Small off grid system questions

squarebob · February 2012

I have been tasked with setting up a small off grid power set up for our Radio Controlled Model Airplane Flying Field. The loads will vary from none most of the time to 1 to 4 - 50 to 250 watt 12 volt chargers plugged into banana jacks used over a couple hours, and not continuously, 2 or 3 days a week. There will also be a 600 watt true sine wave inverter for those who need 120 vac supply. I can't quantify the loads any better as they are very variable. The batteries will be 2 6v GC 220 AH batteries in series. We have a 50 amp, 14.6v engine driven alternator to supply most of the charging for the batteries. I want to include a solar panel to keep the batteries topped off and to supply some of the charging capacity when some one forgets or just doesn't run the gen setup as needed. The plan is to have the genny run whenever the voltage reads below 12.2 volts with a load (is this a good number?). I am looking at a solartech 55 or 85 watt panel from our hosts and a Morningstar sunsaver 6amp controller. As the budget is very limited, would the 55w panel be able to bring the batteries up from say 80% to 100% in a few days and keep them topped off or should I look at the 85w panel. Some one ( me! ) will be sure to run the genny at least once a week to keep the electrolite stirred up and the batteries in good shape. Any input appreciated.

Bob

vtmaps · February 2012

Re: Small off grid system questions

squarebob wrote: »

I can't quantify the loads any better as they are very variable. The batteries will be 2 6v GC 220 AH batteries in series.
<snip>
I want to include a solar panel to keep the batteries topped off and to supply some of the charging capacity when some one forgets or just doesn't run the gen setup as needed.
<snip>
would the 55w panel be able to bring the batteries up from say 80% to 100% in a few days and keep them topped off

Hi Bob,
Your batteries are 220 AH at 12 volt. A 55 watt panel will push less than 3 amps into them. Depending on the battery's self discharge, the panel might be able to keep them topped off. It will not be able to charge them. Rule of thumb: the current needed to charge the batteries should be about 10% of the AH rating. Therefore you will need to push about 22 amps into them.
--vtMaps

waynefromnscanada · February 2012

Re: Small off grid system questions

From my experience an 85 watt panel would certainly keep them topped off, and over a week or so of full sun, with NO loads at all, could charge the batteries, but in the meantime, while the batteries were waiting for that charge (someone forgot to run the generator) I fear the batteries would have been sitting discharged for too long, and thus sulfating. So as squarebob suggested, you SHOULD have more panels. Generally between 5% and 10% of the battery amp hours. If the generator is used as you indicated, than you should be safe in the lower end of that range, or even somewhat less. But regardless, adding the 85 watt panel would be way, far ahead of having no panels at all, and along with the generator, could very well do the job for you. Would not consider the 55 watt PV though. And yes, with that much PV, you should have a charge controller to manage that charge, as continuing to pump 5 amps into fully charged batteries of that size, day after day if no power was taken out, would certainly not do the batteries any good. AND, if they are sealed, or jell type instead of flooded batteries, would destroy them.
All the best with your project.

niel · February 2012

Re: Small off grid system questions

well the guys failed to look at the loads you intend to put onto the batteries. you list up to 4 250w chargers for up to 1000w over several hours. adding at least another 600w possible from the inverter makes this 1600w as a total load possibility. now doing the math 1600w/12v=133.34a. if this is drawn for an hour at that rate it will exceed the 50% dod point. this we don't recommend doing as it degrades battery life more. so even if you draw half of that, but do this for 2 hours, it is the same result. this means lower times of use with lower total wattage consumed to prevent this or go with a bigger battery bank and the larger charging requirements that go with it.

this can be helped with another power source supplying current such as the generator or solar panels. this would lessen the depth of the discharge to the batteries by supplying some or all of the load current. it would seem to me that this is too much of a balancing act that you can't gauge very well so i recommend the use of a battery monitor and even a hydrometer to better gauge your batteries' state of charge. try not to go by the monitor's indicated charge %, but do observe the overall discharged current. knowing what needs replaced helps, but you need to add another 20% to 30% due to charging and other losses. as was said this should be done before wrapping things up for the day so the batteries aren't sitting in a partially discharged state waiting for the next time for you to take them out to start this cycle all over again. if the sun isn't out at the days end for your flying then this means you must run the generator to charge for a few more hours to top things off rather than relying on the solar panel(s). top off charges with generators tend not to be a very efficient use of the generator.

this is going to be something you would have to keep on top of, but if no flying for some time is expected, then the pvs may be used to top off the batteries if not too much is needed for the charge to go to 100% and it should output at least 5% of the battery ah rating. as this would be in the 11a range it may be difficult to bring up a 220ah battery bank to full charge with the pv choices you've cited. with that, chose a pv to help supply power to the battery during the event, use the generator with an appropriate 3 stage charger to bring it up to at least 85%-90% and then let the pv take it the rest of the way. go with the highest rated pv wattage you can afford or make plans on using the generator even more.

oil pan 4 · February 2012

Re: Small off grid system questions

At least with large starting batteries they can take over an amp just to hold the voltage at 14.4 volts.
A 3 amp panel isn't going to give hardly any power for charging.

Small off grid system questions

Comments

Categories