Alternator Performance

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Originally Posted by discolaw

Drifting a bit off topic but being a novice in this area I did a lot of research. The basic info out there was that charging the 2nd or more batteries thru' an isolator system alone (such as traxide) can't charge the aux battery to 100%, at best maybe 75-80%. The Ctek dc-dc charger claims to boost the alternator battery current so that its possible to charge 100% as well as acting as an isolator. I'm hoping its true because I've just put one in It is also claimed it works with the new "smart" alternators.

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Hi discolaw and unfortunately this is nothing more than a blatant lie.

First some background into how lead acid batteries charge.

If you have a battery in a low State of Charge ( SoC ) and you apply a voltage source that is higher than the battery’s SoC, the lead acid battery will charge.

The higher the difference between the battery’s SoC voltage and the voltage being applied, the faster the battery will charge.

This is based on the laws of physics and is the same for all types of lead acid batteries, AGMs, Gel, Flooded Wet cell, all charge the same way.

Now to cause a battery to STOP charging once it reaches 70 to 80% SoC, you would have to drop the alternator voltage to somewhere between 12.3 to 12.4v.

If the alternator voltage was dropped to 12.5v, a battery with an SoC of 80% would continue to charge to 90% SoC.

With an alternator voltage of 13.0v, the battery will eventually get to a 100% Soc or fully charged state.

NOTE that with these low alternator voltages, it would take many, MANY hours of driving to fully charge the battery.

Now to reality. If your alternator is running at 13.6v or higher, you will have no problem fully charging any lead acid battery to 95% in a reasonable drive time.

I state 95% because all batteries from an SOC of 80% determine how quick they will charge to 100% and from around 90 to 95% the charge time required to get them to 100% can be quite long.

When it comes to charging batteries from an alternator with an output voltage of 14+v, as you do with D2s, you are going to charge your caravan batteries as fast as the SMARTPASS will allow them to charge.

The D250S DUAL will then charge the batteries SLIGHTLY faster than what your alternator could do as the DUAL makes compensation for the voltage drop.

BUT, if you were charging the same battery array with your alternator, you would only take between about 15 to maybe 30 minutes more of drive time to achieve the same level of charge, and if you are already going to do those few extra minutes of driving anyway, you have gained nothing by adding both the SMARTPASS and the DUAL.

If as is the case where you have only fitted the DUAL, you are going to need at least a couple more hours driving time before the DUAL can charge your two batteries to the same level your alternator has already reached.

But there is one far more important factor you must consider. Are you actually likely to drive long enough to allow either your alternator, by itself, or with a DC/DC device, to fully charge your batteries in the first place.

The answer here is based on how much battery capacity you use while camped.

The more battery capacity you use, the greater the advantage for direct alternator to battery charging is over DC/DC devices.

Now one more point. You mentioned that charging the 2nd or more batteries through an isolator system alone, the you stated “such as traxide” can’t charge auxiliary batteries.

I do wish that while you were doing your research, you had taken a look at my web site.

If you had, you would have seen that my isolators ( which work completely differently to all other isolators ) actually provide the fastest used battery capacity than any other dual battery setup, including high current DC/DC devices.

Anyway that’s another subject.

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Originally Posted by Mundy

I think I have previously asked this but I now have more data. Do other Td5 owners notice that the voltage output from their alternator changes as the under bonnet temperature increases?

If so, this may be helpful.

I have just returned from 4 weeks traveling in central Qld, including a couple of weeks of bush and other camping. I found I had trouble keeping my second battery (105Ahr) charged when I remained camped for more than a couple of days. What happens is that when the engine is cold, the alternator output varies between 14.6V and 14.7V (I have tested repeatedly with a digital meter). However, when we've been running for an hour or so at high speed (100kph say) and everything under the bonnet temperature is really hot, the alternator output drops to 14.13 or so.

This is a problem because I can't get the second battery back up to full charge. I have a solar panel too, but one tends to camp where there's full or partial shade which limits the effectiveness of the panel.

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Hi Mundy, can you supply some more details about your setup as it sounds like you may have a problem in your existing system.

Thanks Tombie & drivesafe. Most of the info I got from the caravan forums where this subject is debated endlessly by the grey nomads. As mentioned in my original post I use solar as the main charger, the ctek is just a supplement for occasional non solar friendly weather. I'm planning long driving days on a lengthy touring trip to W.A. so the dc-dc charger will have plenty of time to work. I knew it wouldn't be effective on short trips. Have noted all your information.

Hi again discolaw and an additional consideration is that DC/DC devices are grossly inefficient and require far more current going into them than they supply to the battery.

This results in higher fuel consumption than is really necessary with no gain for the additional fuel consumption.

I did the cape in a series with 2 n70z in parallel.
On one stop I drained then both to below 3v and after a tow start, getti g the vehicle into the sun so the panel could put enough in to excite the alternator I had them both fully charged at the end of the days driving using a 35a alternator while running the Engle radio and the two way as well as charging the phone I was using as a GPS.


Using the vechile with the 130A alternator as an example Let's say you car has a 30a draw for fuel pumps and electronics, that leaves you with 100a to charge less total battery capacity( assuming you don't draw anything from the main battery while stopped) than I was with roughly 25amps,

Not sure exactly.why you'd want to limit the potential 100amps you've got available to charge the battery to less than what you can get out of a 1970s era series III.

Hi again Discolaw, I forgot to ask, is your solar a permanent roof top fix and if so, do you feed the it direct to the house batteries ( via a separate reg ) or through the Ctek.

The reason I ask is that if your solar is on the roof, you can use it to reduce your fuel consumption, if the solar has a separate reg.

Yes drivesafe, the solar is a panel fixed on the roof and runs thru' a separate regulator. It has no connection to the ctek altho' the ctek duel does have provision for a solar connection.
Cheers

As Drivesafe has advised before, the alternator is, nominally, a constant voltage charger. This means that the alternator output voltage should remain the same as long as the output load (amps) is less than the capacity of the alternator, which in the case of the Td5, is 120A. For a battery, the charge current will depend on the voltage difference between that of the alternator (say 14.6V) and the voltage of the battery. As a battery is charged up, its voltage increases, which decreases the voltage difference and as a consequence the charge current. Thus it can take as long to charge the last 20% as the first 80% because the charging current reduces as the voltages get closer to one another.

If my alternator were to keep to 14.6V, again as Drivesafe has said, the alternator should charge my second battery more quickly. However, because after a half an hour or so of heavy driving (made worse by hot weather) the voltage of my alternator drops to less than 14.2V, this means that charging time increases dramatically and a DC to DC charger outputting 14.4V should be quicker, I assume.

The ctek charger supposedly has other advantages with a temperature sensor to ensure the second battery doesn't get too hot during charging, a desulphonation cycle and better full charge monitoring and maintenance. I guess time will tell if it does better than what happens now.

Drivesafe, my setup is I have a 105Ah deep cycle battery in the rear, with a 4 gauge cable running from the alternator cable at the fuse box under the bonnet. There is a 40A fuse at either end of the cable, a manual isolator at the bonnet end, and a VSR isolator at the second battery end. I run another 4 gauge earth cable to an earthing point near the front of the vehicle. I also have an analogue voltmeter in the cabin connected to the positive of the second battery. I have a cable from the second battery to a bus bar to which power take offs for water pump, lights, etc are connected.

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Originally Posted by Mundy

If my alternator were to keep to 14.6V, again as Drivesafe has said, the alternator should charge my second battery more quickly. However, because after a half an hour or so of heavy driving (made worse by hot weather) the voltage of my alternator drops to less than 14.2V, this means that charging time increases dramatically and a DC to DC charger outputting 14.4V should be quicker, I assume.

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Hi again Mundy and I hate to upset your analogy of how a DC/DC device works but your Ctek does not charge the battery at 14.4v until the battery has reached around the 80% mark.

This is one of the big deceptions surrounding these miracle devices.

If you have a low battery, with the size cabling you have, your alternator is going to supply a MUCH higher voltage at your battery than the DC/DC device can hope to do.

As such, as you have pointed out, the greater the voltage difference at the battery’s terminals, the more current the battery will draw.

This is the trick in the advertising.

Because the DC/DC device is limited to 20 amps, it’s output voltage will be drawn down by the battery attempting to pull high current, which is not available,something conveniently left out of the advertising hype.

Note, this is how all DC/DC devices, battery chargers and solar regulators work. All of them are current limited.

But because an alternator has a much, MUCH high current capacity available, the voltage at your LOW battery is going to be much higher than your DC/DC device can achieve.

So your alternator will get your battery back up to 80% SoC way quicker then your DC/DC device will and from 80% on, the battery, not the charging device, controls the way the battery charges.

So you are going to gain far better charging using your alternator, even when it’s running at 14.2v.

Furthermore, even Ctek admit this as this is why the sell the SMARTPASS, because they know the DUAL is not so smart and can not charge low batteries anywhere near as quick as your alternator.

BTW even if your alternator was outputting just 13.8v, it would still beat the Ctek, when charging low batteries.

And one more point, if you find you still don’t have enough battery capacity and you need to add another one, you are in real trouble because your DC/DC device will add hours more to the amount of driving time you need to charge the batteries.

Whereas, your alternator will simply increase it’s output current to accommodate the high battery current draw.

So using just your alternator and that cabling you have, you would have already future proofed your setup for more batteries.

Thanks Drivesafe. Yes, I am aware that the dc/dc charger is a current limited device. If I had consulted your previously provided SoC chart and done a few back of the envelope calcs I would have seen what you have said to be true (at 14.2V the alternator always wins). Do you know off hand what the internal resistance of an AGM battery is? I am guessing around 0.08 ohms but is it constant through the charging cycle?