Is there anyway to SAFELY heat a battery box?

[Sunking]

OK Spike now that I know what you are doing and is MISSION CRITICAL here as my recommendations.

Minimum Autonomy = 10 days

Solar Panel wattage = 2 X what is required for winter. That way if you do go deeply discharged say 3 days without sun around Xmas just one day of sun will bring it back up full. Overkill I know, but you cannot afford to have it go down. In summer you are on cloud 9 with abundant power to spare.

So make damn sure you get your daily watt hours down to an art.

Good Luck

SK

[spike001]

[QUOTE=Sunking;71720]I do not have any problem with what C&D is saying, but two points need to be made here.

1. They are speaking for their product, not others. Although most of what they are saying holds true for other manufactures.

2. While it is true that an AGM battery can freeze, no doubt if allowed to be in a discharged state in extreme cold. But bottom line is an AGM can take much colder temps for much longer period of times than FLA. No doubt about it.

Thanks for all of the input. I've just finished reading the technical write-up supplied by Concorde and it does appear that AGM is the way to go given the environment. I have considered the fact that FLAs will produce and maintain heat from the act of charging and the thermal mass and SG of the battery are definite factors. However, my overriding risk factor is that if the computer that will be running shuts down for any reason then it is basically lights out until spring. I therefore have to eliminate, as best possible, all potential risk factors. Basically i'm putting together a solar powered surveillance system from off the shelf components that will allow me to monitor an off-grid camp 300 miles from home. I've completed and tested the camera components of the the project and am now focusing my efforts on the power system. Once the design is complete and I have acquired all of the components i'll be running it for a number of months in Toronto so that I can validate design & funtionality. Next Fall it will move north to it's final home. As has been noted above the amount of sunlight will be a critical factor....so the plan is to ensure that I have more than enough PV capacity for days when there is sun and enough battery capacity for the days when there is no sun. Besides having the ability to check in on the place my other motivation is to see if I can pull it altogether and make it all work as planned. Thanks again for the comments/thoughts and considerations that have been brought forward. Best, mike

[Sunking]

Hi Mike,

Without any implication pro or con about the batteries they manufacture, I have found this description of battery freezing to be helpful.

I do not have any problem with what C&D is saying, but two points need to be made here.

1. They are speaking for their product, not others. Although most of what they are saying holds true for other manufactures.

2. While it is true that an AGM battery can freeze, no doubt if allowed to be in a discharged state in extreme cold. But bottom line is an AGM can take much colder temps for much longer period of times than FLA. No doubt about it.

What I did not touch on and the OP should consider looking at as you mentioned Inetdog; is temperature compensation. Not only with the charge controller as that is easy, but more importantly battery capacity and panel wattage.

If he is in an area that gets to -40 F in winter tells me he is in a area that gets 1 hour or less of solar insolation. This means extremely over sized panel wattage where charge rates are going to approach 1C or maybe even higher. Only 1 battery type can take that, very few AGM's can do that. But only AGM in the lead acid family can possible do that.

Battery Days of Autonomy gets real critical in cold climates. Minimum Days of Autonomy is 5 days, but there is a catch, that is where minimum Sun Hours is greater than 4.5 Sun Hours in short winter months. When you get to less than 2 Sun Hours and I highly expect that is the case here you are looking at 10 days of Autonomy. That may come as a shock to most but you need to understand why.

Let's say you have an application that requires 100 Watt Hours per day and you want a 12 volt system which is doable for such a small system. At 5 day autonomy requires 500 watt hour capacity or roughly a 12 volt 50 AH battery. However that assumes a moderate climate where the ebatteries stay above say 50 degrees F. Well here in TX where I am at no problem, I get 4.6 Sun Hours in Winter. So I would need:

Panel Wattage = 40 watts
12 volt 40 ah FLA battery
5 amp PWM controller.

No problem and dirt cheap.

But here is the catch. As temps go down, so does battery capacity. When you hit -40 F capacity is reduced 50 to 60%. So what you think is 5 day autonomy is now 2 days. In addition you only likely get 1 or less sun hours in that kind of cold weather. Now the system becomes:

Panel Wattage = 200 watts
20 Amp MPPT Charge Controller
12 volt 80 AH AGM battery

This is a much larger more expensive system than one required in TX to do the same thing.

[inetdog]

Sunking, thanks for the response - i'll research AGM batteries. I was hoping to get by with flooded from a cost standpoint. Oh well! Just one - ok two - additional question - are you recommending AGM because they don't gas or can they handle the temperature variance? Also, I'm going with Trojan because of reputation and I believe they build a quality product, in your opinion are there other manufacturers I should be investigating. (My exposure to the solar industry is about 3 yrs out of date.)

As for your question. Although this system will be installed in a forest it is directly next to a clearing and a lake - so I have clear views of the west, south and east.

Thanks again, mike

Hi Mike,

Without any implication pro or con about the batteries they manufacture, I have found this description of battery freezing to be helpful. It covers the variation in freezing point with specific gravity (and the corresponding Depth of Discharge (DOD)), as well as to some extent the heating of the battery which will take place during both charging and discharging. The low internal resistance of the Concorde AGMs is very desirable overall, but will reduce this self-heating effect. Another important consideration for batteries that will not be maintainable for a long period during the winter is the low self-discharge rate of AGM batteries, if the load that they will be providing for is small compared to the capacity of the batteries.

Because of the self-heating, it may be enough or nearly enough to provide very good insulation around the batteries, with thermostatically controlled ventilation to keep he temperature from going too high. The thermal mass of the batteries can then maintain a workable temperature through the night. During the day, the charging process will provide some heat, and the excess solar energy after the battery is charged can be diverted to some form of thermostatically controlled resistive heater.

The main advice in the tech note is to oversize the battery bank so that the SG never drops to the point where freezing can occur. But in addition to this, you will have to consider what will happen when you have an extended period of low light (mid-winter low sun combined with snow or clouds, for example.) You may need to take some specific measures to keep snow from collecting on the panels. People who use PV off grid through northern winters often place their panels in a near vertical orientation for the winter, since that will still be close to the sun angle and will take advantage of the large amount of light diffusely reflected off snow on the ground. Make sure the panels are high enough to avoid drift accumulation.

It is also critical that the Charge Controller (CC) that you use have temperature compensation of the charging voltage and that it use a remote temperature sensor at the battery pack.

You have a lot of major and minor details to provide for in your design, but Dereck has given you a very good starting point. As you progress through the design, please let us know how you are doing and what you have come up with.

[Sunking]

You are welcome.

[spike001]

Yes to both. AGM's are best used where gassing, unusual installation orientation, and extreme cold climates. Think Airplanes, which is what AGM was developed for.



Trojan in reality is just middle of the road quality, not the best. At one time they were excellent quality until they they were sold off to private equity. What made Trojan popular for Renewable Energy applications started some 10 to 15 years ago when off-grid solar started becoming more popular. At that time there were no true Deep Cycle batteries on the consumer market for sale. Trojan golf cart and floor machine batteries were about the only choices one had that was readily available and somewhat affordable. But that is far from the truth today. Even Trojan has redesigned there product line-up for RE applications. For example there is a T-105-RE and L16RE family of products. These designs use heavier thicker lead plates. Those are improved T-105 and LR16 models but in no way top of the line batteries.

For FLA batteries the Rolls Royce of consumer grade batteries is Rolls/Surrette 5000 and 4000 series. They come with a 10 and 7 year warranty respectively, about the best you can get. compare that warrant to Trojans 2/3/5 year warranties. Other notable high quality FLA is SolarOne HUP, and Crown

For AGM the only one I recommend is Concorde Sun Extender series. Concorde pretty much invented the AGM battery and is the exclusive manufacture for the US DOD, and most Aircraft manufactures. They are second to none.

Thank you again....you most likely just saved me hundreds of dollars over the long term AND removed a couple of headaches from my design requirements. Best, mike

[Sunking]

ok two - additional question - are you recommending AGM because they don't gas or can they handle the temperature variance?

Yes to both. AGM's are best used where gassing, unusual installation orientation, and extreme cold climates. Think Airplanes, which is what AGM was developed for.

Also, I'm going with Trojan because of reputation and I believe they build a quality product, in your opinion are there other manufacturers I should be investigating. (My exposure to the solar industry is about 3 yrs out of date.)

Trojan in reality is just middle of the road quality, not the best. At one time they were excellent quality until they they were sold off to private equity. What made Trojan popular for Renewable Energy applications started some 10 to 15 years ago when off-grid solar started becoming more popular. At that time there were no true Deep Cycle batteries on the consumer market for sale. Trojan golf cart and floor machine batteries were about the only choices one had that was readily available and somewhat affordable. But that is far from the truth today. Even Trojan has redesigned there product line-up for RE applications. For example there is a T-105-RE and L16RE family of products. These designs use heavier thicker lead plates. Those are improved T-105 and LR16 models but in no way top of the line batteries.

For FLA batteries the Rolls Royce of consumer grade batteries is Rolls/Surrette 5000 and 4000 series. They come with a 10 and 7 year warranty respectively, about the best you can get. compare that warrant to Trojans 2/3/5 year warranties. Other notable high quality FLA is SolarOne HUP, and Crown

For AGM the only one I recommend is Concorde Sun Extender series. Concorde pretty much invented the AGM battery and is the exclusive manufacture for the US DOD, and most Aircraft manufactures. They are second to none. Concorde batteries is one of the very few AGM batteries that can deliver a 1C discharge rate at the 20 hour specification. This is due to their very low internal resistance. They are also one of th every few AGM's that can take a EQ charge. Concorde would be an excellent choice for your application.

[spike001]

Use AGM batteries. Although I do hear a RED FLAG in your design. Just how do you expect to give the panels full clear view of the horizon to the west, south, and east located deep in the forest?

Sunking, thanks for the response - i'll research AGM batteries. I was hoping to get by with flooded from a cost standpoint. Oh well! Just one - ok two - additional question - are you recommending AGM because they don't gas or can they handle the temperature variance? Also, I'm going with Trojan because of reputation and I believe they build a quality product, in your opinion are there other manufacturers I should be investigating. (My exposure to the solar industry is about 3 yrs out of date.)

As for your question. Although this system will be installed in a forest it is directly next to a clearing and a lake - so I have clear views of the west, south and east.

Thanks again, mike

[Sunking]

Use AGM batteries. Although I do hear a RED FLAG in your design. Just how do you expect to give the panels full clear view of the horizon to the west, south, and east located deep in the forest?

[spike001]

I am in the process of designing an off-grid project that will require 24V to power. Based on my research I'd like to use FLA batteries. ( most likely a pair of Trojan 12 V batteries ). So far, so good. My challenge / question is that they will be running in a fairly harsh climate - lows of -40C in the winter. I'll be building a well insulated/ventilated box to house the batteries but I was wondering if anyone has suggestions on how to safely heat the box. (these will be running in the middle of a forest and somewhat isolated) Eg i'm aware of the battery gassing problem, so some sort of passive or indirect heating capability would be ideal. At this time I'm thinking that i may need to design/implement a second independent PV subsystem to address heating the battery box, which I'm fine with if I need to pursue that route. But I am interested to know if there are any ideas re. a safe indirect/passive mechanism to generate the heat. Thx.....