OT: Car Battery State Metering

[Cursitor Doom]

Gentlemen,

Modern cars don't tell you much about their general state of health. I think it was the 1600E that had all those gauges on top of the dash, was it not? So you could monitor oil pressure, charge, water temp and whatnot very easily. Not so today without plugging in a dongle & a laptop - a piss poor state of affairs I think we can all agree.

I have often thought it would be handy to have a meter that shows the battery voltage, but 'magnified' so as to read just between 10V where the zero would normally be, and 15V FSD. This is the range where all of the really useful detail lurks but it's hard to see sub half volt changes on a conventional analogue meter. If displayed on a digital readout, it's an inferior experience IMV, since I want to be able to see at a moment's glance and I'd like it to be displaying the battery voltage constantly, too.

I thought about using an op-amp to amplify that key differential between 10V and 15V, using a precision adjustable voltage regulator IC for the 10V reference. But am I over-thinking this? Could there be a simpler solution here?

-CD

[Fredxx]

Hardly OT?

There are enough digital modules that read 12V to more than sufficient decimal places with a variation of styles and sizes. I don't agree that it an inferior experience? Especially if you want state of charge where voltage measurement will be, critical.

If you want analogue, then stick a 10V zener in series with a 5/10V meter. You might have to load it by bridging the meter with a resistor so running at the 'calibration' current at say 14V.

If you want a 12V live feed there is a pin on the OBD connector that is a permanently live and inherently accessible.

[tabbypurr]

An analogue voltmeter & zener is simpler, but look at the zener curve & ensure the meter passes enough i to keep the zener doing its job. And expect to need to print a new scale nonlinearly.

Moving iron meters pass plenty of i but aren't linear.

You can of course use an adjustable 'rubber zener'

NT

[Fredxx]

When I say 10V zener, if you want accuracy then I would suggest something like:

Which has versions to give a 10V drop +/-0.1% over a current range of

80uA to 15mA.

[Paul]

You're better off just building something from scratch. The panel meter materials, aren't good enough for any real purpose.

You need to measure the battery temperature and correct the voltage reading for temperature. And that means a microcontroller might come in handy.

I can see at least one ADC board. It's a 16 bit ADC. Which is more than you'd get out of the typical panel meter.

Maybe I'll have to change my processor platform plans so I can have a decent size and number of digits for a LED display.

Paul

[Roland Perry]

In message [e-mail], at 23:54:36 on Sun, 13 Sep 2020, Cursitor Doom [e-mail] remarked:

I have a USB charger which plugs into the cigarette lighter, and has a digital display of battery voltage. The resolution to 0.1v is sufficient for my needs.

[Harry Bloomfield, Esq.]

Fredxx wrote on 14/09/2020 :

There will be quite a variable voltage drop on that pin, so the actual measured voltage will be meaningless.

Best place to measure voltage, is across the battery. I bought a portable 12v battery analyser for not a lot. It reports state of charge, health of battery, health of charging system, cranking volts drop, voltage and graphs charge, cranking and electrical noise from alternator.

[Chris Green]

There are lot's of little digit 0 to 19.99 volt digital meters on eBay for very little money, quite accurate enough in general. Even if not absolutely accurate you will certainly soon learn what is 'normal'.

[Andy Bennet]

I have one of these. Bung it in your lighter socket, job done! Cheaper variants are available ...

[Radio Man]

Noise from the alternator is a feature I?ve not seen mentioned on any I?ve looked at, it could certainly be useful. Could you post details please?

I assume it is Bluetooth and works with a phone etc? Or does it have a remote display?

There are some simpler ones, which don?t sound as versatile as yours, on EBay for as little as £20 which claim to show some of the above on an app via Bluetooth. I think they are all battery terminal voltage based ie they just measure the increase when charging, drop when on load, cranking, etc.

You can get a stand alone tester which claims to measure the available CCA but how good the cheap ones are is anyone?s guess. Then Halfords tested two batteries of mine, both were fine on their meter but didn?t hold a charge even under a light (measured) load. Odd. You would think they would be keen to sell two batteries.

[Dave Plowman (News]

I built a ready made 2 wire LED voltmeter into my rev counter. Reads from

00.0 - 19.9v. What surprised me was the different readings it gave depending on where it was connected to battery positive. Load from other things on that circuit cause quite large fluctuations. So eventually wired it direct to the battery via a small relay switched by the ignition. Since it was battery voltage and charging voltage I wanted to see accurately.

Only complaint is the display is slightly too bright at night - fine in daylight. Have looked for a similar unit where the display can be dimmed without success.

[newshound]

You can get these that go in the cigarette lighter. I keep one in my ATV since that has intermittent useage. It was very useful in that it revealed that the glowplug timer relay was "sticking on". I also carry one in my horse van as a much quicker test than lifting the floor panel to apply a meter. I think I have one in the toolbox too for when neighbours have problems starting. I've never bothered to calibrate one so I expect the decimal point figure might be out by one or two, but definitely good enough for diagnostics. Only about a fiver IIRC.

[newshound]

+1

[Harry Bloomfield, Esq.]

Radio Man has brought this to us :

This one has its own stand alone display, but plugs into USB to upload its test data to a PC, via a downloaded app. It shows the jitter from the alternator and as near as I can tell seems pretty accurate. You set it for the battery type and the CCA, then a second later it shows the data. It shows my new battery at 100%, a really old one which can hold some charge as 30% and my caravans 120amp as 70%. I would not dispute any of those figures...

Ancel BA201 Battery Tester 12V Analyzer 100 to 2000CCA Car Battery Tester Car Quick Cranking Charging Circut Tester PK KW600 $29.57 USD

I ordered from One of the big Chinese resellers, from their stock in Poland, so it was delivered tax paid and in 7 days.

[Harry Bloomfield, Esq.]

Harry Bloomfield, Esq. presented the following explanation :

BTW, I tried to fool it, by connecting it to the main fuse box positive, fed via a very large cable, plus battery -ve. It declared my new battery unfit for any further use. Across the actual battery lugs, it declared it 100% fit.

It measures the battery voltage and resistance in micro-ohms.

[Harry Bloomfield, Esq.]

Dave Plowman (News) brought next idea :

Yep, volts drop is a really major problem at 12v. I tried to use my ciggy lighter socket as a convenient place to plug in a charger regularly, to avoid opening the bonnet to connect across the battery. There was far too much voltage lost via the loom, but it limped along, sort of, until I decided to do something about it.

I added a ciggy socket looped off the large main feed, intended for my towing socket power. That comes straight off the main battery bus-bar, via a 30amp fuse. It's never in use for towing when it's parked in my garage, so now it serves a dual function, with almost no volts drop to the battery.

My caravan has an analogue meter, at almost the far end of the loom, by the door. That drops 1.5v when any load is demanded on the loom - useless for its purpose in life. Not many people understand volts drop, even the so called experts.