Alternator voltage output/Battery charge voltage
 

Hi All,
Just posting this for people who replace their battery with non standard one.

The alternator in the 370z and many modern cars have a variable voltage output which is controlled by the engine ECU.

The charging system is matched to the battery model from factory.

Replacing with non standard battery may lead to shortened battery life due to overcharging.

Normally when car is started voltage is set to 14v and battery is charged until the ecu sees its fully charged via current sensor on earth lead.
Once battery is charged the voltage drops down to 13v where it floats.
This helps increase battery life by reducing fluid loss or something like that.

And sometimes you will see the voltage go higher than 14v which I think is used to de-sulfate the battery or something of that nature. don't quote me on this as i am no battery expert.

Anyways, I replaced my old original battery with a smaller one to help reduce weight in car but it seemed to over charge and lasted only 6 months.

SOLUTION

You can set the voltage to float full time at recommend float voltage by battery manufacture.

This is done by taking control of the PWM signal to alternator and setting voltage.

I have currently installed LifePo4 12.8v 40ah 900cca battery in my 370Z and set the voltage to 13.8v float (13.5v for Pb). 1/2 the weight with more capacity as well.

PWM is 12v at 10kHz at 45% duty cycle.

Hope this helps a few people.

Thanks!
Steve S

Thanks for the astute write up!

Sent from my SM-G955U using Tapatalk

Great write up!!

So how do you set this voltage?

zz

You connect this to one of the wires in the IPDM module.

https://www.ebay.com.au/itm/Signal-G...53.m2749.l2648

Thanks for the write up.
Any more details re which wire(s) and maybe full instructions?
Thanks. :tiphat:

Why did you decide on 10Khz?
That seems unusually high for a pulse width modulated signal.

The FSM shows a PWM period of about 6 ms or 166 Hz.
(Pin 76 of the IPDM/ER in section PCS)

Anyways, thanks for posting this interesting info!

Your correct. Will confirm tomorrow. I didn't even see this page in the FSM.

I first assumed 120Hz at 5v and it didn't work. And then just tried other frequencies till it did. But used 12v supply instead. Strange FSM shows 6v

Been like this for 1 year and still OK.

I will try 166HZ and let you know what happens.

Thanks.

Either way, I think you are fine since the duty cycle controls the output.

Once the weather warms up, I will scope/data log my alternator control line and the output voltage and see if the FSM is accurate. Just over 6 volts is a little odd.

Also, in looking at the PCS section, it looks like the duty cycle is inverted.
What normally looks like 60% (as in 60% of the period ON) is shown as 40%.

It is also my understanding if you disconnect the PWM control line at the alternator, "the normal power generation is performed according to the
characteristic of the IC voltage regulator of the alternator." - From the CHG section.

I use an OBDII reader on an Ipad which has certain live parameters displayed ( Water temp, voltage etc)

You can clearly see the voltage cycle as ECM dictates. You can see it stay @13V or lower but not the amount of the float voltage.

Just tried 166hz and it worked ok. But at 12v. I'm thinking when I tried 5v it wasn't enough voltage.

And the PWN signal is inverted. Reducing the duty cycle increases voltage. Set to 40% gives 13.8v

I first thought is was the PWM module I brought from Ebay.

Very interesting!

With a 5 volt input, the peak output voltage is about 1.3 volts lower, or 3.7volts.
This is due to the voltage drop from the schottky diode (~0.3v) and the 1.0 voltage drop from the 3.3v voltage regulator (HM333B).
(the schematics for these cheap boards are easily found, just google the name on the board: XY-LPWM)

So... with your 12 volt input you are running about a 10.7v PWM waveform, which I don't think will hurt anything.


For anyone interested what the typical power control operation is,
here is the battery voltage on my G37 Sport before starting, during starting and driving for just over 8 minutes (500 seconds):

• Battery voltage drops to 10 volts while starter is engaged.
• After starting the engine, the charging level is higher, resulting in a battery voltage of 14.6 volts.
• After about 6 minutes (350 seconds) the charging level is reduced and the batter voltage is ~13.2 volts

http://www.the370z.com/attachment.ph...1&d=1586702116

14.7 is overcharging.

Is this with the original battery?

It isn't the original battery, but a same size/capacity replacement installed in late 2016.

I agree that typical battery voltages with the engine running is 13.6 - 14.4.
However, in our case the FSM in CHG section, shows the regulated output voltage is 14.1 - 14.7 volts
and the adjustment range of the variable control is 11.4 - 15.6 V.


I don't know how accurate the reported voltage is as it is from my OBDLink scanner.

It could easily be off 0.1 - 0.3 volts depending on the accuracy of the analog to digital converter and the tolerance of the resistors in the voltage divider. (The chip can only measure 0-5 volts and uses a voltage divisor to drop the 12-14v to below 5 volts)

For me, I'm not going to worry about the charging voltage. It was interesting seeing/plotting exactly what it is doing over time.
Well, interesting to me :)