So does that "equation" give you the max amps? How can I find out how many amps my setup draws? Can I just use a voltmeter?

What does 1000 in the equation represent?
Thanks

Its a conversion factor - and its tied back to how C is defined -

"How fast a battery can discharge is it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity."

So a 20C rating that would discharge a batt in 3 min (60/20) is another way of looking at discharge. We are generally more interested in the number of amps batt will produce on continuous basis and look at it this way.

So 3700milliamphours/1000 ma/amps * 25C hr = 3.7 *25 = 92.5amps

Wow. Haha thanks for sharing your knowledge, you definatly seem to know quite a bit, even if it is a bit over my head. I hope we can get lots of useful info in this thread.

Thanks Duster

You're welcome guys!

Just passing the help along that I got a few yrs ago when I started running lipo.

So apparently Duratrax doesn't know specs on their own products. This is the email I sent them:

Hello I recently purchased two of your 3700 LiPo packs to use in my Traxxas Emaxx. I am running a Novak HV brushless setup and I was wondering what the maximum burst discharge rate is on these battery packs. Are these packs sufficient for my application? Thank you for your time.

Here is their reply:

Thank you for contacting Product Support.

25C discharge rate.

I do not know what Traxxas recommends.

Please check with Traxxas.

Ummm ok then. So anyway I'm still waiting for my cut-off's to show up at the LHS. I had another question though, what is the "P" rating? I see packs labeled 2C1P or something to that effect. What does that mean?

So I'll be running two LiPo packs with my Novak HV setup which does not have a LiPo cut off built in. So I know I need to get a cut off unit. My question is, how do I hook it up? The one I was looking at on Tower says it needs to be connected between the receiver and ESC, however I will need two. How do I connect them? Will I need a Y harness as well?

This is the one I was looking at:

http://www3.towerhobbies.com/cgi-bin...I=LXPGM8&P=SM#

The P stands for parallel. Used to increase capacity. For example use two 2000mah cells is parallel to get 4000mah capacity. The other term is S, for series, which raises the voltage. 3.7 volts nominal per. The two numbers multiplied are the total number of cells in the pack.

Ok so then a 2C1P pack would basically be a 3C pack?

2 * 1 = 2

So 2S1P = 2 cells
2S2P = 4 cells

I see most stuff as 2S, 3S, 4S, 5S, etc... so I assume most rc car batteries are 1P.

Oh, duh, well shows how crappy my math skills are, lol.

Thanks to Duster and others for providing the lessons, I sure need 'um. I plugged it into a spread sheet so I can easily compare the different batteries.

What I want to know is how to rate the current draw of a BL motor so you can judge what battery capacity you really need. I haven't stumbled on any formulas for that yet.

I see a lot of people concerned with the ability of a ESC to pass massive current, yet it will only be required to pass whatever is demanded by the motor. I see the motor draw as the primary factor in setting up the system.

Also, I'm assuming even though you motor may only draw ~75 amps max, if you have a 125 amp discharge capacity the extra just ends up as more run time.

It's also not clear which factor (mAh or C) has a larger affect on burst discharge (punch). Forgive me if it should be obvious by the math........

Hard to determine the motor amp draw, a lot of variables, vehicle, gearing, track surface, driving style even, yada yada. You are correct, amp draw is mostly motor determined. (if it’s esc limited you will probably lose the ‘magic smoke’ that makes it work)

More advantages of a larger C rating than what it appears is needed: Will maintain voltage under load better, thus more punch/performance. And better battery life, most anything lasts longer if not pushed flat out all the time.

Theoretically both capacity and C factor determine punch. Multiply the 2 together for maximum current. But from what I have seen, within reason, I’m starting to believe the C factor seems more important. A 30C 4000mah battery seems to have more punch and bench test better than a 25C 5000mah, even though the calculated current of the 25/5000 is higher.

Adding cells in the "P" slot is adding capacity, a 2s1P 5000 mah has 5000mah capacity, a 2S2P would effectively have 10000mah - this is the reason its usually done, increase capacity. I use a123 cells which are 2300mah. I built 5S1P pacs and always run a pair, giving me effectively 5S2P battery for 4600mah capacity. I need the extra capacity as 2300 doesn't last too long in my Emaxx.

dont slow down - I'd pm Novak2, on here to make sure you get the smart stop wired right, I don't know how a HV setup works. With the EVX esc, one batt powers the rx, servos and anything else plugged into the rx (fans...). The typical thing is to wire the smart stop to that batt with the reasoning being it is working harder and will be 1st to go to the lvc, so protectinmg it with teh smart stop lvc will protect the other batt as well. With the EVX, you can test to see which batt is which. That's poor on DTX's part, not knowing the burst rating. You know its not lower that 25C, so 30C???

Amp draw is recorded by a few devices such as an Eagle Tree data recorder. I've seen lots of ET graphs showing run time vs voltage and amps. What you notice is there are amp draw spikes that can run a few times larger than average amp draw. I guess these are caused by trigger finger hitting WOT suddenly and the motor asking for many amps. This is where burst rating comes into play, these spikes have to be covered by your lipos burst rating or if you go WOT repeatedly and the burst rating can't handle it, batt will start to overheat. I have an ET ver3, just haven't had time to do much with it, but its going to get used this spring. I want to know what my setups are doing - ET data will also show you how your lipo is doing - voltage drop versus amps.