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Building An NiMH Battery Pack

Tired of Lugging Around that 7+ Pound Lead/Acid Battery?

(Note: I wrote this for the FMS newsletter UV Waves several years ago.  At that time it was written for regular NiMH batteries, but can be applied to the newer Super NiMH batteries from Eneloop.)

I do most of my glow hounding in Greenland. Not nice and flat – mountains everywhere. The typical "honey holes" are found at 500m above sea-level – and you start at sea-level! With about a 1km climb, I guess that's about a 50% grade? Regardless, carrying hammers, lights, BBQ grill covers (ask me about that sometime), chisels, and a heavy lead-acid battery that's only gonna last for 5 hours is not fun! And you've got to carry it back down, along with all the wonderful rocks you found (until the battery died).

I learned my lesson the first year. Relearned it the 2nd year, and solved it this third year. Adopting a technique used by RC enthusiasts, I built my own battery packs to power my light. I built two "2 hour" versions, each weighing only 11 ounces - and two "9 hour" versions, 4 pounds each (actually two "2 pound packs" to spread the load – more on that later).

NiMH batteries are quite powerful for their size. They make a "AA" 2100mAh battery and a "D" cell 9000mAh battery (and a whole bunch of others). I chose these sizes because at the time they were the highest capacity/lowest weight. Unlike alkaline and nicad batteries, NiMH batteries only put out 1.2 volts (versus 1.5 volts). My Superbright needs 12 volts, thus I needed 10 batteries (10 x 1.2v = 12v).

Perhaps a little discussions about "mAh" is in order; "Milliamp hours" is what it stands for – a 2,000mAh battery would put out 2,000ma (2,000 milliamps – or 2 amps) for one hour before draining. Often, lead/acid batteries will simplify things for you and just list them as 7AH (7 amp hours – 1 amp for seven hours, or 7 amps for one hour). It gets even better with NiMH batteries; they pretty much provide their voltage rather consistently right to the end, then abruptly die. Conversely, a lead acid battery dies a slow death starting from the moment you start using it. The voltage (12v usually) starts out at 12v and rather linearly decreases over the length of time you use it. You may find less and less bright rocks towards the end of the night, until finally you realize your battery has died – what a bummer!

Having selected the batteries I had to figure out how to put 'em together, the tools needed, and the parts. This requires some basic (but good) soldering skills, a solder GUN, some wire, and knowledge of the positive end and negative end of a battery. Luckily, there's an excellent web site that tells you how to do it all: http://www.rccentral.com/guides.asp?ATCL_ID=51. Battery pack supplies can be bought at many places. I used this one: http://www.dynamoelectrics.com/Supplies.htm. Shop around on the internet for the batteries and the best deal. They've gotten more powerful and cheaper since last year.

The difficulty level for this project is:

  1. Boring if you are an engineer,
  2. Easy if you like to tinker and have not electrocuted yourself yet,
  3. Hard if you find assembling your kid's bike on Xmas night a chore,
  4. Impossible if you are a doctor or lawyer (joke!!!! – but, most likely lawyers don't even know what a soldering iron is – find a friend).

nimh-wiring.jpgThe basic theory goes like this: To build a 12 volt battery you need to hook ten NiMh batteries in series (remember – NiMh batteries only put out 1.2v – not 1.5v like we're used to). That means you hook up the positive end (the end with the little bumpy thingie sticking out) of each battery to the negative end (the flat end) of the next battery – over and over again. You do this by soldering heavy-duty wire or braid to each battery.  BE VERY CAREFUL to not allow your solder to plug the vent holes found on the top of the batteries.  Short wires interconnect the batteries while longer wires at the end of the last two batteries are reserved for the connector that goes to the lamp. I built my "AA" packs in two groups of five batteries (a couple of slightly longer wires interconnected the two groups), taped the result together, covered with shrink tubing (a neat insulator that shrinks when heated and holds everything together very nicely (a hair dryer works very nicely to shrink it). The result was a small battery pack consisting of two rows of 5 each "AA" batteries – in a pack measuring about 3" x 2" x 1". This pack provided 12v for 2 hours! The tricky part is figuring out how to hook it into your lamp. I have a Superbright where you can easily solder the output wires from your battery pack to one of those automotive 12v adapter plugs – just make sure you get the polarity right – positive goes to the middle!

Builder's note: Make sure that you "strain relief" the long wires coming out of the pack. Loop them back thru the shrink tubing so that the solder connection does not receive any flexing. This is the weakest part of any system – the part that get's flexed.

anderson-wired.jpgSuperbright 1 Connector - If you are a little more electrically savvy (or know a friend) I would recommend losing (get rid of it, toss it, burn it, stomp on it, sacrifice it to the Gods) the power connector on the Superbright 1 (this was written before the SuperBright 2 which addressed this problem) altogether and simply solder a 6' to 8' coiled wire into the innards of the Superbright), strain relief it, put a Molex style male connector (or another connector of your choice - I use Anderson Powerpoles today) on the other end, and a female matching connector on the wires sticking out of the battery pack (MATCH POLARITIES + to +, - to -). The @#$% connector on my Superbright always kept falling out – and why in the world do I need a connector there anyway? Plus this eliminates at least two (maybe even three) completely unneeded fuses, which always seem to blow at the wrong time (at the top of a mountain).  Of course, all this fiddling will void your warranty I'm sure, but who cares – it's probably already void...

SB-with-nimh-pack.jpgNeat hint: while you're inside the Superbright fiddling around you'll find it rather easy to store a spare UV lamp behind the reflector by just removing a couple of screws. Then you'll always have a spare lamp, well protected.

I built four of these "AA" packs and they were my primary source of power most of the time; light, easy to carry and long-lasting.

But – for more power, simply use "D" cell batteries (again, ten of them) for 9 hours of UV light (9,000mAh – 9 hours at one amp). They're heavier but they last! I built these packs a little differently. Instead of shrink wrapping all ten batteries into one big pack I built two separate packs of 5 batteries each with enough wire between the two packs that I could put one in one pocket and one in another pocket for load balancing. I bought a fly-fishing vest (with gazillions of pockets) and wore it with the batteries nicely tucked into each side – 2 lbs per side, 9 hours of light! And the extra pockets were great for those delicate little rocks I kept finding....

A note on wire: use the heavy 13 gauge, high strand count wire. It is very flexible (means it won't break from continuous flexing) and carries a lot of current. Use red for positive connections and black for negative connections.

Soldering: Make sure your solder connections are good. If you don't know how to solder find someone who does. A cold solder joint (basically a lousy connection) will result in a useless battery. Too much heat on a battery can destroy it, and if you melt the plastic around the battery that'll cause more trouble (shorts).

Final note – Charging. NiMH batteries must be charged by a special charger; even more special because you will be charging ten of them at a time. Here's where a little investment will make a world of difference. I bought a Maha MH-C777Plus-II which easily charges ten batteries at a time (quickly), can be plugged into 120vac or 220vac without an adapter (important for Greenland), has a heat sensor for over-charging, and lots of other bells and whistles. Plus, you can charge all your other rechargeable batteries you have laying around the house.  (I've since replaced the Maha with a Hyperion charger).

I know this all sounds pretty technical (and for many – it is), but the instructions on the web sites really help, and if anyone has any questions feel free to email me.

Happy Glowhounding!

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