[psya]

Deutronic DBL 1200 is the way to go. Or the newest GYS.

[Kentener]

Such nonsensical nonsense is written here.

A power supply unit does not output constant power.

There is a voltage and a load is connected to it. And when the voltage is increased, the current increases and so does the power. All those who write otherwise have no idea at all about electrical engineering.

By the way, I studied electrical engineering. Where did you get your knowledge???

[Kentener]

A power supply unit generates a voltage. If no load is connected to the power supply unit, no power is output.

When a load is connected, a current flows that depends on the electrical resistance.

Ohm's law applies here.

I = U / R (current = voltage / resistance).

Example:

U = 13.5 volts
R = 0.5 Ohm
results in I = 27A

The charger then delivers the power U x I, i.e. 13.5 volts x 27A = 364.5W.

If you now increase the voltage to 14 volts, the current increases to 28A and the output power to 392 watts.

Your thinking error is that you assume that a charger always delivers a constant power, this is not so.

The current drawn depends on the electrical resistance of the load.

[pupu144]

Well, there is studying, and there is knowing and understanding the stuff you are talking about..


You are thinking of "infinite power" power supply (i.e, whole electric grid of a country or region) and not dedicated portable power supply, which have output limits.

By your logic, we can buy 600W PSU, crank up the voltage, and it will be as strong as 1200W supply on stock voltage.

The circuits are protected to not "generate" more than they are safely capable of. You cannot overload transformers infinitely and get away with it..


Also, your calculation are for a load - a PSU will try to match the load, until it hits it's capabilities.
It will try to provide requested current, but the voltage would start to drop. PSU is designed to hold the constant voltage, so it would regulate itself to keep volts steady and current will drop to the point, where there voltage is at steady pre-set level.

[Kentener]

Please simply observe Ohm's law. Current and voltage are proportional at a load. You cannot increase the voltage and decrease the current at a load.

---

Please explain to me how you can increase the voltage and decrease the current on a resistor. Does Ohm's law not apply to you?

---

A power supply unit tries to reach a certain voltage. The current depends on the electrical resistance of the load. The greater the resistance, the lower the current. If the resistance is too low, the current and thus the power is too high and the power supply regulates the voltage down so that the current is also lower. Because when the voltage decreases, the current also decreases.

[ir655]

Edit:

Answer found: Since these battery maintainers and homemade chargers are Transformers, Ohms law does NOT apply.

In general, voltage increase amp increase, yes. 

But this does NOT apply to transformers, which is what we are discussing in this thread.

End of discussion regarding amp/voltage in transformers aka battery maintainers/chargers.

Is resistance dependent on voltage and/or amperage in the system or is R a constant depending on temp, material, length of leads, cross sections of leads etc?

Example: 1000w (max) PSU that is adjusted to supply 13.5V will give these MAX values in theory:

R= 0.18225 ohm
I= 74A
V= 13.5
P= 1000W

Same PSU at 14V:

R= 0.196 ohm
I= 71.4A
V= 14V
P = 1000W

This applies for a DC circuit, AC is a little different. Even charging my 18650 batteries in a Nitecore charger I always see voltage increase as amperage decrease as the battery is nearing full charge.

Of course a car is not going to draw constant current, it all depends on state of charge of the battery and what accessories you are running while flashing. No one is saying the PSU will supply constant output nor max output all the time either. We have limitations in PSU's.

[Kentener]

During a charging process, the voltage naturally increases and the charging current decreases.

This takes place over a period of time.

However, it was claimed that at one point the current decreases when the charging voltage is increased.

And that is and remains nonsense.

[ir655]

In a step-down transformer (220v AC to 12V DC, read chargers) the voltage is inversely proportional to the current so therefore current must decrease if voltage is increased and power is maintained the same.

Otherwise you will have a nice smoke cloud and perhaps a bang from your maintainer/charger unless you have fancy fuses that pop first in the unit.

[Kentener]

Of course. To obtain constant power, current and voltage run in opposite directions.

But we are talking about a connected load here. I cannot increase the voltage on a load without increasing the current.

XULeTZ wrote:
"Increasing the voltage will lower the amps. Lowering the voltage will increase the amps. that`s how is working. So best to keep it at 13.8V with 67A. If you need higher amps, lower the voltage."


That refers to a load and that is still physical nonsense.

[PhreakShow]

No, not in general. That is true for resistive loads. The ecus in our cars often use stop-down converters on their pcb. Given a fully charged or even disconnected battery, the current can easy decrease if you increase the voltage to those ecus.