Number batterys that can be charged from vehicle

Good Jerry,

To add to my answer above. The alternator will output to all batteries connected to it and the amount each battery receives will depend on a number of factors, each battery's dynamics, the cabling resistance to each battery etc. The alternator will be protected from overload by its output current limiting circuitry.

If your caravan is fed from after the Redarc isolator then the alternator will give charging priority to the cranking battery which is as it should be.

Based on my recent investigations re a PX Ranger, I reckon you need a Redarc BCDC 40 amp Low Voltage DC-DC charger. The tech blokes at Redarc are very helpful.

tarbaz,
you just get Ford to turn off the smart charge on the px and they do it for nothing.

Perhaps not "standard" alternators but I understood that D-Max alternators (along with others) utilised the I/R drop in the cable from alternator to battery as a reference to the regulator to limit output current.
Similar arrangements apply to vehicles equipped with Engine Management Systems.
Maybe I am wrong?

I may also be wrong but I believe even with ECU controlled alternators (DMAX etc), the hall device measures the charging current to the battery, ie it is in the battery lead, it does this to monitor the SOC of the battery allowing the ECU to order the alternator to reduce its charge voltage when the battery has attained a reasonable SOC. It also monitors the voltage and current going into the battery to determine if the battery is failing and will then throw a fault code if it determines it is and this is why you can't connect accessories directly to the battery as the extra current is determined to be a battery failure. All accessories are connected to the body earth, ie the current flowing through the body earth and the same point the alternator negative is connected too is not monitored and like any other alternator can be overloaded via this path.

I found the reference. It was in an IEEE journal from a while back attributed to Delco Remy and said in part.......

"Current Regulation is not required with an alternator as the device automatically limits current when the rotor's magnetic flux is exceeded. When the design maximum output current of the alternator is reached, current limiting will occur and both the output voltage and current will be reduced. With appropriate design the alternator is protected from detrimental overload."

So current limiting is inherent rather than measured.

It will depend on the alternator, older smaller ones might but to days high power alternators both Nippondeso, Lucas and Bosch disagree, is from a chat I was having with Bosch's technical expert regarding high loads, multiple batteries winching etc:

"Hi,

Low system voltage (due to having been extremely depleted) equals full alternator rotor current.

Rotor resistance remains fixed and therefore low voltage reduces current draw and weakens rotors magnetic field, this would result is slightly reduced alternator output. However there is also an offset as lower stator current results in less heat (P=VI or RI^2) and magnetic losses decrease. Actual result is that at a lower voltage it is in fact possible to get higher output current from an alternator than at rated 13.5V!

Please note though that system voltage will very quickly rise to plate voltage when charging (assuming winch loads removed)

Consider also-

Battery capacity far greater than designed for vehicle

Load on alternator larger and longer than designed

Rectifier temperatures high due to long duration at high output

No/poor vehicle airflow if stationary

Higher engine rpm often used (engine bay temperatures) and if viscous fan may be poorly engaged

Is it possible to overload an alternator working to replenish large reserves. Yes!

Oh for an editor, should read "the following is from a chat, end of quote after the yes.

Personalty I have not come across an instance of an alternator failing due to overloading but I'm sure it does happen, so better safe then sorry and to manage the load if you can.

I'm not sure that a chat with an "expert" trumps a published paper in the Journal of the Institute of Electrical and Electronic Engineers.
However..............

Allan,

I prefer to go to the horses mouth as I don't believe a blanket statement in the IEEE can be applied to all alternators.

Hi have discussions with the technical design persons of three of the major players, regarding if their alternators are designed to supply the nominated rated output continuously ie longer than 10 minutes duration etc.

Two responded with their alternators are not design to produce their rated output on a continuous basis and should be derated by 20% to 30% under reasonable ambient conditions and operating conditions, following from Denso:

"Leigh,

It will be specific for each DENSO Part Number or application, hyowever, 75% of "rated amps" will generally be OK for continuous operation. "

You have already read the reponse from Bosch, I have had quite a few discussion with them over the past and the only time they will guarantee continuous 100% ouptut is in a free airflow of 25C where the alternator can shed its heat load and this is not going to be the case in a typical vehicle setup, another guote from Bosch:

"Hi Leigh,

I can only comment on Bosch .

An alternator supplies it’s rated current at standard test conditions (25deg ambient @ 13.5V) after a stabilization period, this stabilization period however does capture conditions before they rise to maximum. Windings have an PTC characteristic, diodes an NTC, PTC dominates the losses. It would be not practical to remove all heat loading to maintain the internal test temperatures.

Obviously we need to lock in a standard test procedure, else do you test after what time, what load, what temp, what voltage etc. Alternator temperatures rise at very high output and if an equilibrium point is reached (before damage) then it is very high. Duty cycle in use are infinitely variable and all external influences are a factor, therefore a meaningless metric in the real world if we were to test at a set ratio.


“100A max / 50A continuous, 50% duty cycle”- what time at max?, what time at 50A? (i.e., total time of test period?), voltage deviation due to regulator temp compensation effect on output? Tests period adjusted to a target rectifier temperature? same test periods for all alternator type, range and user application? I can think of many variables that would give me a figure that still requires me to confirm my application in real world testing.

In practice temperatures rise to various levels as subject to differing conditions and to so it is an important consideration. We have seen 190deg rectifier temperature on one local built vehicle with poor air flow design on hot days!"

Three of the main players including the supplier to Toyota all state you need to derate under haevy loads and continuos operation and all also state that is is indeed possible to overload their alternators so i think I'll go with the alternators manufactures in this instance rather than the IEEE:)

Forgot to add, originally I was of a similar opinion to that of IEEE, and was surprised to find that not one manufacture would guarantee that their alternators would supply their rated output on a continuos basis.

It was also my belief from past experience with alternators that the falling rotor current under high loads would self protect the alternator as I have observed in the past. Modern alternators can a do run much higher rotor currents, this together with negative temperature coefficients in rectifier diodes and windings etc can apparently leave them susceptible to overloading under adverse conditions.

I can remember having a dispute with you Leigh a while ago where I made mention alternators have not got a 100% duty cycle and you knocked me down in flames saying I was talking crap....... so why the change knowledge?

From memory it was more over the derating factor and methods to manage the current. You were arguing DCDC chargers are better as they limit the max current, my point of view was not much good limiting the current if you can't recharge the batteries in a typical drive time.

Alternator de-rating is a grey area, as posted above, Bosch indicate their alternators are very conservatively rated and will supply their rated output continuously. The catch 22 is the alternator has to be able to shed its heat load.

Low output alternators have inherent protection inbuilt as with their lower output currents they'll seldom generate enough heat to damage themselves before falling rotor currents cause the alternators output to drop.

With modern high output alternators positioned in a vehicle so the adequate cooling is not available the problem is more the ability of the alternator to shed its heat load than actually being overloaded, the alternator will most likely fail due to rectifier failure caused by the rectifier diodes overheating.

In some vehicles a alternator may run at 100% output all day every day with no issue, the same alternator installed in another vehicle may overheat (note not overload) at 80% output under similar ambient temperatures.

I have had similar discussions with many auto electricians and installers of dual battery systems, interesting not one has ever seen a failure they can directly attribute to an alternator being overloaded and will argue you can have as many batteries as you like so even within the automotive electrical world their is non consensus of opinion!

I myself like to keep things as reliable as possible, so I go will a figure of around 75% and try and keep max alternator loads around that.

No Leight..... The conversation was regarding duty cycles of automotive alternators whereby you said they don't have a duty cycle and as usual I was talking bleep .

According to you if the alternator was capable of delivering 100amps then it can deliver those amps day in and day out and there is no such thing as a duty cycle.

Regarding DC-DC charges I made the comment that they are good because they limit charge rates.

Show me a vehicle with 100% duty cycle from factory........ so heavy vehicles do but no passenger/light vehicles.

As Bosch have written above, there is no such thing as a duty cycle for and alternator, if an alternator is rated at 100A@25C it should be capable of producing 100A 24/7 when operated in a similar environment to that used for testing. No alternator manufacturer I'm aware of specifies duty cycle factor on their alternator specs.

Duty cycle is meaningless without being able to specify the exact operating conditions, ambient temperature, the load on the alternator, how long the alternator will be running, is the load constant, does it reduce over time, does it increase overtime, the maximum permitted temperature rise etc.

If the alternator can't shed its heat load due to environmentals it is operating in then it should be de-rated to keep within the alternators manufactures maximum operating parameters but as they don't tend to publish these that is going to be guess work.