# Flexi-Watt Battery Heater Control System



## PeterH (Mar 20, 2009)

Well... I see 50 views but no replies. I'm going to take that to mean:

1. no one sees a problem with this idea YET...

2. none of those 50 views is by someone who has built something like this already... 

3. everyone else has a satisfactory solution to keeping their batteries warm or doesn't need to keep them warm... probably means they are more concerned about cooling... (not a problem here in far Northern Idaho)

So, I'll go ahead with this idea and document it as best I can as I go along. I might be documenting my mistakes as well... but even that might help someone in the long run.

I think my next test will be to set up one of my battery boxes outside of my rig, insulate it as planned and fill it with batterys and a warming plate. Fire it up and see what happens. But before I do that, I'll need to come up with a reliable way to measure temperature of the plate and the batteries. Fortunately, my Fluke meter has a temperature probe that I can use for that part of the test.

I want to make sure the warming tape doesn't go above a certain temp but only warms the batteries to 50 or so degrees F. I think that means I'll be monitoring this test manually because I don't have a control circuit available yet... at least not one that is ready to be used in the box. That will be the next step.

Pete


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## Wild (Aug 19, 2011)

Hi Peter I like the idea and was thinking of something similar with 11" tape which produces 20 watts per foot. I am sure the 50 lurkers are interested in this project as well


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## PeterH (Mar 20, 2009)

Hello Wild,

I selected the 3 inch wide material thinking I'd have more flexibility (no pun intended!) with how I use it. Also, two 3 inch strips side-by-side will fit my boxes nicely.

I ran a test last night using two strips under one module of 7 CALB cells in one of my battery boxes and it seemed to warm up the bottom of the cells to 60 degrees rather quickly (15 minutes). Although, I think that must have been the air between the cells. 

For this test I was using a temp sensor that came with my Fluke meter to measure the temp. The sensor is taped to the side of one of the interior cells about 1 inch above the aluminum plate.

I've just posted a detailed query on a Arduino forum asking about the best sensor to use. I'd like to be sure to do this right the first time if that is possible!


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## Ziggythewiz (May 16, 2010)

I don't know how valid the results are when sitting in the open air in your garage. I would make a whole box of the pink stuff and see how much it warms up.


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## Wild (Aug 19, 2011)

I think the 3" tape @ 6 watts Ft will work fine Peter the 11" tape may be overkill but I am considering a different application along with a high tech spray on insulation and possible air flow into the passenger area. I think you are heading in the right direction, good work so far!


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## DIYguy (Sep 18, 2008)

PeterH said:


> Hi there!
> 
> I can't find the thread, but someone out there is using the Flexi-Watt heat tape to keep their batteries warm. You gave me the idea so let me start by saying thank you. I think this is the perfect solution for me. I bought 20 feet of the 3 inch wide tape.
> 
> ...


Hi Peter. Looking good. It was likely me, as I think I introduced this stuff. I have built my boxes with it also. I used the 6" wide version and power it with my pack voltage.. . so it does push a bit more heat. One suggestion is to have two temperature sensors. One for temp away from the heat source (ambient) and one at the heat source for over temp protection.
I used less intelligent control system. Two temp snap switches in series with the control side of my DC SSR's. 

Cheers,
Gary


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## PeterH (Mar 20, 2009)

Hi Gary,

Yes, I'm sure you are right, it was you and your build that got me started on this.  Love this Flexi-Watt stuff!

Hope you'll monitor this thread as I go along... 

Just ordered 20 of the TMP36 sensors. Planning to have possibly 3 sensors in each box. Two associated with the heating control system as you suggested and a third in each box as an isolated monitor on a different Arduino controller to backup the monitoring system just in case.

Do I sound paranoid about monitoring this? Starting to feel like it...

Details as they become available. 

Pete


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## PeterH (Mar 20, 2009)

A basic question I must answer is: What is the minimum temperature we should aim to keep our Lithium batteries above to ensure proper charge while parked and connected to power for charging?

An additional question. Between my 5 battery boxes, how much can the temperature vary before I run into problems with uneven charging between cells in the various boxes?

Naturally, I'd like to maintain all 5 boxes at the same temperature, but I don't know just how precisely I'll be able to coordinate the temperature in each box to ensure consistent temperature and therefore charging.

Any thoughts?

Pete


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## DIYguy (Sep 18, 2008)

PeterH said:


> A basic question I must answer is: What is the minimum temperature we should aim to keep our Lithium batteries above to ensure proper charge while parked and connected to power for charging?


I think the right answer is Zero degrees. That said, you should likely shoot for something above this.



PeterH said:


> An additional question. Between my 5 battery boxes, how much can the temperature vary before I run into problems with uneven charging between cells in the various boxes?
> 
> Naturally, I'd like to maintain all 5 boxes at the same temperature, but I don't know just how precisely I'll be able to coordinate the temperature in each box to ensure consistent temperature and therefore charging.
> 
> ...


Good question. Not sure anyone outside of some university lab, has looked into this. . . and maybe not even there. I would shoot for 5 Celsius degrees. . . but have no data to back that up.

One other thought, I used some extra wires to monitor "on" state of heaters. I haven't actually implemented this (I know, I'm behind again), but since SSR often fail on, it's likely a good idea. With your more intelligent control system, it should be fairly easy to do....?? I did pick relays capable of handling much higher current than I will be drawing so, I doubt I will have issues with this.. . ??


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## Wild (Aug 19, 2011)

PeterH said:


> A basic question I must answer is: What is the minimum temperature we should aim to keep our Lithium batteries above to ensure proper charge while parked and connected to power for charging?
> 
> An additional question. Between my 5 battery boxes, how much can the temperature vary before I run into problems with uneven charging between cells in the various boxes?
> 
> ...


This answer should come from the battery manufacturer and there may be a difference in heating depending on the battery casing itself if it has one.

The controller should be able to monitor and control every Battery box separately and DYIguy brought up a critical point there must me a failsafe built into the controller.


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## PeterH (Mar 20, 2009)

Hi there,

You suggested zero degrees... was that F or C? I'm planning to shoot for a minimum of 40F for the battery boxes while they are in my unheated garage. Of course, that will be simple to adjust in the software of the controller.

I don't have any experience with these batteries... so zero sounds rather cold... but I can't argue with experience! I'm guessing you are suggesting 0 C as the minimum. I'll see what I can find in the CALB manual... I don't recall much info on this subject...

Meanwhile, the other question about differing temps across the batteries... sounds like we are back on the bleeding edge of technology again.  Of course I'll try to keep the temps even across all 5 boxes. I don't see why that will be a problem because each box will be independently controlled from the single computer.

With that said, your statement about SSRs failing open is a bit alarming and it suggests a design change. I think I'll put the entire system on a single relay that I can switch off with the software, but that is switched on manually to enable the system. Probably when it is initially connected to the mains.

The computer and sensors will then monitor all 5 boxes and compare the temps. If one starts to fall outside the range of limits I'll establish, ie. too hot, the main contactor for the entire system will be shut down. I just need to find a suitable relay. I called it a contactor a moment ago because I was thinking of the two kilovac EV200 contactors I have on hand. But they won't handle AC so I'll need to find one to work as the master control. Got any suggestions?

These are my knee-jerk thoughts on this matter... I've got plenty of time to let them ferment and evolve. I'm waiting for parts and working on other things on the EV... such as prepping the rest of the heater slabs for the other 4 boxes. They are all cut to size, I just need to apply the Flexi-Watt and solder the connectors... and all that sort of thing.

I'll modify that diagram on the first post to this thread... tonight, assuming I survive a day of skiing.

Pete


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## DIYguy (Sep 18, 2008)

PeterH said:


> Hi there,
> 
> You suggested zero degrees... was that F or C? I'm planning to shoot for a minimum of 40F for the battery boxes while they are in my unheated garage. Of course, that will be simple to adjust in the software of the controller.
> 
> ...


Yes, sorry, Celsius. Charging at lower temperatures results in permanent capacity loss due to metallic lithium formation at the negative electrode. I think the charge rate must significantly reduced at these low temps also. I think your 40F is a good idea. 
As for the SSR, I think the failure is due to high in-rush. (others please chime in), so over sizing them somewhat may be beneficial. While the EV200's will certainly work. . . seems a big overkill. Of course I needed DC rated SSR's so, I went with the Fotek ones from China. You can likely source Crydom relays locally.
http://www.diyelectriccar.com/forums/attachment.php?attachmentid=9541&d=1300451399


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## lowcrawler (Jun 27, 2011)

It is less than 0C here for 5 months out of the year... so I'm following this with interest.

Personally, I'd planned on just a 'little' heating -- just to help. Keeping it at 40F when it's 60 degrees colder out that than seems 'difficult' at best. How much heat does that 11" throw out?


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## PeterH (Mar 20, 2009)

Hi there,

Sorry for the week long delay in any updates to this thread. I've been down with some bug I picked up. While that kept me in the house and out of my unheated garage, I was somewhat productive in the house working on the control system for my battery warmers.

I've received my Arduino Mega and 20 of the thermal sensors. I also received a 4 line by 20 character LCD display that works with the Arduino so I can put a nice display into the system. I have the LCD connected and in a nice project box to house the brains of the system. 

I spent today working on the battery boxes and the Flex-Watt tape.










You are not supposed to cut thru the black part of the heat tape so after cutting them to length, I knotched out the tape so I had a clean end to each section of tape.










Once you cut through the plastic on top of the conducting metal in the tape, it solders very easily.










Of course, it helps if you sand that metal strip a bit so the solder can attach to the metal well.










After everything cools I covered the solder joints with silicon to help with insulation and water proofing. Not that I expect any water issues, but I like to be careful. Once the silicon sets up, it will all be wrapped in electrical tape as some additional insulation.










I'm expecting them to all be ready for installation tomorrow.










This is one of the finished boxes. You can't really see the heating pad in the bottom of the box but the Flex-Watt tape is on the bottom side of that metal plate in the bottom of the box. 

The two conduit connectors on the end of the box are for the 00 welding cables but I don't see any reason I can't also run the heater power lines thru the same conduit.










Hopefully, I'll have more info to post in the next day or three.

Pete


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## DIYguy (Sep 18, 2008)

Looking good. Pretty much the same thing I did. Mine is 6" wide.
I used the aluminum tap to fasten my flex-watt strips to the underside of the Alum sheet.


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## PeterH (Mar 20, 2009)

Just a quick update. I'm still working on this project.

I now have all 5 aluminum plates ready to go. The largest uses 3 strips of the 3 inch wide Flexi-Watt heat tape and consumes 64 watts of power. It will sit under 15 batteries in the rear battery box. The other 4 boxes consume around 35 watts each. So I'm guessing everything will draw less than 2 amps of 125 VAC when plugged in. That doesn't seem like much at the moment... so hoping it turns out to be enough to keep things warm in the insulated boxes.

I bought an Arduino Mega board thinking I'd need all the extra analog inputs for the 10+ temp sensors to be installed. Then I managed to destroy the board with my drill... won't go into details...

Since then, I'm back to my Arduino Uno board which should work just fine since I switched to a different sensor. I discovered that using standard TMP36 sensors won't work because of all of the EMF that will be going on. So I've switched to a DS18B20 Digital Temp Sensor that will communicate temperature digitally to my Ardunio. In theory, I can also put all 10 of the sensors on one digital input. I only say "in theory" because I haven't tested it yet.

Got all the parts together last night only to discover that I really needed a few more bits and pieces before I can put it all together so placed another order on Jameco.com

Meanwhile, I've got all of my battery boxes set up and just waiting for these temp sensors before I load up the batteries. Running conduit for everything under the build is a bit of a pain but I think it will be well worth it later when I'm using this rig in all weather.

More details as soon as I have some concrete progress.

Pete


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## PeterH (Mar 20, 2009)

Sorry that I've not updated this thread much lately... I've been busy with about 5 sub-projects on the EV. One of those was the Flexi-Watt heater control system so time for an update and to ask for some input.

Since I'll be sharing the final results of this effort with the community, it is only appropriate that I ask for input on the design of the software. I've assembled the necessary Arduinio Uno controller and I have a program working to collect temperature information from 11 individual sensors. So far this is all working on a single breadboard except for one sensor that is at the end of 10 feet of Cat3 cable. That Cat3 cable is four conductor, solid copper wire. Very small gauge but appropriate for this project.

My intent at the moment (unless someone has a better idea) is to position two sensors in each of the five boxes. I'd like to place one sensor at the bottom of the batteries and touching the aluminum plate the batteries sit on and that has the flexi-watt tape attached to the other side. 

I'm thinking I'll want to monitor the temp of that plate to make sure it isn't getting too hot. I think that is unlikely because during my bench testing it never got too hot to touch. However, I'm a little paranoid when it comes to heating all that money in the battery boxes. 

I'm also planning to position the second sensor in each box, a couple of batteries 'over' from the the first sensor and about half way up the side of the batteris so it is surrounded on all sides by the batteries. I'd like to think that would give the most accurate measurement of battery temperature. I'm referring to this as the battery temp sensor and the other as the base temp sensor.

I don't recall if I posted this data or not, but I have 3 sizes of heating plates. Smallest draws about 26 watts, the mid size draws about 35 watts and the biggests, beneath the 15 batteries in the back, draws 65 watts.

Now, to the software logic. 

I think one routine should monitor the base sensors (those touching the aluminum plate) to make sure they are in fact working and that they aren't too hot. I'll have one senor positioned under the hood to measure outside temperature as a reference. So if the base sensors aren't warmer than the outside temp, then something is wrong. Plus, I'll program this so that the entire system doesn't even come on unless the outside temp is below a certain temperature. 

I think another routine should be monitoring the temperature of each of the battery temp sensors to make sure they are all staying at nearly the same temperature. I'm afraid if one box gets too far in temp from the others, that they will take on a different amount of charge and get out of balance with the rest of the batteries. So I imagine I might have to switch off the box heater with the highest temp until the others catch up.

I also have a 4 line, 20 character per line LCD screen included in this design that will display the current temperatures. I don't think this needs to live inside the vehicle's cabin only because this system won't be heating unless it is in the garage. However, as I type this, it seems it might also be VERY handy to be able to monitor temps in the summer, just in case things are getting too warm. That isn't likely here in far norther Idaho where I still have a lot of snow in my yard and it rarely sees 100 degrees F in the summer. So, yes, I'll mount this inside the cabin just to add to that NASA look that I'll have by the time all the other instrumentation is installed.

If you have any thoughts about this, please speak up! 

Thanks,
Pete


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## swoozle (Nov 13, 2011)

No thoughts other than it looks great so far, keep up the good work. I'm behind you in my build but had wanted to do something very similar. I picked up the arduino and a display and had got it up and running before you started this thread. Now I'm just getting to the box building stage and so plan on incorporating the flexwatt tape also. I'd very much like to take advantage of your code for the thermo sensors.

Are you still going with the all-or-nothing approach for feeding power to the boxes? It sure would be nice to have individual box control but I certainly agree with the skittishness re individual SSRs.


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## PeterH (Mar 20, 2009)

At the moment, yes, I'm still plannying the 'all or nothing' approach. Basically because during my bench testing, I ran each panel individually for about an hour. I monitored the temperature every 10 minutes or so and never did any of the panels get too hot to touch. With that said, I'm expecting that the system will probably keep each panel switched on for fairly long periods of time because the source of heat isn't that great.

Given that, I'm not expecting to see the Arduino switch the individual SSRs on/off fast at all. Meaning I'm hoping the SSRs last as long as the batteries do.  Each box will have it's own set of 2 sensors and its own SSR to apply power as called for by the software.

I'm going to have one 'master' SSR that provides power to the 5 individual box SSRs. Why? Well... at the moment, I'm thinking I'll have a routine in the program that is watching the 'big picture' and will remove power from everything if it detects anything out of the ordinary in the box temperatures.


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## swoozle (Nov 13, 2011)

How's progress on this, Peter? 
Any chance I can get a copy of your code to start from?


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## PeterH (Mar 20, 2009)

Hi there,

Odd you should ask... I was literally, under my rig not 90 minutes ago installing a water-tight box for the 6 solid state relays for the heaters. I'll post some photos later... just need to take them first.

I've not made any progress on the code side since I've been focused on the hardware side of things. All of the temp sensors are in place and all of the wires are run to the box with the SSRs but beyond that, I haven't done anything with the heaters.

I'm hoping to start working on the code once this thing is actually drivable. Which, might be VERY soon because UPS just delivered my replacement Soliton Jr.! 

I'll be happy to share any code I come up with. At the moment, all I have is an Arduino program that polls each temp sensor and displays the current temp at each sensor.

Pete


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## PeterH (Mar 20, 2009)

It is now the first of December 2012 and I have over 3500 miles on my EV. In the past few weeks I've been working on the battery box heating system and I'm happy to report that it is working just as I'd have hoped. So far, the temps have only reached 32 degrees F. in my garage so I'll continue to test the system as the outside temps drop. I don't heat the batteries while driving, only while they sit overnight in my garage. However, if I had a source of 110 VAC, I could run the system while I'm at work (if I had a job! )

At max load, I'm only drawing 200 watts of power to heat the insulated boxes. My cells are being kept at about 50 degrees F. over night with this system. I might be able to maintain a higher temp but that is my current target temp so the software is controlling the hardware to do just what I wanted. 

I've connected the Arduino Mega to an LCD text display and mounted the display on the dash of my EV so I can see the current temps at any time. Since I have 5 boxes to heat, I really need more than a 4x20 character display, but I'll make do for now with what I have. 

Thanks
Pete


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## corbin (Apr 6, 2010)

Pete.. this is great! I want to start heating my cells...they don't like the cold, even in CA when it is > 50F most of the time. 

Can you post your code to github.com somewhere? 

Also, where did you get the watt tape from?
What SSD's are you using?

corbin


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## PeterH (Mar 20, 2009)

Hi there!

I'm using Jameco part number 162342 for my SSRs. They cost me 18.95 each and I have 6 of them in the system.

I bought the Fleswatt material at this site:

http://beanfarm.com/index.php?cPath=1204_1205

It is dead easy to solder connections to the tape once you have it cut to size.

I have 11 temperature sensors on board. 2 in each box and one outside for a comparison with the outside air temperature. I'm considering adding a few more of those sensors for other areas of the EV where I'd like to monitor temps. Hacktronics supplied the sensors:

http://www.hacktronics.com/Sensors/Digital-Temperature-Sensor-DS18B20/flypage.tpl.html

I've only surfed github.com so if you'd really like a copy of my current code, I'd be happy to send you a copy of the sketch. Just send me a PM. The code needs more work, but in it's current condition, seems to be working just fine.

Pete


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## Zak650 (Sep 20, 2008)

My plan is to use four small animal warming pads.

http://www.ebay.com/itm/K-H-SMALL-A...0801385221?pt=US_Pet_Beds&hash=item4d054c8f05

These are 25 watt each - 120vac. It seems that they have their own thermostat and two wired in series at 240 could plug right in to the charge circuit. Batteries in the battery box could be insulated over the top and around the perimiter with one or two of these as part of the floor. Hard ABS cover to support the weight of the batteries. I haven't gotten any yet so I can't give a direct report. 

They are $23.88 ea. free shipping.

These are the thermistors I'll use to let an Arduino know if the battery boxes are warm enough to allow charging.

http://www.ebay.com/itm/121005378819?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

2 for $3.88


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## PeterH (Mar 20, 2009)

I didn't consider those sort of pads when designed my system. I suspect it was because someone had suggested looking at these flexiwatt devices first. I'm not saying they aren't a good choice or won't work. I'm just offering the following info in the context of this subject.

In my case, the total draw when all 5 boxes (45 cells) are being warmed is only 200 watts. I've left it on over night and the power meter I'm using only recorded about 1.2 kwh of use and the cells were at about 50 degrees F.

I agree that there needs to be good insulation and good support between the warming pads and the cells. In my case, I taped the Flexiwatt ribbons to the bottom side of a 1/8th inch aluminum plate that sits below the cells. The boxes seem to stay warm for a while even when I'm not driving the EV. Of course, all that thermal mass helps retain the heat once they are warm, but it takes a while to get them all up to temp.

If you have more than one box, I'd be concerned about keeping the temp the same, or close to the same in all of the boxes. I have NO data saying this is vital to the cells, but I just have a feeling it is and that is my goal with my computer controlled temp control system. Each box can act independently if needed to warm a specific box to a target temp or to cut off warming once that temp is reached in just that box.

Pete


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## Zak650 (Sep 20, 2008)

Hi Pete,
I like your idea of having an additional temperature sensor seperate from the battery boxes. I might use a dpdt switch so that I can run my system of either 120 or 240. My car will be outside, no garage.


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## swoozle (Nov 13, 2011)

PeterH said:


> Hi there!
> 
> I'm using Jameco part number 162342 for my SSRs. They cost me 18.95 each and I have 6 of them in the system.
> ...
> ...


There's a cheaper way to do this. The Flexwatt takes such low current that there are small relays that will work. I used an Omron G5LE-14-AP3 DC12 ($1.88 at Mouser). It's a relay with a 12V coil that the datasheet shows will take switch .7 amps resistive load at 125V DC. At max volt/amp, the predicted lifetime is still 100,000 cycles. I'm running less than a quarter amp through each of 6 boxes and they stay comfortably warm.
Two of these relays in series for each box are well able to switch that kind of current safely within spec, at least for my 150V pack. For further insurance, I've got another two in series that control power to the whole set. That's 14 relays for 6 boxes at ~$30 total.

And then of course a high volt DC 2 amp fuse just in case.


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## zapyourrideguy (Oct 25, 2012)

I have installed flexiwatt strips just like you did to the underside of 1/8" aluminum and that assembly sits on hard-ish foam pads. I used the 3" flexiwatt. I am thinking about running them in series pairs for 240v. I tested the assembly to make sure the foam could handle the heat. It is great to hear they are working for you. I thought it might not be enough heat. 10 watts per foot. My only concern is to not charge below freezing so I am simply running a thermal switch that closes at 35 degrees and opens at 40 degrees. It will be mounted on one of my packs. This will not be heating the 4 boxes equally but will at least prevent charging below 32 degrees.

The switch turns on the heaters and activates a relay that will interupt my ready charge input to my Orion.

My other concern would be failure of the flexiwatt itself so I should fuse each pad. The heaters have been installed since my build 5k miles ago just never used until now.
which width flexiwatt tape did you use?


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## racunniff (Jan 14, 2009)

PeterH said:


> A basic question I must answer is: What is the minimum temperature we should aim to keep our Lithium batteries above to ensure proper charge while parked and connected to power for charging?
> 
> An additional question. Between my 5 battery boxes, how much can the temperature vary before I run into problems with uneven charging between cells in the various boxes?
> 
> ...


I'm almost done with my battery warmer solution (Farnham heater pads combined with thermocouples and the LightObject temperature controller). I think I'm going to aim for 7 degrees C - roughly 45 degrees F - measured at the center of each pack. The idea being that cells on the outside might be colder.

Before I charge like this, I'm going to run it at 7, and then spot-check cells to see if I need to increase the target temperature.


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## PeterH (Mar 20, 2009)

Hi there,

I used the 3 inch wide strips. I was concerned about uneven heating so I included two temp sensors connected to an Arduino controller to monitor the temp in the boxes and switch the 110 vAC to the heating strips as needed to keep things above 40 degress F.

After 3 seasons and almost 13K miles it is still working fine. However, I do need to keep my 12 vDC system charged because the Arduino and the other 12vDC system that are on all the time, seem to consume a fair amount of power. I leave a trickle charger connected to the 12 vDC system whenever it is parked. At the moment, I'm parked for the season since the OAT is averaging 10F this week and I don't have anywhere to plug into 110 vAC at work for 10 hours daily.

Pete


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## guenni (Nov 30, 2014)

I do not know too much about the standards in the US, but as far as I know e.g. UL and NEC define 110Vac dangerous (live circuit), so special methods for insulation have to be ensured between accessible parts and live parts. Functional insulation is not sufficient, it must be "reinforced insulation", which might be around 3mm air gap (for 110V) or special solid insulation. Or the 110Vac have to be supplied by a safety transformer.
And I would definitely put one fuse in every circuit.

Gunther


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