That is a very valid point. I am looking into reducing the TelePeriod and enabling some energy saving options.
How did you manage to stay within WiFi range during the drive?
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It wasn’t in range. The quick increase is when the engine is switched on and the alternator starts charging, then the car goes out of range (the flat section). The drop is when the car returns in range and is switched off, and the battery starts “calming down”.
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Yes, that is precisely what happened.
Tried clicking on the link to Gitlab but it appears to be dead. I’m VERY interested in making one of your devices to monitor the battery voltage on my solar setup
Does it need to be a Wemos D1 Mini or can any Node MCU be used, I don’t understand the difference and I will just be following your instructions on assembly the unit but here in the UK it seems difficult to get hold of the Wemos.
I want to use your device to monitor 12v batteries on my off grid solar
No the d1 mini is by no means compulsory. The D1 mini is different to a bare esp8266 in that it has a 5v -> 3.3v converter, and a usb port. This means:
- it can be powered by 5v - although the other connections need to be 3.3v.
- it can be programmed without an ftdi adaptor via the USB connector.
NodeMCU is an operating system for ESP8266 chips (running nodejs) but it has become a byword for a board that was manufactured to run nodemcu. In short any nodemcu board can do anything a wemos d1 mini can do.
The benefit of the wemos in this design is that it has a power supply board that just plugs in. The pin settings on a nodemcu may be different, but that doesn’t stop you using exactly the same principle to use any esp board.
There are still plenty of places to buy wemos d1 mini clones on aliexpress, or no doubt other similar sites.
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Did you succeed in reducing the power consumption ?
I’d be very interested in monitoring my motorcycle battery and its capacity is way less that a car’s 
Kind of annoying that @MrDIY has posted three projects here and then deleted them from gitlab, or not maintained his account, or whatever has happened. Hopefully he will sort it.
Posting the information directly to the forum, rather than us having to revert to other sites that may disappear is always preferable IMHO. And youtube is not a howto, it is a showoff!
I know they are more expensive than Amazon, eBay and AliExpress, but they are quick delivery and look to be legit and not cheap clones. I’ve used them a number of times and been very happy - no connection, just had a similar struggle to find them when I was looking in the UK with a quick turn around!
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By the way, this fellow has an instructable too, so if the gitlab resource has disappeared for good, see: https://www.instructables.com/id/DIY-Monitor-Your-Car-Battery-Code-Setup/
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Thanks for that Nick, the NodeMCU thing has always confused me since I started looking to play with these things 
Would be nice to know if there has been any change in the reading from anyone who built one of these.
It’s been about three weeks now and my guess is that the voltage will be about the same now as three weeks ago even if the car has not been started once.
My hunch is that unless you have a history of several weeks in HA then you will not see any change except noise just as you can see in OPs image.
Thanks for the replies guys, appreciate the help.
I don’t really understand most of tech chat about programming these devices but I’m generally OK at following instructions and I don’t think this project is beyond me
So would these be perfect for the job (I’m planning on making 2 and have just bought 2 DC power shields via ebay UK)
the pin-outs look to be the same as the wemos.
Would I do everything the same as the instructions because if buying these meant I would have to deviate from them then I would probably buy a genuine wemos via davefi’s link (thanks Dave)
Is anyone using one of these devices?
They work fine.
Don’t worry too much about the pinout of the brand. Instead make sure it’s compatible with ESPHome.
Everything else is easily fixed.
The only thing that could happen if you buy a board with a different pinout is that you need to change the code from “D2” to GPIOxx, since the ports are labled on most boards it’s just to read off the board and change the code accordingly.
About the programming mentioned, as long as it has a USB port it’s fine.
Keep that as your main priorities instead, USB and compatible with ESPHome.
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Thanks Hellis81, I intend to (attempt) to run tasmota as I already have lots of devices running it via Home Assistant on a Raspberry Pi and it generally does what I want.
So in your opinion will those boards from Amazon suite the job without having to change anything or won’t I know for definite until I physically get them. For £8 I don’t mine taking the chance they will
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Oh right. Forgot that this project was tasmota.
I just know ESPHome has a list of boards buti have never heard of a board that is not compatible.
I even bought one that was not on the list and it worked fine.
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See how it says d1 mini - that means it is a wemos d1 mini clone. Should work exactly the same.
This would woek equally well in esphome, but we won’t confuse you 
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Great thanks. Good idea not to confuse me, its easily done
@MrDIY, your project is promising. Is it doable without tasmotising and just using esphome? With home assistant running off a SD card in RPI4+ 4G.