Chill Out with the Batteries

[AzRoute66]

For many reasons I will need to keep my FLA battery bank in the garage which has no cooling and a large southern exposure. Needless to say, the temperature in there is over 90 and up to about 115 degrees all day during the summer. At night it cools down significantly, the exact amount dependent on when and how long the garage door is open. During the winter, daytime temperature control is not much of a problem, generally less than 85 degrees peak.

I was already planning a large battery box, with a vertical vent, but now I'm thinking that it would be 100% worth my while to make this into a very small, well insulated 'room' and put a cooling unit of some kind on it. I've noted that almost all remote off grid equipment installation case studies make provisions for air conditioning and I think I should too.

The best thing I've thought of so far is a 5,000 BTU window air conditioner. The model I've found is rated 450W, is Energy Star, has 'soft start', thermostat goes down to 64 degrees, and set values are kept and restored after power is removed. I would think the duty cycle would be very small, and only when power availability is more than I need otherwise.

I would make sure it is off during equalization, and put it on a timer from about 0900 to 1700 hours.

Admittedly I haven't been thinking about or looking into this very long, but am I completely bonkers? Is there a better solution for the health of the battery bank?

[Sunking]

OK you are starting to talk crazy. Simple fact is the Air Conditioning will use more energy than your system can generate. You are connected to the grid, last thing you want to do is go off the grid. Anything you take off the grid is going to cost you 5 to 10 times more then buying the power. Why do you want to pay more for power?

[sensij]

I wouldn't call this a recommendation, but if it is properly engineered, something like this seems like a better solution:

http://www.zomeworks.com/wp-content/uploads/2016/01/CoolCell_Flyer.pdf

[SunEagle]

I have to agree with Sunking. Having an AC unit to cool the batteries will just be using more energy then the batteries can deliver. IMO it would be wasting the batteries.

Keeping the batteries cool and dry is important but not if you have to use more energy then they can give you.

[Mike90250]

I think the point is that after the batteries are half way through Absorb, and in Float, there may be surplus PV power, and Only at that time, would the AC be allowed to run,

But I'm i'm only guessing here, that's how I'd do it.

[Wy_White_Wolf]

Instead of an AC unit consider a freezer for your battery box and use a Johnson controller to cycle it at the desired temperature. Since your only cooling a small volume instead of the whole garage it should use less energy.

WWW

[NEOH]

Instead of an AC unit consider a freezer for your battery box and use a Johnson controller to cycle it at the desired temperature. Since your only cooling a small volume instead of the whole garage it should use less energy.

WWW

The FLA Battery Box must be vented, the battery bank cannot be sealed inside of a freezer.

Admittedly I haven't been thinking about or looking into this very long, but am I completely bonkers?

Yes, you have gone completely bonkers.

[J.P.M.]

I wouldn't call this a recommendation, but if it is properly engineered, something like this seems like a better solution:

http://www.zomeworks.com/wp-content/uploads/2016/01/CoolCell_Flyer.pdf

Zomeworks has made some pretty interesting stuff over the years. I've met the founder a few times at conferences, the first time in 1977. A real outside the box thinker and a true solar guru. His name is Steve Baer.

The Zomeworks device for battery thermal protection is certainly a possibility, and seems typical of the Zomeworks approach to R.E ways of seeing solutions, but it seems better suited to outside environments rather than inside as AZ66 seems to be describing for his application.

Looking at AZ66's application, it seems that cooling the batteries is the design goal more than protection from cold temps. One way to do that, as AZ66 has seen and sort of proposed, is to put the batteries in a very well insulated box, provide for safety, service access, venting and other considerations, and otherwise, seal it up. Providing a sufficiently cool environment for the batteries then becomes a calculation to figure out the cooling load of heat gain from the garage through the walls of the box plus any waste heat from battery operations of charging/discharging, whatever those may be, as well as any load from required and perhaps continuous natural or forced convection venting during the day that's in addition to any forced nite time circulation.

Such a box might need 3" or 6" insulation, and maybe not be quite as pretty as the Zomeworks design, and, if needed, getting the waste heat charge/discharge loads might be a bit tricky, but if the waste heat load is small, nocturnal cooling via a fan and a timer, or better yet a differential thermostat, or both, together with the thermal mass of the batteries, might be sufficient to lower the battery temp. enough at night to slow the battery temp. rise during the day and stay within battery temp. requirements that are below unwanted higher (garage) ambient temps.

[Wy_White_Wolf]

The FLA Battery Box must be vented, the battery bank cannot be sealed inside of a freezer.




Yes, you have gone completely bonkers.

So what's stopping a person from putting a vent in the freezer? It don't take much to vent the H2 and since you also want it from the highest point that works out with cold air wanting to stay in the freezer.

WWW

[SunEagle]

So what's stopping a person from putting a vent in the freezer? It don't take much to vent the H2 and since you also want it from the highest point that works out with cold air wanting to stay in the freezer.

WWW

Venting is one way but the best way to keep explosive gases from building up or getting stuck in pockets is to keep the air flow in and out of an area high enough to move and dilute the gases.

[AzRoute66]

sensij, - That is indeed something I hadn't thought of. It starts with something that appears to be a very small, well insulated 'room' with a cooling unit of some kind on it (where have I heard that phrase before). At their website, the photos are all disclaimered with "Cool Cells must have full view of the night sky to function properly." This clearly has more to do with airflow over the lid/radiators than star gazing. Might be a problem, currently not much breeze in my closed up garage. I note they have a Tucson dealer, so will call them to see how many they have sold and perhaps swing by for a look if they have some. Might even dust off my thermo textbook from college. Maybe I could run my air conditioner into it 2.35 hours per day.

Sunking, - From you, I was hoping to evoke some kind of anecdotal point concerning battery lifetime vs temperature, or a mathematical treatise on weeks/degree C. The easy bait was the duty cycle that a 450W unit would require to cool 25 - 50 sq ft 'room' with a battery mass of ____. A rant of why a 64 degree thermostat setting was just plain stupid would have been appreciated. But all I get from you is --- you can't generate enough power to run your part time 450W load from a 4,000W array during a six sun-hour day. Really? Just for you, I am going to treat it as a diversion load for the duration.

It was a chart from Trojan concerning equalizations for batteries in storage that got me started. 10 degrees makes a huge difference. Then I went to some thermal performance spec sheets and started thinking that there must be some reasonable way to keep them cooler than just wishing for it. Then I remembered those off grid case studies that included an A/C unit for the equipment room AND an A/C unit for the battery room. Me being the Rube Goldburg fan that I am went straight to evaporative cooling as it works so well in my house, but decided to shelve that for the moment and keep all options open. Now I am thinking about how amazingly cold the water in the reservoir of my house cooler is, and how hard it might be to run to run two 1/4" lines down to the garage (considering one is already there), but that would require a 10W pump to circulate the water and I would probably need a POCO for that action, would also need some kind of insulated box with a thermal capacitor...

[AzRoute66]

Such a box might need 3" or 6" insulation, and maybe not be quite as pretty as the Zomeworks design, and, if needed, getting the waste heat charge/discharge loads might be a bit tricky, but if the waste heat load is small, nocturnal cooling via a fan and a timer, or better yet a differential thermostat, or both, together with the thermal mass of the batteries, might be sufficient to lower the battery temp. enough at night to slow the battery temp. rise during the day and stay within battery temp. requirements that are below unwanted higher (garage) ambient temps.

Yes, thanks for that idea that the battery mass will assist in retaining any injected cooling. That is the trick, right? The same insulation that keeps outside generated heat out keeps inside generated heat in.

[Sunking]

Sunking, - From you, I was hoping to evoke some kind of anecdotal point concerning battery lifetime vs temperature, or a mathematical treatise on weeks/degree C. The easy bait was the duty cycle that a 450W unit would require to cool 25 - 50 sq ft 'room' with a battery mass of ____. A rant of why a 64 degree thermostat setting was just plain stupid would have been appreciated. But all I get from you is --- you can't generate enough power to run your part time 450W load from a 4,000W array during a six sun-hour day. Really? Just for you, I am going to treat it as a diversion load for the duration.

How big of a battery RU? How many volts? Did not take note you are running 4000 watt panels.

OK for each 15 degrees F you go above 75 cuts battery life in half. For each 10 degrees F you go above 75 adds roughly 5% capacity, and vice versa for every 10 degrees below 75 you loose 5% capacity.

Use a Swamp Cooler rather than a Window Shaker. Not only will it keep things cool, but add a fair amount of humidity which will do the batteries good because that will slow down evaporation of battery fluid. A Window Shaker will remove what little humidity you have making it even dryer. A Swamp cooler should use 200 watts or less last time I looked. Whatever it is will be less than 450 watts.

Ever think of a pit? Or inside your cool home?

[J.P.M.]

So what's stopping a person from putting a vent in the freezer? It don't take much to vent the H2 and since you also want it from the highest point that works out with cold air wanting to stay in the freezer.

WWW

I made a beer fridge once w/ a couple of room mates. It had two holes drilled in it, one for the CO2 line and one for the tap in the door. No reason why they couldn't have been a vent/drain.

[J.P.M.]

Yes, thanks for that idea that the battery mass will assist in retaining any injected cooling. That is the trick, right? The same insulation that keeps outside generated heat out keeps inside generated heat in.

You're welcome. Depending on the volume and weight of the batteries, WWW's idea may well accomplish much the same purpose as constructing a box with less construction and less hassle. I'd just be conservative in the vent/drain sizing however, and see if local codes or common sense issues I'm most likely ignorant about come into play.

BTW, and not to sound condescending, but every good vent needs a good drain. One hole in the top will only let stuff out if an equal amount of stuff can get in to replace it, probably at or close to the bottom. That also brings up the idea of reverse thermosiphoning, or bad stuff going in the wrong direction. Not being well versed in battery containment venting, I'd put holes top & bottom, run them outside, put bug screens on them and dampers as required/allowed by code and common sense, directioned depending on the density of the off gasses relative to air.

On the thermal issues: Insulation works because it slows down heat transfer from where it is warmer to where it is colder.

The better the insulation, and/or the smaller the heat transfer surface is (the container outside surface area, top, bottom and sides), the slower the batteries will change temperature, either warmer or colder.

The more massive the batteries, the slower the temp. change of the batteries.

The least amount of temperature change (note, temperature change, not necessarily heat transfer, although that will be less heat transfer if a lot of insulation is used) will occur when a large thermal mass is heavily insulated. That ratio of thermal mass/overall heat loss is what something called the thermal time constant is all about. Long story. This is the Reader's Digest version.

To a first approximation, and as an example, for the same insulated box, if the contents of the box have a heat capacity of, say 1,000 units of thermal energy per deg. of temperature, the temperature of the contents will change only 1/10th as much over any similar time period as the same insulated box that contains material with only 100 units of thermal energy per deg. of temperature.

Thermally massive stuff loses temperature (but not necessarily thermal energy) a lot slower than non thermally massive stuff. That's one reason people make the mistake of thinking that stone houses are well insulated. Truth is, stone is a crappy insulator, even if it's 2 ft. thick, but because it has such a large thermal mass, it stays warmer/colder longer in response to environmental changes. That's a result of a long thermal time constant (a large numerator in the thermal mass/heat loss quotient) at work.

If you have lead acid batteries, they're heavy, and depending on how much of the volume the acid takes up, and because lead is about as heavy as its specific heat is low, treating the thermal mass of a battery as about the same as an equal volume of water may be a decent 1st approximation. If so, the batteries have a lot of thermal mass and that mass is pretty compact. So, putting the batteries in a well insulated box and circulating cooler air through the box at night may be a possible solution, depending on how much waste heat is generated during charge/discharge cycles.

You were/are on the right track for one possible solution. Just remember safety.