Charging rate question and Critique

**Don H** · 05-24-2018, 05:26 PM

Hi there,

This is my first post. I am thinking of a 3,600watt system with four AGM type 6volt batteries. What type of charge controller would I need? I already have 14 panels rated at 260 watts each.

Regards,

Don H

**Suprasoup** · 05-24-2018, 06:01 PM

Lets use Sunkings sticky calculations for this:

2000wh daily x 1.5 fudge factor = 3000wh

Panel sizing caculation

3000wh/3.9 sun hrs = 769w

Battery Voltage is 24v

Battery sizing Calculator:

2000wh x 5 day reserve = 10000wh

10000wh/ 24v = 416AH battery

Charge Controller sizing

769w/24v = 32A MPPT Controller

So heres a couple of observations. You are planning on paralleling batteries. If you need a 400AH battery just buy a 400AH battery. Avoid paralleling batteries. L16 batteries come in 400+AH. Wire 4 of them in series and you only have one string. Much better.

How do you plan to wire your panels to the charge controller? Since your panels are 36 cell minimum to charge a 24v battery is 2 panels in series. 10 panels would be wiring 2s5p. Problem is that is a lot of parallel strings that each need to be fused. 3s3p might work but you lose a panel and still need to fuse each string. 4s2p might work depending on the Voc limit of your charge controller. You lose 2 panels but no longer need to fuse the parallel strings.

Some things to to think about.

**SixAcreDream** · 05-24-2018, 07:37 PM

Thanks for the response. Forgot about the L16's. They would be pretty close in price to what I was planning and a better choice.

For combining the panel strings my plan was to use a Midnite Solar MNPV6 with a breakers on each string.

So it looks like my math was pretty solid. Only difference I really see is that I calculated in inverter losses into the total load and rounded up some other values.

Thanks for the feedback. Really appreciate.
Six

**Suprasoup** · 05-24-2018, 08:20 PM

FLA batteries have minimum and maximum charging rates. The generally accepted rates are between C/8 to C/12 with most manufactures recommending C/10. So for your 1000W array you could support from 330AH to 500AH. Specialty batteries like AGM can support higher charge rates. But AGMs are also more expensive and dont last as long. Since most batteries die because of undercharging than overcharging you want to skew your charge rate to higher rather than lower. I would not try to charge slower than C/12.

**SixAcreDream** · 05-25-2018, 02:01 PM

Thanks for the follow up. Figured it would diminish battery life but wanted to make sure.

As for the batteries, In my math I made it sound like a specific battery choice was already decided. The AH rating was a number I knew off the top of my head. I looked at several of the L-16 batteries but couldn't find one at the 400AH capacity that didn't have some kind of cranking rating. Most didn't state it up front but after digging I could usually find it. Example: Surrette s-550. If you dig down in their spec sheet the list CCA and one other.

The real plan was for 8 Trojan T-105-re's series to 24v and then finally in parallel using equal length/gauge conductors to a single distribution point rather than bank to bank. That way each bank would receive/distribute the same current. I know parallel is typically considered bad since most people don't balance the banks. What would be the drawback to the fore mentioned approach?

I know I could just series them together for 48v but for now would prefer to keep the voltage to 24v.

**Sunking** · 05-26-2018, 12:10 AM

Originally posted by SixAcreDream

Howdy all. First post but hope to be come an active member.

Currently designing a 24v system and like someone to look over my design and help with a charging rate question.

Our estimated usage is 2000 watts per day (some estimated, some killawatt measured). So here's the design and math.

Usage: 2000w
Inverter efficiency: .8
Total Load: 2500

None of this makes any sense. I suspect you do not know the difference between Power and Energy. There is no such thing as 2000 watts usage. 2000 watts is power not energy. Then you go on to say Total is 2500 something and I suspect you meant watts which is still nonsense.

First step is determine daily Watt Hours which is Energy used in a specified time. Watts x Hours = Watt Hours. Example a 100 watt light bulb illuminated for 10 hours; 100 watts x 10 hours = 1000 watt hours, or 1 Kwh. Power again is watts and is just the rate at which Energy is being used. I have no clue what 2500 is for.

If you meant 2000 is watt hours, than all your math is wrong, if 2500 is watt hours then your math is close. Beats what you mean and I suspect you did not determine watt hours. However I will run the numbers based 2000 watt hours and 2500 watt hours as daily usage with 3.9 Sun Hours

2000 wh x 1.5 / 3.9 hours = 769 watts, round up to 800 watts. Controller = 800 watts / 24 volts = 33 amps, 30 amp controller
2500 wh x 1.5 / 3.9 hours = 961 watts, round up to 1000 watts. Controller = 1000 watts / 24 volts = 41, 40 amp controller.Battery at 20% DOD, depends on what your daily usage is and battery voltage. If you use 2000 watt hours per day x 5 days / 24 volts = 416 AH, and if 2500 watt hours is 520 AH.

From that you made a lot of errors and will cost you a lot more money than necessary. 12 volt battery panels are the last thing you want to use like the Grape 100 watt panels. Use Grid tied panels like 200 or 250 watt panels. GT panels are a lot less money. Secondly instead of 10 panels with racking for 10 panels and a lot of wire you are spending on vs 4 panels. That alone will save you a few thousand dollars.

Why are you going to parallel batteries, you only need 4 x 6-volt 400 to 520 AH batteries depending on what number you used. There is nothing wrong with Roll S-550 if that meets the spec, and carries a lot longer warranty. The S-550 caries a 7 year warranty and the T-105 carries 2 years. However you are comparing an Apple with an Orange. In other words comparing a Chevy to a Rolls Royce. You are comparing Trojans entry level battery to Rolls top tier battery and somehow came to the conclusion Trojan (Chevy) was a better battery (car). If you want to compare the Rolls S-550 to Trojan you need to use Trojans Industrial line.

Last comment is I do not think you determined how many Watt Hours you need in a day. If my assumption is correct, you are heading for a train wreck and you will be a passenger. So you need to make sure you know what you are doing.

**SixAcreDream** · 05-26-2018, 01:12 AM

Thanks Sunking. Was hoping you'd chime in.

After re-reading I agree my terminology is incorrect and confusing.

Using a killawatt and estimating 1 item I don't have yet, but based my estimate upon manufacture specifications (which I doubled just to over estimate), I came up with 2000wh which would be the total over 24 hours which I called usage. The 2000wh vs the 2500wh number is calculating in inverter loss. 2000wh * 1.25 (25% inverter loss) = 2500wh. Since I don't know what inverter I'll end up with I factored in 25%. I said .8 in the original post which was wrong and misleading.

I mentioned in the original post that I already had most of the panels from ham radio club project. I only need 2 more panels which I can buy for $89 each vs buying all new. I also already have a Tristar TS-MPPT-45.

As for the battery assumption. I've seen countless post to stay away from any starting battery. The spec sheet for the Surrette battery mentions Cold Cranking Amps and Marine Cranking amps, so assumed it's an SLI battery. Please see link: https://www.rollsbattery.com/wp-cont...ries/S-550.pdf If it's a better battery then I'm all for it since it mostly fits and keeps design simple and straight forward. Thanks for pointing out that it's a good battery.

The Trojan battery SPRE 06 255 battery (T-105RE replacement) which is in their premium line. I read 5 year warranty with an 8 year expected battery life so on paper it looked like the better option since it's from their premium line and didn't mention CCA.

As you mention above, at 2500 watt hours and 20% depth of discharge I'd need a capacity of 520ah. If I go with the Surrette then I only have a 428ah (4 series) capacity which changes the depth of discharge calculation which was one of my concerns.

Any help you can provide based upon my updates would be greatly appreciated. And if I need reboot this project and use the GS 100 panels for another project I can do that too but would hate to not use what I have available.

**Sunking** · 05-26-2018, 11:28 AM

Originally posted by SixAcreDream

Thanks Sunking. Was hoping you'd chime in.

You are welcome and lets see if we can right the ship.

Originally posted by SixAcreDream

Using a killawatt and estimating 1 item I don't have yet, but based my estimate upon manufacture specifications (which I doubled just to over estimate), I came up with 2000wh which would be the total over 24 hours which I called usage. The 2000wh vs the 2500wh number is calculating in inverter loss. 2000wh * 1.25 (25% inverter loss) = 2500wh. Since I don't know what inverter I'll end up with I factored in 25%. I said .8 in the original post which was wrong and misleading.

OK great I was hoping it was watt hours, and not connected load. Forget the 2500 figure, Inverter Efficiency was lumped in with fudge factor to account for all losses. Erase 2500 from your brain.

Originally posted by SixAcreDream

I mentioned in the original post that I already had most of the panels from ham radio club project. I only need 2 more panels which I can buy for $89 each vs buying all new. I also already have a Tristar TS-MPPT-45.

OK free is a good thing, but since we threw out the 2500 nonsense, you only need 800 watts of panels. But I still have a question. How did you come up with 3.9 Sun Hours? If your winter Isolation is really that high is going to require a bit more panel wattage than calculated to meet minimum charge requirements. With 10 panels also creates a challenge in panel configuration. Ideally you would like to have 5S2P, but I have doubts that can be done without knowing your area lowest temps. SC does not get cold so maybe doable. Otherwise 2S5P is going to get really expensive with combiners, fusing, and wiring.

Originally posted by SixAcreDream

As for the battery assumption. I've seen countless post to stay away from any starting battery. The spec sheet for the Surrette battery mentions Cold Cranking Amps and Marine Cranking amps, so assumed it's an SLI battery. Please see link: https://www.rollsbattery.com/wp-cont...ries/S-550.pdf If it's a better battery then I'm all for it since it mostly fits and keeps design simple and straight forward. Thanks for pointing out that it's a good battery.

Trust me the Rolls S-550 is a great battery. It is a hybrid and thus why you see those ratings. Trojan could list those specs if they wanted, but the thing is Cranking Amps would be low and the battery is not intended for cranking. If you were to compare the S-550 to a similar sized SLI battery Cranking Amps, the S-550 would be weak. Anyway do not rule out Rolls, the S-550 is a solid performer. Let your wallet and availability chime in on the decision.

Originally posted by SixAcreDream

The Trojan battery SPRE 06 255 battery (T-105RE replacement) which is in their premium line. I read 5 year warranty with an 8 year expected battery life so on paper it looked like the better option since it's from their premium line and didn't mention CCA.

OK another good option, now you are starting to compare Apples to Apples. You are correct about the warranty. Like I said Trojan could list CCA and MCA on that battery, but it is not intended for that application. Example if they did specify it would be 650 Amps. Sounds great right? Wrong, dead wrong. Go to any store that sells car batteries and you can find all kinds of 12 volt batteries that exceed 650 amps, and those batteries are physically 1/4 to 1/3 the size and capacity. Moot point, 225 AH batteries are too small for you. You are shopping now for a 400 to 450 AH battery.

Last comment determine and locate you battery selection. If you really have 3.9 Sun Hours, forget all the calculations for panel wattage, you will need to meet minimum manufactures charge rates. So you may very well be looking at 1000 watts.

**SixAcreDream** · 05-26-2018, 01:48 PM

Thanks for the follow up and thanks again for the help. Did a lot of up front research and glad to see it make more sense when I explain better. The fudge factor makes more sense to me now.

The 3.9 comes from a calculator I used during early research. It had an option for Charleston SC which provided the 3.9 value. We' are in Columbia, SC but Charleston was the closest listed.

Just a moment...

https://www.altestore.com/store/calculators/off_grid_calculator/

Did a search and found another calculator. Would I use the lowest value or the average?

https://www.wholesalesolar.com/solar-information/sun-hours-us-map

CITY: Charleston (SC)
STATE: SC
HIGH: 5.72
LOW: 4.23
AVERAGE: 5.06

Is there a calculator or map you recommend?

One of the reasons I get hung up on the Trojan brand is we have a dealer just a few miles away so won't get eaten alive by shipping costs. That being said, my preference is to get it right than have it be a train wreck so open to all options.

In my research I read that some wiring patterns (don't know the right term, I'm a programmer so pattern comes to mind), are acceptable for parallel connections as long as you balance the current to/from the bank. Is that correct? This is for my own knowledge and I'm not stuck on parallel. If it's bad and won't work I won't do it.

**Sunking** · 05-26-2018, 04:37 PM

Originally posted by SixAcreDream

Thanks for the follow up and thanks again for the help. Did a lot of up front research and glad to see it make more sense when I explain better. The fudge factor makes more sense to me now.

The 3.9 comes from a calculator I used during early research. It had an option for Charleston SC which provided the 3.9 value. We' are in Columbia, SC but Charleston was the closest listed.
https://www.altestore.com/store/calc...id_calculator/

Did a search and found another calculator. Would I use the lowest value or the average?
https://www.wholesalesolar.com/solar...n-hours-us-map

CITY: Charleston (SC)
STATE: SC
HIGH: 5.72
LOW: 4.23
AVERAGE: 5.06

Typically Worse case but there is an exception. You still need to meet minimum charge requirements for the battery. In your application you need to meet minimum charge requirements of C/10. A little overkill, but there is no technical consequence, only money. It also means a lot less generator fuel.

Originally posted by SixAcreDream

One of the reasons I get hung up on the Trojan brand is we have a dealer just a few miles away so won't get eaten alive by shipping costs.

Trojan is excellent. My point was to compare Apples to Apples. If I were in your shoes would be using Trojan.

Originally posted by SixAcreDream

In my research I read that some wiring patterns (don't know the right term, I'm a programmer so pattern comes to mind), are acceptable for parallel connections as long as you balance the current to/from the bank. Is that correct? This is for my own knowledge and I'm not stuck on parallel. If it's bad and won't work I won't do it.

That is a work around when you get stuck with parallel batteries. Does not correct the problem, just minimizes the problem. Here is the deal, you cant get batteries in the right AH capacity and doing so is less expensive and will maximize battery life to get the most out of your investment. So ask yourself; Why would you choose parallel over series if it cost you more and shortens battery life? ..........

**Suprasoup** · 05-26-2018, 05:33 PM

Use the lowest #. Better to underestimate how much sun you get vs overestimate.

Ohms law rears its ugly head. In theory if we can perfectly match the resistance of both parallel banks current would flow equally between both banks. Unfortunately the resistance of the 2 banks in the real world are not equal. Each 6v battery has a different internal resistance as well as the cabling for your interconnects. In a single bank not a big deal as the total resistance is just the sum of the resistance of all the components in that bank. Current flows equally in a series circuit.

In a parallel circuit Voltage is the same across all components and current flows based on the resistance of the component. In this case your battery bank.

In a 2 bank system with each bank having different resistance values the one with the greater resistance will receive less current.

Certain parallel wiring configurations are better than others but they still add resistance. You can use it to minimize the difference in resistance between the two banks but it will not be perfectly matched no matter how hard you try.

Im a chemist by training so Im sure I just made all the engineers on this forum die inside and somewhere baby Jesus is crying at my over simplification of Ohms Law but hopefully you get the idea.

**Mike90250** · 05-27-2018, 01:32 AM

Originally posted by SixAcreDream

....

CITY: Charleston (SC)
STATE: SC
HIGH: 5.72
LOW: 4.23
AVERAGE: 5.06....

So you are off grid. How much do you want to run your generator in winter to keep the heat going, well water on, and the essentials ?

Many off-gridders use the next lowest month as the target, and in summer you are overpaneled, but winter, you still cover most, and let the generator fill in the gaps

Charging rate question and Critique

Charging rate question and Critique

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