I finally got tired of having my old dustbuster as a nicd memory-effect demonstrator (it would fade immediately, but then after about 45 seconds come back with some usable power, annoying to wait for). So I decided to replace the battery pack. In the process I realized that I could extend run time and power, hot rodding it to a slightly higher voltage.
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Step 1: Background and Prep.
This particular dustbuster is a wet/dry model, about 10 years old. When I popped out the battery panel I discovered the battery pack was only 3 sub-c size nicd cells. There was room in the compartment for four, so I decided to accept the risk of an extra 1.2V from a 4th cell. This turned out to be a good move so far, although I don't expect the motor to last forever. Given the short duty cycle of typical dust busting, it should be fine. I also marked the leads on the buster so I wouldn't forget which was which.
Then I went online and found 3300mAh sub-c NiMH batteries with solder tabs (these can be found at several battery suppliers, really the only important part is to get Sub-C size NiMH batteries of any reasonable capacity. Getting them with solder tabs makes it easy to make the final pack, so I advise them. I also found out that the stock charger has a reputation for wrecking the batteries, so I'm using my smart charger instead, it's a MRC 959 super brain charger.
Step 2: Battery Pack Assembly.
Once I got the new batteries, a well spent $12 later, I aligned them in the typical inverting pattern. The Pos and Neg terminals are next to each other from one battery to it's neighboring battery.
Given the position of the original packs leads, I used the 3rd and 4th battery as the ends, soldering #4's Pos pole to #1's neg. 1 then goes to 2, which goes to 3, which goes to the other terminal. Unfortunately I thought to take pictures only after I had assembled the pack with some clear tape. Also try not to short the batteries, they're usually shipped charged.
By the way use electrical solder with no lead in it. Lead solder is bad for you, and obviously you need to not breathe the fumes even with the good stuff. Check out the various soldering tutorials if you're new to it. Have aloe vera on hand, because sooner or later you're gonna burn something. Or wear welding gloves.
Step 3: Battery Pack Installation.
The terminal plugs are a small spade type connector, and I found it easy to just use a stripped wire end to plug into it, as you can see in the pictures. The end terminals of the pack are currently (no pun intended) plugged into the charger via the gator clips.
The only last part that was interesting was that I had to use dike clippers to cut out the fins on the inside of the battery cover, so it would close in the new configuration.
Step 4: Close It Up and Make the Wife Happy.
I wanted to use the original charging cradle to charge it, just by putting an adapter on the charging cord for my smart charger, however there seems to be a diode or such in the way, so I had to bypass it with a bit of wire.
As shown here, you can see the charging paddle directly shorted to the positive lead by a loop of wire I also soldered in place. Tight confines, but I was able to do it with a bit of care. Lousy angle, but you can see the diode if you look carefully.
I'm basically going to let the dustbuster run down a bit with regular use, and as soon as I notice it getting slow I'll put the charger on it. This is probably an ok strategy with the NiMH batteries. It's still *very* strong now, and It's been about 2 weeks since it's last charge. It works better than it did new!
Step 5: Charging Setup.
Here you can see my new and improved charging setup. I used an old dead RC pack's deans connector so it would easily plug into my RC charger. This allows me to charge the batteries with more care than the original dumb charger that fries cells over time. Keep in mind that the polarity is opposite of what you expect, since you're charging not discharging. And I did bypass the diode in the charging base to allow unrestricted intelligent charges.