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One of the major problems with most battery packs no matter how good the battery cells are, the connections between the cells and from the pack itself usually suck big time. These crappy connections are typically little tin foil accordions, small gauge wires, and stock Tamiya connectors which simply do not allow free flowing current between the cells and to your RC.

Among my collection of battery packs, I have a couple decent PowerMax Pro battery packs that really seemed to heat up once I moved to a brushless motor system.  Now these are pretty decent packs, the power wires were thin and were initially replaced with Deans Wet Noodle 12 gauge wires and connections to the ESC were upgraded to Deans which was a huge improvement and is a typical upgrade for almost any pack.  Although better than most, the PowerMaxs use a thicker foil accordion connections between the cells, they are still was way way too thin for the current hogging requirements of the Novak 4.5R brushless system.

My specific problem was when I moved to a brushless motor system which sucked more current than my battery packs poor between cell connections could provide.  This issue is typically displayed in the form of a hot battery pack. Another indication was the fact that the ESC was pulling so much current that it would actually brown out the receiver (yeah a lot of current).  This issues was resolved with the addition of a simple 4700mF capacitor wired into the ESC's external battery pack slot (kind of a battery back up for the receiver).  Even with the fix, the system still lacked the power I knew it had. The problem was I either needed better higher quality battery packs or needed to rebuild the packs with better between cell connections so all that lovely current could flow when it was needed and not get bottlenecked at the connection points between the cells.

What are better connections going to do for me? Basically the current will flow more easily from each cell to the next with the least amount of resistance.  Resistance causes the cells to overwork themselves and heat up, heat is bad for a battery. The hotter the pack is the less runtime, current and life it will give and have. You should see a pretty noticeable difference in peak power, overall runtime, all with much cooler running batteries.

Battery Pack Styles - There basically two types of battery packs, shotgun or stick packs which stack the batteries positive to negative like batteries in a flashlight or brick or side-by-side style packs which are just as they sound more on these styles in the articles below.

Buying new cells and making a new pack, no problem follow the same process as described in the articles below.  I would go ahead and buy matched cells if possible. Matched cell packs are built with cells that discharge from a time and voltage perspective almost identically.  They typically will run longer with overall higher voltage than unmatched cells.  However over many uses they will eventually become unmatched cells but will still perform better in the long run for you than un-matched cells.  I would recommend if you are simple a basher (non-pro), going with a high Mh cell like the 3300mh to 4300mh cells for longer runtime.

RC Battery Pack Building and Rebuilding Important Notes:

Before you start tearing open packs there are some things you should know and may need to buy. 

• What type of configuration are your going to use, a standard stick pack or a side by side or some custom configuration of more than 6 cells?  Keep in mind that occasionally side-by-side packs are wider than stick packs.
• Are you pretty good at soldering, soldering battery packs is a little tougher than soldering small gauge wires.
• Do you have at least a 75W soldering iron or more would be preferred with at least a 1/4" chisel tip?  60/40 solder is recommended however a good quality silver solder will work just fine.
• You will need 5-7 battery bars per 6 cell battery pack (7 battery bars are needed if you do not want the power leads to attach directly to the batteries - sometimes clearance dictates the extra bars).  Remember this is the critical connection between your cells, so buy at least Deans ProBar 3 battery bars.  I have seen some out there thicker and wider, but I will let you be the judge as to whether these are overkill versus an upgrade.
• Do you have spare large gauge power wire for the rebuild (sometimes you find that your recycled power lead wires are too short).
• You will need Deans or other low resistance power connectors if you don't already have them for both the male ESC and female Battery connections.
• Big 2 1/2" battery pack shrink wrap.
• IMPORTANT At least 2 cell shrink wraps for each naked un-shrink wrapped cell such as the ones in my PowerMax Pro stick packs. DO NOT ATTEMPT TO BUILD A PACK WITH NAKED CELLS YOU WILL BLOW ALL OF THEM - ONE LAYER OF SHRINK WRAP MINIMUM.  So in my case I needed 12 cell shrink wraps for one 6 cell pack of naked cells.  Naked cells will not have any plastic coating, if in doubt, add a layer of cell shrink wrap.  Also a really great idea in the RC Car action article on having an extra layer on each cell in regard to gluing cells together.
• Custom Logo'ed pack as show above, just glue a 1 5/8" wide logo of your choice onto each cell with simple craft glue before shrink wrapping each cell.
• Some power lead sized shrink wrap tubing for the connection ends.
• A Dean's speed jig for battery building is helpful, however an improvised cell vise will also work well.  I used a strip of masking tape on both sides of the cells to hold the cells together and straight while I was soldering the cells.  A battery jig is very helpful as an extra hand and worth the $10-$15.
• Want out? You starting to think "Dude this is just too much work?"  Well it's actually pretty easy and simple which probably takes far longer to explain than actually building a pack, however there is a process to follow and about $7-$10 worth of materials to get your battery pack back in shape.  These guys, I have heard build good packs and may be an option if either don't have the soldering skill yet or just at the point where it would be worth the money.

There are the articles I would HIGHLY recommend you read them all before proceeding with tearing apart your battery packs.

• Off Shore Electrics.com - How to make stick packs - If you do this remember that you probably are not going to get the cells apart later.
• ff Shore Electrics.com - How to make Side-by-Side packs
• Building a soldered stick pack by Keith Shaw - If you do this remember that you probably are not going to get the cells apart later.
• RC Car Action Battery Pack Building

Good Luck!