Lead acid batteries, once charged, are typically held at a float voltage that is higher than the open-circuit voltage (OCV). The intent of this float charge is to compensate for self-discharge. Not that this is without problems: there is greater water loss due to gas evolution, when compared to being held at OCV. It has also been argued by many that maintaining stationary batteries at float voltage does more harm than good - see, e.g., “Traditional float charges: are they suited to stationary antimony-free lead acid batteries?” T. M. Phuong Nguyen, Guillaume Dillenseger, Christian Glaize and Jean Alzieu http://www.intechopen.com/books/trends-in-telecommunications-technologies. Using a reduced float voltage with intermittent charge can be a superior method of battery management in many applications. One specific context of application for such battery management is in on-grid solar applications. In such applications, the battery is only used as a standby, yet it may represent a sizeable chunk of the investment in the solar system. Therefore, the battery life needs to be extended, and its maintenance and watering needs must be reduced also.
Below we described steps needed to understand how the project was programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.
1. Enhanced Battery Management
On initiating a charging cycle for a lead acid battery, it goes through several states.
We summarize the process very briefly; more detailed information is readily available in the public domain, e.g. at websites such as The Battery University [http://www.batteryuniversity.com]. Charging begins in a constant current (CC) regime called the Bulk phase, which ends when the voltage reaches Bulk Voltage (see Table 1). The regime then changes to constant voltage (CV). The battery enters the Absorption state, in which the voltage is held constant at the Bulk Voltage for a specified period of time. Then the voltage is reduced to the Float Voltage which, in the standard case, signals the end of charging. In this project, we are not content with stopping with that. Rather, we want to hold the battery at the Float Voltage for a specified period of time, then reduce the voltage further to what we call the Reduced Float Voltage, which represents the end of the charge cycle.
Typically, the Reduced Voltage will be close to the battery's OCV. However, a battery cannot be held at OCV for too long because it starts to lose capacity and then sulfate. Therefore, the battery must intermittently be taken to a higher voltage (Float or Absorption Voltage) to compensate for this loss of capacity, then brought back to the Reduced Voltage. This may seem like an obvious feature to have in a solar inverter for example, but in the author's experience it is not always available, even in premium inverter brands.
The state diagram in Figure 1 represents those states and their transitions.
Figure 1. The Battery State Machine
2. Solution Architecture using GreenPAK
In this section we describe how the various building blocks of GreenPAK as well as external elements are brought together to realize the solution. (We will henceforth use programming-style variable names such as BulkVoltage, FloatDuration etc.) The IC chosen is SLG46531V.
Asynchronous State Machine (ASM). GreenPAK 5 was chosen because it offers an ASM that makes it particularly convenient to capture the battery states and transitions. The state and transition definitions are shown in Figure 1.
Figure 2. The GreenPAK Design
ACMP0-2. For this project, we take a case where the nominal battery voltage is 12V. The design can easily be extended to other voltages. We can scale the battery voltage with a resistive voltage divider, but we choose an...Read more »