A battery voltage readout [OCD/autistic problem]

[bob2.0]

This is about me setting up a cell voltage readout, and getting it wrong, but maybe in a good way.

In the 'discussion' thread I posted the lead I made up that links a XT90 plug to the battery, via an ideal diode. This seems to run fine at 20A. This is for charging with my new charger and also the possibility of range extending batteries.

In addition to that lead and diode, I added a voltage divider across the diode output, so I can take off an average cell voltage.

I intended to add 'too much' ground resistance, so that I could then simply trim it with a bit more.

I made an error, got it the wrong way in my head, and put too little in. I did a 21:1 divide with two ~100kOhm in series (redundant protection against a component failure) and tapping that at a 10kOhm to ground. I intended to trim with a parallel resistor, but confused myself, needed to be 15kOhm to do that.

This means there is a ~21.1:1 ratio and always half a mA draining from the battery. ( it might go flat!! )

Anyway, this mistake I made means that it will now read 4.0 for full cell, and 3.0 for flat.

Autistically, I dislike having the wrong cell voltage. HOWEVER! It means if the gauge reads '4' in the first digit, it is overcharged, and if '2' then it is dead.

Or thereabouts. So, easy to read. But even easier to read is the decimals, which now run from '.000' to '.999' from flat to full.

So, I will try it like that for a bit, see if it works for me. 'The Bob scale'. Not volts. 'Bobs'! But I have to get over the OCD issue that it is not 'right'.

1 bob = 1.05 volts. Cell is full at 4 bobs.

Your thoughts welcome on this set of events and errors.

[bob2.0]

One specific question for you guys.

Does the 12V DC-DC run all the time to generate 12V, or only when ignition is on?

If it's all the time, what's the most convenient place to tap 12V?

I'm planning to run the voltmeter off the rear light feed, which is on with ignition, but it'd be nice to see it all the time and not as if it will flatten the battery.

[bob2.0]

I wired it up, to get an idea of what it would tell me.

It turns out this is way out anyway, the meter seems quite a low impedance for a voltmeter, maybe 100kOhm. OK for reading a battery directly (li or LA) but not a high impedance divider like I put in.

So, if I want to change the scale then maybe I will rig up an opamp with a trimmer so that I can tune the scale to where I want it.

I still have come to the conclusion that the 'bob' unit is a brilliant innovation for NMC cells. 1 bob = 1.05 V. On this scale, if the leading digit is '4' then it is a fully charged cell, if it is '2' then it is too low and should not be used. '3' means 'ready' and the digits after it are an approximate percentage. It's, dare I say, brilliant! Slightly troublesome from an OCD-engineer's perspective, but I can get over it.

As it is atm, this reads 3.5 at fully charged, and it'll be discharged by 2.5.

As for whether it is useful or not, all I can say so far is that it is very distracting! You keep watching it for changes and the voltage dips and returns. Eyes are needed for the road, not for watching interesting electrical experiments!