Battery Management

Battery management is an additional function that we would like to add to the golf cart. For a battery management system, the four basic functions are charge control, protection, authentication and fuel gauging. While charge control, protection and authentication still need to be investigated further, adding a battery fuel gauge seems to be a great first step in adding a battery management system.

Currently, the cart is unaware of the status of the battery at any given time. Adding a battery fuel gauge will add more autonomy to the cart by helping it in knowing how much longer it can drive until it needs to be recharged. It will also help in determining when batteries need to be replaced.

Battery Fuel GaugeThe original method considered for monitoring the battery was simply reading the battery

voltage. This would be done by using a voltage divider circuit to knock down the voltage below 3.3V which would allow it to be input into the Arduino DUE. The image below shows the circuitry for the implementation for both the 48V and 12V power supply (lead acid batteries).

These circuits were simulated to ensure the voltage going into the Arduino was as expected and would not exceed 3.3V. The simulations show the need for zener diodes in order to ensure the voltage going into the Arduino stays below 3.3V.

Figure 1 - 48V to 3V Voltage Divider with Zener Diodes

Figure 2 - 48V to 3V Voltage Divider without Zener Diodes

Figure 3 - 12V to 3V Voltage Divider with Zener Diodes

Figure 4 - 12V to 3V Voltage Divider without Zener Diodes

This method was determined to not be the most accurate way to monitor the batteriesdue to the fact that it only looks at the voltage on the battery.

The next method that was investigated was using a TI Impedance Track Standalone Fuel Gauge. This fuel gauge estimates the capacity of the battery using a patented algorithm that

accounts for battery aging and self-discharge. This method would use a TI hardware chip (BQ34Z100-G1). This component supports two wire I2C communication which could be connected to an Arduino DUE, which has two I2C interfaces. The device can be read from or written to using the Arduino DUE and also has many configurable parts which would allow for information on the battery capacity to be sent to the Arduino where the information can be processed. One configurable part of the hardware that would be particularly useful for our desired application is its ALERT output. This output would be connected to the Arduino to notify it of the battery status. Below is a table of the information that would be sent.

The simplified hardware implementation for this device is shown in the figure below.

This battery monitoring chip uses an open-circuit voltage table that stores the battery pack’s open-circuit voltage versus capacity curve. The bq27500/501 algorithm “discounts” this curve by the resistance values (IR drop) to simulate the battery pack’s discharge curve under the present system use conditions (rate and temperature). A resistance profile is the resistance versus state-of-charge curve used to “discount” the open circuit voltage curve based on the present use conditions. QMAX is the low rate or theoretical maximum discharge capacity of the battery pack. This value is again “discounted” by the Impedance Track algorithm to derive full charge capacity (the discharge capacity under present use conditions to a fixed voltage).

TI also provides access to free software that is compatible with this hardware. The details are as follows:

Battery Management Studio (bqStudio) offers a full suite of robust tools to assist with the process of evaluating, designing with, configuring, testing, or otherwise utilizing TI Battery management products. This includes features that provide full access to registers and data memory including support for real-time watching, graphing, and logging, an easy interface to send commands, direct low-level communication and I/O, as well as automated and guided support for configuration, calibration, performing a learning cycle, and generating useful files for taking the device to production. In addition, Battery Management Studio provides automation and scripting support for the Gauge Development Kit (bq27GDK000EVM). 

The TI Impedance Track Standalone Fuel Gauge is a great method for battery monitoring but I believe it will take a while to implement due to the complexity of the system.

Another tool I found for battery monitoring was the TF01N. This device is a coulometer that measures battery voltage, current and power. The voltage and current measured have +/- 1% accuracy. The measured values are displayed on a small LCD screen that can be placed in the front of the cart or in the electrical panel in the back of the cart. The screen displays the remaining battery capacity, battery voltage, battery current and remaining time of charging/discharging. This will be very helpful in determining if the cart can make another trip or if it needs to be charged before. This device also has an output pin so the information can be sent into an Arduino for additional monitoring.

More info on the TF01N

One thing that the team has discussed is how helpful battery monitoring will be during debugging. Having this device will be extremely helpful for debugging because it will be easy to check and ensure that the cart has power and that the batteries isn’t the issue. For that reason, I would like to go with the TF01N. I believe this does exactly what we are looking for and should be quick and easy to implement. Once we begin using this, we can determine if we think we need additional functionality in our battery monitoring system. If we do, I believe the TI Fuel Gauge will fill the gap because it stores all the data in the Arduino which will allow us to go back and retrieve old data to see how the battery behaves over time.