So the logic would be to monitor the main battery voltage, once it's >13.5 (or whatever) for more than say five minutes, connect the aux battery. I'd expect to see at least 13.7 when charging, which would indicate the alternator is running. I'll investigate the accessories power and also do some more research into how other systems handle it.ĮDIT: Thinking this through, I could just isolate the aux battery if the main battery voltage is less than 13.5v. I'd prefer to isolate the battery until the engine is started, but I haven't been able to come up with a method to determine if the engine is running or not. The voltage divider was discussed here: Voltage Divider with Zener Protection Diode - Automotive Application - General Electronics - Arduino Forum The idea being a 16:1 divider removes the need for external protection diodes as it would allow the internal clamping diode to deal with the expected spikes. How are you going to monitor the coolant level? What is the expansion tank level going to tell you? It may be full but the engine empty due to a radiator leak.Īny optoisolator will work for your usage. You don't want to be charging the auxiliary until the main battery is charged. You could simply wait until the motor has been running for a set period & the main battery is up to voltage. You may look at using that for your input as it happens before the motor relay operates. In many vehicles the accessories are switched off when you operate the key to start. Otherwise there may be huge current draw through the MOSFET. I would want the isolation to happen before the starter is operated. I am not sure but I think the voltage divider R in is a little bit high. Please forgive the fairly poor schematic layout, I'm new to using Fritzing. The software side of things doesn't worry me (I work in IT and have done a fair amount of programming including other Arduino projects), but I am less confident with the electronics, and that's where I'm after feedback/suggestions from this community. At the moment I'm considering using either a USB cellphone charger or a buck converter module. I haven't yet determined what is the best way to power the arduino in an automotive environment. If the connection length is an issue, I might put a second arduino in the LCD housing and use I2C or similar to drive the display. The LCD display will need to be mounted about one meter away from the controller, I'll probably use shielded cat 5 cable for that connection, but I will test this ahead of time. This has built in navigation buttons which use a resister divider network to need only one input for a number of buttons. The output will use a Sainsmart LCD shield. The engine ignition on state and starter motor are monitored via optocouplers (can someone recommend a suitable optocoupler for this application). This will allow the internal clamping diode to handle a transient spike of up to 225v. The battery voltages will be monitored using a 1:16 voltage divider with a 1.1v internal reference value. Sound an alert if the main battery voltage falls below a preset value.Sound an alarm if the temp exceeds a preset value Monitor the cylinder head temperature using a thermocouple, display on screen.Monitor the coolant level by way of a sensor in the expansion tank (vehicle uses a pressurised expansion tank), sound an alarm and provide an on-screen alert if the coolant level is too low.Disconnect the auxiliary battery if the starter motor is engaged. Connect the auxiliary battery only when the engine is running.Monitor main and auxiliary battery voltages, display on LCD screen.I haven't fully specified components yet as I don't want to do that until Im fairly confident the idea is sound and free of obvious flaws. I've developed the attached schematic, which is intended as a fairly high level overview of my plan. I'm in the process of designing a monitoring system and auxiliary battery controller for use in a car.
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