Thread: Charging third battery via starter battery

With each of these measurements did you measure the voltage at the battery? As soon as a load is applied the batteries voltage will also drop.

Originally Posted by rufusking

With each of these measurements did you measure the voltage at the battery? As soon as a load is applied the batteries voltage will also drop.

The voltages you can see there are measured at the Anderson plug. What I'm interested in is the relative voltage drop as the current is increased. So for example in the best case scenario, going from 4A to 12A the voltage dropped from 12.30 to 12.00. So I've used V=0.3 and I=8 in the above formula.

I can see your point though -- if the voltage from the battery sags badly under load I am over-estimating the voltage drop in the cable. I would be surprised if the voltage from the battery sagged that badly under load, these aren't terribly high currents after all.

I can see some more testing in my future.

Hi Aidan and as rufusking posted.

While you are partially correct, a 12 amp load is not that much but lead acid batteries are finicky things and a small load may have no major effect on one battery but could easily cause a considerable voltage drop on an identical battery.

One more point, when you measure the voltages at the battery and the Anderson plug, if you happen to have access to a second multi meter, DO NOT use it.

Take all your measurements with the same meter. There can be a small difference in readings from one meter to another, and while this is not that important normally, when trying to measure voltage drop, accuracy is imperative.

Originally Posted by drivesafe

While you are partially correct, a 12 amp load is not that much but lead acid batteries are finicky things and a small load may have no major effect on one battery but could easily cause a considerable voltage drop on an identical battery.

Granted, but the rear anderson is coming off an Optima Yellow Top. Those things can supply (and take) massive currents. Internal resistance should not be a factor.

Originally Posted by drivesafe

One more point, when you measure the voltages at the battery and the Anderson plug, if you happen to have access to a second multi meter, DO NOT use it.

Take all your measurements with the same meter. There can be a small difference in readings from one meter to another, and while this is not that important normally, when trying to measure voltage drop, accuracy is imperative.

Thanks for the heads up. I won't have a chance to re-do the measurements for a while, but when I do I'll post the results.

Originally Posted by drivesafe

While you are partially correct, a 12 amp load is not that much but lead acid batteries are finicky things and a small load may have no major effect on one battery but could easily cause a considerable voltage drop on an identical battery.

Originally Posted by Aidan

Granted, but the rear anderson is coming off an Optima Yellow Top. Those things can supply (and take) massive currents. Internal resistance should not be a factor.

Hi again Aidan, it does not matter what type of lead acid battery it is, the potential of different voltage readings, caused by the current draw will still be the same.

All lead acid batteries react in a similar manor.

Ok, back again!

This time with much more carefully measured results. Voltages were measured at the Anderson plug and the battery (with the same meter) and another multimeter was inline with one of the 50W halogen bulbs to give an accurate measure of current.

All measurements were repeated with the car running

I = current draw
Va = Voltage measured at read Anderson plug
Vb = Voltage measured at battery


Engine off:

I Va Vb

4.00 12.34 12.42
7.86 12.18 12.34
11.61 12.03 12.27


Engine on:

I Va Vb

4.22 13.86 13.93
9.60 13.76 13.91
12.48 13.65 13.90


As drivesafe said, the battery voltage did sag under these relatively modesty loads. The sag making up for half the voltage drop.

As before, the formula for voltage drop is:

V = L x I x 0.017 / Ax

If I use the "engine on" numbers,

V = 0.19V (correct for slight sag on alternator), I = 8.26A, L = 10m (estimated, +ve & -ve cables)

implies

Ax = 7.80 mm2

Engine off:

V = 0.16V (correct for sag on alternator), I = 7.61A, L = 10m (estimated, +ve & -ve cables)

implies

Ax = 8.09 mm2

8 AWG (B&S) is 8.36 mm2, so that looks about right.

Thanks for the tips on measuring that correctly.

Now the question: is it possible to successfully charge a 120 Ah AGM (Fullriver) in a camper trailer with this sort of voltage drop? Even if we use 6 AWG/B&S from the rear anderson to the battery we're looking at another 6m of cable (+ve and -ve). Given that 6 AWG cable is 0.65 of the resistance of 8 AWG, but we running more than half the cable run of the car wiring -- roughly speaking it will drop at least half the same voltage again as the car wiring.

(As noted previously, the D4 never seems to put out more than 14V from the alternator)

So at 8A it's down to 13.7V, probably lower. The float voltage on my Projecta charger is 13.7V.

I'm thinking a DC-DC charger is required.

Hi Aidan and some good feedback plus it demonstrate why the correct size cable is imperative.

As far as the operating voltages go, they are at idle. Once out on the road you will find you will get both lower and higher voltage readings.

It would be a lot cheaper to fit the correct size cable before worrying about a DC/DC device, which will still need decent cable, depending on the size of the device.

Next, does your father-in-law have a 3 way fridge is his set up?

Originally Posted by drivesafe

Hi Aidan and some good feedback plus it demonstrate why the correct size cable is imperative.

You're not kidding! It is doing my head in trying to think about the design "envelope" after the constraints of voltage drop.

Originally Posted by drivesafe

As far as the operating voltages go, they are at idle. Once out on the road you will find you will get both lower and higher voltage readings.

I bought him a cheap (and I'm not joking it is very cheap) cigarette lighter voltmeter to measure the output of the alternator. We confirmed it is reading the same as the voltage at the starter battery, and during driving the alternator never exceeds 14V, and often drops to 13.8V on a long drive.

Originally Posted by drivesafe

It would be a lot cheaper to fit the correct size cable before worrying about a DC/DC device, which will still need decent cable, depending on the size of the device.

You're suggesting we replace the cable with heavier duty stuff? I know it depends on what I'm planning to install, but in general would 6 AWG suffice? An auto-electrician installed it in the first place. I guess we could trace the route and replace it, but to be honest I'm not all that keen.

In any case, if it is the case that the alternator puts out a maximum of 14V, and often 13.8v, there is very little wriggle room for voltage drop.

Originally Posted by drivesafe

Next, does your father-in-law have a 3 way fridge is his set up?

He has a Trailblaza, which, as you probably know, is a very efficient 12v/240V fridge which draws a maximum of 5A, and uses 1A on average.