# Thread: Matching Battery Bank Size to Panels

1. ## Matching Battery Bank Size to Panels

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.

2. BB.
Just some guy
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## 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

3. ## 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.

4. ## 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.

5. Frequent Poster
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## Re: Matching Battery Bank Size to Panels

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?

6. ## 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!

7. ## 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

8. ## Re: Matching Battery Bank Size to Panels

Originally Posted by 54d18
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.

9. ## 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!

10. Noob with a plan
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Dec 2010
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## Re: Matching Battery Bank Size to Panels

Subscribing for future reference

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