Author Topic: Need HELP! 9.1kW solar system charging batteries with my generator.  (Read 4956 times)

Offline tdry253

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I have a solar array with battery back up. I want to know if I add a Generac generator 15kW to my system in the event the sun doesn't shine for hours on end how long will it take for this generator to recharge my batteries to 90%

My system components:

32 - 285watt Solar world mono panels -9.12kW system
01 - four star outback Radian GS8048a 8000
01 - Four star classic 150 double PV center includes MidNite Solar Classic 150 MPPT Charge Controller
01 - Crown battery bank (8 total batteries) AGM 390ah x 48volts = 18,720ah. Controller is set to never allow batteries less than 50% or  9,360ah.

I want to add into the mix a 15kW Generac generator for emergency situations  if the sun doesn't shine for days,  I want to  power on my generator for a short period of time and recharge my batteries. I do not know how to calculate what the run time of the generator will be to charge my bank. I have figured that my solar array can charge the batteries in 1hr 10mis with full sun.

Offline chrisdfw

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #1 on: August 04, 2016, 12:15:36 PM »
the generator can charge the batteries faster than the solar array if you have a charger that can put out the current necessary, a 15kw generator can put out more than a 9 kw solar array.

You will destroy your batteries very shortly either way,     390 ah of battery at 48 volts is very small relative to the size of your solar array. The generator can probably charge the batteries in 45 minutes or less from 50% discharged. This is not good for the batteries

Even your solar array would be charging at a grossly fast rate. you are putting almost 40% of the capacity in the battery per hour, that is too fast for long life. you need either a larger bank or second string of batteries. Whoever designed the system did you a big disservice in undersizing the battery bank. I hope there is note to the story here and the system is not really set up to charge the batteries that fast. 

I see that your inverters are grid interactive so I hope that the charge controllers and inverter are set up to send power to the grid and not into your batteries at the rate the panels are capable of putting out. You really want to charge the batteries at a little lower rate if you want them to last. Without knowing more about how it is set up I can't say.


But the short answer to your question is that the generator can change the batteries from 50% in under an hour with the right charger. If all you are wanting the generator for is to charge the batteries, then a smaller generator would be more than sufficient, something in the 5kw range would be more than adequate.

Offline Carl

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #2 on: August 04, 2016, 12:26:15 PM »
I use a small solar setup BUT To hammer your batteries at a C1 rate will most certainly do damage to the batteries.
I don't believe that your solar panels charge at anywhere near that rate as it would turn your batteries to worthless lumps as batteries are just not designed to take current in such a manner. You should check with the company who sold you the system to discover what they suggest as no wet cells will take a charge at C1 and not suffer as chemical storage just does not work that way.

Your charge controller is rated for 80 amps maximum for 48 volt system...or about 4,000 watts and that should be at least 30% over expected usage.You need at least a 3 stage charger to do your battery bank properly and I would say(but what do I know) a 48 volt at 50 amp charger would be max for that and would actually take 4 to 6 hours or more to bring your batteries to 90% of an unknown starting level(maybe 50%)Also you just as well run on generator power as your inverters are grid tied and will not power anything when the grid is down...though this does explain some of the odd math on your setup.

I suggest you ask an installer,better yet ,pay one to set you up correctly as the math on your system just appears out of balance.

Good luck with your project.This is NOT the kind of project you ask just anyone's advise on.

Offline tdry253

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #3 on: August 04, 2016, 03:30:03 PM »
I greatly appreciate all the help! A lot of this is new to me and by no means do I know anything about electricity. So far I have purchased all the products, but have not had it installed yet. I've got just over 30k in all the equipment.

I do have a hybrid inverter and two MMPT charge controllers that I've purchased. The charge controllers will manage the battery storage and I'll only ever use the batteries in the event of a full power loss. My goal with the generator was to give me the ability to  run a separate panel I've already had wired in and charge the batteries in the rare even the sun doesn't shine to recharge them. My goal would be to run off the batteries as much as possible and use the generator in rare situations in order to conserve propane.

I have all components of the system purchased including racking for my ground mount. My next step was to hire a good installer, but as you've pointed out I may need to look into the system further to make sure it's well balanced. 

Offline cidyl

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #4 on: August 04, 2016, 05:17:25 PM »
I have a solar array with battery back up. I want to know if I add a Generac generator 15kW to my system in the event the sun doesn't shine for hours on end how long will it take for this generator to recharge my batteries to 90%

My system components:

32 - 285watt Solar world mono panels -9.12kW system
01 - four star outback Radian GS8048a 8000
01 - Four star classic 150 double PV center includes MidNite Solar Classic 150 MPPT Charge Controller
01 - Crown battery bank (8 total batteries) AGM 390ah x 48volts = 18,720ah. Controller is set to never allow batteries less than 50% or  9,360ah.

I want to add into the mix a 15kW Generac generator for emergency situations  if the sun doesn't shine for days,  I want to  power on my generator for a short period of time and recharge my batteries. I do not know how to calculate what the run time of the generator will be to charge my bank. I have figured that my solar array can charge the batteries in 1hr 10mis with full sun.

Nice equipment, wish I'd have gone with 48v in hindsight.  You got twice as much system for the same price as I paid not that long ago, I think I hate you :)

You'll be attaching the generator to the generator AC 'in' on the inverter/charger right?  If so just adjust the Radian charging amps to be what Crown recommends (http://cdn2.hubspot.net/hubfs/435823/Brochures/SafetyFirst-AGM-sec.pdf?t=1470065130834), 10% of C/20.  For 8 6v batteries in series, that's about 39A.  To  bulk charge them from 1/2 drained at the recommended amperage it'd be about 5 hours.
« Last Edit: August 04, 2016, 05:31:38 PM by cidyl »

Offline Bad_apple

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #5 on: August 06, 2016, 10:08:19 AM »
as simply as i can put it is there are some math errors:

i only see crown offering 390ah rated batteries in a 6v meaning your battery bank is wired in series meaning your going up in voltage NOT in ah.  this means you have a 48v bank with only 390ah, 16 of these batteries wired series parallel would get you 48v at 780ah and so on.

the solar array should be wired series parallel as well and provide a consistent 4,560watts  and about 147-150 working amps for i.e. charging batteries or covering system usage from the inverter etc. 

HOWEVER,

a weak link is your solar controller.  your current controller is only capable of an 80ah charge rate.  only a bit over half of what your solar array can provide.  this means slower charge and less available power.  adding another midnite solar controller wired parallel would give you up to a working 160A charge rate.  and allow you to utilize the full power available from your panels.  capable of handling slightly more than what you have available from your solar array and would be recommended.

as far as generator charging, the generator charges the batteries using your charger inverter.  the 3 stage charge setup should take almost 2 hours to charge these batteries from 50% and maintain maximum efficiency.  where as your solar also utilizes a 3 stage charging process but currently will take twice as long (at full capacity in bright sun) to charge unless you add another controller.

as far as your question pertaining to ac wiring YES the gen output would wire to the ac input of the inverter as long as you dont already have grid power or any other form of ac power going to the input.  if you do you would need a transfer switch.

recommendations: add a solar controller. and your current battery bank at 48v does NOT have sufficient reserve to run without charging for a long period of time.  there definitely needs some adjustments.  either 8 more of what i believe are currently 6v batteries to make a 2nd bank wired series parallel or even going with the crown 290 12v, 8 of these series parallel would yield 1160ah. adding the 8 you already have gives a total of 1550ah.  but this all requires revisiting the solar controller and redoing the charge time math. 

great setup none the less.  on the right track.  i have done solar installations professionally for about 16 years now.  if you would like more info feel free to PM me im glad to help.  :)

Offline Bad_apple

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #6 on: August 06, 2016, 10:12:22 AM »
addendum, you do NOT need to worry about overcharging or frying your batteries charging from gen power or solar as your inverter charger and solar controller are 3 stage and have over charge protection.  i would be more concerned with the overall ah of the system and with depleting to a 50% charge frequently.  definitely try not to let the batteries get that low before charging unless absolutely necessary.

Offline cidyl

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #7 on: August 06, 2016, 05:36:29 PM »
tdry253 said:

"01 - Four star classic 150 double PV center includes MidNite Solar Classic 150 MPPT Charge Controller"

and:

"and two MMPT charge controllers".

He has multiple AC inputs on the Radian GS8048, for grid and generator, no need for a transfer switch to flip between them.

10% of C/20 is what the battery manufacturer recommends (39A), or a max of 25% of C/20 (97.5A).  The batteries will get more cycles out of them following the recommended amperage (but at the cost of more propane and generator wear if his oversized array can't keep up with the current draw).  But for griddown, if the generator does run at the lower amperage to charge the batteries at the recommend rate anyway, then it's also a good time to run the well pump, turn the frig/freezer up to "high", vacuum the house, make something in the microwave, let the women folk blowdry and curl their hair, watch a movie, etc   

If he configures the Radian setpoints correctly for the batteries it'll start the generator and charge the batteries automatically at whatever charging amps specified and shut it down when the charging it complete - there's no concern the batteries will drain too deeply (unless the genny fails to start and he didn't set the cutout voltage on the Radian correctly).  And contrary to one statement above, the inverter works if the grid is down.

Don't understand the grief everyone is giving tdry253 about the sizing of his system or suggesting he doesn't understand "the math".  If his plan is to sell back to the grid now and have a small battery backup (and generator) to run a few critical items if the grid goes down I think he made fine choices on his gear.  Seems to me that either some don't understand what he has or don't understand how what he has actually works.

Offline Carl

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #8 on: August 06, 2016, 06:28:48 PM »
BOTH the math and the description of the batteries etc have changed since the first post.

Offline Bad_apple

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #9 on: August 06, 2016, 08:20:57 PM »
My maths is based on the items listed and 16 years of certified experience in the industry. I quote by what's listed not what the website says may have been available in a package that he may or may not have. I offered my expertise and assistance and I stand by it. 8 6v 399ah batteries wired in series to get a 48v system is only 390ah no ifs ands or buts. You up in voltage you don't up in ah. That's how it works. And crown does not list a 12v 390ah battery so unless it's an unlisted 12v 390 ah battery then the bank is only 390ah.

C value means nothing. The batteries do NOT control how fast they are charged. The inverter charger and the solar controller controls the charging rate. Both are 3 stage chargers to maintain maximum efficiency.

At the end of the day the battery bank under a nominal load obviously relative  with what he has turned on will yield approx 4 hours before fully depleting the usable voltage and he only wants to deplete to 50% so that's about 2 hours. 

Not really up to debate. If my experience and advice is not wanted I assure you no hard feelings. I do this every day. Free advice was just a Courtesy.

Offline cidyl

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #10 on: August 06, 2016, 11:05:36 PM »
The "double PV center" he said he had in his first post, is a standard offering from different distributors and includes two MidNite Solar Classic 150 MPPT Charge Controllers:
http://www.wholesalesolar.com/1440982/four-star-solar/power-centers/four-star-solar-classic-150-double-pv-center-power-center
http://www.fourstarsolar.com/classic-150-double-pv-center/

He then said, again, that he had two controllers in his next post.  He doesn't need a third, nor a transfer switch to connect both grid AC and a generator with to his Radian GS8048.

C/20 was only referenced my me because it's what the battery manufacturer uses as the basis for the recommended/maximum charge rate for that line of batteries, I posted a link to it.

"At the end of the day the battery bank under a nominal load obviously relative  with what he has turned on will yield approx 4 hours before fully depleting the usable voltage".... sorry?  He didn't provide any details on what the expected aH/Wh draw was, what are you basing "approx 4 hours" on?

What's apparent from any casual reading of the OP's question is that tdry253, as 3 of us here have said or implied, is that he's mistaken when adding the amperage of each battery when they are connected in series.  He came here with a question, how long will it take to charge the bank from the generator?  His conundrum likely resulting from adding the amps of the batteries and it not making sense, which is reasonable enough.  What he got back was that his system was mis-sized, misconfigured, and/or that he required additional hardware.  But the answer to his question is simply: 5 hours at the manufacturer's recommended rate or 2 at the maximum rate. 

His system is not misconfigured or mis-sized based on any information we have about its intended use.  If anyone thinks otherwise, let's see the worksheet/spreadsheet/math based on how he said he plans to use it and why, as designed, it's inadequate for that task.

Offline Unicorn Rainbow Sprinkles

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #11 on: August 07, 2016, 04:49:15 AM »
I might not be very smart with tghis subject, but it looks like the easy solution here is.
1. Run you place off the generator if the batteries get too low and have a trickle charger set up to recharge the batteries. 

This works in my brain, so it probably won't work in real life.

Offline Carl

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Re: Need HELP! 9.1kW solar system charging batteries with my generator.
« Reply #12 on: August 07, 2016, 06:09:34 AM »
CIDYLand BAD_APPLE,it was not your math that was in question,but the battery capacity and speed at which the original post/poster expected to charge the strangely undersized battery ,though now I also understand more his grid/sellback strategy. I don't claim to have expertise on this subject and suggested he seek out expert advise. I do know however that forcing too fast a charge on a battery will damage or severely shorten it's life.