Battery Box design

Sheldon · December 2009

Any pointers to a design that could be scaled to 48V, 400AHR? Would be placed in an outbuilding on a slab floor, ambient is 35-100F

Vic · December 2009

Re: Battery Box design

Hi Sheldon,

This has come up before, but in searching the site, a quick perusal yields nothing valid.

If secondary containment is not a requirement, then I've used the following with Surrette dual-container batteries (where outside case is the secondary container--this meets my requirements):

3/4-inch plywood box allowing 2-inch space around each battery, 3-inch rigid insulation on ALL sides, top and floor. 2X2-inch Fir frame. Front bolts-on using blind T-nuts for the bolts. Piano hinge for the top, and 2X2-inch Alunimum L around the top insulation as a stiffener. Two inlet vents, low on the box, and one outlet at the top on one side.

Works great for me, BUT this has NOT been inspected by a nit-picking inspector.
EDIT: I DO realize that there are valid concerns about flammability, as the foam is one layer of polyisoscyanurate with foil on each side, and polystyrene rigid foam. Have considered skinning the top layer of the foam with Tempered masonite (r). While the masonite is flammable, it would take quite a lotta heat to get it started. AND, of course, the box itself is not fireproof. Perhaps a sheet metal skin on top of the foam, and the masonite on top of that ... any way the box and the foam is all that we have done to this point.

Some have said that above 48 VDC nominal batt voltage, metal box would be required ... etc.

Vic

icarus · December 2009

Re: Battery Box design

Be wary of Poly iso board (thermax) with a foil face. If the board is dropped across the terminals it could short, causing a fire. I personally would use Blue or Red stryrofoam rigid board.

I would also include a voltage controlled Zepher type vent fan.

Tony

PS I think that keeping the batteries cool is way more important than keeping them warm.

Sheldon · December 2009

Re: Battery Box design

I am more concerned about cool than warm, simple use will provide warmth, I suppose I could put a fan that forces ventilation when the interior temp is > 70F, but less than exterior temp and allow for gravity venting of hydrogen.

My boat has fiberglass coated 1/4" plywood, which allows it to be fluid tight. My concern on the foam would be acid resistance, so I may go that way.

BB. · December 2009

Re: Battery Box design

Here is a post I did regarding Acid Resistance of various materials.

-Bill

Sheldon · December 2009

Re: Battery Box design

That's why I would use epoxy resin over some glass cloth to strengthen thin layer of wood, and seal against inevitable drips.

Vic · December 2009

Re: Battery Box design

Tony, the foil-faced foam is covered by glued down PS foam, to the face presented to the inside of the box is PS. It is not perfect, but it is insulated. Iso foam is only 2-inch thick, and the PS is 1-inch.

And on my bank, appears that Asorb or EQ might increase the bank temp about 1 degree C. And some of this increase may just be from an increase of ambient temp.

Vic

icarus · December 2009

Re: Battery Box design

Vic wrote: »

Tony, the foil-faced foam is covered by glued down PS foam, to the face presented to the inside of the box is PS. It is not perfect, but it is insulated. Iso foam is only 2-inch thick, and the PS is 1-inch.

And on my bank, appears that Asorb or EQ might increase the bank temp about 1 degree C. And some of this increase may just be from an increase of ambient temp.

Vic

I'm familiar with foil faced thermax (polyiso board) My concern is if a piece were to come in contact with hot terminals the results could be interesting. My guess is the foil would melt quickly enough and break the circuit, but would/could it heat enough to light a fire?

Tony

Vic · December 2009

Re: Battery Box design

OK Tony,

BUT, the point that I was attempting to make is that the PolyIso foil-faced rigid insulation is COVERED with another layer of diffetent foam. This different foam is called Poly Styrene rigid foam (PS). This PS foam insulation has NO foil on either side. It is glued down to the Poly Iso foam. The Poly iso foam is glued to the plywood box. In the case of the hinged box lid, the Poly Iso foam is glued to the plywood cover, the PS foam is glued to that, and then the two layers of foam are nailed to the PW cover with ring-shank nails ( there are fender washers, which incease the bearing area of the nails.

I do not know what would happen if foil contacted battery terminlas. But there would need to be several failures in order for foil contact to occur.

Is this perfect ? NO ! But is is done, it is adequate for me. It could be made better, but it will have to wait until there are no remaining projects, and that is unlikely to happen. AND, I am not trying to argue.

This is just the way I did it HNY Vic

icarus · December 2009

Re: Battery Box design

Got it, sorry to be so dense,

T

WisJim · December 2009

Re: Battery Box design

My battery box,, for a 24v 1550 amp hour battery made up of 12 cells, is lined with what we call around here "milkhouse board". This is a white smooth or textured plastic/fiberglass material available either as an 1/8" x 4'x8' sheet, or with this sheet bonded to blandex/waferboard. I used the thin sheet, and use the plastic moldings, like panel molding, in the corners. The top of the box is removable, and the front can be removed by unscrewing some fasteners, so that the batteries only have to be lifted a couple of inches to get them into the box, and that is easily done just by tipping them a bit and sliding the upraised corner into the box.

Blayd · December 2009

Re: Battery Box design

My battery bank currently sits on wood over slab concrete at ground level. Cold batteries are annoying in loss of available power, and overheated batteries get expensive to replace and possibly dangerous if they go "thermal" (runaway thermal meltdown).

A potential curative of the too cold/hot problem for soon to be purchased batteries in my particular case appears to be a below grade concrete mini-bunker or vault for the batteries to reside in. Not sure how far below grade to go yet, but deep enough to hit stable temperatures.

Heat is more of a problem than cold, and this area has the potential to hit 120 degrees F. in the summer.

Hopefully this solves a few isues such as cold/hot, keeping explosive/caustic gasses from charging from other equipment, and frees up some room in the power shack for future equipment.

Everyones thoughts on this welcome.

icarus · December 2009

Re: Battery Box design

There is a difference between cool batteries and cold ones. In the real world batteries that are sheltered/boxed and on/in a building will seldom get too cold if they are charged (via PV or other charger) as they will generate considerable heat by charging and discharging. Additionally because they have so much thermal mass, they cool slowly, and in the net/net come close to a 24 hour ambient plus a bit.

So for example, even if the air temp drops to say +10f over night, but the average daytime temp is say +40f the battery will find some temp closer to the 40 than the ten if that makes sense. If it is in an insulated box the temp will even be higher.

So in the real world, unless you are routinely drawing down the battery more than your design draw down, a cooler battery won't hurt you at all. In reality, while battery capacity goes down somewhat with temp, it really doesn't drop off until it gets real cold. For example, at 20f battery capacity is still nearly 90%. At-22f it drops to ~50%.

http://www.windsun.com/pictures/Batt_temperature1.gif

From NAWS battery faq, said better than I can: "Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature.

Even though battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times. Click on the small graph to see a full size chart of temperature vs capacity."

So the long and short is, worry about keeping batteries cool, but don't worry (generally ) about keeping batteries warm.

Tony

westbranch · December 2009

Re: Battery Box design

Blayd, I am estimating that the hole will have to be more than 4 feet into the ground to reach some stable temps. Around here the minimum for footings is 4 feet below grade to get away from frost penetration. you could also 'berm' the battery house with other fill to achieve the necessary depth of soil.
HTH
Eric

Dave Angelini · December 2009

Re: Battery Box design

The problem (and it can be a major one offgrid) is that this is loss of capacity is happening during the time of year that the batteries can often not afford the deeper discharge. The guy comes home one day and there is a failed cell or cells!

Most people do not have a spare battery or it takes weeks to get one. They keep using the system with a generator and it snowballs downhill! Keep them above 40F or have spares because the odds are much better you will need a spare.

icarus · December 2009

Re: Battery Box design

Dave,

My point however, is if you are using a system that is so close to the design limit that a cooler battery puts it over the limit so to speak, then the system is (IMHO) undersized. Using the chart I posted a few back, even at +10'f a fla will still retain ~90% of capacity. Given that few locations get cold enough for a insulated/protected battery box to drop that cold. In my experience, my battery bank that is in a un-heated shed, but the batteries are in a insulated enclosure is probably typical. These batteries are on stand by over the winter so they are not subject to much charge/discharge heating. With the outside temp hovering ~ -10f, the interior of the building averages about 0f, or 10'f warmer, the battery box in turn runs another 5' warmer still, or about +5f. Remember, this is a battery that is just idling, tested in mid winter when we have had temp extremes from-40-freezing or thereabouts. I have never seen battery temps that get much colder than ~0f no matter how cold it gets. Now if they are cycled a bit every day with some charge/discharge they will get a bit warmer as well.

So even at this extreme, and this is probably the most extreme that people are likely to see except in real arctic conditions, the batteries at +5f sill retain ~85% of rated. They are not subject to sulphating as a result, because while their capacity is reduced by the cold, they are essentially fully charged, they just have a smaller capacity.

So I go back to my original opinion, one ought to worry way more about batteries being too warm rather than too cold. (generally!) Rather than worry about doing some exotic battery heating or root cellar scheme (although a root cellar would ultimately be great for both winter and summer!) one would be better off adding a bit to the base capacity, and not worrying about the few days a winter the batteries might get cold.

Tony

hillbilly · December 2009

Re: Battery Box design

icarus wrote: »

Dave,

My point however, is if you are using a system that is so close to the design limit that a cooler battery puts it over the limit so to speak, then the system is (IMHO) undersized. Using the chart I posted a few back, even at +10'f a fla will still retain ~90% of capacity.
Tony

Not to nit pick, but....
I believe that you're reading the wrong numbers on that chart, 10 degrees F would only give you about 65% or so of the rated capacity. 90% capacity would be at 10 degrees C. I think that you have a valid point that one should have some buffer in battery capacity, but depending on local climate, that buffer just might be a lot more expense than finding a way to better insulate the batteries from heat and cold.

I actually think that Dave has a pretty valid point that in an off grid situation, having less capacity during times when there is also much less in the way of sun to charge back up. In our case winter is also our most consumptive time of the year, lots more dark hours that need light, more time indoors than summer, etc. I'd offer some advice on the battery box, but we're still working out ideas for a newer better design ourselves. The next one will be much better insulated, out of concern for both heat and cold.

Sheldon · December 2009

Re: Battery Box design

Had a pre-meeting with the SC county senior electrical inspector, and had generally good results.

Question on battery box, I am planning to use a Midnite Solar MNBE-D or C since its UL listed.

Do AGM's need to be vented? I am not finding any reference in the CA code so far.

icarus · January 2010

Re: Battery Box design

hillbilly wrote: »

Not to nit pick, but....
I believe that you're reading the wrong numbers on that chart, 10 degrees F would only give you about 65% or so of the rated capacity. 90% capacity would be at 10 degrees C. I think that you have a valid point that one should have some buffer in battery capacity, but depending on local climate, that buffer just might be a lot more expense than finding a way to better insulate the batteries from heat and cold.

I actually think that Dave has a pretty valid point that in an off grid situation, having less capacity during times when there is also much less in the way of sun to charge back up. In our case winter is also our most consumptive time of the year, lots more dark hours that need light, more time indoors than summer, etc. I'd offer some advice on the battery box, but we're still working out ideas for a newer better design ourselves. The next one will be much better insulated, out of concern for both heat and cold.

HB,

How right you are, I sit here chastened! Sorry for any confusion. The reality is figuring out your capacity as Dave suggests is therefore a pretty good idea! It seems that figuring some sort of seasonal average, not just the lowest expected temp.

That said, I guess I can justify my experience by observing that when it is that cold, PV output is significantly greater, and coupled with reflection off off snow is even greater still.

Slinking off to repent,

Tony

PS Happy New years to all those that care!

hillbilly · January 2010

Re: Battery Box design

icarus wrote: »

HB,

How right you are, I sit here chastened! Sorry for any confusion. The reality is figuring out your capacity as Dave suggests is therefore a pretty good idea! It seems that figuring some sort of seasonal average, not just the lowest expected temp.

Tony,
I specialize in dilexsia (or however you spell that thing where all the numbers and letters are always backwards), having made just that sort of mistake on a far too frequent basis myself. The thermal mass of the batteries themselves certainly does help some as you say. The only reason I posted was that I have been more affected by temperature swings than I would have guessed in the beginning. I'm also pretty sure that the excessive heat that our batteries have gotten has been partially responsible for their downturn in health.

Sheldon,
I don't think that AGM's need any special venting, and I'm pretty sure that the midnite battery boxes are not really sealed anyways. So this should be ok. Being AGM's have you thought about the possibility to move the battery box into a conditioned space? To my thinking this is one of the big plusses for AGM's, and worth considering...

BB. · January 2010

Re: Battery Box design

AGM's should always be boxed/vented as if they will vent hydrogen. Several of their failure modes do include hydrogen venting (over charging and end-of-life failure of catalyst).

-Bill

Sheldon · January 2010

Re: Battery Box design

Thanks for the replies, its probably best to vent, so that the inspector would be happy...

is there a reference on appropriate methods? do I use metal conduit from top holes, or double walled gas flue?

icarus · January 2010

Re: Battery Box design

A pvc pipe is fine as far as I am concerned, and I see no reason why you couldn't use bath fan vent duct or something similar.

Zepher (sp) makes a nice battery vent fan that couples to 3 or 4" PVC/ABS pipe. Couple that to a voltage controller so that it only comes on when the batteries are at gassing voltage and you only draw ~10 watts if memory serves, and only when needed. My runs most of the afternoon while the batteries are in absorb, and since the CC is ramping down the current, there is really no waste.

Tony

Sheldon · January 2010

Re: Battery Box design

Works for me.. now I just need to deal with the plumbing

westbranch · January 2010

Re: Battery Box design

Sheldon, you might want to investigate the CC you are planning on getting for this function: my MX60 has the AUX power switch, that can be programmed to turn on after absorb is reached. Mighty handy, there have been some mentions here of using it to turn on power to freezers etc during float ..

cheers
Eric

Photowhit · January 2010

Re: Battery Box design

Dave Sparks wrote: »

The problem (and it can be a major one offgrid) is that this is loss of capacity is happening during the time of year that the batteries can often not afford the deeper discharge. The guy comes home one day and there is a failed cell or cells!

True for some, you have to build your system to your personal needs. I run an AC in the summer but use very little in the winter. My batteries live outside (in a battery box) The loss of capacity is not a problem (in general here)/ I've never frozen a battery in the system. Cells don't typically go bad in a maintained system, if your approaching a damaging discharge you reduce use or charge in another manner.

I actually like the idea of a Berm structure, to bury a system I would worry about water intrusion. I intended on building one to help with summer temps here to prolong the life of the batteries, one of last years projects that didn't happen.

Blayd · January 2010

Re: Battery Box design

Dave Sparks wrote: »

The problem (and it can be a major one offgrid) is that this is loss of capacity is happening during the time of year that the batteries can often not afford the deeper discharge. The guy comes home one day and there is a failed cell or cells!

Most people do not have a spare battery or it takes weeks to get one. They keep using the system with a generator and it snowballs downhill! Keep them above 40F or have spares because the odds are much better you will need a spare.

Dead batteries in the winter time are annoying, but at least food can be put outside in a cooler and a few candles lit when the primary system fails. Cooked inverters, baked batteries, fried battery chargers, and blown generator engines in the summer time tend to reduce the quality of life. I have a very nice collection of failed equipment and batteries thanks to the extreme high temperatures that we often have.

Like many on a very small budget, I started off with very little, and upgrade as finances allow. The older equipment and batteries combined with low wattage Pv panels that I got for free are cobbled together to serve as a redundant systems that has gotten me over some rough spots without having to run a generator which on average will cost about $1.00USD per hour to run if the cost of the fuel, oil changes, service on the generator and the generator purchase price. Those generator runnings hours can add up fast if the off-grid system has problems, or if there is a lot of weather related non-charge days.

Dave Angelini · January 2010

Re: Battery Box design

Blayd wrote: »

Dead batteries in the winter time are annoying, but at least food can be put outside in a cooler and a few candles lit when the primary system fails. Cooked inverters, baked batteries, fried battery chargers, and blown generator engines in the summer time tend to reduce the quality of life. I have a very nice collection of failed equipment and batteries thanks to the extreme high temperatures that we often have.

Daniel,
while what you say is true for some offgrid, I would also say that for many of my customers (who I enjoy immensely) failure is not an option. All this stuff is just basic electronics and when installation errors, design errors, & maintenance errors stack up you get failures.

Using candles is not an option so spares are required and in some of the remote places I go they have to be stored at the site. Try snowshoes or an ATV with chains and the desert can really look good!

I use to avoid AC in summer because of the generator thing (damage) and I would just tell you that most of the problems from summer generator use can be avoided by using the mini split AC's with internal inverters. (or similar) Nice slow controlled starts and user limited loading keep all the surging unnoticeable. They will allow you to get rid of the generator where you live! It is a beast!

machineman · January 2010

Re: Battery Box design

I built a box with 1/2 plywood supported by 2x4 frame underneath and legs bolted to the wall. Primed/painted then sprayed automotive rubber undercoating on the inside surfaces.

24v @440ah. The image is before I added the vent to the top lid.

Battery Box design

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