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Our off-grid system & life
This is a discussion on Our off-grid system & life within the Alternative Energy (Wind, Solar, Hydro etc) forums, part of the Off-Grid Lifestyle category; John. Please explain; what do you mean by a C/10 rate?...
John. Please explain; what do you mean by a C/10 rate?
FLA (flooded lead acid) batteries, especially tall ones like yours, need to charge hard enough to stir the electrolyte occasionally. If not it stratifies making accurate SG readings impossible and reducing battery capacity. This means you need to have enough charging current to bubble the electrolyte well occasionally. With your limited number of panels charging your large battery bank you probably aren't capable of really bubbling those cells well. The 7000 watt generator can do it but from your posts I suspect you guys have only used the generator once or twice in the last year.
Simplifying this but.... as I said earlier throughout the day panels rarely put out full rated power. In addition the charge controller loses about 2% of the energy so when designing a system solar professionals assume that panels/controller will put out about 77% of the panels rated power into the batteries.
Your system 2115 watts in panels X 77% = about 1628 watts (68 amps at 24v) going into the batteries.
At 24v your battery bank capacity is 1712 amp/hrs.
1712 / 68 = 25.3 so your solar charging at a C/25 rate
If you had double the panels (136 amps at about 24v) it would look like this
1712 / 136 = 12.6 so you'd be charging at about a C/12 rate.
you can get the same effect by reducing the battery bank size.
Most if not all reputable battery manufacturers have a minimum charge rate to maintain the warranty, usually at least C/20 (Iron Edison minimum charge rate is C/10). This is because it you don't charge the batteries hard enough you can't properly stir the electrolyte (bubble the batteries) resulting in uneven battery charging and stratified electrolyte inside each cell.
A C/13 charging rate is generally the accepted rate for a full time system. Most FLA (flooded lead acid) batteries do not do well if charged at faster than a C/8 rate because it makes them heat up too much. Heat is a battery's enemy. Solar panels put out more power when it's colder outside.
Last edited by 8301; 11-01-2017 at 05:53 AM.
John, you do a great job of making a complex process understandable. Thanks for taking the time to explain so well.
Originally Posted by Slippy
Yea, she may just turn out to be the real deal ...... might have to let her into our good ole' boy club here, huh? (she could bring the bourbon)
I will choose to enjoy the journey that God has prepared for me. Hidden Content
Wonder if she realizes ole
doesn't approve of much. To get his seal of approval is something to be proud of.
Originally Posted by A Watchman
, you drink bourbon?
Thank you John. I use crown golf cart batteries. Just visited their site and yes, according to their guidelines I am not charging the batteries hard enough. They say 10 to 20% of total AH (20 hr rate). I have six 205 a-hr, six volt batteries connected in series/parallel to produce 615 amp-hr of 12 volt power. So I guess I should be charging at 60 amps or greater. My 600 watts of panels probably never do better than 40to 45 amps. Adding panels is not an option at this time. How about charging with a 55amp charger once a month? Still not ideal but maybe better than nothing?
John Galt, first, thank you for taking the time to go into such detail. I wish my husband and I had your aptitude for this, we do not. What I can do, however, is print out your posts and run this by Randy (my solar pro) - for his input and suggestions. There were concerns, when choosing options for the system, that this type of monitoring and maintenance of lead acid batteries was above our skill-set; we did consider silicon-salt, but the cost was prohibitive. If, as things stand, we'll lose them way too soon, perhaps that would have been the better choice - but, that's water under the bridge. So... maybe what we'll do is have Randy come up to handle this aspect. We should have some nice venison steaks for trade.
Originally Posted by John Galt
Thanks again for all your help!
Mountaingirl. John hasn’t weighed in yet but might I make the following suggestion. It should be relatively easy to divide your battery bank into two separate banks with a few switches. Simply alternate, running one bank on even days and the other on odd days. This would double your charging rate while you would still have the same total battery capacity ( just half held in the reserve bank). I may have to do something like that with my bank because more panels are not an option right now.
Hi Chipper, 8 acres out here are probably a lot different than 8 acres where you are. Nearest neighbor is half-mile away (as the hawk flies) and the handful of folks around here (5-mi radius) sing in the same choir, if ya know what I mean. Luckily, there's more wood within reach than we could burn in a few lifetimes...some pine, but mostly fir. Thanks for your reply!
Originally Posted by Chipper
615 amp/hr batteries charging at 40 amps = C/15 rate or at about the 7% rate.
Originally Posted by Chiefster23
Golf cart batteries have some (can't remember the name but it starts with an A) mixed into the lead plates to make them harder and more resistant to being bumped around. But this additional element makes them less wiling to accept a charge. Because of that they require a stronger charging current.
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