Automatic Charging Relay (ACR) is used in vessels for controlling the various batteries available not only for proper charging but also for supporting needs for high currents.

The following figure displays a typical wiring diagram for a boat:

Usually, as you can also see in the above diagram, we need at least two batteries. The first is used as engine – starter – battery and the second as battery for suppling available accessories. Moreover, sometime we may have a third battery only for the radio – for safety reasons. One of the reasons for having more than one supplies are the kind of the load, ie the engine needs high current peak for small duration but the accessories small currents for long durations. So, in the first case we need “starter battery” providing hundreds of amps but not capable for deep cycles, while in the second we need deep cycle batteries. Of course we may combine both in the same battery, ie using an AGM battery, but this will cost. Another reason is related to the safety, we may don’t want to consume the whole stored capacity for playing music and not be able to start engine afterwards, so we need an isolation from the engine path.

So, clearly we need more than one power sources, but how can we manage to charge or consume the power in the right way? The answer is the ACR. The main function of the ACR is to prioritize and monitor batteries. Some of the advantages on using ACR are the following:

  • Prioritizes charging, ACR monitoring the charging voltage and allows main – engine battary to charge first.
  • Supports alternator, usually alternators are not capable to charge more than one battary at the same time.
  • Provide start support, by detecting the shape of the battery can support engine start by sonneting both batteries when this is needed.
  • Secure engine battery, avoid discharging engine battery.
  • Provide automatic fallback to service battery in case of main main battery failure.

Searching the available ACR, we deiced to build our-own, having few additional features. We start the design based on the ESP32 platform from espressif. ESP32 provides cost optimized products having many peripherals that are essential for the ACR design. In more details, we have used 2 SPI master controllers for controlling a graphical display and also acquiring touch screen inputs, 4 – analog to digital channels for gathering the batteries voltages and additionally the engine and exhaust temperature (using K-Type thermocouplers), the WiFi and Bluetooth interface to be able to remotely monitoring the status and also few GPIO to drive the ACR Relay, the alarm buzzer and some status indicators:

Schematic diagram

In more details, we have used the AD8495 K-Type thermocoupler driver for converting the sensor voltage to a ADC readable value. Battery voltage is divide to a range compatible to ADC range (0-1,1V). Display is connected to standard GPIO pins that has been configured as SPI signals. Also, two step-down power supplies are used for providing the 3,3v (using AMS1117 regulator) and 5v (using LM2957 regulator) supplies.

Printing housing
Electronic board
Unit ready
Unit installed
Final GUI

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