it. I have a bunch of power needs because there's actually 3 structures at the property 2 cabins and yurt. They are not all fully wired with power concerns like a fridge but I'd rather have a little more than I need than not enough.

Ok.....Can you explain what forumula you are using to compute the panel size. It appears you simply divided the 440 by a power of 10 to get the 44amps. But why, or is that even right? Also...for the batteries. Two strings, 440 @24 would look like two strings of 4 batts right? so a total of 8 batts?

Thank You...

5,433✭✭✭✭Did you red the battery FAQs I linked in pot number 4 of this thread? it will perhaps explain why and how the relationship between battery size and PV size is related.

Tony

17,615✭✭If the power needs of all three structures are being provided by one system, then the Watt hour and total Watts at any given time are as one load to that system.

This is essentially correct. Most battery manufacturers recommend minimum current of 5% of the "20 hour" capacity for charging. Some can take a lot more, and some require less (those aren't suitable for RE applications really). As a whole, a peak current around 10% usually results in a balanced system when combined with 25% maximum depth of discharge. It's rule-of-thumb, but works most of the time.

To get 440 Amp hours @ 24 Volts from 220 Amp hour 6 Volt batteries you do indeed need two parallel strings of four in series for a total of eight batteries.

5✭Ok.....so you guys are awesome. I got all the formulas and such now but I'm still again stuck with my initial question. They charging controller.

Taking the following set up...... How do I figure out the charge controller

3 parallel strings of 4x6v 220ah batts = 660AH Bank at 24v.

So I need approx 8 of the panels I referenced in this post to provide power. I don't want to ask simply what charger I need but what is the math behind determining it? I know there is more than just the Amps it's rated at.

PS...as a side note....I will need to order appropriate wiring, connectors, etc. Anyone know of a good site, source for all of my wiring needs?

17,615✭✭A good site source for all kinds of solar equipment is the forum's host, Northern Arizona Wind and Sun:

http://www.solar-electric.com/

At the very least it's a good place to start.

The principle of picking a charge controller is that it has to be able to handle the current and the Voltage. Charge controllers are rated by their maximum output current. So if you were going for 66 Amps for that battery bank (about 2kW array on an MPPT controller) you would need to use something like an Outback FM80 (80 Amps) or MidNite Classic (also capable of 80 Amps).

A couple of formulas for figuring the current from an array:

PWM type controller; current = total of Imp of panels @ Voltage.

MPPT type controller; current = Wattage of array * efficiency (usually 77%) / Voltage.

351✭✭Midnite has a Garage Sale section on their forum and currently have a number of charge controllers available at great prices (they're factory refurbs with full warranty)

3,112✭✭✭✭The MidNite Classic is a very capable Charge Controller, IMHO, the best available.

As Rybren mentioned, the MN Forum has a "Scratch and Dent" Section for refurbished, fully guaranteed:

http://midnitesolar.com/smf_forum/index.php?board=30.0

GoodLuck, Vic