Voltage drops, measuring

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nathanlee

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Having been reunited with my long lost multimeter, I've just had a little prod. Now, despite the fact that last time I did this Lensman ended up very kindly fixing my mistake (blown transistor), I've since learned to double check it's set to volts and not amps :p

Anyway, Nasa BM-1 says there's 13.3v at the battery. Interestingly, the multimeter says 13.03v, but still... when I flick the cabin light on, and check the voltage at that end, it's reading 12.75v.

That seems like quite a drop. Is it?
 
It's not unusual. It rather depends on the total length and guage of cable between the battery and bulb AND back to the battery. You should also check the voltage at the battery while the light is on too ..... as that could have dropped ..... although the battery would have to be pretty small to make that size of difference.

0.3V would bother me though if it was voltage lost between an internally regulated alternator and one of your battery banks. I would expect that that battery bank would be regularly under-charged.

In general a 20% voltage drop is indicative of corroded connections and poorly selected cable sizes ..... or corroded cables.

On the other hand you may have a dodgy/inaccurate multimeter ..... or the multimeter battery is poor.
 
When sizing cables to keep volts drop to acceptable levels it is common to take 3% as the acceptable max volts drop

3% of 12 volts is 0.36 volts. On this basis your loss of 0.28 volts (13.03 − 12.75) is not unreasonable.

However you can use your multimeter to look for the missing 0.28 volts. Measure the volts, if there are any, across all the connections in the circuit and in particular across the switch. Don't forget to include the negative wiring as well as the positive.

If you find any significant parts of your missing 0.28 volts at any particular locations then you have found bad connections that you can remake.
 
TradewindSailor said:
It's not unusual. It rather depends on the total length and guage of cable between the battery and bulb AND back to the battery. You should also check the voltage at the battery while the light is on too ..... as that could have dropped ..... although the battery would have to be pretty small to make that size of difference.

0.3V would bother me though if it was voltage lost between an internally regulated alternator and one of your battery banks. I would expect that that battery bank would be regularly under-charged.

In general a 20% voltage drop is indicative of corroded connections and poorly selected cable sizes ..... or corroded cables.

On the other hand you may have a dodgy/inaccurate multimeter ..... or the multimeter battery is poor.
Click to expand...

Brilliant tip. Thank you.
 
If you're comparing the the meter & the NASA, you need to make sure the meter probes are on the same places as the NASA connections. Do that & you probably will see a difference. But, it should be small. Both the meter & the NASA will have errors caused by various components. The NASA certainly isn't a high precision device. You meter may be but if so could be poorly calibrated. Therefore I would not be surprised to see quite a difference between the two.

Voltage drops are created by current flowing through a resistance. Resistance is a parameter of the wire & the connections. Even brand new wires & well made new connections will exhibit some resistance. Most resistance in older systems comes from corroded connections.

So, measure the battery voltage across the battery terminals. Remove a bulb & measure across the socket & you should measure the same. Now put the bulb back & measure again & you will see a drop. The bulb draws current which causes the voltage to drop because there is resistance in the circuit.

To find where the drop is caused, measure the voltage across each part of the system while under load: connectors, wire etc. If you are measuring across a long wire, you will need long wires on the meter. This is no problem. The meter itself is a huge resistance so will draw no current. Hence there will be no drop in the measurement wires.
 
Follow this procedure to measure voltage drops in a systematic way.

You need to create a reference point. Connect a long piece of wire to the battery negative terminal, long enough for you to make measurements at the places you want, size of wire is unimportant. Connect the negative terminal to meter negative. Use the positive meter terminal to measure at points of interest. Switch on as many lights/loads as you can, measure the voltage at the battery terminal and note it. At the positive distribution point (probably a fuse or breaker panel) measure the positive supply, this should not be 0.1V less than the battery terminal voltage. Measure at the negative, the voltage you measure there should not ever be more than 0.1V with a full set of loads (usually lights). Probe along towards a light and measure and you will get a picture of where the drops are occurring. At the light you will measure a small voltage at the negative terminal and something slightly less than full battery voltage at the positive side. Ideally nothing more than 0.3V overall. If your voltage at the distribution terminals starts out low then no amount of beefing up the cabling to your loads will help.

Take care not to short your test wire to any positive connection as it will potentially melt unless you fuse it.
 
Connecting to battery negative & working your way around the system is certainly valid & also fairly easy since you only have to move one wire at a time.

However, when you are in the positive side of the load, you will have to set the meter to measure around 12V. As you move closer to the negative, you will measure close to zero. If you measure across each section at a time, you will have to move both -ve and +ve probes but you will be directly measuring the drop across that section. And, most importantly, your meter will be set to measure a small voltage.

It's all about measurement resolution. You may very well be measuring a few 10s of millivolts. That's not easy if the meter is set to measure 12V. Unless of course you have a very expensive meter. A 3.5 digit meter has a resolution of only 10mV when measuring 12V.
 
In response 10mV resolution is good enough in my opinion. The advantage of doing it the way I've suggested is that a single bad join or leg is easily isolated, if you measure across both legs where you can do it the fault could be in either leg. There is also the problem of making two point readings.
 
pampas said:
And the drop,if any between the lead pillar and the connection clamp, often overlooked.
Click to expand...

... and nearly ALWAYS on the negative side! Commonest cause of slow cranking "... but theres good voltage at the starter terminal. Must be a duff starter" yes but whats happening on the earth side? Corrosion between the engine block and earth strap, and to the battery is a very common cause of replaced starter motors.
 
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