My car's generator is putting out 13.5 voltages when on full load (blower, ac, lights etc) but when i check the voltage on car's cigarette lighter port it it drops to even 11.6 when blower is on, when blower is not on they are between 12.8 to 13.0

My car's battery keeps dying every year because it keeps drying the water inside it.

This is how i think it is happening and correct me if i am wrong,

On battery terminals, voltages are 13.5 which is great and battery is charging and all.

But for some reason there is high resistance between the battery terminal and the wire that is coming inside the cabin to distribution box (which has fuses etc) and when 20amp blower run it drops the voltages to 11.8 or below.

But the main problem why battery is getting damaged is because inside the cabin voltages are 11.8 and of course the car's ECU is getting the voltage reading from inner distribution panel so because of that ECU gets 11.8 and it keeps the Alternator to keep charging battery because it suppose to stop charging when battery is at 12.6 which according to the inner distribution box it never happens.

I know somewhat voltages are suppose to be lost during transmission, but it is practically dropping 1.5 something voltages and these voltages are making differene because ECU is not getting enough voltages, and alternator does have two port signal wire on it which according to some people online are the one that turn on and off the alternator when ECU tells it to be.

And the problem i am facing is low mileage when blower and ac is on, and i am eager to find the problem because mechanics in my country are very dumb and they have practically denied any problem.

  • Duplicate of and I pointed to a similar question anyway : electronics.stackexchange.com/q/371716/152903
    – Solar Mike
    Commented May 1, 2018 at 13:45
  • @SolarMike yes sorry i wanted to post here but posted there first with mistake.
    – ZBT248
    Commented May 1, 2018 at 14:36
  • If your theory is correct why not find where the voltage drop is occurring and fix it? Without knowing the YMM and type of charging system the car uses all we can do is speculate. 2 wire isn’t exclusive to pwm voltage regulators.
    – Ben
    Commented May 1, 2018 at 16:54

1 Answer 1


13.5 V is a bit low even at battery terminals. If you have a two-wire alternator, it is much, much too low at the alternator output terminal. You said your alternator has a separate signal wire. If that is used, it means you don't have a two-wire alternator.

I would bump up the voltage by 1 volt to 14.5 V at the battery terminals, if possible. This may mean you need to change the voltage regulator if it's not adjustable.

13.5V may be barely enough for a battery at room temperature if you intend to continuously float charge in 24/7. If, on the other hand, you have some amount of cycling on the battery such as by starting the engine, and then charge only for an hour or so, 13.5V is too low.

So, in summary, your problem is too low voltage, not too high voltage as you initially suspected. The battery definitely needs more charge, not less charge.

  • I have checked other cars too they also have around 13.5 voltages on terminals, so they all have weak voltage regulator?
    – ZBT248
    Commented May 1, 2018 at 14:38
  • At what temperature is that? Is your multimeter a good one or a $10 El Cheapo? At room temperature, charging voltage should be ideally 14.5 V. Obviously you need a quality multimeter to get accurate readings.
    – juhist
    Commented May 1, 2018 at 15:56
  • i have a good quality expensive multimeter, and the temperature outside is 107F, but i am sure the inner temperature of car engine cabin is more than 107.
    – ZBT248
    Commented May 1, 2018 at 15:58
  • @davidweb That explains it, it's much higher than room temperature. Your 13.5 volts should be okay for your temperature, up to 150-160 Fahrenheit. The high temperature might explain your early battery failure. Batteries just don't like extreme temperatures.
    – juhist
    Commented May 1, 2018 at 16:23
  • what should be done to prevent them? And my real problem is that inside the cabin voltages are low, and don't you think two wires from alternator are connected with ecu and ecu controls the alternator? We have many cars here and non of them evaporate the water like my car do.
    – ZBT248
    Commented May 1, 2018 at 16:25

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