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forescott
08-30-2010, 06:54 PM
I want to run 4-3s turnigy packs in a series/paralell combo. I already have 2-3s 5000mah 35-45c packs. If I purchase 2-more packs, do they have to have the same c-rating?? Or can I get a set of 5000mah 30c packs instead???

chuff
08-31-2010, 07:36 AM
I want to run 4-3s turnigy packs in a series/paralell combo. I already have 2-3s 5000mah 35-45c packs. If I purchase 2-more packs, do they have to have the same c-rating?? Or can I get a set of 5000mah 30c packs instead???

Interpret the C number as indicating the maximum safe current draw from a pack. The safe current is the C number times the capacity of the pack. So your 30C 5000 packs are safe to 30 x 5000 milliamps (30 x 5 amps), while your 35C 5000 packs are safe to 35 x 5 amps. For the sake of what follows let’s assume all packs are 5000 mAhr

For a number of packs (any number; 2, 5, 27) wired in series the “set” takes the lowest C number from the packs making up the ‘set’. In your case a 30C and 35C ‘twin set’ is safe to 30C. (If you wired a 35C, a 30C, and a 25C in series the “triple set” would be safe only to 25C (25 x 5 amps))

Parallel gets more tricky
I guess you now want to run two such “twin sets’ in parallel (4 packs in total?) Well following on from above you now have two 30C sets in parallel. The resulting “4 pack set” will be safe to 60C (60 x 5 amps). This is because in Parallel the packs share the current demand roughly equally. (If you rig three packs in parallel they each deliver about 1/3 of the current seen by the ESC and motor. So three 30C “twin sets” in parallel would be safe to 90C (90 x 5 amps))

HOWEVER… if one of your series “twin sets’ is made from two 35C packs, that “twin set” is 35C. If the other is made from the two 30C packs, that “twin set” is 30C. If you now put those two sets in Parallel, the resulting “4 pack set” is safe to 60C (60 x 5 amps). Note: the answer is double the smaller C number, not the two C numbers added together

Clear as mud??? Phew!!!

I would buy identical packs just to keep everything simple, but you don't have to.

Chris

keithbradley
09-07-2010, 08:57 PM
How do you get 60 X 5A when wiring two 5ah packs in parallel, or 90 X 5A for three? Should be 30 X 10A and 30 X 15A. Parallel wiring packs does not change the chemistry of the packs, only increases the Ah. You get the same numbers doing it that way, it just isnt accurately represented.

JMSCARD
09-07-2010, 11:37 PM
How do you get 60 X 5A when wiring two 5ah packs in parallel, or 90 X 5A for three? Should be 30 X 10A and 30 X 15A. Parallel wiring packs does not change the chemistry of the packs, only increases the Ah. You get the same numbers doing it that way, it just isnt accurately represented.

you only put EXACTLY LIKE PACKS in series or parrallel together.... from everything I have ever ready in every forum its just a bad idea to run anything but exact matching packs........ i am no battery/electronics pro, but I do remember what i read.... just spend the extra $10a pack and have the piece of mind that you have the right stuff....

keithbradley
09-07-2010, 11:46 PM
you only put EXACTLY LIKE PACKS in series or parrallel together.... from everything I have ever ready in every forum its just a bad idea to run anything but exact matching packs........ i am no battery/electronics pro, but I do remember what i read.... just spend the extra $10a pack keith and have the piece of mind that you have the right stuff....

Are you replying to me there or is forescott's name keith too? All my packs are matched :thumbup: I was just replying to the guy who said 3 packs in parallel somehow made them 90C and didnt change the capacity.

JMSCARD
09-08-2010, 12:42 AM
Are you replying to me there or is forescott's name keith too? All my packs are matched :thumbup: I was just replying to the guy who said 3 packs in parallel somehow made them 90C and didnt change the capacity.

ha ha... my bad.... I edited my post..... but good to hear keith...lol...

chuff
09-08-2010, 04:38 AM
Ok, in an effort to avoid further mis-understanding let’s define a few things. Sorry guys, if this is a bit long-winded and I start to sound a bit like a teacher. (I’m not a teacher, I design infrastructure for a living and race model boats as a pastime)

Lets define a “pack” as a collection of lipo cells wrapped in blue plastic with a label pasted on it that reads say, “35C 5000 mAhr”, two very thick wires poking out of it, and several thin wires that we can ignore.

A “set” is two or more such packs with the thick wires somehow connected together, and thence connected to the ESC.

The ‘5000 milliahrs” on the pack label is the total energy contained in the fully charged pack measured in terms of current multiplied by time (eg 5000 milliamps for one hour or 5 milliamps for 1000 hours – both the same amount of energy)

“C” ( that is one C, not thirty five C), is the particular current that if drawn from the pack for a period of one hour would equal the total energy capacity of the pack. So for our 5000 milliamphour pack, one C = 5 amps. IMPORTANT NOTE: If the pack label reads say, 35 C 1800 mahr, then one C = 1.8 amps.

Note that I have not and will not mention the voltage.

The manufacturer’s advice to us, according to the label he has stuck onto the pack, is that the pack he has manufactured for us is safe (and I reinforce the words “manufacturer’s advice” and “safe”) up to, but not exceeding, a discharge rate of 35 C. That is, for the 5000 mahr packs, the maximum safe current is 35 x 5 amp or 175 amps. If the pack was the 1800 mahr pack then 35 C = 35 x 1.8 amps or 63 amps. Take careful note that the safe current depends on BOTH the C number on the label, AND the capacity of the pack, but has nothing to do with the voltage.

Are we Ok so far? I hope so. So a SINGLE 35 C 5000 mahr pack is “safe” up to a discharge rate of 175 amps if we are willing to believe the manufacturer. Think of it as being safe to send up to 175 amps down the thick red wire.

So, if we wire two of those packs in parallel, the ”set” can safely deliver 350 amps to the ESC. If we wire the same two in series the “set” can now only safely deliver 175 amps to the ESC (remember each red wire is safe only to 175 amps). If we wire three such packs in parallel the “set” can safely deliver 525 amps to the ESC. If we wire the same three in series the “set” can still only deliver 175 amps to the ESC. Draw yourself some wire diagrams and think about the current going down each of the thick red wires.

If I put it another way, in parallel the safe currents add but voltage stays the same. In series the voltages add but the safe current stays the same.

What C number you want to assign to the two “sets” I’ll leave to you, but the safe current draws are as I have described and it is the safe current draw we should be thinking about if we want to avoid small fires.

In short, if you want to keep things simple use identical packs.

Chris