volt meter for 2 batteries

Can it be wired to both batteries or do I need one for each?

Cheers

Thank you Mike Harding. Exactly what I'm after.

I have the lo-tech solution, a Redarc isolator. Have used them in previous cars and always had good results.

Louie the fly

And a Redarc tells you the voltage of each battery how?

It doesn't. Thats why I want to put a gauge in.

Louie the fly

My sincerest apologies I didnt realise that you were the original poster - sorry mate should have scrolled back to the top before I posted my 'slightly' sarcastic reply

I have just given myself and uppercut :-)))

Thats basically what the file Mike Harding directed me to shows, except it has a two position switch. I like the idea of the centre return switch though.

All voltmeters use a small amount of current. LCD meters might only use say 8 milliamps, but LED and analogue meters use more. This won't matter overnight, and probably won't matter over a few days, but a voltmeter will run a battery down if left for a week or two. As my vehicle sits unused in the garage during the week, I like the 3-position switch so I can turn the meter off.

Without back lighting electronic meters, typically, have an input impedance of 10M which, at 12V, would draw 1.2uA which, in turn, would take the average car battery about 5000 years to discharge :)

Analogue meters are, typically, 20k per volt so a 15V meter would draw 40uA so we're still talking 150 years.

Mike Harding

Mike,
Perhaps you could get a multimeter out and do some measurements.
My LCD meter with no backlighting was just measured at 7.3 milliamps.
I also measured an analogue meter (2" car meter with no backlighting) uses 87 milliamps. Thats 2Ah per day!!!!

I stand by what I said.

Cheers
Phil

Hi Phil

I can only suggest either your measurement is at fault or the meters are at fault. Anything which requires so much power from the circuit it is trying to measure is a very poor measurement instrument. Imagine using the analogue meter to measure a 9V PP3 battery? It would almost have discharged the battery before you could establish what its needle was pointing at.

Mike Harding

Hi Mike,
The readings are OK - used two multimeters.

"Very poor measurement instrument" might be spot on for the analogue voltmeter. It is a 20years old cheapie that I dug out of my electronic junk box, and maybe they have a big coil for the more rugged environment in a car. But people still use them.

The LCD meter use is an Eflite Power meter - it does a bit more than the average voltmeter and drew 7.3mA. I also recall a Jaycar meter drawing 8mA but it also measured temperature. But they are all meters that are likely to be installed in a vehicle.

i think you'll find that you are both be talking about different aspects of a meter.
whilst the input impedance of the electronic meter may be in the order of 10's of Megaohms with microamp current draw you still need to power the actual meter whih is where the 7 odd mA is drawn.
so you'll need ~7mA for the meter itself regardless of what is or isn't measuring and the device being measured will supply the couple of microamps. the current drawn from the cct being measured is not worth worrying about. the current drawn for the meter itself isn't going to do much damage either.

Any LCD voltmeter which draws 7mA (without a backlight) needs to be redesigned.

My $15 dual temperature meter runs from a single, 1V5, LR44 cell which has a nominal capacity of 150mAh - if the unit were to draw 7mA the cell would last less than 21 hours, instead it lasts for many months. Calculators, watches, central heating controllers etc....

Mike Harding

the figure of 7mA was taken from one of the posts above.

i've got a spec sheet for an LCD panel meter in front of me as i type and it is rated at 1mA ( no backlighting ).
it depends upon the device and one would hope that an appropriate device would be chosen. :)

7mA load is ~ 168mA/day
connected to a 100Ah battery and left to drain to 80% capacity gives nearly four months ( 119 days ) of useful life.
i'm not going to worry about it.
have fun out there..

It's just that you have to consider... if the designers are not competent to design an instrument which meets normal current consumption for such devices... are they competent to design one which produces accurate readings?

Mike Harding

Yep, you get what you pay for.
But I have the choice between spending say an extra $100 on a quality meter or an extra $1.20 to buy a switch that has a "centre off" position.

I too am amazed at the power consumed by the 2" dial voltmeter I measured - thats why it sits in the junk box. I need to keep parasitic losses under control because I ride a bike around town, and the truck sits in the garage for weeks at a time waiting for the next trip.

I prefer to measure at the battery terminals. To measure at the isolator will be fine when you're stopped, but when you're driving and the second battery is taking charge, then voltage drop will not be taken into account.

I use the voltage difference between the two batteries to tell me how much current is being put back into the auxillary battery. And when that difference becomes small, I know that the 2nd battery is nearing full charge.