Car Charging Output

The alternator rating (max output) would probably be in the specs in the handbook.
As you have been told most modern vehicles are around the 90amp mark but cruisers are 110-120 these days and would expect the Prado to be similar. This figure is the total load the alternator can handle, start adding up all the lights, accessories, engine, trans etc etc and you will get to that level eventually ;-))
However your battery will never accept or receive those sort of currents and if they did the battery wouldn't last long. Typical charge rates are in the order of 10 - 20 amps usually which is why a flat battery takes several hours at highway speeds to be anything close to fully charged.
Your 12.8 on the battery readout is volts not amps.

Hi deserter

Your alternator may be able to produce the 70 to 80 amps but the battery pack will only absorb what the battery type, cable size and battery state of charge will allow.

Most batteries will only absorb around 5 to 15 amps in a remote installation.

Please be aware this varies quite a bit. I would need more details.

This is an average and just a guide to give to info.

... :-(

Some people have made it hard to answer a simple question. Email me direct if you like.

Regards

Derek.

Hi Deserter

In addition to the 2 helpful answers above another underrated factor is simply that the alternator while rated at maybe 90 amps just doesn't put this out except at high speed.

Forget exact figures but from Patrol manual the alternator test point was something like 30amps at 2000 rpm.

I.E. even if battery was in a position to except current it couldn't get anything like 70-90 amps at normal cruising speed.

Robin Miller

I just had a look at this today..albeit the camper battery. I have a dual battery setup in the Patrol and another 120a/h in the camper hooked up via an anderson plug.
I have a Xantrex battery monitor in the camper and with the car idling and charging both the Patrols Aux battery and the camper battery, the camper was receiving 12amps. Not sure if this is good or bad but thats what it is getting.

Cheers,
Paul.

Thanks guys - thats good info.

If yours is a 90series Prado, the alternators are 80amp. Not sure about the 120series. And the voltage is regulated to around 14.2 volts at cooler temps. When temperature rises, the regulated voltage drops. This is deliberate.

Derek has mentioned the major factors that affect how much current goes back into the battery. The current gradually decreases as the battery is charged. Theres nothing you can do about this, apart from ensuring you have good wiring, and good earth leads going to the motor.

Most N70 cranking batteries that are discharged to say 12.0 volts (70% discharged), will accept 30+amps at 14.2 volts from the alternator. The battery that is used to crank the vehicle, is unlikely to be discharged (cranking uses little capacity) so within a few minutes of starting the vehicle, would only be accepting say 5 amps, and later this would be 1-2 amps.

If any of your batteries are old or stuffed, they will have a higher internal resistance, so will accept less current, and take longer to recharge.

Cheers
Phil