After using my Olympus OM-D E-M1 Mark III for some time, I've noticed some weirdness with the Olympus BLH-1 batteries that merited some experimentation. In particular, I found out why putting it on the BCH-1 charger at 97-99% would cause it to charge for a few moments then suddenly stop, claiming that it's full when it actually isn't.
From what I can tell, the charger is designed to drive a constant current (CC) of 1.1A to the battery at a variable voltage, up to 8.5V (as marked on the battery), for fast charging, before switching to a constant 8.4V (CV) and a gradually decreasing current to top off. This behavior is inferred from power measurements with the charger plugged into a Kill-A-Watt meter, as well as the fact that the battery is marked for 8.5V charging and not 8.4V despite having a normal 7.4V nominal voltage. (Higher-voltage Li-ion batteries rated at 3.8V per cell are typically charged to 4.35V per cell. With two cells, that would be 8.7V.)
Under normal conditions, the switchover from constant-current to constant-voltage mode can be detected as a sharp drop from about 12W on the meter to anywhere between 4W to 8W, depending on the battery's state of charge (the fuller the battery, the lower the resulting wattage). If the battery is lower, it'll charge in the faster CC mode for some time, then switch to CV mode when it reaches 80% or so. If there's more than that when the battery's put on the charger, it'll charge in CC mode for a minimum of about 20 or so seconds before switching to CV mode.
Apparently, if the battery is near full (~97% to 99% as reported by the camera), that 20-second minimum in CC mode may involve the charger pushing more than 8.5V to the battery at 1.1A and triggering the over-voltage protection on the battery before it can switch to CV mode. When OVP is triggered, the battery goes high-impedance to reject further charging, and the voltage at the terminals will be about 8.00 volts. (The battery will switch back to normal once a load is placed on it. The actual battery voltage should be about 8.30V to 8.40V.) Because of this high-impedance state, the charger immediately signals that the battery is full and stops charging (the light turns green and the meter reads zero watts).
Under normal conditions, when OVP is not triggered, by the time the charger light turns green, there is still some topping off happening: 2-3W load may be measurable on the meter, gradually decreasing to 1W before it actually stops charging. This sudden drop from 12W (CC mode charging, maximum voltage) to 0W indicates that OVP has been triggered.
I was able to confirm this after some experimentation with a jury-rigged variable-voltage supply: if I drive about 8.6V to the battery, it'll charge at an estimated 0.5A to 1.0A for some time, then suddenly reject power and read out 8.00V on the meter. If I put the battery in the camera briefly and remove it for measurement, I'll get something like 8.39V. This means the battery has detected over-voltage and rejected further charging. Now I can say that the stock charger can trigger this behavior if you put a battery on it when almost full.
Not too sure if the charger's fast-charging algorithm is a good idea, but given what I'm seeing, it's probably doing 8.6V and hitting OVP. That kind of charging regime is quite hard on the battery...
Draco
