This circuit prevents over-discharge of a lead-acid battery by opening a relay contact when the voltage drops to a predetermined voltage (lower voltage threshold).
When the battery is recharged to a second predetermined higher voltage (upper voltage threshold), the relay contact automatically re-closes and power again flows to the load. Both lower and upper voltage thresholds are independently adjustable to the desired voltages. Preventing the over-discharge of batteries increases the service life significantly.
Experiment of discharging a 6V 4.5Ah lead-acid battery
Relay contact of solid state switch
While the use of a solid state switch is perhaps a more advanced technology, it tends to introduce significantly higher voltage drop than a relay contact. This voltage drop is exacerbated by the high 20A load current rating. For instance, a very good MOSFET switch has a Rdson of 0.016Ω while a good relay offers only 0.005Ω contact resistance – the corresponding voltage drops at 20A are 0.32V for the MOSFET and 0.1V for the relay contact – the difference is significant!
The main drawback of using a relay is its coil power consumption – any power consumption increases the rate of battery discharge. As a result, I searched for the most cost effective power relay with low power dissipation. The result of the search was a Panasonic PCB automotive relay with 30A contacts, 0.005Ω contact resistance, low 38mA coil current and $5.02 price tag that is available from DigiKey – more details are noted on the schematic.
Not having this relay on hand, I experimented with an Omron G8P series relay – it does the same job, but runs at a higher coil current. Of course, the relay selection is yours – you may not need such a high current rating – just keep these tradeoffs in mine.
Further reducing relay coil power
To further reduce the relay coil current, I employed a relay economy technique described in a previous post. (fig 4). Adding two common components reduces the relay coil voltage and current to 70%. Also, when the relay drops out, the control circuit current drops to about 4mA thus minimizing battery drain.
The occasion of this circuit
If this circuit appears similar to previously posted circuits, you are observing correctly. The reason for this is that the requirements are similar and this basic circuit has exceptional utility, simplicity, straightforwardness and cost-effectiveness – it requires only one inexpensive IC and a few garden variety components.
Setting the voltage thresholds
Note that this is best set up using an adjustable DC power supply. If you do not have an adjustable voltage power supply, but are handy at math, you can measure the attenuation of the R5 & R6 accurately and then calculate the required voltages out of the potentiometers. Keep in mind that the upper threshold may not be set below the lower threshold or the circuit is dead in the water.
Setup using an adjustable DC power supply:
Refer to the battery voltage table on the schematic for determining your best settings – there is no easy “one size fits all” setting – you must evaluate your situation and experiment. I believe that the potential fickleness caused by series battery resistance is the very reason why there is a limited commercial offering for such a product – see next section…
Note that limiting the battery discharge to 50% of depth of discharge will greatly extend battery service life. Increasing battery capacity or paralleling batteries is usually a good idea.
Effect of high battery source resistance – oscillation
Battery cut-off controls are subject to oscillation if the lower and upper limits are set too close to each other. What happens is that when the load is disconnected at the low battery voltage limit, the battery is no longer loaded and the voltage tends to rise – if the voltage rises to the upper voltage limit, the control again applies power to the load which again pulls the voltage down to the lower limit – so if the relay starts clicking and the LED flashing slowly, you will know what is happening. The are four potential solutions: 1. set upper threshold voltage higher, 2. reduce load current (if you have any control over it), 3. use better or new battery that has lower source resistance, 4. parallel batteries to reduce source resistance.
One interesting application is load shedding in which there are multiple loads – some that are vital to keep powered and others that are not. This may require more than one control, depending upon priority and number of loads. In this case, the most critical device would be permanently connected to the battery, but the non-essential loads would be subject to automatically disconnect as the battery discharges.
Battery Discharge Control Photos
Undocumented words and idioms (for our ESL friends)
dead in the water –idiomatic phrase –maritime expression referring to a ship with a dead propulsion system –in electronics, it simply means dead or inoperative