Dave H

Partly because the 4S lipos are at a higher voltage than the power supply, that takes more amps from the supply to generate the desired charge amps.
Some power is needed to run the charger. I normally use 80% efficiency as a safe estimate, might be better but I can’t be sure, varies some for every charger and load level.
And I always recommend some extra headroom for power supply reliability based on a fair bit of experience and typical manufacturers recommendations of 30% extra. Some supplies can do better but not all.

If you have this power supply it’s rated at 300 watts:
TrakPower 25A 12V Racing Power Supply

Let's assume no supply headroom for the moment:
300 watts x 80% efficiency / 2 lipos / 16.8V full charge = 7.1 amps per.

I’d suggest a little less to be safe.

You could charge 1 at a time at double those values.

Stanfsu

iTrader: (294)

So much to learn here. Got it now. I might just get the sky rc 400 and charge one at a time. For $80 seems like a good charger for my needs

Hoese37

iTrader: (5)

Go with the 800 dual.

Stanfsu

iTrader: (294)

On track days I plan to connect the charger to my car battery. Would I get full potential if charging thru the car battery?

Hoese37

iTrader: (5)

Yes.

Stanfsu

iTrader: (294)

Thanks

JiuHaWong

Thanks for your response. I found this: http://www.tjinguytech.com/charging-how-tos which has answered all of my questions in relation to wattage/amps/volts.

Dave H

I agree it likely will work, a couple things I think are worth mentioning though.

Car batteries can deliver considerable current, but sag in voltage quite a bit. Charging a couple 4S at healthy rates might drop it below the 12V minimum from the charger specs at some point. Have no idea when, depends on the size & condition of the battery, how many charges, etc.

More importantly, modern car batteries don’t like being ran down very much. They have thinned the plates to get more short term starting current, not so good for significant discharge. At best it will shorten the service life. At worst they tend to die very quickly and unexpectedly, can even be hard to jump start (they drag down the other battery). At the very least start it up occasionally to recharge (cranky old dude alert, be mindful of the fan & belts and such, and don’t start it while charging).

If using a battery I strongly suggest getting a good sized deep cycle (marine) battery. Bonus is a little more freedom for the pit setup (& some exercise!). Many RC chargers can charge them, or get a ‘smart’ charger from Battery Tender, Schumacher, etc. I have a ~$20 2 amp Black & Decker from Wallyworld that worked fine when I still used a battery, as I could always give it a couple days.

Dave H

Yep, that site is pretty good and contains a lot of useful info.

Stanfsu

iTrader: (294)

+++++1

BulldogSRT

iTrader: (1)

I have the Sky Ultimate 400 charger and using a modified Server 12V 69A power supply. I tried charging 2 2S lipos with a parallel balance board and got a Low Voltage Alarm on the charger. Is this because I need to bump the Input Voltage higher? If so, I can run two Power Supplies but I don't think the charger with tolerate a 24V input voltage............is this problem a Input Voltage issue or am I missing something else?

Thanks,
BulldogSRT

Dave H

Sure could be, assuming it’s not an output low voltage warning for the lipos. If I’m not mistaken that charger has a input spec of 12-18V. A 12V power supply is obviously cutting it real close, especially including accounting for the slight voltage drop of the wires and connectors. The downside of the server-PC power supply approach, especially for higher powered chargers that really like a higher power supply voltage, they have less than ideal voltage. And like you mention, going to 24V doesn’t appear to be an option.