Requesting assistance in off grid system design

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

mike90045 wrote: »

And the IOTA chargers are NOT power factor corrected, so you need 60% more generator to charge with @ max rate.

Yes indeed.
And just in case someone reading this doesn't know what that means, it works out something like this:

75 Amps @ 12 Volts = 900 Watts
BUT ...
With a power factor of 0.85 (for example) you actually need 1059 Watts of generator power to supply that 900 Watts of charging power: almost 18% more.

Cariboocoot · August 2011

Re: Requesting assistance in off grid system design

leftcoastadv wrote: »

There ya go throwing a new word at me.... Power Factor!

Most of these terms are explained in the Glossary here: http://forum.solar-electric.com/showthread.php?t=6136

If you find something that isn't there and needs defining, let us know.

leftcoastadv · June 2012

Re: Requesting assistance in off grid system design

Quick update and some more questions.

Of course my first plan was crap and so things have been re-evaluated.

To start with, I've gone with cheap (er) Johnson controls/Energizer Sam's club batteries. I figured while I'm learning more and more about making and storing my own electricity, I better use the cheaper batteries.
So, here is what I've done.... 6 - 6 volt batteries in series and parallel. 624 ah. 12 volt. 3/000 interconnection wires. (slight overkill, maybe)
45 amp 3 stage charger. 14.4 boost, 13.6 normal, 13.2 float. (I will upgrade in the near future and move this charger to my starting batteries)
1500/2500max watt MSW inverter
old 4kw Onan generator. (fuel hog but stone reliable)
trimetric 2025 (I love this thing)

I've been reading about using the hydrometer and since my batteries are only about 2 weeks old with 2 cycles, I'm gonna dip them to get a base line reading.

I did notice yesterday that not all of the batteries have the same level of water. Obviously the cells within the battery do but I have one battery that actually has a fair amount more water than the rest. Will that cause any issues? I'm thinking it will just bubble off eventually just like the others.

Now for the big question.... I currently have about $500 budget for the beginning of a solar setup. What would you recommend? I know that is not enough money to do things correctly and 100% but would it be worth it to say plan for the full setup but only buy the charge controller and maybe 1 panel. Then in the near future just add another panel, etc, etc.
What do you think?
For now I would be quite happy with a few amp output just to float the batteries.

As you can tell, I'm still quite green and have a ton to still learn but man these last two weeks of making cables, setting up the batteries, trimetric, etc has been a blast. I can see how this can be a bit of an addicting hobby. Or maybe the newness will wear off shortly and I will dread the hassle of making my own electricity.

I really appreciate the guidance and help. Ryan

Cariboocoot · June 2012

Re: Requesting assistance in off grid system design

I'd recommend you save up your money until you have enough to do it right the first time. Trying to expand a solar power system is an exercise in frustration, heartache, and wasted $. The same panels may not be available later, et cetera. $500 doesn't buy much. Even enough current to "just float the batteries" will eat up that budget (6 to 12 Amps means a couple of panels of the 140 Watt variety, and there goes all the money).

You've already got problems with three parallel battery strings; hard to keep current sharing even. You've also got problems with trying to charge a 624 Amp hour battery bank from a 45 Amp charger. That's going to take a while. You will be feeding that Onan a lot of gas to do a little work (charger is less than 1/4 the gen's output).

Sorry that's all bad news.

BB. · June 2012

Re: Requesting assistance in off grid system design

Make sure the plates are covered before charging/equalizing. Only fill after the battery bank is nearly fully charged. Between heat (causing electrolyte to expand) and gas bubbles under/inbetween plates, the water level will be higher at this point. Do not fill the cells to the very top--or you will run the risk of electrolyte being pushed out the caps (usually, at least 1/4 to 1/2 inch below "very full").

You may see slots on a tube--you fill to the base of the tube and the slots stay above the liquid level to let gases escape.

Regarding "14.4 boost, 13.6 normal, 13.2 float", check the specifications for your battery bank... It is not unusual for flooded cell batteries to have the normal charging voltage set to ~14.5 volts or even a bit higher and float somewhere between 13.2 to 13.6 volts.

If this was a "standby bank", you may get away with 13.8 to 14.2 volts of charging voltage (this is the common voltage for the vehicle alternator).

But for deep cycle banks that are cycled overnight/etc., you want the solar charging voltage somewhere in the 14.5 volt range--or it will take forever to recharge the bank (and you will lose lots of available charging current from your solar array as the controller tapers off on the charging current because of low charging voltage--and you only have a "few" hours a day to collect solar energy--unlike an AC plug which is 24x7 and can use the lower charging voltages and a "standby" battery bank).

Back to your loads--Mostly AC, or is 12 VDC going to be used too?

A large battery bank implies you will be needing heavy charging current and a larger inverter. With solar pv systems in general, and RV applications specifically, conservation is usually your first thing you address.

You have "limited room" for solar array and battery bank box. So, any loads you have should be small and of a "critical" nature (i.e., use stove for making coffee, use LED lighting, use a laptop computer, etc.).

If your power needs are "small", a smaller battery bank, inverter, and solar array makes sense.

However, if your loads are "large", then you are looking at a large bank (at 24 or 48 volt), larger inverter, solar array, etc... And possibly a lot of fuel use for the genset (you should be measuring your kWH per day from the generator and how much fuel it uses... You could be running 5kWH per gallon of fuel, or less than 1 kWH per gallon of fuel--all depends on genset fuel usage and your percent of loading--typically, you want to load a genset at somewhere around 50% or higher of rated output for good fuel efficiency, but many times, people will load at 10% of genset which uses almost the same gallons per hour of fuel consumption as 50% rated power does--for many AC generators).

So, before recommending additional hardware, what are your loads... I would say 500-1,000 WH per day is "small". 3.3 kWH per day is an energy efficient cabin with electric fridge, washer, well pump, home loads... And >3.3. kWH per day is a lot to expect from a typical RV installation.

-Bill

leftcoastadv · June 2012

Re: Requesting assistance in off grid system design

Thanks for the reply guys. Let me try to explain our situation a little more and answer some questions.

A year ago this project started off a bit different. Things have changed over the last year so here it is currently:

Our "rv" is a converted school bus. It really is just a mobile apartment.

The wife and I have a line of work that takes us all over the country at the drop of a hat. (We work in the disaster restoration field. Natural disasters, floods, fires, etc) So basically instead of staying in hotel rooms, we travel to our assigned location and live in our bus. Sometimes it is completely off grid and we have to generate our own power but most of the time we're on grid with a 15amp circuit. Pretty much a 40/60 split.

So with that being said, our travel routine when en route to an assignment is generator every other day and camp ground/grid power about every 4 days. Rinse repeat. Generator days only get to about 90% before we shut down. Grid tie days we are able to get 100%.

We have been running this way with just a 208ah house battery bank ( pair of golf cart batteries) and it has been working fine.

DC per 24hr

florescent lights : 400 wh
fans: 225 wh
water pump: 200 wh

AC per 24hr

tv/dvd : 250 wh
stereo: 400 wh
laptop: 200 wh
Inverter: 96 wh

Now we want to add our chest freezer and fridge.

11 LRA
1.5 amp running
They run about 5 minutes every hour.

Currently they are running off our new 624 ah bank via a 1500 watt/ 2500 surge MSW inverter while I'm still testing things. Pulling about 100 ah per day. Per trimetric, 40 amp surge at start up that lasts about 1 second and quickly falls to about 8-10 amps. This is per unit. The inverter can start up both fridge and freezer at the same time.

Hope that helps shed a little light on our situation.

BB.
Thanks for the help. The slots in the tube are exactly what my batteries have. That problem is solved.

As far as the generator goes, it was free. (value wise it might have bought us about 100 gallons of fuel) Secondly and most importantly, we get a fuel/relocation stipend while under assignment. So you can understand the incentive to make our power usage/recharging the most efficient as possible.
The onan uses 1/2 gallon per hour with the 45 amp charger load and the AC loads switched over. With it fully loaded it is right at 1 gallon per hour.
I'd be willing to bet a nice inverter style generator could use half that.

So with only a $500 solar budget, it sounds like we need to wait until it can all be done properly and in one move. Which is fine, we are just currently in a cash flow down turn. It is feast or famine in our industry and so we need to hold off a bit on spending a huge chunk at once. (hence the idea about slowly building the solar)

Instead of solar at this point, can you guys recommend some changes to help make the power usage and recharging more efficient? Would a PSW inverter help make things more efficient? Less standby power usage? It might help the fridge/freezer motors run cooler.
Bigger charger? 60-70 amp?
Obviously a $1200 honda generator would be an option and sure would save some fuel but I'm not 100% sure I want to go that route just yet. If I can get the solar sorted out then the generator becomes even less of a concern.

Thanks again for the help guys. Really do appreciate the opinions.

mike95490 · June 2012

Re: Requesting assistance in off grid system design

You could get a pair of 12V panels (300W) and a simple charge controller, and let the PV top off the batteries, after the genset is run in the AM.

The problem with a genset only solution, is the batteries seldom get fully charged, because you need to run the genset for 2 or 3 hours at very light load, after 3 or 4 hours of bulk charge.

And pure sine inverters are MUCH better for motors, they will run cooler, and longer life.

Requesting assistance in off grid system design

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