Yea I wish they would put a voltmeter on cars these days!
Looks like reading voltage directly is the way to go. Im fairly familiar with lead acids to know that 12.8V is fully charged and 12.0V is fully discharged. However, those voltages are unloaded. The higher the current drawn, the lower the loaded voltage will be even at the same capacity level. Or is there a good rule of thumb for this?
I rather not have to drill out holes and such for a physical gauge. Would be nicer to just have this integrated in my carpc case and hooked up to an internal USB header then have the voltage readings displayed on screen. The Fusion Brain looks like it's exactly what Im looking for. Voltage, current, and even temperature sensors!
edit: Wow this is so cool. "car voltage meter"...basically what soundman98 was talking about above. Im looking for a more integrated solution (and also my cig power is hooked up to the main electrical system, not the auxiliary) but this might be handy for some people.
My Nearly Complete Car:
Micro Control Center... Control Your Car Across the Internet
Website: (It's a work in progress, really. All my projects have taken me from ever really developing it.)
As to typical battery voltage vs %age capacity...
Although that shows a 1.2V range from 0% to 100% (0.12V per 10% capacity), I use a 0.1V per 10% rule. Not only is that easier, but it's conservative and tends to compensate for various conditions & variables.
Under load, I use the 0.1V/10% rule. IE - at (near) no load, read the voltage; the add the load - that voltage is you starting voltage. EG - if 12.7V unloaded is 100% and under load it drops to 12.5V, then 12.5V is the loaded 100% voltage. (I find my car batteries are 12.7V fully charged, but 12.6-12.8V is a typical variation.
PS - definitely an inter-battery isolator. I recommend circuit breakers rather than (the essential!) fuses at each end, and a relay controlled by the alternator/regulator's charge lamp circuit (which is the best isolator you can have for typical applications).
And you would be surprised how many cars have voltage meters built in, you just don't know it. For instance, on all Prius models you can enter a diagnostic screen with the OEM display and one of the boxes is a 12v battery check that shows the live 12v battery voltage on the display. Lots of cars have this ability, but not usually "user" accessible unless you know what you are doing.
Fusion Brain Version 6 Released!
1.9in x 2.9in -- 47mm x 73mm
30 Digital Outputs -- Directly drive a relay
15 Analogue Inputs -- Read sensors like temperature, light, distance, acceleration, and more
Buy now in the MP3Car.com Store
this is the one that i have--i was originally looking for something that is easy to mod to fit my windshield--the ones that i currently use have too narrow of a angle for the lcd to be easily readable from the drivers seat-- i have to lean forward...this is what i get for rushing into a purchase on a business trip...
for $7, i might have to try a couple of those...
The problem with that is it's a cig socket - it should really be across the battery. There can be big differences - especially in fault conditions - though they provide the system voltage indication if that's gone high (ie, battery is 14.4 but IGN is 16V).
It could be bought and butchered for its though it's not backlit etc.
There are lots of voltmeters in eBay starting from a few dollars.
I originally used the guts of the voltmeter in my last reply's link and integrated it into my dash idiot lights (it looked "standard"). It was great by day, but not backlit for night.
So I got an eBay blue-LED 3 digit voltmeter instead. Cool! But it washes out by day (and unlit segments look fugly), and it lacks a dimmer. I could modify it, but I decided LCD FTW.
Problem is getting small lit LCD voltmeters with only 3 digits - I only want an 18mm high display (nearly 3/4").
But if size isn't a problem, then for a few dollars ($2 plus postage), something like .
Whilst an analog meter could sit connected to the battery permanently, digitals may be too high a drain (albethey less than 10mA (less than a Bosch SL alternator LOL!)) so I use a remnant relay from my old charging system - essentially an "ignition relay" much the same as ECU/EMS relays (which me be a good alternative).
But I'm intending a reed-relay controlled by a timer (say 1 minute or few) which delays its off time after the IGN is turned off, and I'll probably add a manual push button to turn it on at any time (it'll turn off after the delay).
Although things like the FB could be used, if that's it's only use, then IMO it's an overly complicated and overpriced system.
And though a voltmeter can be integrated into in-dash screens, it is still well suited as a separate stand-alone meter anyhow.
PS - In my frustration I was considering an FB using small backlit LCDs I got from meat thermometers and multimeters...
update for those looking for a physical battery gauge/display/meter...
i bought one of the previously mentioned cigarette lighter voltage meters, i pulled it apart, and promptly broke off the +12v wire.. all within the first 2 min of receiving it..
after resoldering it back, it is working great! i am not sure that i like the flickering display--it updates at roughly 60hz--so it has a look similar to a led being powered with pwm--but otherwise, it seems plenty bright during the night, and according to my volt meter, it is only off by .03v. there is a pot on the back that i assume is for fine tuning, but i haven't messed with it yet..
i'll have to get my car out of the garage some day to let everyone know how it is during the day though..
Do you mean it is multiplexed at 60Hz?
That is weird because (1) it's a bad choice & (2) that might means 20Hz per digit (unless you mean 60Hz per digit).
I presume the display is beating with AC street lighting etc?
It won't be updating at 60Hz (as in updating measurement ie readings), that's more likely to be from 1/3 to 3 Hz.
(I just got the reed relay and on-off-momentary switch for my display today.)
um, i am not really sure-- the display just flickers if i move my eyes too fast across it, and it bothers me, just like pwm led's...