![]() ![]() You can buy a ‘voltage divider’ sensor, but a couple of resistors (plus some screw terminals) is all that is on it. On the back of the power shield, I soldered up some resistors in a voltage divider arrangement, and connected it up to the 12V input (you can see in a photo that I went though the hole in the power board). I just used some leads with crocodile clips for the power so I could just clip it on my battery (and to a close grounded bolt) and it could be removed when no longer needed. Note that some of my D1 Minis wouldn’t even get a wifi signal without the external aerial, I suspect due to the switching of the power supply components on the power shield. ![]() I wanted the Pro due to the aerial connector (don’t forget to move the 0 Ohm resistor bridge to be able to use the attached aerial). I used a Wemos D1 Mini Pro (or a clone of it.) and an off the shelf ‘power shield’ to power it from the 12V. Mosfet control probably best, but it would need +ve switching. Probably not really necessary as it draws very little in sleep mode at <10mA, but it could be done by having the device switch on with the car ACC circuit) and latch until the voltage dropped below a set level. It would be nice to have the device disconnect itself completely if the voltage was too low – and eliminate all loads when needed. Have a method of complete/automatic disconnection.It might be nice to see if the car can keep up with charging the 12V battery when other systems are on. Doing this would mean going away from something simple like core ESPHome though… but there have been a couple of feature requests and discussions about having ESPHome log to SD. It would be nice to be able to push historic data to influx DB (haven’t investigated this… and not sure whether time series databases like this idea). Having SD storage and logging to that would be a good idea. Measure 24/7 when the car is not at home.It would be great to see just how much current is flowing in and out when driving and charging. An esp32 would probably be a better idea, or maybe an i2c adaptor. Using an ESP8266 is a bit of a pain for trying to do this as it only has one Analog input though. 100A range would likely do, and it gives 0-10V out. I have bought a range of current measuring sensors, and something like the hall effect CT is what I’ll be using. ![]() Measure current in and out of the battery.This is no doubt similar to other EVs though, and maybe newer cars just charge the 12V quicker when charging is possible. It is sad really that they have to keep the HV and 12V system completely separated, as there are times when the 12V battery (around 1kWh of storage) can go flat even though the car main drive battery (24kWh) is full. Leaf 12V batteries seem to quite commonly be the cause of issues, and especially when they have been sitting unused for a while. Obviously this should be better than ICE vehicles, as they also only when the engine is on. the drive battery is on charge) and also when you are actually moving. They charge when the car is charging (i.e. I have a Nissan Leaf, and they seem poorly designed to keep the 12V battery topped up. Use low power (and deep sleep mode where possible) to avoid draining the battery further.Sending regular reports to MQTT (say, every 10 mins).A calibrated measure of the battery, which could be up to 15Vdc.Using the Analogue input on an ESP8266 and voltage divider to monitor the status of a 12V battery in a car. V1.2 Minor changes to the ESPHome yaml & notes… (I built a second one for another vehicle) Summary V1.1 Addition of some graphs showing voltage measurements
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