Question regarding size of gen and battery charge time

MaineOFFGRID · July 2011

I just got in my john Deere 60kw diesel gen shes brand new... cost me 12k with a 250gal base fuel tank.

So will get straight to the point. I have a 2000sqft off grid home and i dont want the battery banks to ever run out of juice so i bought this over sized massive diesel genny figuring that i could hook up 8-10 5000w charge controllers (or whatever the max size is for the best charge controller) my goal is to charge up the battery bank as quickly as possible.

what should i be bringing the battery bank down to before recharging? proper %?

and how big do you think my battery bank should be? on my electric bill im using around 1500kwhrs a month id like to be set for 2000kwhrs a month just to be on the safe side.

eventually i will add solar and or wind. ideas? questions? comments? please i need to hear what the experts have to say

Cariboocoot · July 2011

Re: Question regarding size of gen and battery charge time

I think you may have a few terms and numbers mixed up.

First of all, battery banks are sized according your Watt hour needs: Watt hours / system Voltage = Amp hours needed (roughly). Since you don't want to discharge most types of battery below 50%, you'd need to multiply that number by at least two.

Once you have a battery bank size, you want to recharge it with a potential peak current of 5% to 13% of that size. So a 450 Amp hour battery bank would be looking for 45 Amps peak current at charging Voltage.

Now, generators do not power charge controllers, they power chargers like these: http://www.solar-electric.com/bach2.html Charge controllers regulate the power from solar panels or wind or hydro turbines for recharging.

Your usage seems to be 50 kW hours per day. You should be examining that to see where you can get the power down. My old all-electric house used about half that. My off-grid cabin uses 1/20th of that. The key to successful off-grid design is to get the power use down as much as possible first, then come up with a plan to supply it.

BB. · July 2011

Re: Question regarding size of gen and battery charge time

Note, I am not a diesel/genset expert by any account--so below is my generic understanding of the issues around generators/battery charging systems. While I try to be accurate--please confirm with your supplier/manuals any of the assumptions.

A 60 kW/99 HP genset would (guessing) use somewhere around 3-8.5 gallons per hour.... Not a small amount of fuel (assuming ~15-60 kW load).

And for a diesel genset, you should plan on running around 40-60% minimum load to reduce coking/wet stack/be in the fuel efficient power band.

And, depending on charge controllers, they can run down to 80% efficiency and 0.6 power factor--Or:

0.8 * 0.6 = 0.48 derating of charger's output power vs Generator rating

Smaller generators are rated at Watt output and VA rating is same (i.e., 5kW and 5kVA output)... Larger genset may be rated at 60 kW and 75 kVA (random numbers) to indicate that the genset can supply power factor loads less than 1.0 at full rated power...

And, for battery banks, generic flooded cell battery banks can be charged around 5% to 13% cost effectively with solar panels... And probably up to 25% of 20 Hour Rate... However, it is possible to have thermal run-a-way when charging battery banks over ~13% (my SWAG) rate of charge--so you should have temperature monitoring on any bank that you plan to charge at high rates (as batteries heat up, their charging voltage goes down--charge controllers can get confused and continue charging instead of ramping down charging current--causing thermal run-a-way).

So... Based on the above, we can make some guesstimates about the size of your battery bank to "fit" your genset performance requirements.

Assuming that we want to load your genset to 50% of rated load (remembering that, at worst, the maximum battery charger you could put on your genset is ~48% of genest kW rating). And assuming a 48 volt battery bank with 59 volt charge setting:

60,000 watts * 0.50 max charger output derating * 1/59 volt charging = 508 Amp battery charger

Assuming a 500 Amp battery charger and 5%, 10%, 13%, 25% rates of charge (with 10-13% being a good selection):

500 amp * 1/0.25 rate of charge = 2,000 AH @ 48 volts (minimum battery rating)
500 amp * 1/0.13 rate of charge = 3,846 AH @ 48 volts
500 amp * 1/0.10 rate of charge = 5,000 AH @ 48 volts
500 amp * 1/0.05 rate of charge = 10,000 AH @ 48 volts (maximum battery rating)

So--the "optimum" battery bank for your genset with "generic" AC battery chargers (non-power factor corrected or PF~0.6) would be around 2k-10kAH at 48 volts

Now, lets look at your battery bank sizing for your home's needs... We generally recommend 1-3 days of "no-sun" and 50% maximum discharge (for longer battery life)...

A 2,000 kWH per month at 2 days and 50% maximum discharge works out to be:

2,000kWH per month / 30 days per month = 66.7 kWH per day
66,700 WH per day * 1/48 volt battery bank * 2 days * 1/0.50 maximum discharge = 5,558 AH @ 48 volts

So, overall, it would appear that a ~5,000-6,000 AH @ 48 volt battery bank would be a nice fit as you currently describe your needs/existing equipment.

6x of these batteries:

Crown Industrial Battery - 24 Volts, 1720 Amp-hours Price: $6,528.00

Would give you 5,160 AH @ 48 volts for ~$39,000...

There are still lots of details to discuss--but I hope this gives you some basic sizing information. (note: Moderators here are volunteers and not connected in anyway with NAWS or any other solar vendors--These are not quotes--just handy links from our hosts).

-Bill

Cariboocoot · July 2011

Re: Question regarding size of gen and battery charge time

Short version of what Bill said; your generator is huge.

Even if you do use 50 kW hours per day that does not mean you need a 60 kW generator. The difference between a kW and a kW hour. Basically that gen could supply your needs of 24 hours in one hour.

In other words, based on the numbers given, it's about 20 times the size it needs to be.

RCinFLA · July 2011

Re: Question regarding size of gen and battery charge time

15kW to 20kW generator would have been about the right size.

Minimum lead-acid recharge time is about 4.5 hours from 20% SOC. Would be less for less discharge on battery but time it will still be about 2.5 -3 hours even if only discharged 50% of rated capacity.

Since you have money to burn you could go with something like lithium iron phosphate batteries that can be recharged in less then 2 hours at 0.5C recharge current. These are the batteries used for electric vehicle for high discharge and recharging rates.

MaineOFFGRID · July 2011

Re: Question regarding size of gen and battery charge time

BB. wrote: »

So, overall, it would appear that a ~5,000-6,000 AH @ 48 volt battery bank would be a nice fit as you current describe your needs/existing equipment.

6x of these batteries:
Crown Industrial Battery - 24 Volts, 1720 Amp-hours Price: $6,528.00

Would give you 5,160 AH @ 48 volts for ~$39,000...

There are still lots of details to discuss--but I hope this gives you some basic sizing information. (note: Moderators here are volunteers and not connected in anyway with NAWS or any other solar vendors--These are not quotes--just handy links from our hosts).

-Bill

Thanks Bill pretty much everything i needed right there.

So if i get 6x of those batteries and run them down to 50% load how long would i have to run my gen to power all them back up to 100%? heres the genset by the way... http://www.generatorsales.com/order/JD1PH60.asp?page=JD1PH60

her maximum continuous load is 500amps @ 120v so i believe i would take 5160 and divide that by 500 which is roughly 10hrs but that would be from 0%-100% charge so roughly 5hrs if i ran them down to 50% charge correct?

Cariboocoot · July 2011

Re: Question regarding size of gen and battery charge time

MaineOFFGRID wrote: »

Thanks Bill pretty much everything i needed right there.

So if i get 6x of those batteries and run them down to 50% load how long would i have to run my gen to power all them back up to 100%? heres the genset by the way... http://www.generatorsales.com/order/JD1PH60.asp?page=JD1PH60

her maximum continuous load is 500amps @ 120v so i believe i would take 5160 and divide that by 500 which is roughly 10hrs but that would be from 0%-100% charge so roughly 5hrs if i ran them down to 50% charge correct?

Voltage difference. Charger efficiency. The two missing factors.
Bill's proposed 6k Amp 48 Volt battery bank is 144 kW worth of power @ 50% DOD. (That is so mongo-huge it defies all off-grid logic.)
Charge rate of about 600 Amps @ 60 Volts = 36 kW of power. How you're going to come up with a charging system that supplies 600 Amps @ 60 VDC from your 120 VAC output I don't know, but depending how you do it's bound to have a power factor that will increase that 36 kW need.
As for time to replace used Amp hours, that varies with the Peukart effect of the particular batteries used, but in general the "first" Amp hours get put back fairly quickly; the last 20% or so takes longer. I'd suggest getting some manufacturer's charging info specific to the batteries you intend to use. Very important for a mammoth bank like that.

MaineOFFGRID · July 2011

Re: Question regarding size of gen and battery charge time

Cariboocoot wrote: »

Voltage difference. Charger efficiency. The two missing factors.
Bill's proposed 6k Amp 48 Volt battery bank is 144 kW worth of power @ 50% DOD. (That is so mongo-huge it defies all off-grid logic.)
Charge rate of about 600 Amps @ 60 Volts = 36 kW of power. How you're going to come up with a charging system that supplies 600 Amps @ 60 VDC from your 120 VAC output I don't know, but depending how you do it's bound to have a power factor that will increase that 36 kW need.
As for time to replace used Amp hours, that varies with the Peukart effect of the particular batteries used, but in general the "first" Amp hours get put back fairly quickly; the last 20% or so takes longer. I'd suggest getting some manufacturer's charging info specific to the batteries you intend to use. Very important for a mammoth bank like that.

very true my genset is single phase 120/240... i wonder if i can even bring her to 48v

Cariboocoot · July 2011

Re: Question regarding size of gen and battery charge time

MaineOFFGRID wrote: »

very true my genset is single phase 120/240... i wonder if i can even bring her to 48v

Your gen set is also Alternating Current, not Direct Current.
Usually this is used to power loads while charging and the charging system.
Typically on a very large system you'd use an inverter/charger - a DC to AC inverter that has a built-in battery charger. Such as Magnum, Outback, or Xantrex (to name the better-known brands).
But those chargers sure don't do 600 Amps to my knowledge. The XW6048 has a charger rating of 100 Amps (0.98 PF): http://lib.store.yahoo.net/lib/wind-sun/XW-Inv.pdf

BB. · July 2011

Re: Question regarding size of gen and battery charge time

But, yes, roughly AHout / Amps in = time on charge... And add about 2-3 hours for the last ~10-15% charge (Charger is at "absorb voltage of ~58-59 volts, and current slowly tapers down from Imax-charge to ~1%xBattery-AH Rating...

You do not need to charge to 100% every day--Charging to >~90% state of charge several times a week should be fine. That will cut down on your genset fuel/time.

Also, if you have a 5% or larger solar array--You can "Bulk" charge to ~85%+ charge on the genset in the early AM, and let the solar bring the bank back up the rest of the way when the sun is up...

-Bill

Cariboocoot · July 2011

Re: Question regarding size of gen and battery charge time

I'll just say that this is a pretty "sideways" way of designing a system.
Normally you determine how much power you need per day: maximum Watts & total Watt hours.
Then you pick an inverter and battery bank size based on that.
Then you figure out what you need to recharge the batteries (panels & controller, back-up gen).

Starting out by buying a whonking big generator is a bit ... unusual. :roll:

wild01 · July 2011

Re: Question regarding size of gen and battery charge time

So in case you missed it, we are all telling you not to get this generator.
if you insist on going with a generator this size then I highly recommend a 480v 3 phase model
use 3 1000amp hour plus 48v forklift batteries and 3 240amp industrial battery chargers. this will allow you to run the gen at about 50% capacity (which is usually where they are most efficient)
this option will run you about 15to 20k for batteries and 4-8k for chargers. charge the batteries at 50% dod and they should last quite a long time. again I WOULD NOT RECOMMEND THIS ROUTE. but you asked and I answered.

(if you want to use your generator to full capacity then you would really need 5 batteries and 5 240 amp chargers. ) oh and you will really need to pick up a forklift to move the batteries around with. above prices do not include any kind of install or wiring

techntrek · July 2011

Re: Question regarding size of gen and battery charge time

And in case you missed wild01's first comment:

Your genset is multiple sizes too big.

Then to touch on something Bill said:

You will waste so much fuel each day that doesn't give you any extra killowatts you can use, you could power a properly sized genset for a week. Again, that's just in the fuel you aren't using for any actual purpose.

Then to go back to another comment Bill made which is more important than you realise:

Diesel gensets, unlike LPG, NG or gasoline gensets, must be run at a minimum load to prevent something called "wet stacking". Usually this is in the 40%-60% range but it varies by model. This is great for commercial loads which are usually steady. But most homes need less than a thousand watts each hour over 50% of the time when you aren't there or are asleep. End result - your genset will self-destruct very quickly unless you load it up. Even running your house while charging a battery bank its unlikely you'll load it up enough to prevent this.

Summary: send it back and get a smaller one. You will save yourself thousands in the purchase cost. You will save yourself thousands more each year in fuel. Then you will save yourself the cost of a whole new genset in a year or three.

Cariboocoot · July 2011

Re: Question regarding size of gen and battery charge time

Guys;

He's already bought the big capacity gen - needed it for other purposes and now is trying to include it in a solar plan. I think he's satisfied it's sufficient for both applications - and too large for one. We'll see where he wants to go from here after he figures out how much "regular power" he needs.

"No wonder my PM inbox is always full." :roll:

Blackcherry04 · July 2011

Re: Question regarding size of gen and battery charge time

techntrek wrote: »

Summary: send it back. You will save yourself thousands in the purchase cost. You will save yourself thousands more each year in fuel. Then you will save yourself the value of a whole new genset in a year or three.

I sure wish someone had given me this advice when I bought a 20 kw ( 83 amps / 240 v ). It was too late when I figured out how much overkill it was. I will agree that having too little reserve can be a issue with high amp loads. Most homes will do great with a 12-15 KW and run anything you want. A little load shifting and charging during off peak times will help on usage.

mike95490 · July 2011

Re: Question regarding size of gen and battery charge time

Blackcherry04 wrote: »

I sure wish someone had given me this advice when I bought a 20 kw ( 83 amps / 240 v ). It was too late when I figured out how much overkill it was. I will agree that having too little reserve can be a issue with high amp loads. Most homes will do great with a 12-15 KW and run anything you want. A little load shifting and charging during off peak times will help on usage.

Most homes would do fine with 5KW, in an emergency. A 12KW genset just takes a little longer to wet stack and die, when it's only loaded with 3KW of load.

Vic · July 2011

Re: Question regarding size of gen and battery charge time

I must have missed Coot's reference to the fact that this HUGE genset was purchased to do some specific large other task, and now is a bit of a push-fit for battery charging ... perhaps there was some PM traffic, or mabe I am just too blind to see that reference.

Would certainly agree, that if this genset could be traded-in for something more appropriate, it would be great. Seems that this gen is not Tier four, and therefore not quite the correct one for Stationary applications. This is a bit of an additional nit. As othrs have also noted, a generator in this size range would normally be three-phase. Perhaps it is re-strappable to three phase, as many of the larger alternators are easily configurable.

I DO like large machines, altho, would like to run them on someone else's charge card! JMHO, Vic

BB. · July 2011

Re: Question regarding size of gen and battery charge time

And it comes down to the cost of fuel too... Looking at $4+ per gallon in the near future--Large gensets really suck down the fuel (even a 10kW unit will take 0.5 to near 2 gallons per hour).

If you can get down to a 1,600 watt Honda eu2000i, you are no in the range where 1.1 gallons of fuel will run it 4-10 hours (1,600 watt to 400 watt loads).

-Bill

techntrek · July 2011

Re: Question regarding size of gen and battery charge time

Blackcherry04 wrote: »

I sure wish someone had given me this advice when I bought a 20 kw ( 83 amps / 240 v ). It was too late when I figured out how much overkill it was. I will agree that having too little reserve can be a issue with high amp loads. Most homes will do great with a 12-15 KW and run anything you want. A little load shifting and charging during off peak times will help on usage.

I wish I had the same advice when I purchased my 12 kw unit. With all the conservation changes I've made since and future conservation projects still in the works I could have bought something half as big. With all the same benefits as before.

Blackcherry04 · July 2011

Re: Question regarding size of gen and battery charge time

Buying a stationary Generator is like a easter egg hunt. Some manufacturers use the same propulsion engine on a 8kw,10kw,12kw and sometimes a 15 kw bored out.

Example : Onan uses the same parts list on a 3.5 kw and 6 kw. The only difference is a jet in the carburetor and the circuit breaker.

The Fact that a generator has a John Deere industrial engine means little. Who makes the generator end ?? About the only thing a Manufacturer does is maybe make a manifold and motor mounts and add a radiator and a control unit they buy and bolt up the Generator end.

I'v been looking at a 5.5kw and 7.6 kw, both use the same engine with identical specifications. The 7.6 uses a AVR and a Mecca-Alt generator end that I consider much superior to the capacitor type regulation. It uses .33 gallon more per hour at full load, over time that could be a deal breaker.

The question would be, if coking and wet stacking occurs on a Generator loaded less than 50 %? Would it occur on the same Engine rated with more KW or less KW on the same engine ?? They are both turning 1800 rpm, the one pulling more power would have the governor giving it more fuel.

techntrek · July 2011

Re: Question regarding size of gen and battery charge time

That's why I said "Usually this is in the 40%-60% range but it varies by model." In the case you describe, maybe they would specify 3.5 kw as a miniumum load for both. Which works out to 63% and 46% loads for your 5.5 kw and 7.6 kw models.

Vic · July 2011

Re: Question regarding size of gen and battery charge time

I completely agree with Blackcherry's comments regarding the hunt for quality gensets at a low price.

Many, many of the "integrators" are looking to meet a price-point. The lower the selling price, the lower the cost of components used. A brand-name engine does not make a brand name genset. The lower cost units just cannot use cheaper patrs.

Sometimes you get what you pay for, but you seldom get quality that you do NOT pay for. It does happen, but not on a regular basis, IMHO.

Regarding wet-stacking and coking, I am not certain the extent that this applies modern Diesel gensets. The rental diesel generators regularly go 10 K or more hours before overhaul, and have seen many of these sets on construction sites, very lightly loaded for many hours per day, but are there for some heavier, occasional intermittant loads. Perhaps I am deluding myself. My larger diesel genset gets very occasional use for battery charging. Am adding a 50 gal electric water heater to add some load for at least part of the run cycle of the genset to get it up to about 75% for the middle or toward the end of the run cycle, with a cool-down at the end.

My Kubota genset is a bit too large, but it is well made and quiet, and have discovered A/C, and do have some larger three phase loads coming on-line, which will help provide heavier loads. Hope thet the OP finds a good resolution. Vic

henry1 · July 2011

Re: Question regarding size of gen and battery charge time

I was looking at the Efoy Menthol fuel cartridge system gen set up for rechargeing the small battie bank i put together for the cabin ..Plus the unit can be used dureing the winter time plus it small and compact unit and can it be used on the well vented indoor area without alot of problems as long as you follow the safety rules when you use the unit ..

I was looking at the months dureing the winter time when the sun was not shining enough to make power with the solar panels and wiind and i started looking at small gen set to use with the wind and solar chargeing system to add to the triangle of power makeing

My power needs at the cabin are never go over 3500.watt's a day at the max time and that with the use of Rv washer & dryer unit once a week of 4 hours of doing laundry on a sat morning ..The rest of time is the load on the battie bank is not taxed alot of the time with the Appliance's beening used are the low wattage draw from the battery

Question regarding size of gen and battery charge time

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