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Old 04-09-2015, 08:05 AM   #31
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The key is the voltage of the pack being charged as Howard and several others has mentioned. Not everybody uses a 2S pack. For the ebuggies running 4S (or more) packs additional power supply current is needed to achieve the power required at the higher voltage. While the 1S guys need far less power supply current.

It's similar to all the threads asking why a common 50 Watt / 6 Amp changer will only go to about 3 Amps on a 4S pack. Not enough power. Yet the current draw from the power supply is about the same in both cases, 2S at 6A compared to 4S at 3A, despite the change in charge current.

With many of the higher powered chargers available a higher voltage supply is needed for the larger packs at high charge currents, due to charger input current limitations. It's far easier, cheaper, and more efficient to use higher voltage to achieve the high power levels.

In the '80s chargers like this Tekin BC100L were common. It came with a large power resistor to reduce the supply voltage when charging 1/12 scale 4 cell nickel packs, or a 10V or slightly lower power supply could be used (typically an adjustable voltage power supply). To prevent burning up the charger when charging these lower voltage packs. On the other hand it was simply not capable of charging a 12 cell pack with a 12V supply. Modern switch mode chargers have certainly increased the flexibility of chargers, but haven't altered the power relationships involved.

You missed the point I was only talking about 2s lipo packs nothing else. The point is the charger input was always 12v whatever it did to the voltage is not what I was talking about
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Old 04-09-2015, 08:39 AM   #32
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Now I'm really asking this question cause I dont know.... All of our equipment for racing has and will always be based on a 12 volt power DC. Back in the 80's we carried 12 volt car batteries wherever we raced or charged off the car battery whenever we could Park close to the track and had no AC.
The chargers themselves were always based on 12 volts and to this day has never changed.
That is the constant and the other constant back then was that a car battery could produce any current you needed. And of course the other constant is that we are talking about 2s packs.
Why does everybody bring 12 volts up when we know that is the constant?

Isnt how I think about it just easier then doing all these calculations? You want to charge at 6amps max then you get a power supply that will give you 6 amps or more depending on what you can afford. More current more money. Simple
99.99% of the time charging a 2s pack at 1C you would be exactly right.

While it might not be a factor or matter, anything above 1C on a 2s pack and you should probably start to factor in watts. Or at least look at them.

When you start charging a 2s pack at 20, 30, 40 amps. 12v may no longer be a constant since some/many chargers require increased voltage input to achieve the higher amps on the output side.
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Old 04-09-2015, 08:42 AM   #33
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99.99% of the time charging a 2s pack at 1C you would be exactly right.

While it might not be a factor or matter, anything above 1C on a 2s pack and you should probably start to factor in watts. Or at least look at them.

When you start charging a 2s pack at 20, 30, 40 amps. 12v may no longer be a constant since some/many chargers require increased voltage input to achieve the higher amps on the output side.
lol, I had a 6amp charger that would only put out 5amps with a 12v power supply. I called the company, and they told me I needed 15v for 6amps... I laughed and sold the charger
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Old 04-09-2015, 09:45 AM   #34
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That's why I said "99.99% of the time"

12v really isn't the norm anymore.
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Old 04-09-2015, 09:51 AM   #35
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That's why I said "99.99% of the time"

12v really isn't the norm anymore.
nope, but 12v is cheap
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Old 04-09-2015, 10:58 AM   #36
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All chargers today are using a switched voltage regulator. Because these regulators do not make a loss into heat you can talk about power in watts. Charging 8.4v with 5A is 42 watt, on the 12v side the current will be 42/12 = 3.5A

12v is still the norm for most RC car use, as mentioned before there is a lot of equipment using 12v. With current flying machines using 6S batteries it will take a huge power, taking that from 12v will mean a huge current. That is why high power chargers now want to have 24v to keep the current down.They still work on 12v because the use on a car battery but with high power chargers the power is most of the time limited to half.
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Old 04-09-2015, 11:38 AM   #37
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All chargers today are using a switched voltage regulator. Because these regulators do not make a loss into heat you can talk about power in watts. Charging 8.4v with 5A is 42 watt, on the 12v side the current will be 42/12 = 3.5A
Switching regulators still have losses. Typical efficiencies are somewhere around 80 to 90%. The rest comes out as heat.

The power supply should be rated at higher power than the charge voltage times charge current. An extra 30% is a good rule of thumb. In your example that would work out to 12V at about 4.5A.
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