Heat-shrink for battery packs: ideas from RCU
Red Scholefield
FOLLOWING ARE SOME ideas for using heat-shrink material with battery packs that I found on the RC Universe (RCU) online forums. Rickochet (RCU name) wrote:
“To measure your cells tape them together then wrap a piece of paper around them and mark where the paper touches. Tape the paper together at the mark then press the paper loop flat. Measure the flattened piece of paper and select the next size of shrink larger than the paper measurement.
“Although you can use larger, I have found that the smaller you use the better the ends look. I get mine from Radical RC—great service also. I have about five different sizes I got for various size packs. You should look under ‘Batt Packs and Heat Shrink’ then ‘Pack Parts and Heat Shrink’ then scroll down to ‘Pack Heat Shrink.’ They also have all of the other pack making parts.”
From simplecj (RCU name):
“Here’s a cool idea. I just rebuilt my 6-cell 3s2p pack in a triangular 2-cell long pack. I used a plastic water bottle for the shrink wrap. Worked great, I just covered the ends with some cardboard and electrical tape, cut the top and bottom off the bottle and the pack fit right in. It shrunk up real nice. I think you could probably fit a sideways flat pack into it too.
“Larger packs you can try the one liter pop bottles. Just be sure to not heat it too fast or too hot or holes will open up. If done right this could work for all sorts of packs.”
Is it A123 or not?
The following is from Bret Martin at StickZERO RC Hobby Products. He wrote:
“Isn’t it true A123 is a brand and not a type of battery? In your articles you compare A123 to Ni-Cd, Li-Poly, NiMH, etc. Wouldn’t it be better to compare A123 to ABF, Panasonic or Sanyo as brands? Or compare Ni-Cd, Li-Poly, LiFePO4, etc. as to types of batteries?
“Maybe that would save some customer confusion in the future as all the major players will soon have LiFePO4 batteries. I was hoping you could straighten out the market (and give other companies besides A123 a chance) with a note or sidebar that not all ‘facial tissue’ is Kleenex.
“As we move into the LiFePO4 market we don’t want LiFePO4 to be synonymous with A123. Customers will not want LiFePO4, they will want A123 even if they cost more. I think the customer should have the knowledge and option of shopping the local hobby shop, BP Hobbies, Cellpro, ABF, or us, as they choose any brand.”
You are correct, but the brand A123 has become synonymous with LiFePO4—Lithium Iron Phosphate—technology. If I were to write “LiFePO4,” most modelers wouldn’t have a clue about to what I was referring to.
As the technology is more widely used, maybe we can educate the modeling public to accept and understand what we are referring to as LiFePO4 (LiFePoo?). For how many years has plastic shrink covering been available? And even today, many modelers call all shrink covering MonoKote.
With difficulty obtaining LiFePO4 cells, other than the limited line that A123 racing (Enerland) offers, I feel that any other brand that is offered will have a reasonable chance if its quality is acceptable.
Look at the variety of Li-Poly batteries that are on the market with all kinds of claims of superior performance. Some is crap, some is good stuff, depending on the day and production lot, as well as the manufacturer.
So far, A123 has an excellent reputation (at least for those scrounged from the DeWALT 36-volt power packs). Other providers have yet to gain that reputation, and they haven’t been reported on to any degree by modelers who have run test and evaluation programs.
Someone who is visiting a Web site in search of LiFePO4 packs has to wonder who the manufacturer is. As with NiMH or Ni-Cd, if it is not Sanyo, it is a tough sell to the knowledgeable modeler. It doesn’t help when a battery vendor has to attach his cute name to the battery to make it seem different when it is the same technology that others are offering.
I'll have more about this subject in the January 2009 column.
Parallel Operation, One More Time:
Pete Richiuso wrote:
"I just finished reading your battery article in the July '08 Model Aviation. Aside from the convenience of using plug-in wiring harnesses for wiring packs in parallel for higher amperage packs, rechargeable batteries of any type should never be charged in parallel. The reason for this is due to the fact that the charging current through each of the parallel legs cannot be controlled.
"If you are using a voltage sensing 'smart' charger, the charger will shut down as soon as one of the legs reaches full voltage, thereby under charging the other legs. Since all of the Li-Poly chargers are voltage sensing, this would lead to a serious undercharge of one or more of the parallel legs during discharge, probably destroying the leg.
"For Ni-Cd and NiMH packs it's not quite as bad. Over discharging one leg of a parallel pack could induce a reverse charge on one or more cells in that leg. Also using a high current uncontrolled charger to charge parallel Ni-Cd/NiMH packs can lead to serious overcharging of a parallel leg and the heat generated could destroy that leg or even the entire pack. Thanks."
Thanks for your note. You might want to review my article about parallel operation on The Battery Clinic Web site. See the source list for the address.
From the beginning, I have advised against charging Ni-Cd or NiMH packs in parallel for the reasons you wrote. Li-Poly and Lead-Acid, on the other hand, are essentially current-limited, constant-potential charge systems; they charge to a specific cutoff voltage while the current tapers off as they approach that voltage.
Two packs of equal numbers of cells in parallel will, by definition, have the same voltage under charge and discharge. Each will accept current as required to attain the cutoff voltage. We get into problems charging Li-Poly packs, because a multicell pack could have out-of-balance cells, where the low cell doesn't get a full charge and the good one may be overcharged to the point of damaging the cell. Since the cutoff voltage is the sum of the individual cells, subsequent cycles make the matter worse if the cells are imbalanced on the charge cycles.
As I wrote online, leaving Li-Poly packs in parallel makes it more difficult to diagnose individual cell problems if the cells cannot be isolated. Yet some Li-Poly vendors employ series-parallel systems in their larger packs.
The more cells that go into a pack, be it series, parallel, or series-parallel, the greater the risk of something getting out of balance. It's best to keep the packs separate at all times except when you are flying them.
Current Probe:
Another trick you might find useful when you want to check the current drain when using cells in a holder is to make a probe to go between the cells. It is made from two pieces of thin brass or nickel tab material with an insulator (card stock) between them. You clip your multimeter onto the tabs and read the current.
Simple Charger Holder for Better Viewing:
Bob Peiser offers a neat solution to wire clutter and module placement. He wrote:
"I have been using the TME Xtrema for a year now. I had a hard time reading it on the bench and in the back of my car so I built a stand from scrap Depron to be able to view it at a 45 degree angle. Then TME comes out with the Balancer and Balancer Interface Module which was a bunch of unruly wires.
"The BIM is mounted on the back of the stand, held in place with Velcro. The Balancer is on the front also mounted with Velcro with the balancing cable in line with the temp probe and battery connector. I'm only charging 3-cell packs so the Xtrema never gets hot but larger packs would require the stand to be built from ply or an even better heat resistant material."
Wheel Pants:
What does this have to do with batteries or chargers? For the benefit of the few modelers who seem to rail against the AMA in general and this magazine in particular for the slightest deviation from what they perceive is the way to run things, I'll explain.
A significant number of modelers find that flying early in the morning, when the dew is still on the grass, is a good way to avoid the heat of the day. Many airplanes are designed in a way that the aileron servos are mounted directly over the wheels, where they get the full benefit of the dew and wet grass that the wheels throw up.
That wet grass and water directly hitting the servos is not the best environment for them and could cause a problem. And any problem with the servos becomes a problem for the battery serving said servos; hence the justification for this discussion.
Find a couple of plastic containers that are roughly 1/2 inch in diameter larger than the wheels you are using. Follow the steps shown in Figure 3.
- In the center of the bottom of the container, drill a hole that will fit snugly over the axel.
- Mark off roughly 1 inch on the container.
- Cut away the rest of the container.
As shown in Figure 4, measure and cut an arc that is approximately 225° (180 + 45), leaving roughly 3/4 inch of the center portion.
Figure 5 shows the next two steps.
- Mount the completed wheel pant on the axel between the landing gear and the flange on the axel.
- Install the finished product.
Figure 6 shows the results (with and without wheel pants). Painting is optional.
Is Your Building Season Looming?
If it is, don't forget to remove those Ni-Cd and NiMH batteries from your model to guard against black wire disease.
You can store Ni-Cds at any state of charge; just make sure there is no load on them. According to the manufacturers, Li-Poly and LiFePO4 batteries should be stored in the 50%-60% charged state.
If your building season is particularly long, it would be wise to check the Lithium family every month or so, to make sure they are in the 50%-60% charge range.
I'll be back to you in the January 2009 MA. You can write to me via postal mail, but E-mail is faster. MA
Sources:
- RC Universe
- Radical RC
(937) 256-7727 www.radicalrc.com
- SlickZERO RC Hobby Products
(352) 489-2376 www.slickzero.com
- The Battery Clinic: Parallel Operation
www.rcbatteryclinic.com/parallel.html
- The Battery Clinic
12219 NW 9th Ln. Newberry FL 32669
Transcribed from original scans by AI. Minor OCR errors may remain.
