Sizing Battery Bank

Arkansasoffgrid

Looks like my loads are around 7.362 kwh daily. Was able to use KAW meter for everything except mini-split ac which hasn't been purchased yet. Data plate on mini split states 500w on low, so I allowed 8 hrs= 4kwh. To get battery size do I use 7362w/48v= 153.3 ah @ 100% dod, and 306ah @ 50%dod ?

vtmaps

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

Looks like my loads are around 7.362 kwh daily. Was able to use KAW meter for everything except mini-split ac which hasn't been purchased yet. Data plate on mini split states 500w on low, so I allowed 8 hrs= 4kwh. To get battery size do I use 7362w/48v= 153.3 ah @ 100% dod, and 306ah @ 50%dod ?

Usually we aim for daily use to be 25% of battery capacity, thus 612 ah at 48 volts. --vtMaps

Cariboocoot

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

Looks like my loads are around 7.362 kwh daily. Was able to use KAW meter for everything except mini-split ac which hasn't been purchased yet. Data plate on mini split states 500w on low, so I allowed 8 hrs= 4kwh. To get battery size do I use 7362w/48v= 153.3 ah @ 100% dod, and 306ah @ 50%dod ?

What vtMaps said about the DOD.
You cannot go to 100% discharge because ... well you won't come back.
50% DOD is pushing things for battery life and increases recharging needs.
25% average DOD works best over-all.

So the basic math is right: 7362 Watt hours / 48 Volts = 153.375 Amp hours used.
But this does not include the conversion efficiency of the inverter or the inverter's consumption.
Thus we take the 7362 Watt hours AC and factor the efficiency (90% say) and get 8180 Watt hours DC. If the inverter draws 20 Watts and is on 24 hours that's another 480 Watt hours DC that must be supplied.

Total: 8660 Watt hours DC / 48 Volts = 180 Amp hours used. Multiply by four and you get a minimum battery bank size of 720 Amp hours. Round up to the nearest available battery configuration, then calculate the PV array size from there.

Arkansasoffgrid

Re: Sizing Battery Bank

The previous loads were "worst case" like washing/drying a load of clothes daily. So if I use a 4k array and 600ah batt bank, the math would RESEMBLE this, 4000 x .77 = 3080w / 48v = 64 charging amps. 64 x 100 = 6400 / 600ah = 10.6 charge rate. Is this in the "target range"? For reference the array is sitting on side of a south facing overlook in Hot Springs AR that gets sun with absolutely no shading/interference. If these #s are in the ballpark, what brands inverter, charge controller, & panels would you purchase for your full time residence? Do have a new Honda eu20000i, and a like new TroyBuilt 6500 running & 9k surge generator, so I'd like something with "gen assist". Looking at the MagnaSine 4448pae, but need input from the knowledge of this forum. Thanks in advance

Cariboocoot

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

The previous loads were "worst case" like washing/drying a load of clothes daily. So if I use a 4k array and 600ah batt bank, the math would RESEMBLE this, 4000 x .77 = 3080w / 48v = 64 charging amps. 64 x 100 = 6400 / 600ah = 10.6 charge rate. Is this in the "target range"? For reference the array is sitting on side of a south facing overlook in Hot Springs AR that gets sun with absolutely no shading/interference. If these #s are in the ballpark, what brands inverter, charge controller, & panels would you purchase for your full time residence? Do have a new Honda eu20000i, and a like new TroyBuilt 6500 running & 9k surge generator, so I'd like something with "gen assist". Looking at the MagnaSine 4448pae, but need input from the knowledge of this forum. Thanks in advance

Yes, that math is correct and 10.6% maximum charge rate is indeed a good target.

I can tell you that at that rate if you want to make full use of the array you would need an 80 Amp MPPT controller and that means either the Outback FM80 or the MidNite Classic. I'm not aware of any others that can handle >60 Amps (except the very pricey 600 Volt XW unit).

Panels: any 'name brand' is going to be about the same quality, as they are basically all made of the same stuff in the same way. It comes down to price per Watt, mostly.

The MagnaSine is certainly a good inverter. If you need 4kW of power it's probably the best choice for the job.

Others will have their own opinions.

Arkansasoffgrid

Re: Sizing Battery Bank

Thanks, finally feel like I'm getting somewhere. Last ?? for tonight, advantages & disadvantages of roof vs ground mounted array? If building a ground mounted rack, I'm skilled enough to fabricate/design my rack. If roof mount I'd rather not be welding together a rack on my new roof so that means buying a aluminum rack system. What other things am I overlooking?

Cariboocoot

Re: Sizing Battery Bank

Up on the roof: out of the way, no extra real estate used up. Also harder to get at so safer from theft and damage but not so easy to build or work on. You have to be sure the roof can take the weight and the fasteners aren't going to start leaks. Needs to be removed when re-roofing.

Down on the ground: easy to build it, no ladders to climb, no roof leaks to worry about. But easier to get damaged or stolen, may need fencing around it to meet local regs, takes up space where your putting green was going to go.

Arkansasoffgrid

Re: Sizing Battery Bank

Anyone heard of this battery http://www.powerstridebattery.com/us-l16hc-xc-6-volt-420-ah-solar-storage-battery ?

Cariboocoot

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

Anyone heard of this battery http://www.powerstridebattery.com/us-l16hc-xc-6-volt-420-ah-solar-storage-battery ?

"US L16" - possibly made by US Battery (East Penn/Deka).
Be a bit wary as it sounds like an industrial battery which is not always suited for RE use due to expect high charge Voltages/currents/times. See if you can get the recommended charging specs before buying.

Arkansasoffgrid

Re: Sizing Battery Bank

Just spoke to engineer at US battery, their recommendations are 58.8v bulk/absorb, 52v float, and 61.2v equalize 1 to 3 hours.

Cariboocoot

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

Just spoke to engineer at US battery, their recommendations are 58.8v bulk/absorb, 52v float, and 61.2v equalize 1 to 3 hours.

Nothing wrong with that.
Some maker's batteries a 48 Volt system would be Absorbing at 60 Volts, for example.

Arkansasoffgrid

Re: Sizing Battery Bank

Just for my education, what would some unacceptable numbers have looked like?

Cariboocoot

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

Just for my education, what would some unacceptable numbers have looked like?

When batteries have to charge at higher Voltages you have to get them to that Voltage and hold them there until Absorb is complete. This presents problems such as having sufficient maximum Voltage available from the array to begin with (Vmp - temperature Voltage loss - wiring Voltage loss), and being able to achieve that charging regime in the time limits of solar (unlike with plug-in charging there is a finite amount of time available and the power during that time rises and then falls again).

So if you need 15 or 30 or 60 Volts or more to the batteries it can be hard to achieve the charging using solar with its inherent losses and limited time frame. What's more some of the less expensive charge controllers don't have the ability to adjust to high enough Voltages to begin with (such as certain PWM controllers which have a choice of only "GEL-AGM-FLA").

If you were to try an use some of these higher-Voltage batteries on a simple 12 Volt system you'd run in to all these problems, and also an inverter screaming and shutting down from "over Voltage" when it goes over 15 Volts.

Anything requiring a multiple of 14.8 Volts or less for Absorb charge is usually okay.

vtmaps

Re: Sizing Battery Bank

Cariboocoot wrote: »

If you were to try an use some of these higher-Voltage batteries on a simple 12 Volt system you'd run in to all these problems, and also an inverter screaming and shutting down from "over Voltage" when it goes over 15 Volts.

Also, I hope you have a remote battery temp sensor for your controller and your inverter/charger. What temperature will your batteries be in the winter? If they are cold, your charger and controller will raise their voltages to compensate. When you set your absorb to 58.8 volts, you may see an actual voltage of 61 or more if the batteries are cold. This 'over voltage' may upset your inverter, but it is necessary to have the voltage up that high if the batteries are cold.

--vtMaps

Arkansasoffgrid

Re: Sizing Battery Bank

Battery box will be inside, so it should remain between 60 and 80°F. What's meant by "anything requiring a multiple of 14.8 volts or list for absorb should be okay"? What formula does 14.8 V reference?

Cariboocoot

Re: Sizing Battery Bank

Arkansasoffgrid wrote: »

Battery box will be inside, so it should remain between 60 and 80°F. What's meant by "anything requiring a multiple of 14.8 volts or list for absorb should be okay"? What formula does 14.8 V reference?

On a 12 Volt system the maximum Absorb Voltage for most flooded batteries is 14.8 Volts. A few require higher than that, and those are the ones that can cause trouble. So 14.8, 29.6, and 59.2 would be normal "upper limits" for Absorb setting.

As vtMaps said cold temperature compensation causes the charge Voltage to go up, so the higher it starts out at the higher it goes. Whereas this is mainly a problem on 12 Volt system (where the inverters tend to have maximum Voltage of 15 and minimum Voltage of 10.5 - both too low) it can also be trouble for higher Voltage system. Somewhere on the forum there is at least one account of someone who bought industrial batteries for a 48 Volt system and did not get the charge specs right; he kept having to crank the V up and run them longer and longer to save the batteries. Not a happy camper he.