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We're recently off grid, maybe 4 months in. When I calculated the load for the house I was very perhaps overly cautious in estimating total daily amp hours used. The array is 12 solarworld panels @ 280W. The batteries are 8 Rolls 6CS21S @ 683 AH. 48V system. I had originally planned for 3-4kw daily usage with a 3 day reserve @ 50% dod. The actual usage is far less than we anticipated because we don't have all of the usual appliances yet (refrigerator mainly). drawing from the batteries. My question is this: am I damaging the batteries in any way by only cycling them with such a shallow daily DOD? They charge to 100% every day but I am a bit worried about not 'exercising' the batteries to a lower DOD more frequently. I have not seen any problems with the low DOD yet but I'm a bit worried about the cumulative effect of this plan. any thoughts? We're only using about 1500W daily as it stands and I don't forsee using more than say 2500W in the too near future. The batteries we chose are robust (understatement) and their price was equally robust. Would hate to be doing something stupid unintentionally. Intentional stupidity something alltogether different. Thanks.
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Howdy jt,
Is you system only 4 months old? Was it done by professional? If its only new and was designed and set up right I wouldn't be worried about it but there are some very experienced members around who will hopefully drop by, cheers -
System is only 4mos old. I bought everything except the bateries as a package. Magnum 4448pae inverter with a midnite xclassic 150. Midnitw e panel. I installed it all myself and it all works as it ought to. My concerns are the somewhat shallow disxharges. We only draw around 28ah daily. We are in the winter doldrums and I am an offgrid greenhorn....hence the xoncern.Originally posted by solar pete View PostComment
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No problem except you should allow then to draw down to 50% 10 to 20 times to form the plates. Once plates are formed no problem and will actually increase cycle life.MSEE, PEComment
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I have a question. How do you charge such a large capacity bank of 683ah with only 3360 watts in panels? As even assuming full rated output from the panel, without thermal losses etc, this is still way short of 68 amps i.e C10. Seems the best you could hope for out of those panels would be just shy of C8 and that's at pretty much full output.Originally posted by jtbartlett View Post
Perhaps as you are cycling them very shallow, less amps are needed to get them back up?Comment
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What is the problem? 3360 watts @ 48 volts is 70 amps a perfect C/10 charge rate. C/12 to C/8 is the range you want to be. C10 would be 6830 amps and would cause the batteries to explode.Originally posted by Jemplayer View PostMSEE, PEComment
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I meant c/10 sorry. I do get turned around here. That's why I ask the questions.Originally posted by Sunking View Post
So you multiply the voltage by the amps, but the voltage is changing all the time as the batteries bulk charge. Therefore if you take the reading higher as the bulk charge progresses it thus takes more Watts to make the same amps?
And also, very seldom a string of panels produce its full rated output everyday. As an example, I have 2400 watts in panels. Only on cooler cloudy days do I see the full 2300-2400 output as the sun comes out onto cool panels.
On most days, esp in summer, I see 1900 max. The reason I am asking these questions is that I want to put a 460 ah bank on my existing panels , but would seem to me that with the power I am getting I can't effectively charge the bigger bank of batteries.Comment
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You use nominal battery voltage, not finishing voltages when current is low. Open circuit voltage on a 48 volt battery at 50% Depth of Discharge is 47.9 volts. So for a 100 AH battery you apply 10 amps current only raises the voltage .1 to .2 volts or roughly 48 volts. As the battery charges the voltage rises slowly and current will taper at a giver wattage input.Originally posted by Jemplayer View Post
So here is the consequence. Lets assume you used 54.6 volts (floating voltage) rather than 48 volts with a 2000 watt input. You would go look for a 2000 watt / 54.6 = 36 amps and would likely buy a 35 amp controller. Well 35 amps x 48 volts on a discharged battery is only 1680 watts. What you just did is clip 320 watts. You would not reach full panel power until your battery voltage reaches 57 volts right near the end of BULK mode and would take a lot longer. You should have been looking for a 2000 / 48 = 42 amps and should be shopping for a minimum 45 amp amp controller.
Personally I use 80% DOD voltages but manufactures use nominal.MSEE, PEComment
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Originally posted by Sunking View Post
Ok so I feel I am on the right track in my understanding. I put in a 60 amp mppt so it's not the limiting factor here. I occasionally, under the right condition actually produce over the rated output I have seen as much as 2800watts.
But like I said, the average peak power is 1900 odd on a 2400 watts set. Can I add the 460ah bank? I would be discharging that bank by around 20% on average.Comment
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Do you mean replace? or just add to the existing battery bank? if you want to replace the bank and the daily DOD is not deeper than 20%. you should be ok. From my limited experience I notice the charge current don't has much to do with the gassing, it is the high voltage that does the trick. with 48 volt bank when the voltage reach to 60 volt that is the time the cells start to gas. How ever the C/8 to C/12 charge rate do help to get the battery to absorb volt quickly and start the difficult absorb charge state. If you have enough solar power to charge the battery to finish the absorb charge daily, you should be ok.Originally posted by Jemplayer View PostComment
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Originally posted by paulcheung View Post
I mean replace. I would have two new banks of 6v volt 225ah batts. Total 450ah being charged by my current panel array of 2400 watts.Comment
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Why don't you just get a bank of Trojan L16RE-B? they are rated 370AH and can be PSOC. 20% DOD on 450AH is around 25% on 370AH that would be much better than use 2 strings of battery. Or you can get a bank of Rolls S-550 which is rated 428AH at 20 hours rate.Comment
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Originally posted by paulcheung View Post
Thanks for the suggestions. I did consider this, but one thing keeps me on the two banks of 8 solution. COST. It is the most cost effective AH for the money. Atleast here is SA that is.
Don't even know if you can get Rolls here, and likely would be very expensive.Comment
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Quit looking at initial cost, and look at long term cost. parallel cells will shorten cycle life significantly.Originally posted by Jemplayer View PostMSEE, PEComment
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Really. I didn't realize it was that significant. By what percentage would parallel banks loose cycle life?Originally posted by Sunking View PostComment
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