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Old 08-19-2007, 03:57 PM   #1
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Default 7 cell packs??

alright i know this is a marginal question with lipo's being out, but, is anyone running a 7 cell nimh pack. any pros or cons? just courious.
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Old 08-19-2007, 05:01 PM   #2
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The cons would be.. having to buy an extra cell, not allowed in stock or spec classes, extra heat buildup in esc and motor. You won't gain any runtime for it if that's what you're after. You will gain power and speed due to more voltage, but you could just buy a faster motor and use your current 6 cell packs if that's what you have.
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Old 08-19-2007, 05:55 PM   #3
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Default 7-cell packs

Actually you can get more runtime out of 7-cell packs than 6-cell packs simply because there are more watt-hours (a measure of total energy in a battery pack) in 7 cells than 6 cells. Adding a 7th cell adds 16% more watt-hours but rarely adds 16% to total vehicle weight. You do need to change gearing and other motor parameters to take advantage of the added power.

I run some 7-cell 3300 packs in a rustler with a 19T handwound motor. The gearing has been adjusted to account for the higher voltage and timing set very low (5-10 degrees) to cut back the RPM and increase efficiency. I get 10-13 minutes runtime at a very nice street bashing pace.
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Old 08-19-2007, 06:13 PM   #4
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iam running a lrp shpere with a vector x 5.5 w/sintered rotor. on my IB4200 6 cell packs its nice and comfortable, iam getting like 15-20 min runtime on my track at my house. depending on how hard iam running. i know that speed isnt everything as we have talked on here about making 5cell packs to cut pwr and add driveablity, but i disagree with that saying if you cant take the pwr, gearing is your next best thing. or just cut down your throttle EPA if you can. but i would like to see a little more power so adding another 1.2 volts would put my motor rpm at 68,040. which at a 13.9 final drive(on a T3 with a 15/87) it would put top speed at around 62.5mph, where on a 6cell its doin 53.6mph. so gaining 9mph by gaining 1cell, i think its worth it. iam also running a fan on my motor and esc. so heat isnt an issue.

if anyone wants the formula to get mph ill pm it to ya just ask.
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Old 08-19-2007, 07:28 PM   #5
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[QUOTE=rc10guy;3595295]alright i know this is a marginal question with lipo's being out, but, is anyone running a 7 cell nimh pack. any pros or cons? just courious.[/QU
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Old 08-19-2007, 07:39 PM   #6
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skeery, what are you trying to say?
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Old 08-19-2007, 08:29 PM   #7
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you will get more power but no extra run time. (not significant anyway)

instead of dumping at 5.4volts, you will dump at 6volts.
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Old 08-20-2007, 02:41 AM   #8
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I don't think that's how it works.

Adding cells to a pack wired in series will add voltage (power) but not capacity (run-time). A 7 cell pack will have more voltage (8.4 vs. 7.2) but may actually have less run-time, because of extra resistance from an extra solder joint and the internal resistance of the cell.

I'm not positive of the physics, but I believe that because the voltage of the pack will be higher, it may actually cause the pack to discharge faster.

To add cells in order to add run-time, you will have to wire the cells in parallel, which will not add voltage (again, power).
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Old 08-20-2007, 03:16 AM   #9
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you will not get less run time with more cells, it will be pretty much the same. the resistance from an extra joint really would not make a difference.
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Old 08-20-2007, 03:41 AM   #10
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To the guys saying you get less run time with higher voltage, go look at the forum on the 1/8th buggy conversions. They use at least 4S LiPo packs (14.8V) because there is more total power in the higher voltage packs than in the same capacity lower voltage packs. These conversions use motors that take advantage of the higher voltage to gain efficiency and runtime.

More voltage because of more cells equals more total power out of the pack. If you properly adjust your motor setup to use that power, you will get more runtime. If you leave the gearing and timing the same, you could (probably will) go faster and consume the power faster. But adjusting motor parameters for the same speed, you will get longer runtime because you will be using less amps out of the pack to get the same power.

Ask some of the 1/8 electric conversion guys. They know more about this than anyone else - because they need to.
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Old 08-20-2007, 06:16 AM   #11
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It depends what motor you use. back in the eighties and early ninties the battery capacity was lower. So to go fast and make runtime you ran a mild modified motor(16,17, 18 turns) with 7 cells. The higher voltage made the higher wind motor faster, the higher wind motor gave you more runtime with the low capacity batteries.

Higher voltage lets you use a more efficient motor and still go fast.

Its all relative to race length and battery capacity really. Since most electric races are currently only five minutes long. The current 4200 capacity has more than enough capacity to run a very fast motor (7 turn in touring car for instance) to go fast with 6 cells.

If you make electric racing longer, like 20 or 30 minutes, and you would like to run the same 4200 capacity battery, you would be better off with a higher voltage battery so you can use a more mild motor to make runtime and still go the same fast speed.

If the only thing you changed was an increase of voltage and nothing else, you would get less runtime. Running a 10 turn motor at 8.4 volts will get less runtime then running at 7.2 volts using the same 4200 capacity battery, assuming you could use the extra power and go faster.
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Old 08-20-2007, 05:08 PM   #12
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well who is up to trying this to acually see what happends, all the theorys are right. its going to come down to doing it. i dono what else to say.
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