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Good video, I'm a technician and what we were always told is that the amp rating on a vehicle alternator is a maximum amp output. The manufacturers only test 80% of that output for 10 minutes to meet that ampacity. Anything above that rating or the time frame is a gamble. Also that F terminal signal you're seeing is the fielding of the windings. The belt spins a internal magnet with windings in it. When the vehicle voltage gets low it increases the voltage to this internal magnet increasing the magnetic field in turn inducing more voltage into the stator windings. The real issue I see is at low rpm and high fielding the fan that cools these windings is driven by the belt so not much cooling action. This can cause the varnish on the windings to melt and short out. Good by alternator. Great testing tho, and love your in depth testing videos.

I remember one of the first times I charged my powerbank by DC-DC. Pulling 600W late one winter evening, I joined the back of a traffic queue and my headlights DIMMED. I dialled that back. 🤣 Thanks for giving me some understanding of the reasons why.

A very useful video. The key here is to use an appropriately sized DC to DC charger. On your vehicle you can just use a 1000w but on smaller vehicles 500 or even only 200 would be the correct choice. So it's not that these chargers are bad, it's just picking the right size.

Well done! I installed a 20 amp Renogy DC-DC charger on my Ram Promaster City van conversion. The alternator is 180 amps, and I determined that I didn’t want the charger load to be any more than a 50% load on it for exactly the scenario you demonstrated: full load at idle. That meant 50% X 180 amps = 90 amps alternator output. Typically, a DC-DC charger can draw up to 50% more power to deliver its rated power. So, 50% x 90amps = 45 amps. I could have gone with up to a 40 amp charger and still be reasonably comfortable with that draw. I decided that 20 amps was plenty for my system, and I know that my alternator won’t get anywhere near 100% load even in the most extreme conditions.

There are six diodes inside the alternator - the alternator is 3-phase, so the ripple after rectification is 4%. Any overload will cause those diodes to overheat and become either open or shorted. The signal you read on the OBD port is proportional to the field excitation voltage, used to control the alternator output. At lower engine speed, the control must compensate with higher excitation. I found out that cars with start-stop technology have an upgraded alternator. I also recall that the cars from the '70s had a 14.4 V - 30 Amp alternator, and the engine idled at 900 rpm. My first car had a dynamo which started generating at 1200 rpm. My suggestion for the power station - charge it only if you really need it, and while driving on the motorway. The alternator is much more efficient at 3000 rpm, and the higher rotation speed cools the diodes and the copper windings much better. If you don't exceed 500 W continuously, there should be no toll on the alternator. Greetings, Anthony

Alternator upgrades are easy and fairly reasonable. We installed a pair of 50 amp Victron Orion XS and upgraded the single alternator on my diesel super duty to a 350 amp version by Tucson alternator. This one upgrade totally changed our Truck camper life. We can now run the AC all night at a harvest host or boondocking and can easily recharge our 1000 amp hrs of lithium while driving to our next spot. Prior to this, our 600 watts of solar didn't cut it and we're forced to plug up every third or fourth day. We routinely get 85 amps at idle and 100 at just above idle. After installing the system, I checked it with a heat gun because I didn't have a thermal imager and only saw about a 10-15 degree rise at idle.

If you want to measure stress on the alternator, monitor the temperature of the windings and diodes. The F term voltage is somewhat misleading. It is simply part of the regulation for voltage and not due entirely to load. As the RPMs decrease, the voltage on the field winding is increased to maintain voltage output.

Finally, was waiting for someone to make a video about this. Thank you.

tnx, really an eye opener, it is now time for smart DC-DC chargers that take into account the alternator loads, temp, etc. and optimise their draw.

Excellent presentation. Thank you. I have a 2004 Duramax with dual batteries that came with a 105a altgernator from the factory. I have the new 700w Victron Orion XS 12v charger on the truck. I have done most all of the same testing that you have at various loads with the Victron. I can adjust the power draw on the unit and can control how much load it will put on the truck. One thing you need to check is the "temp" of the alternator, especially if you idle the truck much when charging - like at a campground or whatnot. There is less air moving under the hood and with the alternator turning slower, it also moves less air though the windings. I think that is the real killer of them. I normally cap the load on mine to around 30-35amps to keep the alternator temps down. If you pull a 1000 watts from the alternator at engine idle and parked, that bad boy is going to HOT regardless of what it is rated at. It would be interesting to see some studies on the "duty cycle" of various alternators both factory and aftermarket. It would not be wise to assume that they will all do 100% rated output "forever" - or even 60, 70 or 80%.

We install victron dc chargers. We keep it simple. Whatever the rating of the alternator, we use a charger that is no more than half of the alternator amp rating. 80 amp alternator, orion 50 with amp output set to 40 amps. What we'd really like to see is a temperature sensor attached to the alternator and tied to the software of these devices. But that really is just another point of failure and issues. We've not burnt an alternator in 5 years of just sizing half the rating.

About a decade ago I built a diesel powered charger for my off grid battery bank using an old school Ford 1G alternator. The field is connected directly to a 12 volt lead acid battery that is maintained by a small battery charger during use. In old school terms, we call that "full-fielding the alternator", in other words, the alternator is running full-out (balls to the wall), and by my last calculation based on the total amount of fuel I've burned, there is over 2000 hours of run time on the unit and still going strong. I set the rpms on the diesel so she's putting out 60 amps at about 55 volt (it's connected to (4) 16s 3.2v lithium battery bank, or roughly 3300 watts continuous. BTW, that alternator was a used Ebay find for like $70. They are way stronger than you'd think. Take that for what it's worth.

That high idle load is exactly why I installed a small, 20 amp DC to DC charger. I also installed a switch, so I could turn the charger on after full warm up. 20 amps, plus 600 wats of solar on the roof takes care of most charging in my Ram Promaster (400 amp hour battery bank). I want my alternator to last. Just my take.

Thank you so much for doing this video. A huge help for the van community with this exact question.

Your suspicions are right on! As the engine RPMs drop but demand stays high, output voltage drops, field voltage maxes out but your alternator still cannot meet demand at low RPM. The alternator starts heating up and will eventually damage the diodes and/or windings. Many professional installers will just ignore this problem if the alternator is big enough because the alternator likely will not fail until the conditions are right i.e. high alternator demand loads, high ambient temperatures and prolonged low RPM (note that when your AC is on the engine idle is stepped up), On my boat I use two Victron Orion DC to DC chargers; max net load ~112A from a 175A alternator (90A at idle). The Orions have a low voltage cutoff so if the alternator cannot keep up. the output voltage drops and the Orions will taper demand and shut off if necessary. A secondary protection is provided by using a Balmar programable voltage regulator that reduces the field voltage if the alternator temperature exceeds 100C. Some will suggest just getting an alternator thar is twice the size of your demand. Not so sure this will work in my case. I can be running for several hours at very low RPM and my cooling is not so great. Other improvements I discovered; Increase alternator cooling, increase pulley ratio, use a programmable voltage regulator that reduces the output at low RPM. A quick look at the alternator output curve will explain the relationship between RPM and alternator output.

Great video, thank you. I pull an RV about 7,000 miles/year. It has 200 W of solar going into a Victron charge controller into only a 100 amp/hr lithium battery. On cloudy days the solar didn't keep up (mainly 12 volt refrigerator load) so installed a Victron Orion 18 amp 12vdc to 12vdc charger. I was losing over 1.5 volts from the truck battery back to the camper DC-DC through the truck factory wiring into 7 pin trailer plug. I ended up running a dedicated #6 gauge wire (both the +12vdc and ground) with heavy duty connector to solve the voltage drop. This system works great and brings my battery up in only a couple hours of driving. The Victron 18 amps (actually about 21 into Victron to produce 18 into the lithium) shouldn't do any damage to the vehicle charging system. My tow vehicle is a 2023 Ram 1500 with the E-Torque mild hybrid system. That system charges the 12 volt truck chassis battery and a 48 volt battery behind the rear seat. On deceleration this motor/generator E-torque setup charges batteries and supposedly reduces brake pad wear. The motor generator also provide added additional take off torque during first acceleration from an idling stop. Plus it starts the engine if the start/stop fuel saving perimeters are met. (I keep that disabled). The big question for me is what is this E-torque system rated at for charge current?? Most trucks spec out what their alternator can do. Ram E-Torque?? I do like the system but some have reported problems with early failures. Most get along fine but I bought the 125K mile warranty to sleep better.

I remember as a teenager when you had these guys running massive CB amplifiers in pick ups and suburbans. They would have fancy belt setups to run 4-6 alternators. Yes I said four to six alternators to be able to swing these giant amplifiers. You can google CB several alternatives and see some of the photos. It was crazy and you would have guys meet on a Saturday morning in an empty parking lot or park and have “key down “ contests. They would rev the hell out the engines and key up the CB radio and see how many KILOWATTS they could product.

gooday from australia.. i use a bluetti charger 1 to charge my bluetti max200 in my camper. i also monitor via the ecu the parameters of the alternator output in amps , duty cycle, and volts at all times while driving. the charger 1 has 500 w output but like others can be dialed into your requirements, i drive a toyota hiace 2.7 petrol, im sure the alternator is rated at 120 or 170 amps (yet to varify ) the variables while using the chager1 and without is what i focus on. not charging the hiace sits around 25 % duty cycle at anything over 1000 rpm. if i switch on the charger1 to 500w i see the duty cycle climb to 50% with no accesories on. i also see a 50amp load . the charger 1 has a quiet mode switch that turns off the internal fan and reduces the output to 200w. this is the setting i use most. it draws 20 amps. one point of interest, the charger one has a cutout voltage of 13.4v i think. so when idling and battery volts drop , the unit turns off automaticlly.. once voltage increases with rpm, the unit turns back on. a great feature to stop drain on the alternator at low rpm.

I have the Bluetti charger which outputs @ 500 and change . My overland rig (Jeep JL Rubicon) has the tow package 240 amp alternator. I’m only going to use it when the sun isn’t out and I’m travelling and I want my Bluetti 200 to get to full charge . I won’t be running any other accessories. I find the sun and my solar panels do pretty good ( even when cloudy the panels put something in) … no I’m only going to use the alternator charger when I really need it . Great test though 👍🏻 Cheers Wild Bill , overland exploring the remotest Regions a of northern Canada 🇨🇦.