Thread: Charging third battery via starter battery

Originally Posted by drivesafe

Hi again Aidan and cigarette volt maters can have up to a 1 volt error, caused by other devices being connected to the same loom.

Things like your sound system, will cause voltage drop and you should see voltage readings in the high 14v, 14.8v is common place, particularly when you first start your drive.

We've now measured the starter battery voltage when the car is running, and confirmed that it is the same as that measured by the cigarette volt meter (within error -- the cigarette one is out by 0.1V sometimes). We measured the starter voltage from the anderson plug in the rear load bay -- it is connected directly to the starter battery. It had no load, so the voltage was the same as the battery.

So to recap, the Disco 4 alternator appears to output 14V only when first started, and then not always that high, and quickly drops to 13.7-13.8V.

[QUOTE=Aidan;1799784
So to recap, the Disco 4 alternator appears to output 14V only when first started, and then not always that high, and quickly drops to 13.7-13.8V.[/QUOTE]

Mine will run at around 14.5V if the battery is a little low, but you won't generally see the higher voltages at idle - only when driving and on overrun. The alternator output is reduced at idle and when under power to improve economy, unless really needed.

Cheers,

Gordon

Originally Posted by gghaggis

Mine will run at around 14.5V if the battery is a little low, but you won't generally see the higher voltages at idle - only when driving and on overrun. The alternator output is reduced at idle and when under power to improve economy, unless really needed

These voltages were measued on a 35 minute trip with variable driving conditions - some stop/start city driving and including up and down some reasonable grades.

This was done to check if the cheap cigarette lighter voltmeter was giving a true indication of alternator voltage. It was confirmed that it was.

According to a months worth of driving with the cigarette lighter voltmeter the alternator NEVER produces more than 14v, and typically it is 13.7-13.8. Admittedly I don't think the starter battery has been heavily depleted during this time.

Originally Posted by Aidan

According to a months worth of driving with the cigarette lighter voltmeter the alternator NEVER produces more than 14v, and typically it is 13.7-13.8. Admittedly I don't think the starter battery has been heavily depleted during this time.

That's probably the difference. Mine and most of the cars I've installed dual battery systems into generally run quite a few accessories, often permanently wired. I can confirm that under those situations, the charging regime will easily see over 14V

Cheers,

Gordon

Back again again!

Now we're trying to figure out what the voltage drop will be in the camper trailer. I got 4m of 6 B&S cable (will probably only need 3m), attached a 50A Anderson plug to one end, stripped 50mm of insulation back on the other end and measured the voltage drop with the engine running.

I = current draw
Va = Voltage measured at cable attached to rear Anderson plug
Vb = Voltage measured at battery

I Va Vb

4.07 13.59 13.68
8.06 13.44 13.64
11.97 13.30 13.60

First up, that is a low alternator voltage! Turns out the air-con was on the whole time (only noticed after we'd packed up), but this is still relevant. He wants to be able to use the air-con. Additionally, the alternator dropped 0.08V when the load increased by 8A. In the previous trial (with air-con off) it only dropped 0.03V. Does that mean the alternator will struggle when under load?

Secondly, the additional voltage drop wasn't too bad, only 0.21V for 7.9A extra draw, compared to 0.18V ** for 8.24A when we measured at the rear Anderson plug. This translates to a 0.027V drop per amp, compared to 0.022V / A, so approximately a 20% increase in voltage drop.

If I want to run a 20A Ctek DC-DC charger, it will pull at least 24A to make up for the voltage drop. If the alternator output was similar to that measured above I'd be down to 12.81V at the charger. This is just marginally above the voltage where the CTEK D250S dual starts to throttle the current (I emailed CTEK -- apparently the charger will drop to a 10A max current, and if the voltage is still below 12.8 it will shut off). It is unlikely the CTEK would shut off completely, but it would also mean a low charge current, and therefore an unreliable SOC.

If the alternator output was closer to that observed in my previous post (13.93V @ 4A, 0.03V sag @ 12A at the alternator), the voltage at the CTEK would be closer to 13.19. This would be ok -- the CTEK powers on at 13.1V, but this is with no current flowing.

I'm going to advise my father-in-law to keep an eye on the alternator voltage -- to see if 13.7 is typical now that he is using the air-con more.
If it is, then we're going to have to go for a switched DC-DC charger which doesn't shut off with this extra voltage drop.

Cheers

Aidan



** I made a simple error in the other post and said this was 0.19V, but

A very intereesting discussion and I've learnt a lot - thanks.
A question: What is the effect of the D3/4 having a 'smart' alternator regulated by the ECU? Does that mean the voltage will be adjusted to demand, and does that reduce the effect of voltage drop? How would that affect the situation being discussed here, and how would it be different to that of vehicles with normal alternators?
Just trying to get my head around this electronic stuff...

Originally Posted by DiscoMick

A question: What is the effect of the D3/4 having a 'smart' alternator regulated by the ECU? Does that mean the voltage will be adjusted to demand, and does that reduce the effect of voltage drop? How would that affect the situation being discussed here, and how would it be different to that of vehicles with normal alternators?
Just trying to get my head around this electronic stuff...

Not sure I can help alot with specifics, but I'm trying to reconcile my father-in-law's experience with what gghaggis reported about seeing over 14V from the alternator.

I'm guessing (and this is pure speculation) that the "smart" alternator is only different from traditional ones in that it won't always boost the voltage when there is a load -- it will boost, but other criteria come into play.

When we did this latest test the air-con was on, but the car was idle. The alternator didn't boost output, and in fact the output sagged worse when we loaded it more. If the car had been doing 100km/hr downhill the alternator output may well have been much higher.

Plus this car has never had seriously depleted batteries -- the starter and auxiliary have yet to be "used in anger".

We're going to do some more testing and observing -- going on a camping trip this weekend -- to see how the alternator behaves. He's going to turn the air-con on and off, under different circumstances to see what effect that has on the alternator output. Plus the auxiliary will be drawn down somewhat as it will have powered the fridge overnight.

Hi again Aidan and you have done a lot of work there but it’s not going to give you the info you need.

The only way to get the relevant info is while the vehicle is on the move.

You need some form of data logger, to monitor both the voltage and current at the house battery.

Measuring voltage anywhere, while the motor is idling is not going to give you the operating info you need to make a decision about what will work the best.

The D4 has a 180 amp alternator and you are not getting anywhere near the potential while the motor is idling because the D4’s own electrical requirements are anything up to 50 amps and at idle, you will have very little left for any accessories.

I have a system that is capable of producing 60 amps battery to battery and works from as low as 11.0v but it’s of no use if there is nothing available from the alternator and at idle, thats what you have, or more correctly, don’t have.

Aidan, you need operating info while on the move because the info achieved at idle does not reflect to true ability of any vehicle’s charging capability.