Building a big battery

Building a big battery
Putting it all together is a simple soldering job. If you’ve never soldered, this is a great time to learn; an iron, a stand and a big reel of solder will set you back less than $40 (Australian dollars - even less in US currency). If you don’t want to learn, your local electronics technician will be able to do it for you in minutes if asked (and/or paid) nicely.
Make sure you get the polarity of the connector right. Most barrel plugs are wired "tip positive"; the outer, "sleeve" terminal of the plug is the negative. If you’re uncertain, use a cheapo multimeter to check the output from the plugpack. You can now buy digital multimeters quite good enough for basic tasks like this for less than $20 from various electronics stores.
Even D size alkalines, though, aren’t particularly efficient when driving digital camera loads. NiCds are much better. Really, really BIG NiCds are better yet.
(Note: When I first wrote this piece, NiMH cells weren't yet a good option for this sort of application. Nowadays good NiMH cells are easy to get, and so are hobby chargers that understand them. If I were building this same pack today, I'd use D-sized NiMH cells. Bear that in mind as you read the NiCd-plugging below.)
The pack I use with my DC120 is made out of NiCds so large that most people don’t even know they exist. They’re Sanyo F size cells, one size up from D, with a capacity of 7Ah. There is only one larger standard size, M, with a 10Ah capacity. I got my F cells from the USA; in Australia you can expect to pay something like $30 each for them. The much more common 4.4Ah D cells sell for about $15 each, and are a rather more sensible proposition. M cells, in case you’re interested, cost the thick end of $70. Each.
I’ve GOT a mains adaptor for my DC120, but thanks to the 7Ah pack I practically never use it. I plugged it in for the first time in months in order to get the pictures of my battery pack cabling! Doing things like serial data transfer and LCD screen image reviewing under battery power is a terrible idea if you’re using AA cells, but with the monster pack – no problem. I have never run the big pack flat.
Nor, by the way, do I intend to in order to "maintain the pack’s capacity", because I do not subscribe to the myth of "memory effect". If you think your camera, laptop, cellular phone or cordless drill battery suffers from memory effect, you are wrong. Check out http://bargain-batteries.com/camera-batteries.htm and the full NiCd Battery Frequently Asked Questions file at http://bargain-batteries.com/camera-batteries.htm before you flame me about this. I ramble on at greater length on this subject here.
Eagle-eyed readers will note that my pack contains not four but five cells. The five cell pack gives the camera 6V, with which its motorised zoom works noticeably faster than when it’s running from the 4.8V it gets from four NiCds.
Like many electronic devices, the DC120 works from a range of voltages. It will run just fine from four NiCds, but it’s happier with a higher voltage – its mains adaptor is rated at 8V. A NiCd pack that delivers less than the rated voltage of a camera’s mains adaptor should cause no problems; mains adaptors do not deliver less than their rated voltage unless they’re delivering more than their rated current, which you shouldn’t force them to do. When delivering less than their rated current, most plugpacks deliver MORE than their rated voltage. Only "regulated" plugpacks maintain the same voltage at all times.
To build a custom battery pack, you’ll need to solder the cells together. Soldering to cells can be tricky – you need a soldering iron with a fairly large tip, and you need to roughen up the ends of the cell a little with sandpaper or a file to encourage the solder to stick – but you can get around the problem by using "tagged" cells, which cost maybe 25 cents more each and have a solder tag sticking out from each end. The tags are very easy to solder to, and can handle currents much higher than a digital camera will ever draw. Do not attempt to solder any kind of cell straight to another cell – use wire in between.
If you MUST solder to untagged cells – and you may have to if you want to use really high capacity NiCds for your camera – practice on a few non-rechargeable cells first. Roughen the ends, apply the iron and some solder to "tin" them, then press the wire you intend to use into the solder blob with the iron. If you roughen the ends properly and have a decent iron, it should all be over in five seconds. If you find yourself heating the end of the cell for ages, you’re doing it wrong.
Some resellers of NiCd cells, like for example http://bargain-batteries.com/camera-batteries.htm, offer solder tags as an option on their entire range. The cells don't come from the factory that way, but the resellers weld them on themselves. Done correctly, this is every bit as good as a factory-installed tag.
Once you’ve got your tagged cells or learned to solder to untagged ones, arrange your cells as you like, and stick ‘em together. Hot melt glue is a decent adhesive for battery pack assembly – it holds quite firmly, but not TOO firmly, and gives you some time to get the line-up right. The cells should be aligned so it’s easy to wire the positive of each cell to the negative of the next, with the power cable attached to the terminals on the ends of the line. What exact arrangement you use depends on what size cells you’re using and how you intend to carry the pack; shuffle the cells around until you’re happy.
You can, if you like, use a sealed lead acid (SLA) "gel cell", as used for standby power in alarm systems, to power your digital camera. Gel cells come in 12V and 6V versions; the 6V batteries will power many digital cameras just fine. I prefer NiCds, though, because lead acid batteries are rather heavier for a given capacity, can’t be charged nearly as fast as a NiCd, and don’t like being run flat. If you flatten a gel cell and don’t charge it fairly promptly, it will die. For this reason, always make sure any gel cell you buy is charged – another use for that cheapo multimeter!
If you use alkaline cells in a holder, you only need to solder your cable to the holder. If you go for a rechargeable pack, the battery lead will need a plug and socket arrangement in it somewhere that you can unplug, so you can connect the battery pack to a charger. You could also wire a barrel socket matching the plug to the charger if you liked. My own battery pack has a couple of Anderson connectors with specifications many times higher than are needed for this application. Anything that won’t come unplugged when you don’t want it to will do, but remember the golden rule of all connectors – if there’s a male and a female, the FEMALE connector is the one that should be on the "live" cable that comes out of the battery pack. Otherwise, unplugging the connector will leave a live male connector dangling around, and male connectors are so called because they have protruding terminals, which can easily short out on any metal object. Short circuits are especially bad with NiCd and NiMH cells, because their low internal resistance lets them deliver enormous currents when shorted, and they can and do start fires.
If you’ve never charged your NiCd or NiMH cells, they should be flat; that’s the way they come, and that’s the way they should stay while you’re building a pack out of them. These kinds of cells, unlike lead acid batteries, are perfectly happy to stay flat for as long as you like, and putting the pack together while the cells are flat removes the danger of short circuit excitement.
Once you’ve built the pack, give it a bit of a charge, check its voltage with your multimeter, and if all is well plug it in and see if it works. Once you’ve verified that it does, mummify it. I wrap large packs with several layers of cloth-backed "Gaffa" tape ("duct tape", to Americans; actually, the Gaffa brand stuff is inferior; Nashua 357 tape is the real gaffers’ choice). You can also get heat-shrink tubing in surprisingly large sizes (again, ask at the hobby shop!), but it’s hard to beat cloth-backed tape for durability and price.
Incidentally, because of the huge current capacity of NiCd cells, it's not a bad idea to put an in-line fuse in the power lead from any NiCd packs you build. If you don't, and the lead shorts out, the cells will probably still not be damaged - but the lead can easily get red hot, then melt. This is probably not exactly what you want to happen in your camera bag.
A 5 amp slow-blow fuse should give plenty of headroom for even power-hungry digicams, but will blow long before your cable overheats.