|Feb 15, 2014, 11:07 AM|
Here's my LiPo heater box
I didn’t know this LiPo heater box was going to consume so much of my spare time over the last couple of weeks, but it turned into a slightly larger project than I imagined. That’s okay because it’s winter here and there’s no flying going on anyway.
In F5D electric pylon racing, battery size and the loads imposed on them are contradicting factors. For the packs to fit into the streamlined fuselages and the planes to comply with maximum weight limits, the batteries are small (in the 1800mAh range in a plane that reaches speeds of approximately 170mph). But the planes are flown at full-throttle continuously, so stress on the batteries is high—we’re trying to get the most speed and efficiency possible out of the smallest package possible. This means battery performance is crucial.
These demands, combined with the cost of LiPos (not to mention the relevance of all this to my job in R&D at Hobbico) has piqued my interest in the care, handling and performance of LiPo batteries. This has resulted in considerable time spent perusing the Batteries and Chargers forum on RC Groups to continue to learn from sources I trust.
Recently, I learned that the best way to determine the maximum discharge Amps a battery can realistically withstand is by measuring its internal resistance, then using that value in a formula to calculate the realistic, maximum Amps (rather than simply going by the manufacturer’s oft-exaggerated C-rating). The lower the resistance the healthier the batteries and the better they will maintain voltage during discharge—not to mention the longer they will last. Conversely, the higher the internal resistance, the less the batteries will be able to maintain voltage and the more they will be stressed when pushed, resulting in even more stress and a decline in performance.
There are a few things that can be done to alleviate stress on batteries including switching to larger capacity batteries, shortening flight time or decreasing the load (propeller size). However, in F5D racing these are usually not viable options—at least not to a degree that will help the batteries. That’s where internal cell resistance comes into play. We can lower the internal resistance of a battery by warming it—or pre-heating. This practice effectively raises the C-rating resulting in less stress on the battery (in addition to delivering consistent voltage during the flight where the pack will be as “fast” at the beginning of the race as it is at the end of the race when the battery has naturally “self-warmed”).
Where racing is concerned, pre-heating LiPos is also a means of controlling the airplane’s speed whatever the weather conditions dictate. Warm temperatures, high humidity and high altitudes decrease air density causing the plane to fly slower resulting in more time to complete a race. As I understand it, this would call for less pre-heating in order to conserve energy so the race can be completed before the on-board energy limiter automatically cuts the motor. Cool temps and low humidity increase air density resulting in faster speeds, less time to complete the race allowing for more LiPo pre-heating—the better the air the more efficient the wing and propeller so the more rapidly energy can be released from the battery.
Further reading suggests that optimum LiPo operating temperature is in the 100F – 125F [40C – 50C] range with an absolute, maximum ceiling of 140F [60C] and pre-heating target in the 85F – 100F [30C – 40C] range.
As if F5D racing wasn’t already expensive and technical enough, looks like I’m going to have to kick it up another notch and build myself a LiPo heater box!
More reading and searching on RC Groups found a LiPo heater box by Gerben. I think he’s from Great Brittan, maybe Holland, but I’m not sure. Gerben’s heater box uses RC car tire warmers sandwiched between two aluminum plates serving as the heating element with a couple of cooling fans to circulate air.
So now it’s time to get to work on my own LiPo heater box – the perfect winter project!
Here’s a link to the thread on Gerben’s LiPo heater box:
Coincidentally, my dad had a transmitter-type case he was no longer using which he offered to me. This would make the perfect LiPo heater box!
First, I had to remove the factory lining and the additional padding my dad put in on top of that. This was quite a mess, but eventually it cleaned up well enough.
To make my box like Gerben’s, I purchased a 12V RC car tire warmer that came with four warmer pads and a control box with a temperature sensor. I also purchased four fans for air circulation inside the box. The tire warmer and fans can be found at just about any on-line RC car outlet.
Mini digital thermometers are all over the place on eBay, but it took a little more searching to find one shipped from the States. I wish they made one with an on/off switch and that displays in degrees F, but I couldn’t find one. To shut off power on mine you have to remove the batteries, but now that my box is completed I’m leaving the batteries in to see how long they will last.
The insulation for the box took a little more shopping. I wanted something with foam backing (for insulation) and adhesive-backed. This narrowed the choices a little. I found what I thought might work at my local Menard’s, but it turned out to be a failure, so avoid this stuff…it has an aluminum skin on a gummy foam backing, but the foam padding stuck to both the box and the aluminum skin poorly.
After MORE TIME on eBay I found several sources for foam-backed, reflective insulation intended mostly for automobile panels. I found what appeared to be a leftover roll for about $9.00 and it seems to be perfect. It’s 3/16” thick and the reflective skin is a durable, flexible fabric, so it’s easy to work with. I’ve had it stuck inside the box for a few days and so far, it seems to be sticking well.
The aluminum sheet took a little shopping as well, but I found what I think is perfect. First I went down to my local big-box home-improvement stores, but they didn’t have anything thick enough to suit my taste. On-line I found MSC Industrial Supply Company where I purchased a 12” x 24” sheet of .063” 3003-H14 aluminum sheet:
Fortunately, I have the advantage of working in the R&D department at Hobbico, so drew patterns for the plates in AutoCAD. Those without the resources could make templates from cardstock or manila folder material. I drew my pattern and laser-cut out a template from lite-ply to make sure I had everything placed correctly including the cutouts for the fans and the rounded slots for the Velcro straps that will hold in the battery that powers the system.
|Feb 15, 2014, 11:08 AM|
Once everything was confirmed, I printed out paper patterns, then glued them to the aluminum plate with 3M 77 spray adhesive.
The edges were cut with a metal-cutting band saw (another perk of R&D), then the holes were drilled. I contemplated whether to cut the 1” holes for the cooling fans with a hole saw or to perforate them with a drill. Perforation won out as I figured Menard’s wouldn’t have a hole saw for metal. Plus, it was the weekend so I didn’t have access to the R&D shop where a hole saw would have required a drill press. Drilling all those holes by hand was a little tiresome, but got the job done.
The paper templates were soaked off with Naphtha (lighter fluid), then lots of filing by hand and countersinking the holes to remove all the burrs. Cleanup around the small holes was done with small hobby files and around the large holes with a half-round metal file (another trip to the hardware store!). Then, the plates were wet-sanded to a uniform finish with 320-grit, then 400-grit.
The bottom insulation was cut and a wood framework from Ĺ” x Ĺ” basswood was glued around the inside of the perimeter.
If you’re in the RC hobby a Dremel grout-cutting bit is indispensable and came in handy for cutting the hole for my digital thermometer.
I had already lined the top of the box and wasn’t thinking ahead, so I had to peel back the insulation to conceal the cord from the thermometer sensor. I wound it around the inside of the top and it peeked out one corner.
|Feb 15, 2014, 11:08 AM|
Next, it was time to put all the electrical components together. I used Deans 2-pin connectors for everything so I could disconnect if necessary. The fans were wired together and will be powered by the main power battery via a Great Planes BEC to reduce the voltage. The warmers were also wired together to a single 2-pin Deans and connected to the controller box.
The tire warmers were trimmed down to the internal heating element wires, then positioned on the plate and sandwiched over the bottom plate.
I also made a small tab for securing the temperature sensor from the controller box.
On a whim, I designed these little cooling fan covers to protect them from the LiPo battery wires during heating.
So here’s the complete heating element ready to go with the box ready to accept it.
Here’s the view from the top with the final insulation strips around the inside all ready for action.
I guess the cat approves!
|Feb 15, 2014, 11:09 AM|
Finally, letís take my new LiPo heater box for a spin! First I just want to see if it worked. Turning the dial on the tire warmer control box up to about 80C [176F] heats up the aluminum plate enough so that you canít even hold your hand to it for more than a couple of seconds. So far so good!
Next, I need to see how long it would take to heat batteries and correlate the temperature setting on the control box dial to the temperature inside the box (as read on the digital thermometer) to the temperature of the batteries, so time for some serious testing. I used an EagleTree data logger with a temperature probe inserted down between the cells on a 2200mAh 4S LiPo. The battery itself powered the logger.
I set the dial on the control box to 50C [122F], plugged in my 3S 6400mAh LiPo to power the system and closed the lid. Eventually the temp on the external micro digital thermometer stabilized to about 37.5C [99.5F] for the 2 hour trial.
Time to read the data on my data logger. According to the graph, it took about one hour, 20 minutes for battery temp to fully stabilize at about 42C [107.4F], but was pretty close in about 60 minutes. According to this, a dial setting of 50C correlates to a battery temp of about 42C and an internal box temperature of about 37C [122F, 107F, 99F respectively]. The sudden dip in the graph at about 102 minutes is where I opened the lid to show my friend. Also according to the graph, the thermostat seems to cycle every approximately 6-1/2 minutes. Battery temp varied approximately 1.5 degrees (+/- .7 degrees from average).
After two hours the individual cell voltage of the 3S 6400mAh LiPo Iím using to power the heater box was 3.81V/cell and it took 3627mAh to recharge. This calculates out to an average consumption of 30mAh/minute and an average current draw of 1.8A, so I can go about 170 minutes (basically three hours) on a 6400mAh battery if I want to keep it to 80% capacity used.
I conducted a 2nd two-hour trial with the dial set slightly lower to 45C. This time the temp on the digital thermometer hovered around 35C and once stabilized, the average battery temperature was approximately 39C [113F, 95F and 102F respectively]. Same as before, it took about one hour, 20 minutes for the battery temperature to fully stabilize, but was within about one degree in about 60 minutes. This time though, the thermostat cycled every approximately 8 minutes causing battery temp to vary approximately 1.7 degrees (+/- .75 degrees from average). I suppose this makes sense considering the lower temperature setting.
As for my 3S 6400mAh LiPo battery Iím using to power the system, after two hours cell voltage was 3.84V/cell and it took 2973mAh to recharge. This calculates out to an average consumption of 25mAh/minute and an average current draw of 1.5A meaning I can go about 210 minutes (basically three-1/2 hours) on a 6400mAh battery if I want to keep it to 80% capacity used.
I also used a Wattís-Up Watt meter to measure the current when the fans and the heating elements are on and when the heater was off and only the fans are running. The current draw for both is 6.7A. When the thermostat kicks off the current drops to .35A for the four fans only.
Where the rubber meets the road;
When I first got my Wayne Giles I.R. meter I measured the I.R. readings of all the cells of my F5D batteries:
I then entered the resistance value of the highest cell into the F.O.M. (Figure of Merit) calculator to calculate the maximum, recommended current draw and therefore the true C of the pack:
I put two sets of my F5D batteries (each set comprised of one 2S + one 3S 1800mAh battery) in the box and set the dial to 50C [122F] looking for a battery temp of around 100F. Box temperature stabilized at around 36C [97F] and after about an hour I measured the battery temperatures, this time with a DuraTrax IR thermometer where they measured around 38C [101F].
The data was entered in a spreadsheet.
The chart shows the original I.R. readings of two of my batteries, plus additional readings again at room temperature (72F), and two more readings of the same batteries heated. You can see the significant rise in maximum Amps current draw when the packs are heated.
Now the only thing to do is put it to practice which wonít happen until flying weather returns here in the Midwest. Then, Iíll heat my batteries (probably to around 100F), make a flight, then measure battery temp after the flight to make sure it didnít get too hot. Itíll also be interesting to compare the voltage and current with the warmed batteries to that of not warmed batteries. I suspect it will be about the same toward the end of the flight, but the voltage should be higher at the beginning of the flight for warmed batteries.
Iíll get back to you in a few more months!
|Feb 15, 2014, 11:39 AM|
GREAT IDEA....might do one myself....are you planning on traveling (by air) with it?...I can see some stupid TSA agent tearing that beautiful idea completely apart or worse yet blowing it up in the parking lot
|Feb 15, 2014, 12:50 PM|
Hey Tim that's some good work! You will definitely see an improvement in performance at the field. I tried using one of HK's kits in my field case and threw it away after it started melting things. Looks like your approach is the way to go.
|Feb 15, 2014, 10:26 PM|
Thank you gentlemen.
I do plan to bring my LiPo heater box with me to Austria. I considered disassembling it so it doesn't look like a device.
|Feb 16, 2014, 01:50 AM|
Nice work Tim, you'll see the difference when flying. For me in F5B the biggest difference comes from having consistent motor run times.
Back in the old days when 30c (claimed) batteries were the latest thing without heating we would see a increase of over 1kW between climb 1 and climb 5
One suggestion is you might want to electrically insulate the box, you seem to have a lot of metal in there waiting to short the batteries out. You only need a dodgy balance lead or heat shrink on one of your power leads and that's quite a big event waiting to happen. Something like Kapton tape on the plate and Depron insulation around the sides would be good.
When I made my second box I went for the metal plate too and this is a improvement over the first one which just had the tire heaters laying in the bottom, batteries heat up quicker. I also made my own heater element out of carbon which works well.
Even with your plate make sure the batteries are in the box for a good hour before flying to give them a chance to heat soak properly and then keep them in there for the whole flying session.
If your struggling for space with your luggage a good alternative is to put the tire heaters in a small cool bag, packs down small and will do the job as lipo heater when at international comps.
|Feb 16, 2014, 06:05 AM|
Joined Dec 2004
Looking good Tim, glad to see my thread inspired you
If you're interested in a few tweaks, here are some points I observed during testing of the heaterbox:
You may find moving the heater's thermostat sensor to the outer edge of the bottom plate allows the box to heat up a little quicker (just put the sensor-clamp und the nut that holds your plate down).
Moving your thermometer sensor around can change the readings to better reflect actual battery temperature. Mice gets hot air blown straight onto it (it's right above a fan), which makest the thermometer show the same temperature as I measure with an IR-themometer on the Lipo itself.
If you want to reduce the temperature variation of the batteries, you could modify the thermostat as shown in my thread, but it seems pretty stable as is. 1.5C seems accurate enough.
Neither are essential, but if you enjoy fiddling around...
PS I'm from Holland / The Netherlands
|Feb 16, 2014, 01:25 PM|
Hey, thanks Steve!
Yes, I am looking for more consistent run times (from beginning of flight to end) and also longer LiPo life as the C-rating should rise with a little warming.
Thanks for the tip about insulating the plate. Iíll probably go ahead and do that.
Yes, I learned through the data logging I posted in my thread it takes about an hour and twenty minutes for battery temp to stabilize, but about an hour gets them within a degree or two.
Thanks for the tips and taking the time to comment.
|Feb 16, 2014, 01:33 PM|
Yes, your thread certainly inspired me. Another thing you did that also inspired me is how you were able to post images within the thread, not all just at the bottom. It took me a while to figure this out, but I learned that the images need to be uploaded on a separate site. I used Photobucket. I then copied the URL in the text. This gives better continuity to the post as you can view the images as you read. So thank you for that too!
I did see your modification to the thermostat, but couldnít quite understand it fully, so I skipped that part. Maybe Iíll open up the box and try it now.
Got it Ė Holland!
Okay, thank you again!
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