# Matching Battery Bank Size to Panels

Posts: 100Solar Expert ✭✭
I have a small solar system for an off-grid cabin, used to power a few DC water booster pumps and small DC lights once a week (supplemented by a generator when needed). I have (2) 205 W Kyocera panels and an MPPT controller. I'm thinking of changing out my batteries to (4) Trojan T105-RE 6V 225 AH batteries (wired as 24V in series). All my calculations indicate that the batteries and panels are adequate for my power needs. However, I want to make sure that the panels are sufficient to charge the size of my batter bank. I'd prefer to not have to add an additional panel. Are there standard minimum calcultions I should be looking at? Thanks.

• Posts: 25,261Super Moderators admin
Re: Matching Battery Bank Size to Panels

The recommended charging current is between 5% and 13% for standard batteries (AGM can be more/less--but it still does not hurt to follow the above rule of thumb):
• 225 AH * 0.05 = 11 amps minimum
• 225 AH * 0.13 = 29 amps maximum
From 2x 205 watt solar panels, the average charging current for a 24 volt battery bank would be:
• 2x 205 watts * 0.77 panel/charge controller derating * 1/29 volts charging = 11 amps
So, you are at the low end of solar panel charging current... But if you watch the battery state of charge (don't deficit charge the bank with your loads)--you should be fine.

By the way, if your MPPT charge controller has the option--highly recommend to get the Remote Battery Temperature Sensor Option for optimum battery charging.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 17,615Banned ✭✭
Re: Matching Battery Bank Size to Panels

I hate to have to point this out, but you've done what so many people new to RE do; planned backwards.

Battery bank should be sized to supply your power needs, and then the array matched to re-charge the batteries.

As it is, you have a 225 Amp/hr 24 Volt battery bank. The rule-of-thumb for charging is that the charge current should be 5-13% of the Amp/hr rating. In this case 11.25 to 29.25 Amps. I personally favour 10% (22.5 Amps) as a charge rate for off-grid, remembering that such systems are usually drawing power back out of the batteries to run things while the panels are charging. That means your true charge rate is Amps In minus Amps Out for loads.

Panel-wise you've got 410 Watts peak, so your charge current would be approximately 17 Amps under good conditions. Keep in mind that's approximate: there are factors such as the panels' actual Vmp, line loss, et cetera.

With the nominal numbers (not dialed in to the specifics of your install) it should work, but it's a bit on the low side for my preferences.
• Posts: 100Solar Expert ✭✭
Re: Matching Battery Bank Size to Panels

Thanks, all, for the responses - very helpful information. I realize I'm going about it the wrong way, but given where I am in the process, I'm trying to do all I can to make it work. I'll definitely add a remote temperature sensor to the Morningstar MPPT controller.
• Posts: 59Solar Expert
Re: Matching Battery Bank Size to Panels

Just reading about system sizing, I am running
150w of solar through a Rogue MPPT Controller
into a bank of 8 6v 225Ah connected for 12v.

I use my system mainly for weekends, all year long,
but not necessarily every weekend, it runs
a 22" LCD TV, Sat receiver, assorted CFL lights 5-13w.

I pretty much run generator free, unless totally crappy weather.

Am I to small on solar?
• Posts: 17,615Banned ✭✭
Re: Matching Battery Bank Size to Panels

You've got 4 'banks' of 2 each; total 900 Amp/hrs. At 10% charge current, you need 90 Amps going in to that bank to keep it 'happy' (charged and un-sulphated). 150 Watts of panel isn't going to supply more than about 10 Amps peak (at a charge Voltage for 12V system). Rough calculations without exact numbers or efficiency losses, but you can see it's way, way off.

This is exactly the situation that leads the user to think everything is okay until plates are so sulphated that there's no longer enough Amp/hr capacity to supply the minimal draw. By then it's too late.

But the real question is; are you 'under-paneled' or 'over-batteried'?

I suggest you get a Kill-A-Watt meter (or similar) and get some 'real world' figures on your usage in Watt/hours. That will determine how big a battery bank you need to handle loads between charge periods, and that will determine how much solar panel you need for charging.

Technically you can replace used Amp/hrs over a longer time, but if the current never hits that 5% - 13% 'ideal' rate the electrolyte will not be stirred up enough to prevent sulphation. Gotta make 'em bubble a bit!
• Posts: 5,070Solar Expert
Re: Matching Battery Bank Size to Panels

Just for illustration, since both posters here have similar situation to mine.

We run ~400 watts of panels (depending on how you count the name plate). We run that through a Rogue 3024 controller, into 4 225 ah T-105 trojan batteries wired for 12vdc, leaving ~450 ah of battery.

On an average day, we draw the battery down ~60 ah by morning, sometimes as little as 30, almost never more than 75 ah. So on average we draw down ~ 15% every day. In addition, we do significant load shifting so that we recharge laptops and do web work during the day, pump water during the day etc.

Our panels get into full sun ~ 8:00 AM between the equinoxes. The panels are essentially dard by ~2:30 PM between the equinoxes. Because of odd peculiarities in our panel arrangement and shading almost never do all the panels get full sun at the same time. The front wall mounted panels get sun very early, but have partial shading through out midday. The roof panels get full sun ~ 10 and loose it at 2 or so, but have no shading. Depending on the season, they get some late evening (4-9pm sun) but I don't count that in any calc. So when everything is firing right, the rough puts ~300 watts/~ 20 amps for the bulk of those hours. I sometimes see ~375 watts and 28 amps, but that is the exception. On an average (full sun day) we harvest ~ 1.2 kwh/day (or ~80 ah/day).

So the 20 amp average charging rate is only ~ 4.5% of the 450 ah capacity of the batteries. Would I like more? Of course, but my system works just fine. I do think that any less would be net/net not enough, but that said, we lived for years with only 200 watts, and the batteries have lived 10+ years.

Bottom line, I wouldn't get too hung up on the % charge rate (within reason) I would pay attention to the batteries on a regular basis.

Tony
• Posts: 10,311Solar Expert ✭✭✭✭
Re: Matching Battery Bank Size to Panels
54d18 wrote: »
Just reading about system sizing, I am running
150w of solar through a Rogue MPPT Controller
into a bank of 8 6v 225Ah connected for 12v.

I use my system mainly for weekends, all year long,
but not necessarily every weekend, it runs
a 22" LCD TV, Sat receiver, assorted CFL lights 5-13w.

I pretty much run generator free, unless totally crappy weather.

Am I to small on solar?

you are too light on the solar charge as you are around the 1% rate. this is fine to float the batteries, but to charge them you better be at least at a 3% rate with no loads on at the time imo and preferably a 5% or better charge rate. light loading is ok and icarus may be thinking you are just running off of the pvs essentially, but when one gets down to the nitty-gritty of it those batteries are sliding backwards from being 100% charged as you would need the power to run the loads plus maintain the batteries as any draining of power will not be able to be replaced by a 1% rate of charge.
• Posts: 100Solar Expert ✭✭
Re: Matching Battery Bank Size to Panels

I'm resurrecting this old post for additional advice - I'm still having difficulty wrapping my head around this concept. I'm plannign on increasing my solar system with a planned remodel/addition to our off-grid cabin. All the solar array calculators I've used indicate I should have a 2050W panel array. I think I'll use (10) 205W Kyocera panels - 26.6V Vmpp and 7.71A Impp. I'll run those through an Outback FLexmax 80 charge controller to charge a 48V bank of (16) Trojan 6V 370 A/H batteries. I'll wire the batteries in two strings of 48V each.

So my question(s) - is that panel array large enough to charge the battery bank? The panel array is more than large enough for my energy needs, so I don't want to go any bigger than I have to. The battery bank gives me 3-days of storage, which is as low as I want to go.

I understand that I need to be, at a minimum, at 5% of the battery AH - wouldn't that be 37 Amps (740*.05)? Each panel puts out 7.71 Amps at 24V, which would be derated at 77% to 5.9 A. Since I would be using 10 panels (at 24V), for a a 48V system, they would be wired together to have 5 strings. Do I multiply the 5 times 5.9 A to get ~30 A? If so, that would indicate that I'm still too undersized and that I would have to increase my panel array to 14 panels (2,870 W) at a bare minimum. That seems like overkill (and way too much excess cost), given that my daily watt consumption is less than 2,000. Further, if I went to 10%, would I have to go to 24 panels (4,920W)? And if I wanted to go from 3 to 6 days battery storage, I would have to double that again? Those numbers seem outrageous - am I doing something wrong with my calculations?

Any direction that any of you can provide would be appreciated!
• Posts: 292Solar Expert ✭✭✭✭✭✭
Re: Matching Battery Bank Size to Panels

Subscribing for future reference
• Posts: 5,070Solar Expert
Re: Matching Battery Bank Size to Panels

Your numbers are roughly correct, at least in simple calcs. The real reason for the 5-13% charging rate is to charge fast enough to get the batteries reasonably full during very charging cycle. For example, if you are routinely drawing your batteries down 20%, the 5% rate would take 4 hours to get full (not counting any basic battery chemistry efficiency which would add another 20% to the total required to fully charge). The other reason is to charge at a high enough current to keep the electrolite in the battery well mixed.

If you are drawing ~2 kwh/day I'm guessing that you have too much battery capacity for your average loading. For example 2 kwh out of 48 v battery is ~ 42 Ah. If I am reading you correctly and my math is right, you have ~ 740 ah of batteries @48 volts. If you are drawing only 2 kwh (42 ah) daily that is ~ 6% of the total .

While it is often nice to have extra battery capacity, too much is not always a blessing. Often times I have seen people (including me) let the loads grow without growing the charge capacity because "I have lot's of battery". In a normal situation, you might well be money ahead to have half the batteries (ergo half the cost) draw them twice as much, (that would still only be ~ 12%!) and then your current charge capacity would be just about right. (~ 30 amps) Over the life cycle of the batteries you might find it net/net cheaper.

As for the three day "reserve"; You 720 ah of batteries, and 42 ah of average daily load, you have at least 4 days reserve to draw to ~ 75% SOC, ~ 8 days to draw to ~ 50%. If you reduced the battery bank by half you would have 2-4 days reserve respectively, a pretty good number given all the circumstances.

I may have mentioned it in a previous post on this thread, and have certainly mentioned elsewhere. When we built our new house and system, we did a calculation that told me that 4 225 ah T-105s were way cheaper than 4 l 16 over their expected lives. Cheaper to buy, cheaper to replace. Given our loading (5-800 wh/day) and our PV ~ 400 watts it made a nice balance. 4 years in, I am very happy with the choice.

Bottom line,,, I think your battery bank is too big!

Hope this makes sense.

Tony
• Posts: 100Solar Expert ✭✭
Re: Matching Battery Bank Size to Panels

Tony-

Thanks for the response - your posts are always very helpful. I think I misspoke in my previous post. My solar array is 2050W (10 X 205W), but my estimated daily watt consumption is 8,210 watt hrs/day (accounting for efficiencies). I've attached the worksheet I used for my calculations. If you wouldn't mind - could you take a look at it to determine if my calculations are correct, and if my solar array is sufficient for my 740 AH battery bank? The calculations indicate I need a minimum of 677 AH (at 48V) - 266 AH per day, with three days of storage.

Thanks
Mike
• Posts: 10,311Solar Expert ✭✭✭✭
Re: Matching Battery Bank Size to Panels

i'm thinking you may need a bit more in pv as the charge % is hovering around the 5% area and if you get 5hrs of full sun you may break even with your daily loads. if not more solar then supplement as needed with a generator.
• Posts: 5,070Solar Expert
Re: Matching Battery Bank Size to Panels

Mike,

It looks like at a bare minimum, you need ~ 45 amps of charge current, (assuming a 4 hour window of good charging sun) Your panels are likely to put out ~ 33 amp into 48 vdc, so out of the gate you are light, unless you are prepared to use the generator nearly every day.

Here is (and I'm sure you have seen it before!) is my quick calc for off grid PV.

Take the name plate rating, divide it in half to account for all system loses from PV to inverter outlet(s) then multiply that number by four to represent the average hours of DAILY (there you go Neil!) good sun one can expect over the course of a year. One can then adjust the 4 hours to suit their own experience, but in the real world, between shorter winter days, partial cloud cover, rainy days, 4 is a pretty good average in most places.

So your PV 2050/2=1025X4=4100 watt hours/day or call it 4kwhs. Once again you can see that you are too small by half, not counting any reserve for cloudy days. I would certainly consider trying to reduce your loading, and or increase your PV. Failing that, consider running the gennie for a designated time during the day, where it can do some big bulk charge (as well as carry some of the loads, like bulk water pumping, laundry etc.

The other thing is to try to time shift the loads such that as many of them can be powered directly by PV instead of by the battery. By doing this, you save ~ 20% battery charging loss right out of the gate. For example, we turn off our pump after the sun goes down (unless we really need extra water!) We also will work on the computer using the lap top battery in the evening, but plugged in during the day.

good luck and keep in touch,

Tony