ANT BMS

Background

Although I presently have no use for an ANT BMS, I know two guys who do.  One wants to build a custom battery and another needs to replace a failed BMS.  Both are motorcycle applications.  I offered to be the guinea pig, and ordered an ANT BMS that supports 7 to 16 series cells and is rated for 80 amps continuously (200 amps for 10 seconds).  It comes with built-in Bluetooth for setup and monitoring.  Although it looks great on paper (especially for the price) you don't really know what you'll get until you start working with it.

Prior to ordering from the AliExpress store IC GOGOGO, I downloaded the user manual, Windows software, and Android APK file from ANT's website.  The PC software requires a separate USB interface board.  I bought this because my old eyes like working with a computer rather than a phone.  I never thought I say this, but I actually prefer the Android app to the PC software (more on that later). 

To make my evaluation easier, I used a Greenworks battery pack that was configured 20S1P.  This meant I did not have to spot-weld any cells.  My electronic load is only capable of handling a maximum of 60 volts, so I just tapped the positive end of the 20S pack at the 14th cell.  The cells are LG Chem ICR18650-HE4 with a nominal 2.5 Ah capacity.  Although the Greenworks pack is rated for 180 Wh, it was decommissioned back in June 2023 when it could only deliver 128 Wh.  This evaluation experiment is the perfect final use for it.

Preliminary Testing

Prior to removing the heatsinks for photography, I wanted to ensure the BMS worked.  The unit derives power from the battery via the Black and Red cell connection wires.  So I just powered it up by connecting a bench supply between those wires.

Since it's specified to operate with as few as 7 cells (which may only provide 21 volts, or less) that's the voltage I used.  The BMS draws only a milliamp or so when it's sleeping.  It may be “awakened” in one of two ways.  The simplest method is to momentarily short the “S” and “V” terminals together with a push button.  (Note that this push button must be accessible in your final packaged solution.)

Alternatively, providing about 4 volts between the “+” and “-” terminals at the communications interface also works.  This is the method used by the computer/USB interface.

This minimal configuration allows communications with the Android tablet application.  Although the ANT BMS app is available via Google Play, I preferred to sideloaded it.

Hardware Observations

• The following observations relate to the two photos below.

• The components are covered with a conformal coating.  This make reading part numbers almost impossible.  I think the 44-pin QFP on the top of the PCB is the analog front end IC.  This would make the 44-pin QFP on the bottom of the PCB the microcontroller.

• The separate blue PCB on the bottom is the Bluetooth module.

• The copper-colored components are 0.2 mΩ shunt resistors for measuring current.

• Balance resistors are 43 ohms in a 6032 package and rated 0.6 watt.  An LED is associated with each balance circuit.  The LED illuminates when a given cell is being discharged. 

• The black square near the upper righthand edge of the PCB is a piezoelectric buzzer.  It sounds when the BMS awakens from sleep, when a command has been accepted (and likely when a fault condition arises).

• The heavy black (C-) and blue (B-) connection wires are made from two flexible #10 AWG cables connected in parallel.  They are each roughly 10 cm long.  I measured the resistance of each parallel pair at 0.18 mΩ.

• Thermistors are nominally 14.5 kΩ at 20 °C. 

Low Side vs. High Side Switching

A BMS may either switch on the high side (positive rail) or the low side (negative rail).  There are advantages and disadvantages to both methods.  Typically, high-power systems (automobiles) use high-side switching via an electromechanical contactor (relay).  Whereas low-power systems (bicycles) switch using MOSFETs on the low side.  Electric motorcycles fall in the middle, and either system may be used.

The 80-amp ANT BMS switches on the low side using SVG104R0NS N-channel MOSFETs.  They are manufactured by Hangzhou Silan Microelectronics.  They are rated 120 A, 100 V and have a 3.4 mΩ on-state resistance.  

Sixteen such MOSFETs are connected in parallel to form the discharge switch.  This provides a combined on-state resistance of 213 μΩ (3.4 mΩ / 16).  For some context, current is measured using two 0.2 mΩ shunts in parallel (100 μΩ). 

At the rated continuous discharge current of 80 A, the MOSFETs dissipate less than 1.5 watts.  At the rated 10-second current of 200 A, the MOSFETs dissipate 8.5 watts. 

Additionally, a single SVG104R0NS MOSFET is used for the charge switch (which obviously needs to handle much less current than the discharge switch). 

All MOSFETs are in thermal contact with the aluminum cover plates (heatsinks) by way of a thermal pad (which is unfortunately fairly thick).  Thermal resistance is directly proportional to the thickness of the pad.

Wiring Diagrams

Software/Operating Observations

1. These observations represent my best understanding at the time of writing, but there may be errors.  You should consider ANT's reference material as the definitive source.

2. The BMS is always connected to the battery, but is only operational after it has been awakened by either of the methods mentioned earlier.  The BMS remains awake as long as it is Charging, Discharging, or Communicating (via either Bluetooth or USB).  When all those activities cease, the BMS goes back to sleep within about 30 minutes.

3. In addition to there being one LED associated with each balance circuit, two other LEDs are present (one on each side at the connector end of the circuit board).  When the BMS is asleep, no LEDs are illuminated.  When the BMS is awake, both LEDs flash.  When communications have been established, the LED near the communications interface connector turns to solid-on.

4. Interestingly, you can communicate via the Bluetooth/phone app and the USB/computer app simultaneously.  The Bluetooth works over a fairly substantial distance (several meters at least).

5. The charger must limit its output current to whatever your battery can safely accept.  The BMS's current measurement resolution seems to be 0.1 amp.  Towards the end of charging, the displayed value would alternate between 0.1 and 0.0 amps.  However, my charger was actually supplying about 0.2 A during that time.

6. Note that any BMS must completely cut power in response to a protection event (over-current, over-temperature, etc.).  This can result in a sudden, complete loss of power.  It is unlike a motor controller's protection which can “throttle back” to a lower current/power.

7. The firmware's default parameters may allow your system to function, but it's likely many settings will need to be optimized.  In fact, some default settings may be quite inappropriate for your particular system configuration. 

8. It's important to understand the concept of hysteresis as it relates to parameters.  Take the cell voltage spread as an example.  AutoBalancing can commence when the voltage difference between strongest and weakest cell exceeds 20 mv.  AutoBalancing stops when the difference drops to 5 mv.  If there was just a single number (say 10 mv) the algorithm oscillates around the setpoint -- which would not be good.  

9. Also note that AutoBalance cannot be locked in the enabled state.  It automatically reverts to being disabled when charging discontinues.  This makes sense because you don't want to constantly perform balancing as that needlessly drains the cells.

10. There are no hardware signal lines for the user to operate the charge and discharge MOSFETs.  Initially, I thought this meant they had to be manually controlled each time via the use of an app.  But it would appear that both the Charge and Discharge functions can be simultaneously enabled (and the BMS just figures out what to do based on the circumstances).  The way the app works is to have buttons called ChargeON / ChargeOFF and DischargeON / DischargeOFF.  It would be more understandable to me if the states were called something like “allow” and “disallow” for both charging and discharging.

11. To further complicate things, the nomenclature between the Windows app and the phone app is sometimes inconsistent.

I'll probably add to this section as I learn more.