We really wanted this week’s Fail to work. [Michael] wanted more juice for his Nikon D3100 camera. The idea he had was to replace the cells of the battery with a Buck converter and add leads for an external battery. This opens up the possibility of running from a wide range of voltage sources; an attractive prospect for devices using specialize batteries. Specifically, he wanted to swap out the stock 7.4V 1030 mAh battery and use an 18 Ah lead acid one instead.
The biggest hurdle to get over in a project like this one is the logic the camera uses to communicate with the battery. For this reason — and for the ease of hitting the right form factor — he scrapped an old battery pack to reuse the logic board and enclosure. His power supply is a free-formed circuit which fits nicely in the allotted space.
The circuit powers up, but only to about 6.4V. This isn’t enough to run the camera, which means this was just an expensive way for [Michael] to practice his soldering. After the jump you can read his recounting of the experience. You’ll also find a few of the build images, and the two hand-drawn schematics he used during development. His Dropbox has the entire collection of images.
The hack was to build a buck converter into the aftermarket battery and run leads out of the battery bay such that the camera (7.4V 1030mAh) may be run from a much larger external Sealed Lead Acid unit (12V 18Ah).
The motivation for this project was realised after I noticed the genuine battery was almost flat after 30mins of HD filming. It was anticipated that the hack would also be very useful for extended runs of time-lapse photography. Whatever the case, it is worth noting that the 216Wh SLA costs about $40 while the genuine Nikon battery (at 9.6Wh) costs about $50 on eBay!
Even with pessimistic efficiency figures for the buck converter, it is plain to see the gross gains in Wh/$.
But I digress.
The fail rises from the disappointing circuit performance. The circuit failed to output the calculated voltage (7.3V nominal) and was instead only able to supply around 6.4V, too low for the camera to even power up.
I’m still unsure what the problem was;
- Miscalculating the component values / inductors not in range?
- Current draw too high?
- Noise in the dead-bug style architecture?
- Incorrectly soldered parts?
- Incorrectly identified how the donor battery’s circuit board functioned?
- Conductivity in the hot glue? (I doubt it)
The photos originally were to serve as a build-log, but are now resigned to merely showcasing some neato dead-bug soldering.
Whatever the fault, I’ve since scrapped the arguably more elegant method that I attempted, and instead plan to use a $5 variable buck converter that one can find on eBay. This unit will be mounted on the large external lead battery and have the output run into an empty shell aftermarket battery. Being a pre-assembled circuit, all I’ll have to get right is the input and output polarities!
You can also see on the schematic .pdf that I was thinking of implementing input polarity protect (the two mosfet circuits). These were from an application note that I now cannot remember how to find, but I’ve found a similar one (PDF). An elegant way to avoid dropping that 0.7V on your garden-variety input diode.
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