Thread: 12 volt power efficiency.....

Hi again Feral, some more questions.

Although it sounds like you have run both cables, do you have both a positive cable and a negative cable run from the cranking battery to the auxiliary or have just run a positive cable between the two batteries and earthed the auxiliary battery to the chassis?

The .2 volt drop over that length of cable is quite acceptable provided this is while there is some form of load, even just the load that a low charged auxiliary battery will cause.

No load and there should be no voltage drop.

Driving those sort of hours should have at least near fully, to fully charged the batteries.

To have low charged batteries after that length of driving means there is a major problem ( obviously ).

Is your cranking battery low and causing starting problems?

Cheers.

This has been a very interesting thread. It really is a significant problem using a dual battery system with a modern multi-stage charging system that in some circumstances will output too low a voltage to trigger the dual battery system.

On the surface, it seems that Feral's first suggestion of using an inverter and a separate charger is the only real solution to the problem.

Of course, the savvy operator could when hot-starting after a very short break, simply run their headlights with the engine off for a few minutes to get the battery flat enough to demand full output from the alternator......

Considering older single stage alternators it is also true that many deep cycle batteries DO charge more quickly and more completely if charged at over 14 volts.

Some (single stage) alternators have a remote sensing lead so that the alternator regulates to the actual voltage at the battery without getting tripped up by voltage drops between the alternator and the battery.

In this case, putting a simple ten-cent diode in series with the sense lead will increase the alternator output by about 0.6 volts - ie, from 13.8 to about 14.4 volts. This then gives an excellent charge for both the auxiliary battery and the car battery. Most car batteries are happy at 14.4 volts.

This is a cheap and nasty trick in some ways, but it works pretty damn well and costs bugger-all.

All food for thought.......

Cheers all,

Paul

Just a thought..... and Drivesafe I hope will respond with any corrections as I don't have any experience with his product.

Could this be a combination of issues rather than just 1?

I am wondering if the cranking battery has been load tested? My reason is in the past I had a cranking battery that when a multimeter was put on it (engine off) it read around 12.4v so seemed OK. However I was having cranking problems, especially when cold (Diesel BJ40 Landcruiser)

The auxillary was causing problems with not appearing to hold a charge. After much anguish and searching for a bad earth etc etc we tried a new cranking battery, problem solved. No more cranking issue, and the axillary was now also holding charge!

Unfortunately we didn't have access to any "load testing" type battery testers but what led us to trying a new cranking battery (which was only about 8 months old at the time) was isolating the aux battery then using the headlights to gauge battery condition after a couple of very short turn overs it was apparent the battery wasn't holding up well.

The battery isolator was a simple system of a solenoid which was triggered by oil pressure.

Just some food for thought. Some of the other more knowledgeable may be able to "explain" this a little better!

Hi Wilbur, actually, all automotive type batteries, Flooded Wet Cell, Gel Cell and AGM Batteries will charge quick, the higher the charge voltage, providing the current is also available.

The BIG problem is that both Gel Cell and AGM batteries can easily be irrevocably damaged by high charging voltages and additionally, AGM batteries can be damaged by high charging currents.

Those old type diode isolators which never allowed batteries to charge properly, actually work quite well in vehicle fitted with an alternator that requires remote sensing, for the very reason you described.

In this case there is a fault of some kind as it is not normal for any alternator to run at voltages as low as 12.8 unless there is a particularly heavy load being applied to the vehicle’s electrical system. This does not seem to be case.

Poor earth connections can cause this problem but with a poor earth connection at the alternator, you usually get voltage run away where the alternator produces anything up to 18 volts and cooks batteries.

But, a poor earth elsewhere can easily hamper a battery’s ability to charge properly.

The negative cable from the auxiliary is wired in combination of both to earth and to the battery.

The cable comes off the battery and is earthed at the point under the floor near the filler cap in the rear of the D2. The cable then continues on and goes all the way to the engine bay to the earth point near the fuse box. This earth point is 25mm away from the Landrover earth point that wraps around the negative cable which leads 75mm to the negative battery terminal.

In regards to the load test, it was tested at Alice Springs and it was deemed to be OK for use. All the testing equipment that Landrover used suggested that the battery was fine.

The Traxide controller appears to be working. If you observe when you start the car the yellow light continues to flash for about a minute and then it transfers to full on.

Does this make sense?

Originally Posted by Feral

The Traxide controller appears to be working. If you observe when you start the car the yellow light continues to flash for about a minute and then it transfers to full on.

Does this make sense?

Yes and this means that the vehicle voltage is rising to at least 13.4v otherwise the SC40 would not cut in BUT, under normal operations, the SC40 would cut in in about 10 seconds. The longer delay in cutting in is indicating a vehicle voltage problem of some sort.

The earth sounds fine.

All this is not much help but it is eliminating things and the problem is still looking like being the alternator.

Cheers

So ......................
What is a normal range of voltages for:

1. a properly charged battery, (disconnected)
2. an alternator that is working correctly

I know different batteries/alternators will most likely have different voltages but when is it time to get a new battery/alternator.

eg when the battery is less than 11.5 volts or an alternator that puts puts out more than 16v or less than 14v or whatever, (just examples).

Depending on how mechanical you are it looks to me like time to pull out the alternator and do the bushes and bearings and give it a good clean up.

Or off to an auto electrician (assuming your out of warranty of course) and get your alternator done.