Parallel Charging...or get a Multi Charger? Math hurts my brains.

[SkinnyDave]

Hey Guys - well, my tricopter is on it's way (yay!) - but, my new dilemma is how to charge the 4s/2700mah batteries. Sure, my trusty little Accucel-6 can do it...but doings some simple math means I can have 4 batteries charged in...roughly 6 months

So - I know parallel charging is kind of a nice option...charge 4 batteries at once, bada-beep, bada-boop all done. And I know multi charger (like the one at HK that is basically 4 turnigy's stuck together) has some upsides (charging different batteries sizes simultaneously, etc)...but, where I'm getting majorly confused is what kind of charger I'll need to get to parallel charge 4 batteries at once and still be able to charge at a 5C rate!

I think I did the math correctly and I think I need something that does at least 13.5amps, at 800watts...but let me know if I'm waaaay off on that. So with that in mind only a few chargers qualify - something like a iCharger 3010B or Turnigy MEGA 1000w. I think I got that far...but, the power supply is really what is boggling my mind! I know I can get the standard power supplies that HK sells...but, most of them are rated higher amps then both of those high wattage chargers take...is that OK? Or does the charger just cap out and not use those extra amps? Also - are those converted server/ATX computer power supply chargers a decent option? I've seen some of them priced really well just didn't know if people have had good/bad results with them...or is a power supply simply "a power supply"? My company has lots of old servers laying around that *might* be worth me taking a look at next week!

Thanks for any wisdom anyone can offer...I've definitely gone over the edge from my little 2S mini quad days...I guess this is the part where I just start throwing money out the window, right?

-Dave

 

[trigger]

I don't know much about chargers and power supplies, except this :

A good charger isn't cheap, a good power supply is even more expensive.

Unless you have hundreds of dollars to spend, or are willing to risk burning your house down, forget about charging 4 4S 2700mAh in parallel.





Ok, still reading ?

For the math :
Charging 2700mAh at 5C is 13.5A
For a 4S, i.e. 16.8V at full charge, that's 226W
To charge 4 packs in parallel, you'd need ~900W

So, to do what you want, you'll need at least a 4x226W charger, and a 900W power supply.
AFAIK, HobbyKing doesn't have such a charger (they have 2-channels chargers with enough power, or 4-channels ones with lower ratings).
Furthermore, as much as I like Turnigy stuff, I'm a bit wary of their chargers. They're cheap... but maybe too cheap.
I don't want to risk a fire in my house because of dodgy QC.


The power supply unit must have a higher rating (or at least equal) to the charger. The extra amps are "unused" so to speak (the PSU will only supply what the charger is asking for).
If the charger "asks" for more amps than the PSU is capable of outputting, then things starts to overload, heat up and burn (or, more likely, a fuse will go off in the PSU)

I know some people use server power supplies (as in blade server PSU, NOT classic desktop one), but you'd still need a balanced lipo charger. As you said, a power supply simply is "a power supply"

 

[Cyberdactyl]

The $24 Turnigy single battery charger is actually well above average for the price. Here's a fairly prolific electronic video blogger I watch reviewing it.

Also, for a power supply, I use an 80watt very ancient PC power supply. However, with little effort, it should not be too difficult to find a 450-650W power supply out of an old abandoned PC. You could then merely spend another $24 for another Turnigy charger. While it falls short of charging more than 6S, two chargers for $48 would easily handle two 6S, with the 650W. Another advantage, similar to using two batteries in parallel providing higher mAh at only roughly half the cost if one of the batteries fail, you also have the same with having two smaller chargers if one is fried.

 

[SkinnyDave]

I'll have to see which is cheaper right now, my home owners insurance coverage or buying a new charger! The old computers I have laying around our office are old blade/high end servers which is why I might just take a peak! That's the joys of working in a bankrupt post production companies former space...lots of old toys to play with!

Yeah - I do think I'm in the upper echelon of "better then hobby king" charges when you get to the kind of charger I'm looking for...but I just pulled up their site to see what was readily (ish) available! There are some things that qualify...but I wouldn't mind a charger that could maybe parallel 2 at a time...that might reduce my needs a wee bit...save me a few dollars here/there.

I may or may not have started a spread sheet that helps me calculate charge time, watts needed...and price of each layout

I guess I got mixed up with 4S being 16.8 volts, so my math was a little off but...am glad to see I was kind of close! I'll keep checking around and see what else is out there...but also good know know you want to go over the watts/amps needed, I guess if I sat down and thought about it for a few seconds that would make sense.

I guess the 4xAccucel-6 type chargers work well (saves me a few dollars over buying 4 separate chargers) too...that might be the "cheaper" solution, though still around the same price and not too much longer charge time in the end.

Appreciate all the input guys - I'd love to hear if anyone has any direct use and/or likes/dislikes of some of the beefier chargers! I really do love my little Accucel-6, not a bad little guy for $25!

 

[trigger]

Be careful with desktop PC power supplies and their ratings.
When a PSU is branded as, say, 450W, it's 450W across all rails (+3.3V, +/-5V and +12V)
Each rail actually supports a load which is much less.

For instance, I use a 350W PSU for bench testing, but the +12V rail alone can only output ~150W.

That's why people who wants high ratings (for instance, 800 or 900W as you mentioned) use servers PSU, which, since they are able to provide +12V for several blades, have much higher ratings for each rails.


As for your math, it wasn't exactly wrong The truth is a bit more complex. While it's true that a fully charged 4S lipo is 16.8V, which in turns translate to 225W for a 2700mAh pack, in practice you don't have to output 225W all the time.
What happens is that the charger will apply a constant current (13.5A) while the voltage slowly increases as the lipo charges. At some point, it will switch to a constant voltage (16.8V) while the current slowly decreases until the lipo is full.
When does this switch happens, I'm not exactly sure, but I think that considering the maximum for each parameters (13.5A and 16.8V) gives a better estimate of the required power.


Anyway, I still think that getting a reliable 4x200W (for instance) charger would be very expensive.
A 4x50W charger would be much more affordable, and could charge 4 packs at 1C. I.e. everything ready in about one hour, which imo isn't so bad

 

[SkinnyDave]

Interesting - I know the rating was a little different for PC PSU's - but that's good to know. I'll have to check some of these companies that specialize in selling them...looking at the things that can go wrong, spending $40-$70 on something *might* be worth it!

Then again, if I see some of those hi-end supplies at the office tomorrow I might spend a night or two tinkering. Thanks for the feedback guys, really appreciate it!