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Old 7th February 2014, 01:10 AM   #1
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Default batteryless voltmeter?

Due to a recent incident where a car wouldn't start, I want to get a couple voltmeters to leave in my cars.
- Cheap would be nice
- Doesn't need to be ultra precise, but on the other hand I need to be able to distinguish between 11V vs. 12V vs. 13V
- No battery would be very nice, so I can just leave it in the cars forever.

Is there such a thing? Or am I on crack? I searched Amazon for "analog voltmeter" and realized most of them probably have a battery for the ohms function.

Or should I just get a cheapie digital and then what-have Google calendar remind me to check the batteries? (And be sure to get an auto-off model?)

Any likes/dislikes for battery cables, spew forth as well!
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Old 7th February 2014, 01:33 AM   #2
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Battery cables: don't skimp on the copper. Get 'em long and thick so you'll (almost) always be able to reach the other car's battery.

Heat's very bad for batteries, so stow your unit accordingly. Lithium batteries do seem to last the longest, although alkalines are not too shabby for shelf life.
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Old 7th February 2014, 01:39 AM   #3
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Hmm, how many amps might flow through the battery cables? Then one could calculate the voltage drop for a given wire gauge. For home extensions, I like to just get the biggest gauge, but in the car a big gauge could make a huge coil of wire.
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Old 7th February 2014, 03:07 AM   #4
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100 amps would not be surprising, depending on the size of the engine being cranked and the size/condition of the battery supplying the juice. Don't be caught short-cabled.
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Old 7th February 2014, 03:12 AM   #5
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The problem with light battery cables is that when you need them you have no battery, so All the cranking current goes through them. If you can wait a few minutes maybe your battery will rake some charge and help out. A little four cylinder engine with a gear reduced starter might take only 30-50 amps average, but a big bore engine can ask for many times that when a cylinder nears TDC. I think 8 gauge or better is OK, thicker if longer. The quality of the clamps are no joke, neither is strand count if you think you might use it more than once. A cheap movement type meter can be accurately interpreted if you "calibrate" it before you throw it in the car.

Last edited by Andrew Eckhardt; 7th February 2014 at 03:18 AM.
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Old 7th February 2014, 03:52 AM   #6
DUG is offline DUG  Canada
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Quote:
Originally Posted by Andrew Eckhardt View Post
The problem with light battery cables is that when you need them you have no battery, so All the cranking current goes through them. If you can wait a few minutes maybe your battery will rake some charge and help out. A little four cylinder engine with a gear reduced starter might take only 30-50 amps average, but a big bore engine can ask for many times that when a cylinder nears TDC. I think 8 gauge or better is OK, thicker if longer. The quality of the clamps are no joke, neither is strand count if you think you might use it more than once. A cheap movement type meter can be accurately interpreted if you "calibrate" it before you throw it in the car.
Exactly, let the "dead" battery get some charge and then disconnect right after starting.

But don't load the electrical system of the car with the "dead" battery as there may not be enough charge to run things with heavy electrical loading...happened to a neighbor of mine.

As for voltmeters...search for "expanded scale" voltmeter.

Two or three resistors, a zener, (or a LM7805) and a meter movement will get you a good 9-14V scale... you will be able to tell the difference between 10, 11, and 12 Volts.
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Old 7th February 2014, 05:42 AM   #7
Sonce is offline Sonce  Macedonia
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Quote:
Originally Posted by head_unit View Post
Due to a recent incident where a car wouldn't start, I want to get a couple voltmeters to leave in my cars.
- Cheap would be nice
- Doesn't need to be ultra precise, but on the other hand I need to be able to distinguish between 11V vs. 12V vs. 13V
- No battery would be very nice, so I can just leave it in the cars forever.

Is there such a thing? Or am I on crack? I searched Amazon for "analog voltmeter" and realized most of them probably have a battery for the ohms function.
Any analog voltmeter will measure voltage without battery, just choose one with good resolution display in the 12 V range.
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Old 10th February 2014, 06:14 AM   #8
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LM3914 with few components will make you one and the resolution u want.

Gajanan Phadte
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Old 11th February 2014, 07:37 AM   #9
JMFahey is offline JMFahey  Argentina
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Will LM3914 run batteryless?
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Old 11th February 2014, 10:14 AM   #10
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How much current will it draw from the car battery if powered from it... plus will be rugged and small. If the car battery cannot drive the LM...!!!

Gajanan Phadte
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