I thought it might be useful to do a complimentary set of articles on useful circuits and hardware we have employed in some of our projects. This one was used as a battery back-up for a monitoring system monitoring Azipods on ships.
The system had a fair few requirements regarding materials, explosion and water ingress. One of the requirements was coping with a dodgy and interrupted power supply. Why not use a UPS I hear you ask...we couldn't and it was neater to package our own is the answer.
From a design perspective this would be a bit of kit that could be deployed on various ships, monitor various things and have very different power requirements so putting the batteries in a separate box worked nicely.
Having a nice wide range of inputs on the PSU makes life easy and this can both charge the batteries and power the equipment.
One useful piece of kit was the monitoring relay (Carlo Gavazzi DUA52). The blurb from the manual describes it like this..
"DUA52 is a voltage monitoring relay that measure its own power supply. The measuring ranges are 8-28 VDC and 38-58 VDC. It has separate potentiometers for setpoint and hysteresis. Typical applications are monitoring of backup batteries, batteries on diesel generator sets and the like."
In short it measure the power supply and switches on at a certain voltage and off at another voltage, there's some potentiometers on the body that allow you to adjust these values. You set the value and the hysteresis. In our case we set the relay to turn off at 18Vdc and back on at 22Vdc.
I'm told this scheme is good for the lifecycle of the battery. (2 off Yuasa NP 12-12 in series, lead acid was a requirement).
Also the cRIO doesn't like having power removed in an unplanned manner, if you find you're having reliability problems it's almost always to do with this. So it's a good idea to shut down the cRIO before the monitoring relay goes off. A monitor line was added to allow this. Even if you don't do this it is good practice IMO to drop the power out at a low voltage and only apply it again when the voltage is higher.
If this type of article is of interest, the next one will be about custom keyboards for cRIOs.