Solar Battery Management System Interface

The interface card is used to perform the basic current and voltage measurements for the battery. The current measurement must allow for current to flow in either direction to the battery. The simplest solution for doing this and to allow for high-side measurements is to use a special current monitor chip (INA170). This device makes high-side current measurements in which the common mode voltage is above the chip supply voltage. For voltage measurement a simple single-supply op-amp can be used (LM124). The outputs are scaled to 3.3V max and passed through a zener diode protection circuit to the microcontroller. The computation of the resistor values in the battery voltage measurement is somewhat messy and is explained here.

The circuit breaker was adapted from one of Rob Paisley's Fast Circuit Breaker circuits. These use the LM393 comparator and a small amount of positive feedback to switch very rapidly and remain latched until an external reset is applied. In our case this can be done remotely through the microcontroller. Note that the reset button is applied through a pulse circuit rather than directly. The signals are intercepted by the central controller card and used to instantly disconnect the problem load or the source. The batteries do not have this disconnect feature as their current is determined only by the loads and source. It is of course possible that a failure inside the system can result in heavy currents, but the handling of this event is complex and needs further attention. The simplest approach would be to place independent circuit breakers in series with the batteries.

A circuit derived from the voltage measurements is used to disconnect the source in the event that the input voltage drops too low. This is useful as the MOSFET circuit used in the power switch can overheat at low voltages.

The circuit uses an LM336-2.5 to provide a stabilized 2.5V offset voltage for the voltage measurements. The measurements are scaled to give zero to 3.3V over n inputa range of about 9 to 15.5V. The current measurements are offset to provide bidirectional measurements. These are buffered and applied to a comparator that sets the trigger level. The trigger only activates on rising current levels and therefore is only relevant to positive currents. For a battery this is current flowing to the load. Charging current will come from the source to the battery and will register as negative current. It is assumed that the power source will have its own circuit breaker (probably not necessary for a solar panel or other current limited source). The second comparator provides a low voltage indicator back to the processor board.

This is the third version of the board.

First created
30 August 2013
Last Modified 2 january 2015
Ken Sarkies 2013