Cooling the batteries

WaterWheel

I'm in a hot area and the batteries are in an even hotter garage.      I've considered moving the batteries outside into a hole dug 18" into the ground but testing shows that 12" deep the ground is not much cooler than on top of the ground.       I've also considered making my battery box into a water bath that can be filled at lunchtime and drained every evening.      That's the route I was moving towards when I had a different thought.

What about using a 48v thermoelectric module installed in the wall of the battery box?       A 100 watt unit (draws about 3.4 amps with 2 fans) makes about 340 btu of cooling; not a lot but it would go a long way towards pulling some of that heat out of the box so the box doesn't just sit hot all night.

A 200 watt thermoelectric module is also available but draws over 7 amps.

Perhaps use the AUX function to start the unit when battery voltage is in absorb and cut off when battery voltage drops below 52v.   Since it takes about an hour for the battery voltage to drop below 52v after the sun is off the panels the cooler would continue to run for about another hour with minimal drain on the batteries.

A 48v timer could be used but with a timer you've got the cooler coming on when it's cloudy some days.

Any thoughts on this?

mike95490

If you are in an area with low humidity, you could rig up a swamp cooler system to chill the battery water bath.

WaterWheel

Hot and humid here in the south east

mcgivor

Water has much higher heat transfer rate,  ~25 times higher if I'm not mistaken, the water bath is very effective, i experimented with one using a water cooler with a 90W compressor circulation by a pond pump before building a permanent one, I would expect a daily change of water would work as long as the water temperature is significantly lower than the batteries.

WaterWheel

The tiny water cooler is an interesting idea.         But I was just going to use the slightly cool well water filling daily when the batteries are in absorb and drain during the evening.       I figure a automatic cutoff like I use in the donkey's water trough means I wouldn't have to wait around as it fills and with a drain in the garage floor a few feet from the battery box would work for empting the box.

But building the water bath battery box and running the water lines along with daily filling is a lot more complicated than just hooking up a 48v thermoelectric cooling unit.

But is roughly 300 btu enough to even slightly lower the temperature of 2700 lbs in batteries?   I may be doing the math wrong but it looks like that 300 btu cooling power would take 9 hours to produce enough cooling to lower the battery temp 1 degree fahrenheit.     

It was an interesting idea.     

mcgivor

Complexity is one factor with the water bath arrangement, stagnation is another, at least in a closed loop as I experimented with, perhaps a water bath with a feed into the bottom with overflow drain top would simplify things somewhat, but in my case the ground water is at 29°C, so not enough to be of advantage. The water cooler method was the best at keeping the batteries cool, the switch to Lithium was one of the reasons I abandoned the project, as LA is not that efficient. Having said that I'm sure there will be new challenges I'll have to deal with.

jonr

Thermoelectric coolers are horribly inefficient.   

Also consider a fan moving air from outside and across the batteries - but that is only on when outside temp < battery temp.

Wheelman55

Waterwheel. Desert Rat in the Big Bend has been putting his and several other battery banks in water baths that simply evaporate to cool the batteries. 

Check his post #26 in this thread.

http://forum.solar-electric.com/discussion/350970/my-1-8kw-off-grid-set-up-what-to-do-with-excess-power/p1  

softdown

This has been discussed a few times though I can not recall a successful solution being realized. My 4" "button fan" consumes 10 watts while claiming 8 watts. It may be possible to direct such a fan at a battery cooling "pond" and lower the temperature. A 3" fan would likely consume much less energy - percentage wise.

Insulating the garage? Somehow shading the garage? 

Estragon

FWIW, a pit 7-8' deep would stay pretty close to the local mean temp. In most places, the bank would be happy with that.

In my mostly insulated (not excavated) crawlspace, batteries only recently got over 20°C, despite a relatively hot (lots of 30°+ days) summer. 8' down, they'd stay pretty close to the annual mean (~8°C IIRC).

It's one approach I've thought about to prevent charging lithium batteries in freezing temps if/when the time comes. In my case I'd have to insulate the top of the pit better to prevent cold air settling in the pit, but the general idea is the same.

jonr

The day you dig the 8' hole, it will be close to the annual mean.  But as soon as you start adding heat, it will start warming up.   To some extent, soil is insulation and blocks removal of heat from batteries.

Estragon

If the batteries are adding enough heat that the surrounding dirt is acting as insulation, it's time to replace the batteries IMHO. Mine go up maybe a degree at the end of absorb. Most of the time, they're pretty close to ambient.

jonr

Let's say someone is putting 20 kWh/day to/from a lead acid battery bank that is 85% efficient.  That's 10K btu/day of battery heat that needs to be removed. 

1) Say the batteries are 6C/10F above outside air temp and you want to fan cool them over 5 hours (say 1am to 6am, when it's cooler outside). Depending on heat transfer from batteries to air, you might need 400+ CFM of outside air.    Maybe a 100W fan or .5 kWh/day (from the batteries).  Works best where nights are cool.

2) Using a small  water bath with a chiller (say 200W), it should take about 1kWh to remove the heat.   But this can be done during the day, so no load on the batteries.   Plus the water bath can get the batteries colder than ambient (vs always above it like #1)  - better for battery life.  Say one extra solar panel to get 2x the battery life.  Works everywhere, even year round hot/humid places.

Maybe something like https://www.ssbrewtech.com/products/glycol

3) Evaporation from a water bath - would be quite efficient if you can evaporate about 1/gallon day.   About right if you put all the batteries in a plastic "kiddie pool" and provide a little bit of air flow.  But I doubt it will go much below daily average ambient.  

4) I can't calculate how well underground batteries will work.  A large area (like a basement) may stay reasonably cool even with some heat load.   A small pit might not.  Has potential in climates with cold Winters.

5) Well water cooling - say 125 gal/day flowing into the water bath.  Maybe .5kWh/day in pumping costs (depends on well depth., etc).  A good system if well water is cold enough (maybe < 18C/65F?).


Overall, there is no good reason for warm lead acid batteries.   Remember, 8-10C/16F warmer is 1/2 the life.  As far as I know , there isn't a big downside to fairly cool - like 15C/59F.

softdown

jonr said:

Let's say someone is putting 20 kWh/day to/from a lead acid battery bank that is 85% efficient.  That's 10K btu/day of battery heat that needs to be removed. 

1) Say the batteries are 6C/10F above outside air temp and you want to fan cool them over 5 hours (say 1am to 6am, when it's cooler outside). Depending on heat transfer from batteries to air, you might need 400+ CFM of outside air.    Maybe a 100W fan or .5 kWh/day (from the batteries).  Works best where nights are cool.

2) Using a small  water bath with a chiller (say 200W), it should take about 1kWh to remove the heat.   But this can be done during the day, so no load on the batteries.   Plus the water bath can get the batteries colder than ambient (vs always above it like #1)  - better for battery life.  Say one extra solar panel to get 2x the battery life.  Works everywhere, even year round hot/humid places.

Maybe something like https://www.ssbrewtech.com/products/glycol

3) Evaporation from a water bath - would be quite efficient if you can evaporate about 1/gallon day.   About right if you put all the batteries in a plastic "kiddie pool" and provide a little bit of air flow.  But I doubt it will go much below daily average ambient.  

4) I can't calculate how well underground batteries will work.  A large area (like a basement) may stay reasonably cool even with some heat load.   A small pit might not.  Has potential in climates with cold Winters.

5) Well water cooling - say 125 gal/day flowing into the water bath.  Maybe .5kWh/day in pumping costs (depends on well depth., etc).  A good system if well water is cold enough (maybe < 18C/65F?).


Overall, there is no good reason for warm lead acid batteries.   Remember, 8-10C/16F warmer is 1/2 the life.  As far as I know , there isn't a big downside to fairly cool - like 15C/59F.

A 200 watt chiller? I've sold and installed a lot of chillers that were rated between 1/5 and 1/2 HP. Wish I knew the wattage but 200 watt chillers were not available in the 90's. Apparently much has changed.

Overall - cooling batteries during the day when solar power is readily available seems quite logical.

WaterWheel

Jonr,
Where do you get your calculations of 10k btu per day heat from 20kwh cycling the from batteries?    It sounds about right but I'd love to play with that calculator.        During the summers with the AC I pull about 15 kwhs/day from the battery and less during the rest of the year.
Then in point 2 you seem to be saying 1 kwh energy would be sufficient to keep the batteries cool or remove that 10k btu which seems awfully low to remove that much heat using our less than efficient coolers available.

My water line is buried 18" deep and runs about 60 yds underground from the pump.      Tonight after running for a few minutes water coming out of the sink is 82 degrees so not so great for cooling batteries.        This matches up with my 8" in the ground temperature measurements (84 degrees).        I'm confident that if I dug 7-9' deep ground temps would be cooler but it's not realistic unless I dug a 8'x8' room 8' into the ground so I could add water to the batteries. Rainwater drainage that deep could be done but just a stupid amount of work and money to build that battery box.

My current thought is something like you and Softdown suggested.       A 1/4 hp water chiller for fish aquariums.    It draws 340 watts and cools about 1000 btu depending on the model.       Pump draws another 27 watts.       I could set it to run from 1-6 (summer hours) when I'm usually well into absorb and float with the house AC running.      Prices for a 1/4 hp unit run from $340 to well over $800.         More cooling power would help but I only want to run it when the panels are making extra power so I figure 1/4 hp is a good compromise.

Not enough to keep the batteries chilly but since it's power that I'm not using I could use it to keep the batteries a tad cooler.  I'm already putting a water fill and drain system in so replacing the water occasionally as Mcgivor suggested would be easy. 

jonr

20 * .15 * 3.4 where 20 is kwh of cycling, .15 is the efficiency loss (I assume this ends up as battery heat, but this could be wrong) and 3.4 converts to btu.  Your numbers will vary.

Agreed, a horribly inefficient  thermoelectric cooler won't work so well.     But the one you mention isn't efficient either (340W to remove about 300W of heat is a very poor COP = .86.  In theory, COP around 3 should be possible.

1000 btu/hr is probably too low to get the job done in < 6 hours.  Consider more like 2300 btu/hr (it can cycle on/off as needed).

If you are going to cool below ambient, then it makes sense to insulate the water bath (not doing so will require more btu).

A $1500 cooling system to double the life of $5000 of batteries makes sense to me.

Dave Angelini

Duct the garage/battery room with air from the cool house. It is the cost of doing business.

WaterWheel

Since I use a mini-split and there is a large unheated room (old garage that was later enclosed) between the current garage and the living room it is about 70' from the closest point of air conditioned room that I'd need to run some sort of powered duct.      Since all those floors are concrete that ductwork would be running through hot crawl spaces above the hot rooms.

WaterWheel

Jonr,       Thanks for explaining the math.    But i doubt any cooling effort I make is going to double the life of the batteries, maybe more like a potential 20% life increase since the batteries are generally below 80 degrees about 4-5 months of the year and I've only going to get so much cooling effect in the 2-5  hours of excess energy that can be used for cooling per day.

Dave Angelini

Anything can be done. I get it is work and not going to pay-off with LA batteries.
With a battery that might outlive you, it might be worth the work as the good ones log temperature and will not be there if you cook them

jonr

As far as I know, the benefit is still there at lower temps.  Say 65F down from 80F.    Agreed, you have limited hours - which is why a larger system (more btu/hr) + water storage (no battery hit)  is better.   Or in some cases, get  more panels.  They are getting pretty low in price.

Estragon

One downside to cool/cold batteries is the capacity gets reduced, so get taken down to a lower %SOC with a given load. FWIW.

mcgivor

The real problem is when ambient temperatures prevent any cooling effect, each charging produces heat due to inefficiencies inherent to lead acid batteries, this manifests over time. When ambient temperatures do drop, the battery temperature remains elevated, dropping slightly each day however with cyclical use, each day means a little more added heat, two steps forward, one step back. Unfortunately in hot climates  the only time the temperature drops is overnight so using a fan means deeper discharge with the corresponding heat generated to compensate. The refidgerated  water bath experiment was very effective in that it used surplus energy. Hopefully will LFP batteries being more efficient will at least reduce the self heating, considering 4Kw generated 0.8 Kw end up as heat with LA, using 80% round trip efficiency, as opposed to 0.32Kw for LFP  (92% RTE) ~0.5Kw difference, albeit with the added  complexity involved.

Dave Angelini

As most of you folks are not using LI, I would add to this discussion that it is during the charge time that is critical to their life. Do not park a hot car in the garage.......Go thru charge as fast as is recommended bu the manufacturer, maybe a a bit less.
 Still at 99% Sol here after almost 2 years. They started at 99% BTW

WaterWheel

Dave,     In many ways I agree that LI is a better choice and if the companies who made what ever lithium battery I chose had been making their models for over 5-8 years so I had a bit more confidence in their ability to have replacement BMS parts available say 12 years after I purchased the battery I would probably have gone with lithium just for the east of maintance.

BB.

One of the problems with batteries--Even if the battery mfg. has a good history (>5-8 years), they do sometimes seem to lose the "recipe" -- Even for the 100+ year old Lead Acid tech batteries. 

-Bill

WaterWheel

Put the batteries in the water tight box yesterday, wired up this morning, and have some numbers to report.     
This is a commissioning charge so not a full blown bulk and absorb.      Batteries averaged 1.252 and 3.3v which seems a tad low for "fresh from the factory" batteries.      Since 5000 series batteries are double walled Rolls wants the outer cap of 1 battery temporarily removed so the temp sensor can be placed directly on a 2v cell.       I chose the middle cell and put the sensor where it was not touching the outer battery wall to assure accurate cell temp reading when cool water is surrounding the batteries.     There is about a 3/8" air gap between the outer wall and the temp sensor.       I haven't measured but the box holds about 80 gallons of water when filled 13.5" deep. 
  
Batteries were wired up, went through a short bulk and after an hour of absorb water was added:     Water chiller was not installed at this point.
pulling 48 amps, battery 90 deg,     just added water in box was 72 degrees, air was 89 deg.
30 minutes pulling 33 amps battery was 84 deg
60 minutes pulling 22 amps battery was 86 deg
90 minutes just went into float, battery was 86 deg      
after 90 minutes the air was still 89 degrees, water temp had risen 4 degrees to 76, batteries dropped 2 degrees.

While the 1st test was going on the 1/4 hp chiller was installed.      Batteries remained in float and chiller was started.     
start,             battery 86 deg, water 76 deg, air 89 deg
45 minutes    battery 84 deg, water 74 deg, air 91 deg
90 minutes    battery 84 deg, water 73 deg, air 90 deg
150 minutes  battery 82 deg, water 71 deg, air 90 deg
180 minutes  battery 81 deg, water 70 deg, air 89 deg

Not a hard charge cycle so not much heat produced.       Batteries in absorb using water chiller dropped 5 degrees in 3 hrs (about 2 deg per hour) while the air temperature remained about 90 degrees.       Water dropped about 2 degrees per hour.     The chiller is small but helps and the water will continue to cool the batteries for another few hours.

Update 3 hours later,     No chiller, just resting batteries in previously chilled water stagnant water (no pump running).       Batteries have dropped 1 degree to 80.      Water is up 3 degrees to 73.    Air temp has dropped 5 degrees to 84.

Dave Angelini

Many of you do not know that the OP here makes some of the best WaterWheels i have ever seen. Some amazing work and I am a bit let down that he did not use one for this application   

Nice job and still no leaks? Don't let them freeze....You should link your web page?

WaterWheel

Years ago I picked the general area I wanted to live the rest of my days and rented a house in the area for 2 years while I searched for what I wanted with a good hydro source being highly desirable.        But as most of us know good hydro sites are rare, especially considering the requirement to bring an 18 wheeler in for the business occasionally and a desire not to spend every penny I had (need to save for retirement).

After 2 years searching I found a beautiful patch of 10 acres with no neighbor's houses in sight (nearest neighbor is 300+ yds away).       House is on a bit of a rise with fields extending at least 200 yds in all directions so fantastic sunsets with two 2400 sf buildings for my business with a huge parking area for trucks.     A nice farmhouse with a large 3 car garage and an additional 400 sf room (old garage that was enclosed but not heated) I call the junk room and is perfect for little projects.      1100 sf of deck space means plenty of room to watch the deer and sunsets from.      But sadly no water.       Still, after 2 years of looking I had to buy it. 

But if I ever win the lottery I'm buying that 60 acre mill site 40 miles North of here.       800 gpm  with 18' of head.  

WaterWheel

Day 2 results:    
Light 11% DOD discharge last night so after sitting in the chilled water overnight and going into absorb before 10 am the batteries were 78 degrees at noon with the air temp at 82 degrees.       After sitting overnight the water temperature was up to 78 degrees.     Timer turned on the water chiller at noon.

4pm air is 89 degrees, batteries have been in  float (less than 2% amp draw ends absorb automatically) for about 2 hours and are 75 degrees, water is 67 degrees. 
On a day like this without the chilled water bath I would normally expect to see the batteries around 93-95 degrees.

6pm  air still 89 degrees, batteries are 73 degrees, water is 62 degrees,  timer will turn the chiller off in a few minutes.

I'll take the batteries down at least 20% tonight and see what the temperatures with a heavier charge are tomorrow.

softdown

Keeping your charging batteries well below ambient air temperatures is a significant success. I wonder what the charging battery temperatures would register without the cooling apparatus.

Also wonder what to expect of typical well water in your location. Well water may be cooler or warmer than ambient air though I doubt there is too much truly cool well water in most of Texas. Not a geologist, I could be wrong. 

There may be a general formula for expected ground temperatures  - say 10' below the surface. Think I may have heard that 55F is typical here. My root cellar is currently sitting at 59F after a hot summer drove it up to 61F for awhile.