Battery discharge and lifespan for limited use cabin

Steve961 · January 2010

I am trying to take a realistic, and balanced, look at my power needs vs. battery life. I have read the FAQ and seen the charts that recommend limiting battery discharge to 50%. I understand the logic for that, but I am not sure it it applies to what I will be doing. This system is for a small hunting cabin that will be occupied about 30 days per year. The needs will be small, mostly CFL lights, a small water pump, and my laptop computer. I will be starting off my system with a single Concorde AGM 104 AH battery. The battery will be stored at my home in the off season on a small battery tender. If I follow the 50% rule, I will probably want/need to add another battery before too long. If I allow for an 80% discharge, I may not need to add another battery at all.

My thoughts are the following:

1. If I only cycle the battery 30 times per year, and a 50% discharge rate provides for 1,000 cycles, will I really get a 33 year lifespan out of my battery? Can a battery even last that long without any cycles. If not, why would I want to limit my discharge rate to 50%.

2. According to the charts, an 80% discharge will provide for 500 cycles. With 30 cycles per year, will my battery then last 16 years?

I'm not trying to be cheap here, although the batteries are ~$300 each delivered, I just really want to get the most efficiency out of my batteries for what my needs are. I can't see limiting the discharge rate to extend the lifespan if there is no way I can ever reach that lifespan.

As always, thanks for your help.

Steve

BB. · January 2010

Re: Battery discharge and lifespan for limited use cabin

I agree--let the battery go to 80% discharge. I believe that Concord's warranty is to replace/credit when it does not meet 80% capacity any more.

The only downside is that you will have no margins if your power usage increases/spikes. Just from a "human factors" point of view--it is not a bad idea to double your known/planned usage to allow for growth and bad weather.

However, if you have a reasonable plan for back up power (small genset+charger) and/or just turn off the power when you run out--why not.

My two cents, I would like to see you use a battery monitor... Taking the battery down past 20% state of charge runs the risk of cell reversal (especially if your loads are like lamps and radios that will draw past 10.5 volt dead battery). Granted a battery monitor is not perfect--but if it saves you one early battery death (at $300 a hit)--the monitor will have paid for itself. Also, if you have guests/family at the cabin--it is easy to tell them to turn of the loads and/or crank up the generator if you see <50% or <20% State of Charge.

-Bill

Steve961 · January 2010

Re: Battery discharge and lifespan for limited use cabin

Bill:

I'm not too worried about running out of power. I have a small generator (Honda eu2000i) and kerosene lanterns. I consider this solar system a luxury item, as I have done without it for the past 20 years. It's nice to be able to just flip a switch for light in the middle of the night without having to start the generator or light a lantern.

To prevent the battery from going too low, I have a Blue Sea 8003 volt meter, and will utilize the 11.5 volt low voltage disconnect on my SureSine inverter. If you have a better suggestion for the meter, within reason of course, I would like to know.

Thanks.

Steve

icarus · January 2010

Re: Battery discharge and lifespan for limited use cabin

Sort of an age old debate. I went from 4 L16 batteries to 4 (cheap) t-105s) I did a calc, that if the cheapies lasted 4+ years against the l-16s lasting 6 years I was way better ahead with the smaller battery bank, even with a bit more discharge.

That said, I never discharge mine below ~70% SOC. I expect the T 105s to go 5-7 years. (I am on one set that is in year 11 with seasonal service).

What I would consider, is buying two batteries, and instead of dropping them to 20%, drop them both to only ~40-50% SOC. That way you stay within the (good) range of battery management. (Or run the Genny every day to keep it from going below ~50% daily)

Or, as Bill suggests, try doing it your way, and see what happens. My guess is that over the life of the battery, it isn't going to make any statistical difference.

Tony

Kamala · January 2010

Re: Battery discharge and lifespan for limited use cabin

I would probably double the AHrs. But... I see no mention of how the battery is being charged. I assume by PV. If so, how much power (watts.)

Steve961 · January 2010

Re: Battery discharge and lifespan for limited use cabin

Kamala:

If my calculations are correct, a 104 AH battery discharged at 80% should provide about 900 watt hours through my SureSine inverter. My power needs are going to be very small in this cabin - the largest load will be CFL's for lighting. I figure I could have 60 watts of CFL's burn for 15 hours - more than enough for a typical weekend getaway.

In regards to solar, I will probably work my way up to around 130 watts. I already have a single Uni-Solar 64, and will probably get another one later. I figure the battery will have anywhere from 5 to 12 days to get recharged while I am away.

The one thing that could cause me to change this is refrigeration. I am on the fence right now between a propane or electric refrigerator. Propane is probably the smarter choice though considering how little it will be used.

Steve

westbranch · January 2010

Re: Battery discharge and lifespan for limited use cabin

Hmmmm...

64 W panel x 50% (derating) x 4 hours /day = ~120 wHrs/day x 5 days = ~ 600 wH

you will probably have to run that gen set if you shoot for your batteries getting back to 100% charge before you start to use them the following week.
I suggest you think about another panel &/or using the genset before you leave for the week, to get the battery back to 90% + SoC and let the panel do the finishing while you are gone. You did not mention what Charge Controller you are using. ???
For your set up a propane fridge is a good choice due to the short periodic use.

HTH
Eric

mike95490 · January 2010

Re: Battery discharge and lifespan for limited use cabin

Why pull the battery? The PV and charge controller will keep it happy thru the winter, unless you expect it to get < 30F indoors.

BB. · January 2010

Re: Battery discharge and lifespan for limited use cabin

If you have propane (such as for cooking)--A used RV propane powered fridge would make the best use of money (vs PV panels, batteries and inverters) for a fridge that is only used on weekends. You only pay for propane when you are there.

Electric Fridge makes more sense if it is used 9 months or more per year.

-Bill

Steve961 · January 2010

Re: Battery discharge and lifespan for limited use cabin

Eric:

I pretty much figured I would eventually need another panel. I have been looking for another US64, and am just waiting for the right deal to present itself. Used ones of these seem to come up on Ebay fairly often. In regards to the charge controller, I have a Morningstar SunSaver SS-10L.

Mike:

My cabin is in northern Wisconsin, and the outdoor temps can go below -40F on occasion. Although whether or not that would translate into -30F indoors, I am not sure. Another concern is that the panels would get snow covered for a long time and not send any power to the battery. Either way, it's not that hard to transport one or two batteries twice a year just to be on the safe side.

Thanks for all your insight guys.

Steve

icarus · January 2010

Re: Battery discharge and lifespan for limited use cabin

Couple of thoughts,

First, the decision whether or not to use Propane or a compress fridge is usually made by how much use it will get on an annual basis. The more you use it, the more the balance tips in favour of compressor. That said, even a cheap LP fridge will burn only ~1500btus of propane/hour and burn perhaps 50% time on a 24 hour basis. A gallon of propane has ~79,000 btus per gallon.

As far as leaving the Pv and batteries in place. A fully charge battery won't freeze until it gets to ~-60f if memory serves. Even partially charged they won't freeze until it gets really cold. http://www.batteryfaq.org/ for more info.

One simple way to keep the battery topped up, unattended over the winter is to install one or two panels (what ever you need to keep up with self discarge) mounted on the wall so that snow cannot accumulate on them. I have a couple of small sites where we keep the batteries on all winter this way, at temps that get to -40 on a regular basis and have ALWAYS returned to fully charged batteries. We even have one set, with a single panel on the roof. Over the course of the winter the panel self clears often enough that that battery is fully charge each season as well.

Finally, I wouldn't bother with an Ebay panel, unless you can get it for less than the price of a new panel. (I have almost never seen ebay panels that cheap in the last few years!) With new panels as low as ~$1/watt why would you pay as much as $3-5 that I see ten year old panels. I have even considered selling off my old ones on ebay for $3-5 and by new ones at $2 and end up with twice as many!

Good luck,

Tony

Ps Over winter indoor ambient will run ~10f warmer than outside average in a reasonably insulated building. Putting your batteries in a insulated close fitting box will result in substantially higher battery temps, since the charging/discharging will generate substantial heat, further reducing the risk of batteries freezing.

Kamala · January 2010

Re: Battery discharge and lifespan for limited use cabin

Steve961 wrote: »

... a 104 AH battery discharged at 80% ...
Steve

I'm confused. "Discharged at" is ambiguous to me. "Discharged to" or "discharged by" would be more explicit. In other words, are you giving a SoC (state of charge) or a DoDc (depth of discharge?)

Depleting the battery bank to an 80% SoC is perfectly OK. Bringing it down to an 80% DoDc (which is equivalent to a 20% SoC) is going to significantly reduce the banks lifespan.

Steve961 · January 2010

Re: Battery discharge and lifespan for limited use cabin

Tony:

I like the idea of a small vertical panel under the eaves for winter maintenance - I will give that a try. The reason I am shopping on Ebay is so I can get another Uni-Solar US64, which has been discontinued by the manufacturer. I have experienced a fair amount of vandalism at my location, and I really like the resistance to damage the thin film Uni-Solars offer.

Kamala:

I will be bringing the battery down to a 20% state of charge. Read my original post and Bills response after that. Since the battery will only experience maybe 30 cycles per year, I felt it was not necessary to limit it to 50% SoC.

Thanks.

Steve

icarus · January 2010

Re: Battery discharge and lifespan for limited use cabin

If you look at my avatar, you will see a space on either side of the main windows. I have a 55 watt siemens panel mounted one on each side. They mount on hinges so that during the winter if I am away, I can fold them back into the building. In other seasons, I swing them out on props to the proper seasonal angle. They are high enough off the ground so that you don't generally run into them.

You can see them on the attached picture

Tony

Kamala,

Read to the begining of the thread. The conversation is about battery size vs DOD as it relates to ultimate life span of a battery that gets used rarely

Battery discharge and lifespan for limited use cabin

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