Reason for same mah per battery in a 5s setup?
I'm thinking about running a 5s setup in my stock summit (I've read it's a little risky, but I'm a fairly conservative driver) and I've read a lot of "you need to use the same mah and same discharge rating for each battery".
Well, that doesn't make a ton of sense to me. If I have a 2s 5000mah 50c lipo paired with a 3s 5000 mah 50c lipo, isn't the 3s going to run down faster than the 2s considering it's delivering more voltage for every given amp? Wouldn't I want a 7500 mah 3s lipo on the other side (Basically each cell installed in the car is 2500 mah)?
Now, given that the 2s lipo is going to be running the BEC side, I guess I could probably get away with 6200 ~ 7000 mah on the 3s battery and have the two batteries hit 3.3v at roughly the same time since the BEC side always drains a bit faster.
Anyway, can somebody tell me why my logic is either correct or flawed?
And if it makes any difference, while I'm relatively new to the hobby, I do have a bit of an engineering background (ME, but we had to take ECE classes of course). I researched my first RC for about a year before I finally bought one.
Both a 5000mAh 3s and a 5000mAh 2s will run for the same amount of time. This means you don't need a 7500mAh 3s. If this was true, people would have destroyed their 3s LiPos becaus of over discharging. So, use LiPos with the SAME mAh.
Last edited by snowslider; 02-12-2012 at 10:27 AM.
Reason: i no god att spelyng
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Are you sure they run at the same amount of time? Seems to me both batteries contain the same amount of total energy. . . 5000 mah, but the 3s battery will be delivering its power at 11.1 volts, whereas the 2s battery will be delivering its power at 7.4 volts. If you have the same capacity, but one pack is delivering a higher voltage, it should run down faster correct?
Let's say I was running a MMM system and I had 2x 2s 5000 mah batteries, and 2x 3s 5000 mah batteries. Of course the 6s system is going to run faster than the 4s system. The reason being is it's delivering more power (watts). Well, if you draw more power on packs of the same total capacity (10,000 mah in this case) the higher voltage packs will give shorter run time. This is assuming you run each setup to the capabilities of the batteries. If you putt around on a 4s system and a 6s system at equal speeds, your run time should be similar because the same total amount of work is being performed by the truck. Then it just comes down to the efficiency of the motor and electronics to determine if they prefer to produce the same wattage at higher or lower voltages.
So in a 5s setup, if the 2s pack and 3s pack are each delivering 7.4v and 11.1v respectively, the 3s pack should run down faster than the 2s pack, thus requiring a higher capacity 3s pack to match the run time of the 2s pack.
You need the same mah. Yes the 3s battery puts out power (read watts) but amps (thats based on mah) are used the same in both batteries.
Now if you measure watts, you are correct. The 2s would need to be 50% larger to deliver the same power. However the mmm is wired in series, so it uses the same amps (mah) from both batteries, not watts.
I just figured it out by looking at the dimensions of the turnirgy batteries. a 2s 5000 mah battery is a volume of 135, 632 mm^3.
A 3s 5000 mah battery has a volume of 243,375, so it's a larger battery.
I was under the impression the way the amp hours were measured for batteries was regardless of voltage, but this is not the case because the 3s battery is larger to deliver the same mah.
Since I haven't put my hands on any 3s batteries, I had not yet conceptualized the larger size.
Last edited by CornUponCob; 02-12-2012 at 11:38 AM.
I don't see a delete double post option.