Charging Large Battery Bank

HadesOmega · April 2009

I am an Aviation student from San Jose State University. I am making a remote airfield lighting model and was going to use solar panels to charge the battery bank in the daytime.

I was wondering how I can find out how many amps I would need to charge my battery bank for 12 hours with a charge regulator. The battery bank is in parallel for a total of 1575 AH. I want to charge the battery bank within 12 hours (assuming its discharged, probably not so realistic for solar panels) so would this mean that I would need to charge the battery at 131.25 amps? Would the battery get really hot charging at that current?

I've been reading that the batteries should be charged at 5%-13% of their capacity. So this would be 78.75A-204.25A respectively. So my number fall in between I guess.

Here is some more of my calculations:
12V X 1575 AH = 18900 watt-hours

Charge in 12 hours: 18900 WH/12 HOURS = 1575 WATTS

1575 WATTS/12 VOLTS = 131.25 AMPS

To charge

I'm going to be using 200 WATT panels so:
1575 WATTS/200 WATTS = 7.875

so I would need about 8 panels to get my battery charge assuming they'll be putting out 200 watts all the time.

icarus · April 2009

Re: Charging Large Battery Bank

You also have to figure in panel efficiency, (~80%) charge controller efficiency (+90%) and ultimately battery charging efficiency(~80%). My general rule of thumb is if you take ~50% of name plate rating for Pv panels, multiply that by the expected number of hours of good sun per day,, and that will give you a pretty good number of how many ah or wh you can draw in a day.

This number would be a bit better if you are using battery voltage rather than an inverter because there would be no inverter losses.

Tony

niel · April 2009

Re: Charging Large Battery Bank

it might be better to feed it more current (and thusly the watts too) because the final stages tend to taper off somewhat. the 10 hour (10%) rate of charge will do for this as the extra 2 hours could top of the battery.
do note that 12 hours is unrealistic in terms of solar being used to charge the battery in a day or 2 as it will most likely take 3 and possibly more if you are getting less than 4 full sun hours a day. for other consistent charging sources 12 hours will be fine.
adding here that i based this on 100% dod when starting and we don't recommend going beyond 50% dod. this would only change the number of hours needed to reach a full charge and not the required pvs to charge it.

halfcrazy · April 2009

Re: Charging Large Battery Bank

There was a post somewhere i believe it was here on NAWS about this type of project but the guy found a radio that would activate the inverter so the Pilot on approach could call on a certain frequency and it would activate the runway lights for a certain time span. This could be a huge savings for you as the lights could be off the majority of the time.

I will see if i can find it but maybe someone else remembers it as well?

halfcrazy · April 2009

Re: Charging Large Battery Bank

Ok here it is
http://forum.solar-electric.com/showthread.php?t=4&highlight=Runway+lighting

This may make a lot of sense with the cost of solar panels?

HadesOmega · April 2009

Re: Charging Large Battery Bank

Sounds good, so it looks like I'm on the right track but need to fine tune my calculations. And yes I figured 12 hours was unrealistic, I will have to dig up some weather information for my area. I'll just need a lot more panels and more amps

Another questions is that if my charge regulator can only output say 80 amps and I needed more than 80 amps to charge my battery bank, is it ok to install more than one charge controller?

oh cool looks like someone has already done a similar project. Yes my model will be using PCL also its good for airfields that don't have a lot of operations. Its nice to over engineer a bit just in case.

niel · April 2009

Re: Charging Large Battery Bank

sure you can add more, but the outback i believe would need the mate. also know that battery banks of higher voltages will allow higher wattage abilities for many of the major controllers out there. controllers limit the current, but when outputting a higher voltage like 24v or 48v it doubles and quadruples the wattage capability respectively. this higher voltage battery bank will also need to operate items like an inverter that will work at those same voltages as the battery bank. 12v inverters will not work on a 24v battery bank and could damage it. many like the higher voltages to work with as it can reduce resistive loss percentages, allow smaller wires to be used as these wires can get very huge, and that some may not need as many controllers under some circumstances.
in addition to this any conservation efforts help tremendously in reducing the strain or possibibly even the size of the system.

mike95490 · April 2009

Re: Charging Large Battery Bank

Radio activation of lights is SOP at un-staffed airfields. The critical sizer for your battery bank, is "non-sunny" days. PV needs direct sun to produce power. Knowing how much time the lights will be "ON" and the # of days to reasonably expect cloud cover that prevents charging, will resolve to how much battery bank to plan for. Depending on the airfield security, the batteries could be housed under the panels. If you have enough weather data, you could bias the panels for AM or PM clouds/fog, if AM cloud cover is common, aim a bit more west on the panels. You should only plan on 5 charging hours a day, other times, the sun is not in prime power position. You can also bias panel angle to allow for faster charging in AM, when batteries are low, and can accept the higher rate, and off-axis lighting later when the charge controller throttles back. Depending on your budget, you can trade Flooded Cell 80% recharge efficiency, for expensive AGM Cell 95% recharge efficiency, saving a bit on PV array cost (15% smaller PV array)

HadesOmega · April 2009

Re: Charging Large Battery Bank

For those non sunny days I was going to incorporate a gasoline generator into the system. If the voltage dips below a certain point my inverter/charger will fire up the generator and start charging the batteries and take over powering the lights.

However with PCL I would assume the airport will be able to last the night. The only thing I do have to leave on though is the airport beacon which consumes like 400 watts. I'll be using a photocell to turn it on at night. Fully automated

Yes security is another concern, this is just a paper project though so I don't have to worry about it just make up hypothetical scenarios about it. I'm targeting private airports on private property. For added security though I would build a fence around the solar panels and put most of the components in a secure building or something. Probably get some good insurance plan on everything also, never know what hooligans are out there.

Thanks for everyone's input electronics is not my strongest subject

mike95490 · April 2009

Re: Charging Large Battery Bank

Nix the gasoline generator. Propane for the genset. The carb won't gum up in the summer (when the engine never runs), and the propane dealers make scheduled runs to top off tanks. You get by with less oil changes too.

icarus · April 2009

Re: Charging Large Battery Bank

mike90045 wrote: »

Nix the gasoline generator. Propane for the genset. The carb won't gum up in the summer (when the engine never runs), and the propane dealers make scheduled runs to top off tanks. You get by with less oil changes too.

Even better,,, since this is a theoretical exercise,,, Natural gas.

Toony

mike95490 · April 2009

Re: Charging Large Battery Bank

400W beacon, wintertime dusk - dawn, call it 14 hours. (or adjust as needed)
that's 400x14 = 5,600WH round up to 6KW for ease.
12V = 500AH
24V = 250AH
48V = 125AH (at a 14Hr rate)

Batteries must be at least 2x that size, one night takes them to 50% discharge.

HadesOmega · April 2009

Re: Charging Large Battery Bank

thanks for the input, bleh I went over the calculations for the whole night last night. I decided to go with a 24 volt battery bank because like everyone was saying it'll be easier on the wiring and it'd be less batteries. I went from Optima Blue Top to Deka batteries because I didn't really know much about solar batteries before. I would think the the Optima batteries would do ok but the battery I decided to use had more than double the capacity. Plus I was calculating for 80% dod which would not make happy batteries. The Series-Parallel wiring sure was a pain electronics and avionics was not my strong suit in school.

Going with 50% dod was hard the battery bank got huge and the panels got really expensive so I did some tweaking with the PCL. Since I'm using a make up airport I adjust the operations to 1 per hour, it hard to tell the night ops for an airport that didn't have night ops or is a made up one of course. I got the battery bank to a decent size now and I separated the beacon and lighting system runtimes to trim the AH down a bit more.

You really learn how much energy conservation plays a part in an off grid system. I'm using the MOST energy efficient lights (mostly LEDs) and being very conservative with equipment and its still very hard to get the system tuned well. I'd imagine thats what is holding solar power behind, when panels become cheaper like DVDs, CPUs, etc... get cheaper it'll go mainstream. But solar power has come a long way in past decade or two. I guess the same goes for batteries as well. Just not enough resources to go around I guess. All that lead in the batteries.

Thanks mike for the beacon info. Its kind of late in the project but I will bring it into consideration at the conclusion of the proposal.

I will look into the propane (is that like CNG?) generator as well, like I mentioned it kind of late in the project now so I can add it as an option at the end. Too many hardware changes already. But if its FAR superior I'll go for it.

Thanks everyone things are starting to take shape, I got 2 weeks to work on this, I'll be glad its finally done. Maybe when I get it done I will put up my airfield diagram with all the power components on it.

mike95490 · April 2009

Re: Charging Large Battery Bank

You should not spec a gasoline engine for irregular, un-attended operations, as the fuel goes stale in a month.
http://en.wikipedia.org/wiki/Propane Liquid at 200PSI
http://en.wikipedia.org/wiki/Compressed_natural_gas liquefies at 3,000psi
BIG difference in style of tank needed.

dwh · April 2009

Re: Charging Large Battery Bank

HadesOmega wrote: »

Maybe when I get it done I will put up my airfield diagram with all the power components on it.

If you would that'd be great. I'd really like to see it.

(Soloed in 1976 on the morning of my 16th birthday.)

Charging Large Battery Bank

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