Olympus PEN USB charging

Olympus E-PL7

Olympus E-PL7

For several years I've been shooting with Micro Four Thirds cameras, first an Olympus E-PL1 and now the E-PL7. I find them to be a nice tradeoff between image quality and size/weight, especially the Olympus PEN models. They're great cameras for hiking/backpacking, cycling, and inconspicuous street photography.

Like most cameras in this class, the PENs ship with a separate box-like battery charger and power cord. Whereas most compacts support charging the battery inside the camera, this feature is lacking on most “serious” cameras. The manufacturer probably assumes you'll want to charge as quickly as possible, and not tie up the camera while doing so. For someone like me who travels light, however, it's not a great fit. First of all, I'm unlikely to have AC power access except overnight, when I'm not using the camera anyway. Second, I already have an AC-to-micro-USB adapter and cable for a mobile phone, tablet, etc. It would be nice if I could use that instead of lugging around another heavy power cable.

The USB-charging question comes up from time to time on the micro-4/3 discussion forums, but there aren't any ready-made, lightweight solutions. So, while contemplating a trip to New Zealand last spring, I decided to make my own.

Circuit design

The circuit consists of a TPS61085 boost converter running at 650 kHz, providing about 9V to a pair of MCP73842 7.4-volt Li-ion linear chargers. The boost converter is connected directly to pins one and five of a micro-USB connector. The circuit does not perform any kind of negotiation for USB power levels; I knew I'd be running this off a 2A plug-in charger, not a computer. I settled on dual charging because ten watts is enough power to charge two of the 1100-mAh batteries at once, parts are cheap, and there is space for one spring-loaded battery connector (TE 1717838-1) on each side on the board.

Finished micro-USB charger

Finished micro-USB charger

The thermistor (temperature sensor) inside the battery is monitored by the MCP73842 for safety, but the fourth contact on the BLS-5 and BLS-50 batteries is not connected. I believe, but cannot prove, that this duplicates the circuitry inside the older Olympus charger, which works just fine with all of my batteries. The charge current is about C/4, so charge times are a bit longer than the stock charger. It could be set higher or lower with a resistor change.

The batteries don't bring out the inner connection between the two series cells for balancing purposes, but with low charge currents this is apparently OK as Olympus' own chargers [obviously] face the same limitation.

Mechanical

There is room for one battery on each side of the PCB

There is room for one battery on each side of the PCB

Together, the populated PCB plus a hair band for holding the battery in place weigh 13 grams. I haven't built an enclosure yet because when backpacking I usually have a safe nook where I can stash the bare board.

In use

Low-tech battery holder

Low-tech battery holder

I carried this charger for three weeks in New Zealand as my sole camera-battery charging solution, and it works great! I brought three batteries and took advantage of the dual-charging capability on several occasions.

This sort of thing probably doesn't make sense if you are packing a suitcase or carrying your gear in a car. But if you are backpacking or cycle touring, it's a win. When I travel I already have to bring a USB wall adapter and micro-USB cable for my mobile phone and/or tablet, so that weight is “free” and I get to save the weight of the Olympus cord and charger.