Parallel 7.2 volts

[mr_lister999.]

I know its possible to connect 2 7.2V packs in parallel. The question is does this double the run time (2X3000 = 6000mah)(or there abouts) or double the force the power comes out of the cells. (sorry cant think of the technical terms [] e.g. if i puts out say 50Amps does 2 put out 100 Amps at the same time and then overheat the speed control? ) I only ask as i have set up my emaxx to a 7.2 volt system and now have a spare battery compartment.

I am running a novak super rooster if that helps.

(7.2 volt with 2 top spec 27 stock motors is just sooooo much faster than 14.4 volt with standard motors. Much more torque too!)

Hope thats not too confusing.

[Stu.B]

This parallel idea sounds risky. Although the voltage would still be 7.2 volts the current would be doubled. It is not high voltage that damages speed controllers but instead high currents. The super rooster is rated at 10 cells max and I would imagine that novak thought long and hard before rating the unit. I guess Novak discovered that it could run with 12 cells at a push but only 10 if you want it to last. If they could have rated it at 12 cells they would have and I think 10 cells is the maximum safe number. Why dont you try two different speed controllers linked independantly to each battery and motor. Thats what I use in my truck.

[TWO STAR]

yes if you put the cells in parallel the cells capacity will double

if you put them in series the capacity stays the same & the voltage

doubles.i would not recommend running 12 cells on the super rooster.

stu is correct the max cells rated for this spd controll is 10.

[mr_lister999.]

yes i knew about series and series and parallel voltages, it was just about the current issue really. I used the super rooster as thats what came with the second hand maxx i bought. This control looked like it had come straight out of the box. Not a mark on it. The last owner fried the EVX and bought the rooster as a replacemnt but didnt realise the 10 cell limit.

I ran it on 10 cell with the standard motors... not great. Bought 2 fantastic stock 27's and it flies on the 1 battery pack. I set out especially to get high torque stocks and wired them in parallel. I was really surprised that after a full 3000 pack they were barely warm. I was expecting them to be too hot to touch. Just shows you.

With that controls limit i am worried that connecting 2 packs in parallel will put out more amps than the control can handle. And at £125 (the cheapest i have seen) i dont want to break it. ) Guess i will just have to pull over and swap connectors to the other pack that i will install.

[terry.sc]

There seems to be a lack of understanding here in the replies. The equation here is

Voltage=Current X Resistance

With a single pack the voltage is 7.2v. With 2 packs wired in parallel the voltage is still only 7.2v. The resistance of the motors stays the same, so the current is about the same (There is a slight drop in overall resistance because of the parallel batteries). With parallel connected 3000mah packs you end up with the equivalent of a 6 cell 6000mah pack. The speed control has a maximum rating of 10 cells, that's 12 volts. Too much voltage will instantly destroy your speedo. Do not run your nicads in series!! Current is not really a problem with a high spec ESC, your Super Rooster can handle 400-500 Amps easily.

To show this, the minimum number of cells are 4 to generate enough voltage to power the electronics. If you connect 4 sets of 3 cells in parallel (12 cells, 3.6v) there is not enough voltage to run the ESC.

The current drawn is governed solely by the motor or motors in the car. Don't try this at home, but if a fully charged battery pack is shorted out the low resistance means it can put out hundreds of amps, which can weld whatever is shorting it out onto the end of the pack. I still have the screwdriver to prove it. When the power goes through a motor the resistance of the motor governs the amount of current drawn by it.

What this means is that if you have 2 motors wired in parallel you will then have twice the current draw. Remember, most ESCs are designed to run with only one motor. I would be more worried about the 2 motor setup myself. As the motors are not getting warm they are having an easy time in the car, so as long as you do not replace them with modifieds I don't think you are overloading the ESC. If it runs fine with one battery pack it will run the same with 2 packs, but it will last twice as long. (But if it all goes wrong don't quote me on it[])

I would say go ahead and do it, just make sure that if the motors or ESC get warm after a run let them cool down before going again - a sound piece of advice for any car.

BTW the EVX speed controller was not up to the job, and all new E-maxxs have a purpose made Novak speed controller in the box. If you do want to replace the Rooster I would look for one of those instead.

[TWO STAR]

the manual states that you can oly use 6-10 cells only.the rated foreward current is 320 amps.the spd controll with twin motors in series will handle down to a 8 turn motor.the transistors in the

super rooster are part #IRL3803ND.the rating on the transistors are

30 volts @120 amps.due to the battery eliminator curcuitry,6 volt output you cannot go above the 10 cell rating.

[DJTheo]

quote:the manual states that you can oly use 6-10 cells only.the rated foreward current is 320 amps.the spd controll with twin motors in series will handle down to a 8 turn motor.the transistors in the

super rooster are part #IRL3803ND.the rating on the transistors are

30 volts @120 amps.due to the battery eliminator curcuitry,6 volt output you cannot go above the 10 cell rating

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Note that all above cell statements assume that you use cells serially (adding their voltage), if you use them in parallel no problem, as terry.sc said, as it is as if you use a battery with a bigger capacity, wouldn't give more power (except a small difference due to less internal resistance of both packs in parallel) but double runtime.

[bholio]

I believe there is a risk when running 2 parallel packs if one of the packs dies before the other. In this case, the other pack may try to charge the dead one with bad results. Make sure both packs are charged fully, and don't run the car until it's totally dead. Anyway, that's what I read somewhere. I'm no EE, so I can't comment on voltage/current. Besides, terry.sc did that already.

If you want to run 2 motors in parallel, the super rooster will do it, and has a motor limit specified somewhere for 2 motors in parallel. It's not unlimited like the single motor rating. You'll have to look it up.

[mr_lister999.]

quote:I would be more worried about the 2 motor setup myself. As the motors are not getting warm they are having an easy time in the car, so as long as you do not replace them with modifieds I don't think you are overloading the ESC.

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Dont worry about that. The control is designed for parallel motors. It tells you how to set them up. * turn limit means 2 16 turn motors max. I ran 2 packs of 3000's one after the other about an hour ago and the motors were still almost room temperature. (based on a 3 tooth smaller spure than standard with standard pinions.)

Thanks for the detailed descriptions guys, that helps alot. Just wanted to make sure as i didnt want to wreck anything.

Also dont worry about different states of charge in the packs. Both are the same age, run the same number of times, charged on my Novak millenium charger and discharged to the same voltage on my cs electronic discharger. (I know you dont need to discharge those batteries but i like to know how long i had left in them [] )

Thanks again.

[miramar]

quote:Originally posted by bholio

I believe there is a risk when running 2 parallel packs if one of the packs dies before the other. In this case, the other pack may try to charge the dead one with bad results. Make sure both packs are charged fully, and don't run the car until it's totally dead. Anyway, that's what I read somewhere. I'm no EE, so I can't comment on voltage/current. Besides, terry.sc did that already.

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I'm an EE, and also I calculated and sold industrial Ni-Cd and Ni-MH battery systems for a couple of years, so I have a little experience on parallel systems.

The "danger" of parallel systems and reversing cells is very exaggerated and very often misunderstood by RC-modellers.

Always, when cells are connected in SERIES, tolerances will lead to a certain imbalance in a battery (pack), so some cells will be discharged before others, and will be "charged" with wrong polarity as the resulting cells discharge. On very old and heavily used batteries, this is sometimes a problem, causing cells to vent (who cares, the batteries are old and sluggish anyway! []), but with newer packs which are well maintained and charged with a proper charger, this is rarely a problem.

(For open type Ni-Cd batteries, reversed cells is not a problem under any circumstances, and they are brought back to life without damage with a proper charge/discharge/charge cycle.)

When connecting batteries in parallel, imbalance is not a problem at all. Of course you will have to use batteries of the same voltage and quite similar quality and capacity and they would have to be equally charged (not one "half full" and the other "full"). As the batteries are discharged, voltage will gradually drop, and the battery which runs flat first will just have its voltage level stabilized by the other battery. Remember that sealed Ni-Cd batteries need a charge voltage of about about 1.6-1.7V per cell to get charged, so the weakest battery won't be charged by the other, and even if it did (it doesn't, believe me!), it would just mean that run time would be shortened as the best battery would spend some of its energy charging the other.

You can't quite expect double run times with two batteries connected in parallel, as the added weight and slighty higher currents during full acceleration and full load will take its toll.

Others have pointed it out already, but it doesn't hurt to repeat that two batteries in parallel is like one battery with the double capacity, and just the lower (normally half) internal resistance of two batteries in parallel compared to one single battery will lead to slightly higher discharge currents. The rest of the circuit (motor(s), ESC) stays the same, so Ohm's law applies. Only short circuits and heavy loads on the motor(s) (acceleration, steep inclination, swampy surface) will lead to considerably higher currents with two batteries, and shorter run times (than the combined run times of two batteries run separately).

Simple rather non-technical conclusion: in light fast models with hot motors and/or gear ratio, running two batteries in parallel will have more cons than pros. In heavy "slowish" models with moderate motors and/or gear ratios, running two batteries in parallel will have the pro of almost the double run time and slightly improved acceleration and low rpm torque, but the con of not being able to charge one battery as you run the other. If you have enough batteries and chargers, you wouldn't mind though! []

Parallel systems are SAFE.

[mr_lister999.]

quote:You can't quite expect double run times with two batteries connected in parallel, as the added weight and slighty higher currents during full acceleration and full load will take its toll.

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No worries there, its an EMAXX so these things weigh a ton to start start with. One more pack is only a small percentage.

[WillyChang]

quote:Originally posted by bholio

I believe there is a risk when running 2 parallel packs if one of the packs dies before the other. In this case, the other pack may try to charge the dead one with bad results.

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No, this never happens. You can try it yourself... take 1 fully charged pack and connect it to a flat pack of the same # of cells. Even if you wait till kingdom comes your flat pack will not become charged in any way.

The problem here is there's no potential difference (=voltage), thus no current will flow.

[WillyChang]

quote:Originally posted by mr_lister999.

... connect 2 7.2V packs in parallel. The question is does this double the run time or double the force the power comes out of the cells. e.g. if i puts out say 50Amps does 2 put out 100 Amps at the same time and then overheat the speed control?

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Double packs in parallel will give you 7.2V but twice the "force".

If 1 pack can pump 50A into a dead short then 2 packs will be able to pump 100A into the same deadshort. Note I say dead short, ie with Nil resistance in the circuit. The mega current flowing will be controlled only by the internal resistance of the battery/s thus if you halve the resistance (2 packs in parallel), you'll double the current.

I have a starterbox for nitro cars made for 2x 7.2V packs.

After some fiddling... I've wired up 'special' packs. I use slabs of 6 cells in series (=7.2V), and I use 3 slabs per pack (=3x capacity). Made up 2x of these packs to be connected in series (=14.4V)... and they run the twin 550 motors just fine.

Yes... 6 x 3 x 2 = 36 cells...! []

Did I mention they're AA cells?? []