Panels and Battery Bank Draw and Refill..

ywhic

Well I ordered 3 of the 145 w (12v) (7.75 & 17.XX) panels and hopefully tommorrow I will be in the position to order 3 more for a total of 870 watts of PV panels.. which should provide 452.4 watts (@ 52% eff.) to 669.9 watts (@ 77% eff.).. (and yes I know the numbers will vary with my wiring).. I also found out my location gets 6-7.2 hours of sun from March - August..

If I were to run my Air Conditioner (550 watts) only at peak times (aka the sun is out and hitting the panels for max volts and amps) for 3-4 hours on a 330 AH battery bank..

I should only draw either 98 watts per hour (aka 9.8 AH) (@ 52% eff) or 0 watts (0 AH) (@ 77% eff) off/out of the battery bank for each hour I have the A/C on.. (based on the above panel effeciancy wattage numbers).

Also draining 30-40 AH (3-4 hours * 9.8AH draw above for the A/C) from a 330 AH bank to run the A/C doesn't sound like much draw.. and pumping 45-67 AH back in with the panels in the last 1-2 hours of sun sounds like a stable plan..

Am I on the right track with this logic??

I'm also thinking without any load (aka no A/C running) my small 330ah battery bank will be in FLOAT unless I have something running, or turn off some of the panels..

45.2 amps or 67 amps into the bank at peak times sounds way MORE than the 33 amps I would need for the 10% charging rate to replenish the bank.. am I correct on these basic numbers??

Suggestions or tips appreciated..

Cariboocoot

Re: Panels and Battery Bank Draw and Refill..

I think you're getting lost in the numbers.
The 77% efficiency we talk about is panel & controller average.
The 52% efficiency is for over-all system operation (name plate rating of the panels to AC Watt hours out the door).

So 870 Watts of panel would produce about 670 Watts DC; important for determining charge rate with an MPPT controller.
In 6 hours of equivalent good sun it would produce 2.7 kW hours AC, providing you can make use of it.

Your battery bank at 330 Amp hours @ 12 VDC can supply up to 1980 Watt hours DC, about 1600-1700 Watt hours AC.

Your A/C unit uses 550 Watts. From the battery alone that would be up to 3 hours total run time (at full power consumption).
From the panels only it would be perhaps as much as 4 hours; a lot depends on how depleted the batteries are when charging begins and thus how much panel power has to go to recharging before the A/C can be used "for free". This is not too far off from what you expect.

The difference between the A/C's 550 Watts when converted to DC consumption and the panels' output ability is not very much. At best it would be 612 Watts (550 @ 90% conversion efficiency) from 670 Watts (870 Watts @ 77% efficiency) or a meager 58 Watts to spare. But don't panic:

A couple of things: first, the A/C is not going to use exactly 550 Watts all the time it is running. The start surge will be much higher, and some times the compressor will not be running. Second, if the batteries aren't drawn down over night they may not need much charging in the morning.

Usually, as a guide to keep it straight for discussion purposes, it helps if you think of Amps only when dealing with the DC side, and Watts on the AC side. Although both exist in either realm, it's easier to consider it as "Amp hours" (as in the battery rating) DC and "Watt hours" (as in what the utility bills you for) AC.
In between the available DC Amp hours @ Voltage and the AC Watt hours is the inverter's own consumption and its conversion efficiency.

ywhic

Re: Panels and Battery Bank Draw and Refill..

Cariboocoot wrote: »

Your battery bank at 330 Amp hours @ 12 VDC can supply up to 1980 Watt hours DC, about 1600-1700 Watt hours AC.

Your A/C unit uses 550 Watts. From the battery alone that would be up to 3 hours total run time (at full power consumption).
From the panels only it would be perhaps as much as 4 hours; a lot depends on how depleted the batteries are when charging begins and thus how much panel power has to go to recharging before the A/C can be used "for free". This is not too far off from what you expect.

The difference between the A/C's 550 Watts when converted to DC consumption and the panels' output ability is not very much. At best it would be 612 Watts (550 @ 90% conversion efficiency) from 670 Watts (870 Watts @ 77% efficiency) or a meager 58 Watts to spare. But don't panic:

So if the batteries are full and ready to go at 12pm (noon) and the panels are maxed out (for bringing in the watts) can I expect to make the A/C run about 3-4 hours??

That 58 watts extra.. does that mean I can produce enough juice to run the A/C and not draw much from the battery bank and pump that 58 watts worth back into the bank??

And that 1980 Watt hours DC ^^^ , is that like a 50%, 60% DOD number?? or is it "I've murdered my battery down to 20%" ??

I only plan on running the A/C with full sun and full battery juice and try to use the incoming power to offset the draw..

More than likely I will go to 440 ah bank as well..

Now for the OFFTIME (aka little or no draw from the bank).. how would you suggest I handle that??

ETA:
I re-read your post.. and I think your saying with full batteries and panels bringing in the max.. I will have 58 watts +/- that will be unused and going into the bank.. 612 used by the A/C and 670 coming in from the panels.. is this correct for my overall usage??

Cariboocoot

Re: Panels and Battery Bank Draw and Refill..

ywhic wrote: »

So if the batteries are full and ready to go at 12pm (noon) and the panels are maxed out (for bringing in the watts) can I expect to make the A/C run about 3-4 hours??

Yes, so long as you still have full sun on the panels.

That 58 watts extra.. does that mean I can produce enough juice to run the A/C and not draw much from the battery bank and pump that 58 watts worth back into the bank??

Pretty much. It's the difference between what you could expect the A/C to draw and what the panels can produce. Keep in mind it does not include what the inverter uses, which may be 20 Watts all on its own.

And that 1980 Watt hours DC ^^^ , is that like a 50%, 60% DOD number?? or is it "I've murdered my battery down to 20%" ??

Rest assured I'd never give numbers for "and then you throw the batteries away" scenarios. That maximum DC Watt hours is for 50% DOD, which is as low as you'd ever want to go.

I only plan on running the A/C with full sun and full battery juice and try to use the incoming power to offset the draw..

Good plan. I only turn the water pumps on when I've hit Absorb or Float and have power to spare.

More than likely I will go to 440 ah bank as well..

Also a good plan. That 870 Watts of panels could easily recharge that much.

Now for the OFFTIME (aka little or no draw from the bank).. how would you suggest I handle that??

How do you want to handle it? Off is off; no loss, no gain. If you're talking about nighttime use you may want to shut the inverter off completely.

Eric L

Re: Panels and Battery Bank Draw and Refill..

So if the batteries are full and ready to go at 12pm (noon) and the panels are maxed out (for bringing in the watts) can I expect to make the A/C run about 3-4 hours??

In principle yes; possibly longer since the end part of the absorption charge to bring the batteries up to full charge uses very little (allowing you to start the A/C a bit sooner). You won't really know for sure until you try it.

That 58 watts extra.. does that mean I can produce enough juice to run the A/C and not draw much from the battery bank and pump that 58 watts worth back into the bank??

As someone who also runs opportunity loads 'near the edge' of the system's panel capacity, as you're proposing doing, I can say that you can't really count on the 58 watts, since there will be so many variables that change output day-to-day. High haze that you can barely see, or high heat can quickly chop off power output from the panels, for example.

And that 1980 Watt hours DC ^^^ , is that like a 50%, 60% DOD number?? or is it "I've murdered my battery down to 20%" ??

330 amp hours x 12 volts = 3960 watt hours. Divide by 2 for the 1980 watt hour figure at 50% D.O.D. But here too, so many things like heat and rate of draw will change what's actually available; you won't really know for sure until you're out there in the sun. :cool:

I only plan on running the A/C with full sun and full battery juice and try to use the incoming power to offset the draw..

A couple of things to keep in mind. Even a passing cloud will stress a relatively small battery bank like this if a 550 watt A/C unit is going, and could run it flat quickly. You'll have to shut off the A/C if it clouds up. I bet your compressor will be on much/most of the time.

Also, your equipment (charge controller and inverter) will be running pretty hot. Have you thought about equipment cooling? I suggest a pack of these at a minimum. They have a tiny draw and you can wire them into the DC side of a 12 volt system. Some for the batteries, one on the inverter, one on the cc, etc.

Also draining 30-40 AH (3-4 hours * 9.8AH draw above for the A/C) from a 330 AH bank to run the A/C doesn't sound like much draw.. and pumping 45-67 AH back in with the panels in the last 1-2 hours of sun sounds like a stable plan..

Here's a problem though; if the controller goes into float before you add the load, it won't necessarily fully recharge the batteries after you shut off the A/C/; it will just lift them to the float voltage. Some controllers let you 'force bulk' a second time in the day, which restarts the cycle. But remember, you're cycling your batteries more doing this.

or turn off some of the panels..

One of the great things about solar is that they don't produce if there's no load. So there is nothing to turn off and no need for a diversion load if you have too much potential power.

ywhic

Re: Panels and Battery Bank Draw and Refill..

Thanks Cariboocoot...

Though my numbers were skewed and I confused Amps, AH, & Watts abit I was still somewhat correct on usage..

I can run my A/C with using mostly the panel wattage coming in.. or possibly have a few watts to spare to still load the battery bank during the usage hours..

I would think by 1pm (when its get unbearably hot) I will be in full sun and should be GTG to run the A/C 3-4 hours and not kill my batteries.. of course for more than 1-2 days in a row I would definatley need a bigger bank as I may not be able to recover any negatives by that same 1pm deadline..

At least I'm on the proper 'path'.

Eric.. I have previously thought about those small fans for the battery box and the controller area.. there convienent as there 12v..

The controller is the MorningStar TS45 (PWM).. and in the book I think it does say something about reverting to absorb from float if needed though I am not 100% on this...

I know the real test will be when I get 'out there'... just want to try and have some 'ducks in a row' before that day..