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	Sanyo's USB powered charger can charge one or two AA or AAA rechargeable batteries from your computer's USB port.

Review: Sanyo's USB Powered NiMH Charger

Just over a year ago, I wrote and posted an article describing how to build a USB powered AA battery charger. At that time I needed such a thing, and none of the commercially available solutions were satisfactory. When I later reviewed Sanyo's Eneloop rechargeable batteries, I caught the attention of a senior Sanyo Europe executive who, in addition to providing some feedback, informed me that Sanyo was about to release a USB powered charger that had none of the shortcomings of the other such devices available. He offered to send me one, and I gladly accepted.

Why Would You Need a USB Powered Battery Charger?

The main advantage of charging your batteries from a USB port is portability. When I go on a trip, I bring a lot of gadgets: digital camera(s), PDA, cell phone, GPS, and laptop. I used to bring along AC powered chargers for all of these, which really filled up the carry-on luggage with heavy and bulky items.

Over time, I've made and/or purchased charging cables so I can charge all of these items from my laptop's USB ports. All these cables are either retractable or very short, and thus take up very little space. I've also built a USB powered charger for the AA NiMH batteries in my GPS, but I've never taken it anywhere by air because I'm leary about taking a home-made gadget through security. Now Sanyo has come to the rescue, and the only AC adapter I need to bring is the one for the laptop itself.

Clearly, charging from your laptop's USB port should only be done when the laptop is plugged in. If the laptop is running on batteries, you're just draining one battery (the laptop's) while charging another.

	When not in use, the charger folds up into a very compact package.

The Sanyo USB AA Charger

Sanyo's new charger measures approximately 3.7" × 1.9" × 0.7" and weighs maybe an ounce or two. It has a short (4 inch) USB cable which stores neatly in the side of the charger when not in use. It can charge either one or two AA or AAA Sanyo Eneloop cells at the same time (it can also be used with regular NiMH cells).

To use the charger, just plug it into any available USB port and insert one or two AA or AAA rechargeable batteries. Since AAA cells are shorter than AAs, the positive end of the battery compartment contains a flip-up connector. Flipped up, the compartment is long enough for AAs; flipped down, it fits AAAs. The polarity is marked in the battery compartment, and if you try to put the cells in the wrong way around, they simply won't fit.

Once the cells are inserted, an internal blue LED will start to blink to indicate that charging is in progress. When charging is complete, the LED stops blinking and stays on continuously.

How Does It Work?

Wanting to know how things work, I monitored the operation of the charger using my battery analyzer in monitor mode. What I was surprised to find out is that it is only the average charge rates that are about 850mA and 450mA respectively. In fact, the charger uses short pulses of higher current, and monitors the battery voltage between the pulses for charge termination peak detection. The length, rate, and current of the pulses are approximately the same whether charging one cell or two, but in the latter case, the pulses are sent alternately to one cell and then the other.

Charging time depends on the state of charge the cells had when you put them in, their overall capacity, and whether you are charging one cell or two. The following table gives the estimated charge time for fully discharged cells:

Cell Type	One Cell	Two Cells
800mAh NiMH	1.0h	1.8h
2000mAh NiMH	2.5h	4.5h
2700mAh NiMH	3.3h	6.2h

The charger uses a charging rate of 850mA when charging a single cell and 450mA when charging two at the same time. If you are aware of the 500mA limit imposed by the USB standard, you might be surprised at the 850mA charging rate, but fear not. The charger never draws more than 500mA from the USB port. It uses a very efficient power conversion circuit to convert the 5V DC input to a lower voltage at a higher current, much like a transformer can do with AC power.

Charge termination appears to be done by detecting the voltage peak (rise and fall) that is characteristic of NiMH and NiCd batteries when they reach full charge.

This charger isn't as fast as some AC powered ones, but I haven't found that to be a problem. It can easily fully charge two AA cells overnight. If you need to charge more, you could purchase two chargers and run them from two separate USB ports.

How Does It Compare with My DIY Charger?

• It can charge both AA and AAA cells.
• It can charge either one or two cells at once.
• It uses peak detection, and thus is easier on the cells, since they don't have to get warm before the charger shuts off.
• It automatically detects cells inserted backwards, or attempts to charge non-rechargeable cells.
• It can simultaneously charge two cells of different capacity or different state of discharge. If one cell finishes first, the charger stops charging it and continues to charge the other.
• Being a commercial product, it is not likely to cause you grief while going through airport security.

Important: If you've built my home-made charger, take note that the orientation of the two cells in Sanyo's charger is different. In my charger, the cells point in opposite directions, whereas in Sanyo's charger they both point the same way.

Where to Buy It

In the USA, this charger is available from Amazon.com. Unfortunately, it is not yet available in Canada, and Amazon.com won't ship it to Canada (The Source by Circuit City carries the rest of the Sanyo Eneloop line, but when I enquired about this charger, they had no idea what I was talking about). Here's the link to Amazon.com:

• USB Powered Charger with 2 AA Eneloops

For purchase in Europe, please see the Sanyo Component Europe web site.

Other Articles of Interest

If you found this article useful, you may also be interested in:

• Build a USB Powered Charger for Two AA NiMH/NiCd Cells
• Review: Testing Sanyo's Eneloop Rechargeable Battery
• Choosing and Using Nickel-Metal-Hydride (NiMH) Rechargeable Batteries

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Last updated Saturday April 12, 2008.	E-mail Stefan
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