? how to calculate current needed for DC/DC conversion

[cudaboy_71]

i'm in the market for a power supply and charger. i really like the looks of the estation 902 w/ dual outputs.

so, if i'm to be charging 2 FP4900 6s packs, what kind of power supply would i need to reliably charge both simultaneously at 1C? 2C? what about calculating for the future (dual 8s or 10s packs).

is the current draw linear in the DC/DC voltage conversion, exponential, logarithmic???? anyone have any formulas. i slept in that day in class.

[flyinfool]

To charge a 10S pack at 6A will use a little over 18.5A at 12V input.
The 10S pack at 4A will use just over 12.5A. So you are right on the edge charging 2 10S packs at 4A off of your 25A power supply.

To calculate the amps required from your 12V power supply or battery.

Pv X CA ÷ 12 = SA

Pv = Pack Voltage
CA = Charge Amps
SA = Power Supply Amps

This is assuming 100% efficiency which will never happen in electronics.
I have no idea what the efficiency of a 1010 or any other charger is, so I would add an extra 20% to your required power supply amps as a safety margin.

[Pinecone]

My 1010C pulls about 3.3 - 3.4 amps at 13.8 volts (normal PS voltage) to charge a 5S pack at 2 amps.

22V x 2 amps = 44 watts
3.4 amps ax 13.8 V = 47 watts, so less than 10% overhead for the conversion.

And that 3.4 amps is just in the 21.5 - 22v range at 2 amps. As soon as the charge amps drop the supply amps do too.

[cudaboy_71]

thanks flyinfool. that's exactly the formula i was looking for. i take it the ÷12 in the formula is supply voltage?

and, thanks for the empirical data pinecone. now i'm off to scour the web for deals :D

[flyinfool]

Yes the ÷12 is the power supply voltage.
Most 12V power supplies will output near 13.8V.
The 13.8V is because that is the voltage that an automobile has with the engine running.
Most 12V things are made for use in a car.
By using the 12v in the formula you end up with a little extra safety margin.

[Pinecone]

Actually cars run at 13.8 volts because that the the proper charging voltage for a 12V lead acid battery. And actually most cars run above 13.8 volts right after starting.

Just like the voltage needed to charge a 11.1 volt lipo pack is 12.6 volts.

[flyinfool]

You are correct.
I just didn't go into the why to avoid confusion.