Oset 24.0R Battery

Background

The Oset implementation is the “cleanest” example of a production electric motorcycle battery I've seen so far.  Two factors likely contribute to this:  1. The space available for the battery is fairly unconstrained.  2. The battery was designed by Yuasa (and I'm a big fan of Japanese engineering).

Unfortunately, I don't have this battery available for firsthand inspection.  All my observations are based on photographs and communications with the owner.  

The battery would no longer supply power, and simple diagnostics did not reveal the trouble.  The main fuse was intact, and the on/off switch was operational.  A few LEDs could be seen flashing on the BMS.  All cells were nearly fully charged and well-balanced.

Design Evaluation

The battery comprises two layers of cells.  The lower layer is 9 × 8, and it extends under the BMS.  The upper layer is 4 × 8.  This gives a total of 104 cells.  The configuration is 13S8P.

The cells are Samsung INR18650-25R.  These have a capacity of 2500 mAh and are rated for a 20-A maximum discharge current.  The 18650batterystore has these cells listed for $3.50 each in 10-piece quantities.

Thirteen series cells each providing 4.20 volts (fully-charged) yields a 54.6 V (maximum) pack.  At 3.6 volts (nominal) per cell implies 46.8 volts.  (Oset rates the motor as being 1400 watts and 48 volts).

2500 mAh (2.5 Ah) x 8 parallel groups yields a 20 Ah capacity.

20 A × 8 parallel groups yields a 160 A maximum discharge current.  Ultimately, the motor controller decides how much of this current (and for how long) to provide during acceleration. 

The main fuse is rated 250 A, which seem excessive for the application.  But its purpose is to prevent a fire should the electronic current limiting fail. 

Current is sensed via five 2-milliohm shunt resistors (bright copper areas in the above photo) all connected in parallel.  They yield a combined resistance of 400 micro-ohms.

The large 64-pin quad flat pack (U7) is undoubtedly a microcontroller.

The cover to the battery box contains several connectors.  The largest connector provides power to the motor controller.  The middle-size connector is for the charger.  

I don't know what the smallest connect does, but it's probably for communications with the BMS.  The connector's five wires terminate near the microcontroller.  It may even be possible to re-flash the microcontroller's firmware via this connector.

The circuit area outlined in yellow is for cell balancing and voltage monitoring.  A simple circuit is replicated 13 times (one for each of the series cell groups).  An LED is associated with each of the balancing circuits.  The LED illuminates when its related cell group is being discharged during balancing.  These LEDs are not wasteful because the current consumed by the LED is part of the overall balancing current draw.  This is another example of design excellence in that the battery cover can be removed to observe these LEDs simply by undoing a few screws. 

U5 (44-pin SOIC) is probably the ubiquitous Texas Instrument BQ7694003 Analog Front End (AFE).

Switching of the charge and discharge functions is performed on the low side (battery negative) via MOSFETs located under an aluminum heatsink plate.

The square inductor just to the right of the aluminum heatsink is for an SMPS (switched-mode power supply).  The SMPS is a high-efficiency converter that bucks battery voltage down to 3.3 – 5 volts to run the BMS.

Note the green locking agent on the screw in the upper right-hand corner of the heatsink.  Unfortunately, that screwhead is only accessible via the underside of the circuit board.  This necessitates removal of the PCB in order to remove the heatsink for MOSFET testing.