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Archive for June 25th, 2012

Canon NB-5L Battery Teardown, Cheater, and Voltages

The motivation for gutting that Dell laptop battery was to find out if the cells could become a higher-capacity external battery for the Canon SX230HS camera. Those discharge curves suggest they can’t, but I also want to know what voltage levels correspond to the various battery status icons, which means I must feed an adjustable power supply into the camera… so I need a fake NB-5L battery with a cheater cord.

The first step: crack the case of the worst of the eBay junkers. I squeezed it in the bench vise to no avail, then worked a small chisel / scraper (*) into the joint. The lid was firmly bonded to the case, but it eventually came free:

NB-5L Battery - opened

NB-5L Battery – opened

The protective PCB sits at one end of the cell, with a strip of black foam insulating the components from the nickel strips:

NB-5L - protective PCB

NB-5L – protective PCB

It turns out that the cell’s metal shell is the positive contact, which I didn’t expect.

The component side of the PCB has a 10 kΩ resistor connected between the center and negative contacts. That should be a thermistor, but it’s a cheap eBay knockoff and I suppose I should be delighted that there’s not a gaping hole where that contact should be. The PCB fits against the small notch in the case and is held in place by small features on the top and bottom. The negative contact is on the far left:

NB-5L - PCB interior view

NB-5L – PCB interior view

Canon sells an AC adapter for the camera that includes an empty battery with a coaxial jack that aligns with a hole in the battery compartment cover. I soldered a pair of wires to the PCB, drilled a hole in the appropriate spot, added some closed-cell foam and hot-melt glue to anchor the PCB, and made a cheater adapter. For the record, the orange wire is positive:

NB-5L - gutted case with pigtail

NB-5L – gutted case with pigtail

It turns out that the camera battery cover must be closed and latched before the camera will turn on, but the sliding latch mechanism occludes the hole. This cannot be an inadvertent design feature, but I managed to snake the wire out anyway.

Connecting that up to a bench supply (with a meter having 0.1 V resolution) produces the following results:

Voltage Result
3.8 Full charge
3.7 2/3 charge
3.6 Blinking orange
3.5 “Charge the battery”

The camera draws about 500 mA in picture-taking mode, about 300 mA in display mode, and peaks at around 1 A while zooming.

The Genuine Canon NB-5L is good for 800 mA·h to 3.6 V, as are the two best pairs of the Dell cells. The latter remain over 3.7 V for 500 mA·h, which suggests one pair would run for about an hour before starting to blink. Maybe that’s Good Enough, but … a new prismatic battery is looking better all the time.

(*) Made by my father, many years ago, with a simple wood handle that eventually disintegrated. I squished some epoxy putty around the haft and covered it with heatshrink tubing, but (now that I have a 3D printer) I really should print up a spiffy replacement. I’ve been using it to pry objects off the printer’s build platform, so that’d be only fitting…

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