Low Voltage Disconnect
The
life of lead-acid batteries depends on the depth of discharge.
Typically, the state of charge of a lead-acid battery is linearly
proportional to the stabilised voltage between 11.75 and 12.9 volts. A low voltage disconnect prevents
the voltage dropping below a set level, thus providing a minimal form of
charge management. In the case of the
solar bucket, a low voltage
disconnect is desirable for two reasons. First, there is no need to
monitor the discharge phase to prevent the stabilised voltage falling below
11. 75 volts (i.e. not worry about oversleeping). Secondly, it is
desirable to start each day with the battery in more or less, the same
state of charge.
Design
The circuit described below is not intended to be a definitive design, rather a discussion of how such a device can be constructed. Component sizing will depend on the application and the components available.
The components and functionality are:
VR1 - This is a voltage regulator (e.g. 7805 in the test design). It serves two purposes, first to provide a 5V supply to IC1 (and any other integrated circuits that might be incorporated as the design evolves) and a reference voltage against which comparisons can be made.
IC1 - A voltage comparator (e.g. a single channel of an LM339 in the test design). When the voltage on the non-inverting pin (denoted by a + sign) is higher than the voltage on the inverting pin (denoted by - sign), the output of the comparator is high (in this case close to 5 V). When it is lower, the output is low (close to zero).
R2 & R3 - These form a voltage divide, the output of which is fed to the inverting pin on the voltage comparator (IC1) and which provides a reference voltage (in this case approximately 2.5 volts).
R1 - A potentiometer which acts as a voltage divide. The setting of which determines the disconnect voltage. (In the solar bucket application, the disconnect voltage is set at 11.75 volts, giving a stabilised voltage of around 11.85 volts).
R4 - A pull-up resistor which most comparators require (see datasheets etc. for more details). In the test application, a 10K resistor was selected because one was available.
T1 - A switching transistor. This uses the output of the voltage comparator to disconnect the load. In the Solar Bucket application an IFR540 mosfet is used, when the voltage on the gate is high, current flows from source to the drain, when it is low, no current flows.
RLoad - In the Solar Bucket application the load is provided by one or two 10 watt 100 ohm resistors which provide a discharge current of approx. 120 or 240 mA respectively. As the load is purely resistive, there is no need to mess with things like flywheel diodes.
Operation
The circuit appears to draw approximately 10 - 15 mA. The stabilised voltage can be higher than the disconnect voltage. When the voltage falls below the disconnect level, the voltage across the battery rises resulting in reconnection to the load, followed by subsequent disconnection. Thus it can take some time before the stabilised voltage approaches the disconnect voltage. In the case of the solar bucket, it has been found expedient to switch off the LDV and provide at least half hour for the voltage to stabilise before measuring the battery voltage.
Development
The short term plan is incorporate a latch into the design, thus when the battery has been disconnected from the load, it stays disconnected until reset. This will allow the battery voltage to stabilise. In the longer term, similar capability will be added to disconnect the solar panel when the maximum float voltage of the battery is reached, thus preventing overcharging. This is seldom a problem in winter, but around the summer solstice it is necessary to monitor the voltage.