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Mike Trautman

Roll Ur Own.... Batteries that is

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CAUTION CAUTION! MAKING BATTERY PACKS CAUTION! CAUTION!

I've been cycling batteries... per my own advice given earlier this year. The results have been equivocal at best and disappointing on several counts. My giant scale planes have large batteries. That should guarantee security, no? The Tigermoth has two 3000ma 6v NMH batteries, expensive, and often on back order. Well, the last “cycle” resulted in one accepting 3000ma and the other 179ma. That's not a good result. I'm still trying to sort out the settings on my two different charger that are supposed to cycle batteries automatically. Where's that darn manual?

Meanwhile, I'm probably looking at replacing some batteries this year. I like large capacity 6v batteries. Often pricey and frequently back ordered. I am a ham radio guy. I know how to solder. I should be able to buy some rechargeable batteries cheap and make my own battery packs... right? Sure! Here's what I learned.

I accept no responsibility for anyone else trying to make battery packs. Whether you burn down your house, or just crash a couple planes, It's not my fault. I'm not saying you should do anything the way I do. I'm just describing a few things that worked, and some things that didn't, in my hands, with my equipment, for my own use.

Batteries:

Tenergy Centura “Low Self Discharge” NiMH rechargeable batteries, 2000ma, Amazon, $32.99 for a 24 pack of “AA” size. Tenergy “Sub c” batteries are available for $5.45 each. I've seen other Sub c's available for less, but they carry a lot of of negative reviews for lower capacity or reliability. Stick with the good stuff! It's still much cheaper then ready made packs. For 4.8v you need four 1.2v batteries in series. For 6.0v you need five 1.2v batteries in series. If you don't understand the difference between batteries in series and parallel, stop now before you hurt yourself.

Soldering:

Connecting batteries commercially is usually done with a spot welder. The weld is strong and heat transference to the battery chemicals and structure are minimal. Prolonged, gradual application of heat should be avoided. Use a low melting temperature solder and a large soldering gun or iron so contact with the battery is brief. Test each solder joint vigorously.

Connecting batteries can be done with wires or metal straps. Do not try to solder aluminum connectors. Special flux and solder are needed for aluminum and the technique is tricky. Copper, brass or plated ferrous metals can be used more reliably. Test before use! Appropriate flux (resin only please) is helpful. Cleanliness is next to godliness!

Pre-Assembly:

I strongly suggest assembling your battery pack ahead of time, using a strong adhesive. I have had failures with hot glue, not to mention the possibility of high operating temperatures affecting it. Your solder joints are not enough to hold your battery pack together without help. Epoxy, or “Shoe Goo” seem to work well. A linear arrangement can flex and break your solder joints more easily then a stacked “hump” arrangement. I go with a humped pack when I can.

Design:

Depending on your voltage requirements and the number of batteries you're using, you need to think about the path of the wiring and connectors. I like to start with one battery and add more, visualizing how the electricity will travel along the circuit through each one. A mistake here can be dangerous and costly. 6V doesn't seem like a lot, but I have suffered 3rd degree holes burned into my fingertips, just by momentarily placing a strap on the wrong two contacts. Place and hold your straps with a tool, not by hand. The brush/spark test can be useful too. Lightly touch (brush) the strap between the contacts. You should not get get any spark or heat. If you do, you have a short and that's bad, very bad.

Test, test, test:

Make sure you have a volt/ohm/continuity meter that you know how to use. Once you have made all your connections, you should have the expected voltage at your battery cable connectors. Go back and look for cold solder joints and loose connections before you wrap your pack in tape! I once spent a maddening hour trying to figure out how I could have the right voltage at the battery pack, good continuity from the battery connection to the power connector, and yet still no juice flowing. Turned out that... 1. My glue joint had failed, allowing a solder joint to flex. And 2. I had a cold solder joint with intermediate contact... times two!... The straight line pack had separated both end batteries due to flexing. That is why I always go with the “hump” design when I can. Do yourself a favor and provide some strain relief for your power cable to solder joint connections too.

Cables and connections:

Years ago I bought a good quality cable making kit from Hansen Hobbies at E-Fest. I've never regretted it and have enjoyed making my own custom length cables and connectors since then. Most receivers use a JR or Hitec style connector on 24 to 18 gauge wire. 22 gauge is the most popular and sufficient for most uses. For battery connectors I like BENTECHGO 18 Gauge Silicone, 150 strand copper wire. Amazon, $6.48 for 20' (10' red, 10' black) I have used 16 gauge by stripping it to 18 gauge, but the 18 gauge is really the largest practical size to use with the standard R/C servo connection. Seriously, if you need more amps than high conductivity silicone 18 gauge can provide, you need professional design and fabrication services.

Wrap it up!:

I wrap my finished packs in self bonding electrical tape for insulation and physical protection when I'm done. Have fun! Don't blow yourself up!

Mike Trautman

Here's some pics:

"Low Self Discharge" Ternergy AA batteries:

 

AAbatteries.thumb.jpg.d0bf07d05f01686dd290f5b64f712c7d.jpg

Hump versus Linear arranements

Hump_Line.jpg.144f21dd91f0c9c7e8791a2cc76ff658.jpg

Big, Ugly soldering iron, but quick!

Soldering.jpg.ac785a3843d01c6595c2dfd8e15a0ff2.jpg

Failed glue joint, failed solder joint!

Failed.jpg.d518154e963c56874c4813e4bac08836.jpg

Some finished packs wrapped in self bonding electrical tape. 

FinishedPacks.jpg.5221e56c4acd05004d4c5c171a678149.jpg

 

Always

Testing.jpg

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