I’ve been searching for a bigger better battery to power the Raspberry Pi battery board V2. Based on past experience, I’ve found that the least costly sources for Li-Ion batteries to be inconsistent in how well the measured capacity of the battery matches up with what was claimed by the manufacturer. The battery that I have been testing is normally used in a Samsung Galaxy Note 2. The Galaxy Note 2 OEM battery is rated at 3100 mAh. Doing some web research I came across a page called Battery Analysis & Reviews that listed a number of recommended batteries for the Galaxy note 2. One of the batteries recommended on BA&Rs was manufactured by a company called Anker and was available through Amazon. I ordered two of these batteries as a set that included a wall charger for $26 US. The batteries were selling individually for $14.
I haven’t posted any of my battery testing data yet. My testing to this point had shown that all the batteries that I tested were measuring well under the rated capacity. The fact that I had made my own circuit on an Arduino to take the measurements made me question something with my test setup. When I first tested the Anker batteries I couldn’t get a measurement greater than 10500 mWh (milli Watt hours) which for a 3.8V battery works out to be 2763 mAh vs. the rated 3100 mAh. According to the BA&R site, I should be measuring a bit more than the 3100 printed on the label.
My battery tests were run after charging the battery from my home built charger circuit that uses the MCP73831 chip. I repeated the test using the supplied wall charger from Anker and measured 11962 mWh or 3147 mAh. The good news – I finally had some data that showed my Watt hour measurements were believable. The bad news – my homebuilt charger only appears to be charging to about 88% rated capacity of the battery.
There are two reasons that I believe may explain why my circuit is not fully charging the battery. The first possible reason is that the MCP73831 I am using is built for batteries with a 3.7V chemistry so during the constant voltage part of its charge cycle is run at 4.2V vs 4.35V as should be for a 3.8V cell. The 4.2V and 4.35V setting on the MCP73831 is set in the chip itself and I haven’t been able to source the 4.35V version of the chip in a SOT23 package. The second possible reason for the under charge is that (from what I have read) some chargers only charge to 90% of the rated capacity of a battery for the sake of improved charge/discharge cycle life – I will probably have to get my hands on a MCP73831-3 (4.35V version of the chip) to find out.