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Info only: 2021 Tacoma amp draw and alternator loads tested

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Dave in AZ

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This thread is just for reference for those looking to use excess alternator capacity to charge a battery. Other cars should have similar loads to this Tacoma for lights, airco, radio, etc.

21 Tacoma OffRoad, 130A alternator, 9k miles, running perfect, zero issues battery or alternator. I decided to test how much of my alternator output capability was being used, before I came up with a plan for a dcdc charger off the alternator to charge a 2nd house battery.

I used a Klein tools clamp ammeter, the one Will Prowse recommends. It has been tested and verified for good calibration, using Kill A watt meter and cross checking ac and dc amps for devices both.

I tested on the main negative battery cable. The positive starter battery terminal has 2 cables to it, one from alternator and one to fuse box. At idle, alternator charging tested on alternator to battery positive was only 4 to 5 A. I was surprised the positive to the fusebox showed -4A, which I didn't understand, thought all the power from battery would flow through that to fusebox. However, the main negative battery cable was the highest readings and seemed correct, and I know the SmartShunts go there also, so... The battery negative to chassis ground cable was about 4A less than the main negative.

Truck running idle, nothing on, 11.2A
+ lowbeam 55w ea +11.1A 22.3A
+ door open light and dome +1.2A 23.5A
+ air-co full cold and fan +10A 33.5A
+ fog lights, only avail on lowbeam 19w +3.4A 36.9A
+ turn signal, 21w x 2 +1.6A 38.5A
+ highbeam headlights, 65w ea 45.5A
(+7A from lowbeam&fog to Hibeam)
+ Qi pho e charger +0.2A
+ radio full loud +3A
Bed lights +0.4A

Likely driving setup Snapshots:
Truck alone 11.2A
Lowbeams 22.3A
Full draw: Hibeams, full airco, turn signal, full radio 46.5A

With Inverter: 125v measured, supposedly 400w in park and 100w in gear. Should be 31.2A of 12.8v at battery. However, tested way lower.
+ 465w pull on 400w inverter, +22A
--- In Lowbeam, dome, full airco, 33.5A without inverter, 465w on inverter 55.0A
--- In Full Draw (hibeam, full airco, full radio) 46.5, with 465w on inverter 65.5A

Conclusions: test was done parked in idle, normal driving 2500rpm load unknown.
130A alternator appears able to support 400w inverter full time. With all loads off but inverter, amps are 33.2A. Driving with lowbeams and turn signal gives +12.7A for approx 55A. This is a safe setup for driving, at 42% alternator stated max.

Most I could load truck in idle WITH inverter, 65.5A. This is 50% of alternator stated max.
If running full 400w while driving, can reduce alternator load by turning off devices, 11A for lights, 10A airco, 3A radio.
 
@sunshine_eggo @justinm001 saw you guys talk about alternators before... am I testing this right for power draw? I figured the ammeter clamp around the alternator to battery positive cable would show everything, but it was just 5A or something low... does the battery negative show everything? These amps were more reasonable than any of the other 3 cables that touched battery. It just seems positive would make sense? But then the Victron shunt and all go on battery negative too so...?
 
Dave in AZ said:
@sunshine_eggo @justinm001 saw you guys talk about alternators before... am I testing this right for power draw? I figured the ammeter clamp around the alternator to battery positive cable would show everything, but it was just 5A or something low... does the battery negative show everything? These amps were more reasonable than any of the other 3 cables that touched battery. It just seems positive would make sense? But then the Victron shunt and all go on battery negative too so...?
Click to expand...
Yes an ammeter clamp on the large alternator positive cable would show everything as long as its the only cable.

Negative would too but on an alt it's likely also engine/frame grounded since metal on metal so you wouldn't get the full reading.
 
Nice work. That answers a lot of questions we wonder about but never think to investigate.

Your alt is not meant to be able to run at its max capacity for too long. There are 100% alternators, but will not come stock on a vehicle. Alternators will burn out if run at 100% for too long. Check that info for your particular one...

Consider also that a running vehicle with a fully charged battery, and healthy battery, will draw less than one with a battery that was barely able to crank the motor and in need of being recharged. Recharging will be a big amp draw. Would be interesting to measure how many amps are drawn on a depleted battery.

Big draws as you show are the blower fan, the ECU (always running) and other Xcu's, then on down the list. Dont blow air cond on full when moving if you want to save power, let the air pressure move air. Air con or not it is the fan, not the compressor that needs electricity. The compressor runs off a belt, not electricity.

If a vehicle has an electric radiator fan then everything you do will heat the water temps up. More alt output means more engine horse power which heats up engine which needs more air to cool the radiator which needs more electricity to run the fan which needs more power from the alternator. luckily at speed the fan is not needed, the speed moves the air.

Alternators are not free electricity. When loaded more and more the engine has to provide more and more horse power to turn the ever increasing output of the alternator, this uses gas.
Typical alt uses about 6 horse power if I remember correctly, not a lot, but it adds up.

**edit** the electric or even mechanical radiator fan will be controlled by a temp sensor in front of the fan and behind the radiator. When you are at 60mph the air flowing through can be cool enough that the fan is off and lets the 60mph wind do the work. Electric fans are easier to hear on or off bc they turn off when off, but mechanical ones always spin a little, and worn ones/old ones never spin hard - when they spin hard you hear a roar. If your mechanical clutch fan does not roar the first 15 seconds on start-up then it is worn. Might need repair. Normal for them to wear out at 60,000-100,000 miles. Some can be rebuilt, some cannot, well, all can, but some are going to be a lot harder to rebuild.
 
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@0truck0 those are some excellent points I didn't evem consider. The belt driven airco compressor, and the radiator fan. Great info on how to mitigate power use on stuff too. Thanks, very helpful!
 
I would also confirm the wire sizes to make sure they can handle the amps. Consider the "big three" wire upgrade.

1. Ground wire from battery to frame

2. Ground wire from engine to frame

3. Power wire from alternator to battery
 
featherlite said:
I would also confirm the wire sizes to make sure they can handle the amps. Consider the "big three" wire upgrade.

1. Ground wire from battery to frame

2. Ground wire from engine to frame

3. Power wire from alternator to battery
Click to expand...
Always good to have those upgraded for sure. In this case, I am just using the factory inverter, and will use this amp info to be smart and shed unneeded loads before using that inverter power too.
 
This post is great, thanks for testing and providing results. Couple tips, you need to average the draw over a few minutes just because of battery draws the load then alt charges the battery. Also just after starting the battery would be recharging so can affect it. Typically standard car alts are rated at 70% of load continuous and when idling its likely only able to provide 50% of power. The RPM of the alt isn't the same as the engine RPM. This depends on the pulley size on the engine and the alt itself, typically the alt is around twice the RPM of engine for gas and can be 3-4x for diesel as alts can withstand much higher rpm than an engine can. Diesel engines typically have a PTO or high idle switch and should be set to provide RPM needed to run the alt and other devices at idle without overloading.

1HP creates 745watts so if you're running your alt at 70% (91amps) you're adding around 1.5 HP load on the engine which affects heat, mpg and everything else. Its minimal impact but just something to consider especially when testing at idle where you don't have much demand. Added alt load increases heat which kicks on the electric rad fan which draws more power. Rad fans are 80-400w each and some are multi-speed.
 
scrubjaysnest said:
Tow/haul on my Dodge Ram changes the alternator output.
Click to expand...
You sure about this? Do you know if it has a smart alternator? It sounds like marketing decided to add that phrase into its promo to sell the tow/haul option/feature. I'd bet it adjusts the idle RPM and the transmission shift points to help. I know when I had a 16 powerwagon it adjusted the shift points
 
justinm001 said:
This post is great, thanks for testing and providing results. Couple tips, you need to average the draw over a few minutes just because of battery draws the load then alt charges the battery. Also just after starting the battery would be recharging so can affect it. Typically standard car alts are rated at 70% of load continuous and when idling its likely only able to provide 50% of power. The RPM of the alt isn't the same as the engine RPM. This depends on the pulley size on the engine and the alt itself, typically the alt is around twice the RPM of engine for gas and can be 3-4x for diesel as alts can withstand much higher rpm than an engine can. Diesel engines typically have a PTO or high idle switch and should be set to provide RPM needed to run the alt and other devices at idle without overloading.

1HP creates 745watts so if you're running your alt at 70% (91amps) you're adding around 1.5 HP load on the engine which affects heat, mpg and everything else. Its minimal impact but just something to consider especially when testing at idle where you don't have much demand. Added alt load increases heat which kicks on the electric rad fan which draws more power. Rad fans are 80-400w each and some are multi-speed.
Click to expand...
You keep thinking up more smart things I didn't consider. Triggering more radiator fan thus load...

I have a switch I can turn on/off the factory invertor, which is what I'm goint to use to supply 100 or 400w alternator charging to my rear truckbed power station. After reading all this and your good comments, I will only use that invertor if I can depower everything else while driving, so no airco, fan blower, headlights, radio, heater. Well, maybe headlights at night ;)
 
Dave in AZ said:
You keep thinking up more smart things I didn't consider. Triggering more radiator fan thus load...

I have a switch I can turn on/off the factory invertor, which is what I'm goint to use to supply 100 or 400w alternator charging to my rear truckbed power station. After reading all this and your good comments, I will only use that invertor if I can depower everything else while driving, so no airco, fan blower, headlights, radio, heater. Well, maybe headlights at night ;)
Click to expand...
If you have a dash voltage gauge just keep an eye on it while driving. You can overload the alternator for some time and it'll just dip into your battery voltage, but if you keep it overloaded it'll get very very hot and can eventually fry. Easiest way is if your voltage is the same at idle as it is at normal speeds then you gotta be under that 70% utilization since alternators are like half output at idle than while driving.

Wakespeed alternator controllers put a temp sensor on the alternator and default option is for it to start derating output at 200F. IDK of a good temp sensor you could put on there permanently while driving but that would be ideal. Then you could run that thing as much as you can as long as its under temps and you trust the sensor.
 
justinm001 said:
You sure about this? Do you know if it has a smart alternator? It sounds like marketing decided to add that phrase into its promo to sell the tow/haul option/feature. I'd bet it adjusts the idle RPM and the transmission shift points to help. I know when I had a 16 powerwagon it adjusted the shift points
Click to expand...
That is correct idle changes and shift points are changed.
 
Oh wow, what a great post. THANK YOU for testing this and posting. I was just about to post a question on how to "ball park" these figures but you did the dirty work! I also have a 130A alternator.

I'm planning on running a 500W inverter to run an 30A 12v AC-DC charger instead of running a DC-DC charger. My math says the draw from the alternator would be about 44A, counting for 72% efficiency. My math is likely on the conservative (high side) end. Cheaper all around (both inverter and wiring), only real downside is the reduced efficiency.

The only thing I don't know yet is the amp draw when the electric trailer brakes are engaged. Google says about 16A.

I estimate that at max load we'd use (we rarely use the stereo), 46A+44A+16A = 106A, which is about 81% of the alternator's rating, which would not be running that high consistently anyways. On long mountain downhills, I would likely turn the inverter off to not stress the alternator too much. I can also adjust the 30A charger to any rating all the way down to 7.5A if I wanted less stress on the alternator, too, but that is too technical for my wife to appreciate if she happens to be on a trip without me, lol.
 
Great Post

I drive an 09 Silverado 2500HD 6.0L gas truck.

I am working on adding a second smart alternator (125 amp) to just charge my house batteries.

You can get to needing 175 amps when driving with everything on. Everything is heated now a days.

Half the rated amps are what they will normally charge to keep the alternator heat down.
I will have gauges for amps, volts, and alternator temp. My charge rate will be controlled by an adjustable signal generator in the cab giving me full output control.

Running a 50 amp DC/DC controller charger I should be fine.
 
From my observations testing multiple vehicles, if you turn on all the 'standard' equipment that basically every car has (headlights/tails, wipers, full blower speed, rear defrost) it is usually around ~55amps. That doesnt include recharging the battery which drives up the current in the first few minutes after a startup. ~55amps would be the 'steady state' number. Of course theres a ton of wiggle room in such a huge generalization but generally an alternator will only put out 60-80% of its max output at idle, and the max load of the car with everything turned on is generally a little lower than what it can make at idle.

So, if you wanna ballpark it REALLY roughly, take your max alt output, drop it to 60%, and a little less than that is probably what the vehicle pulls with all of its electrical equipment on. Thus the reason why things that have heated everything (seats, outside mirrors, steering wheel, front and rear defrost grids, etc) tend to have much bigger alternators in the 160+ amp range.
 
On some other forums, I have seen amp draw from the radiator electric fans ( on vans ) and the numbers were surprisingly high.

They were so significant that I really wondered it would be a good idea to setup a relay so that battery to battery charging stopped when the radiator fan turns on, especially for small alternators, such as anything under 250 amps.
 
Well, it does depend on the vehicle but MOST fans are on no more than a 30amp fuse. I think most DC-DC converters (actually this is an assumption, maybe wrong!) have an adjustable low voltage cutout so that if your starter battery system falls below a certain voltage (ie you're 'overdrawing') it would cut out until voltage rises again.

If you wanted to add that functionality separately you could put the power feed to the DC-DC on a voltage sensing relay aka VSR which typically cuts out at 12.8 and reconnects at 13.3. They are ~$20 on amazon and typically rated to 140a. If you put it at the battery end of the power feed it would not be subject to the voltage drop of the wiring and would work 'accurately'. If you put something at the DC-DC end of the battery feed then you have to account for the voltage drop of that circuit which is both largeish and variable and will take some observation and adjustment to get right.
 
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