need help in selecting battery

[mechaboy]

hi we r going to Nasa's 4th annual lunabotics mining competition.
we have to build a mobile robot. we selected 8 motors for it
motor name is Rs 775 from www.banebots.com.
http://banebots.com/p/M7-RS775-18

Motor Specifications


Performance

Model   M5-RS775-18

Operating v   : 6v - 20v
Nominal v   : 18v
No Load RPM   : 19500
No Load A   : 2.7A
Stall Torque   : 166.4 oz-in   1175 mN-m
Stall Current   : 130A
Kt   : 1.28 oz-in/A   9 mN-m/A
Kv   : 1083 rpm/V
Efficiency   : 78%
RPM - Peak Eff   : 17040
Torque - Peak Eff   : 23.99 oz-in   169.4 mN-m
Current - Peak Eff   : 18.7A
   
Physical
Weight   : 11.9 oz   (337g)
Length - for motor   : 2.81 in   (71.3mm)
Diameter (with flux ring)   : 1.85 in   (47mm)
Diameter (no flux ring)    : 1.66 in   (42.1mm)
ShaftDiameter   : 0.2 in   (5mm)
Shaft Length   : 0.3 in   (7.6mm)
Mounting Screws (2)      M4

for 8 of this motor i need batteries. i want to use li poly battery. the battery should run at least 15 minutes.
please tell me which battery should i use? also give the link
weight should be as light as possible

mechaboy
[email protected]

[Soeren]

Hi.

for 8 of this motor i need batteries. i want to use li poly battery. the battery should run at least 15 minutes.
please tell me which battery should i use? also give the link
weight should be as light as possible

Impossible!
You cannot find a battery without knowing the average load current and you cannot find that without knowing all the details of the entire 'bot and it's working environment. So mount them and test the thing to find the average current while it does what it's supposed to do.

You didn't mention anything about gearing and you cannot drive the wheels directly from these motors, if that's what you were going for.

[jwatte]

We would have to guess what you're trying to do (how much load it is.)
Free running, these motors draw 3 A; stalled, they draw > 40x as much.
Let's say that you will be running them at the half-way point, which works out to about 65 A.
Similarly, you need a battery that doesn't over-volt the motor, so fully charged, it shouldn't be over 20 V (unless you use an inverter, but you probably aren't.)
Fully charged, a LiPo is 4.2V per cell, so a 5S LiPo fully charged would be 21V. However, when drawing actual current, the LiPos sag in voltage, so I think they'll actually be safe. Fully discharged (assuming you do safety cut-off,) a 5S would be 16V.

So, 5S LiPo, capable of giving 8x65A, for 15 minutes. That's a total of 16500 mAh per motor. You also need high-discharge-rate batteries, because you're going to be loading at > 2 times the mAh capacity of the battery.

Thus, I would recommend something like three of these in parallel, per motor (so, 24 total):
http://www.hobbypartz.com/98p-25c-5500-5s1p.html
Or maybe two of these in parallel per motor (so, 16 total):
http://www.hobbyking.com/hobbyking/store/__18209__Turnigy_nano_tech_8000mAh_5S_25_50C_Lipo_Pack.html

If you don't know what your load is actually going to be, you could start by buying one per motor, and see how far that takes you, and then multiply out by however much runtime you need.

Some more comments:
- You need one battery circuit per motor, because the maximum stall draw of the motor would otherwise destroy the batteries (akin to shorting them out.)  The "C" rating of the battery times the capacity of all the batteries in parallel for a single motor need to be at least as much as the maximum (stall) current of the motor.
- You need some kind of battery management. When a LiPo goes below 3.2V, you will ruin the battery, and if you load it so it gets too hot, it may even burst into flames and possibly explode! Heat sensors, voltage sensors, and fail-safe cut-offs are important. Either a large number of very low-Rdson MOSFETs, or a relay with very high-amperage rated contacts, can do the job.
- You also need to think about motor control. What controllers are you using that won't fry out at > 130A current draw?

[mechaboy]

Hi.

for 8 of this motor i need batteries. i want to use li poly battery. the battery should run at least 15 minutes.
please tell me which battery should i use? also give the link
weight should be as light as possible

Impossible!
You cannot find a battery without knowing the average load current and you cannot find that without knowing all the details of the entire 'bot and it's working environment. So mount them and test the thing to find the average current while it does what it's supposed to do.

You didn't mention anything about gearing and you cannot drive the wheels directly from these motors, if that's what you were going for.

We are gonna use 1 of this gearbox
http://banebots.com/pc/P60K-S7/P60K-44-0007


and 7 of this gearbox
http://banebots.com/pc/P60K-S7/P60K-444-0007

[mechaboy]

We would have to guess what you're trying to do (how much load it is.)
Free running, these motors draw 3 A; stalled, they draw > 40x as much.
Let's say that you will be running them at the half-way point, which works out to about 65 A.
Similarly, you need a battery that doesn't over-volt the motor, so fully charged, it shouldn't be over 20 V (unless you use an inverter, but you probably aren't.)
Fully charged, a LiPo is 4.2V per cell, so a 5S LiPo fully charged would be 21V. However, when drawing actual current, the LiPos sag in voltage, so I think they'll actually be safe. Fully discharged (assuming you do safety cut-off,) a 5S would be 16V.

So, 5S LiPo, capable of giving 8x65A, for 15 minutes. That's a total of 16500 mAh per motor. You also need high-discharge-rate batteries, because you're going to be loading at > 2 times the mAh capacity of the battery.

Thus, I would recommend something like three of these in parallel, per motor (so, 24 total):
http://www.hobbypartz.com/98p-25c-5500-5s1p.html
Or maybe two of these in parallel per motor (so, 16 total):
http://www.hobbyking.com/hobbyking/store/__18209__Turnigy_nano_tech_8000mAh_5S_25_50C_Lipo_Pack.html

If you don't know what your load is actually going to be, you could start by buying one per motor, and see how far that takes you, and then multiply out by however much runtime you need.

Some more comments:
- You need one battery circuit per motor, because the maximum stall draw of the motor would otherwise destroy the batteries (akin to shorting them out.)  The "C" rating of the battery times the capacity of all the batteries in parallel for a single motor need to be at least as much as the maximum (stall) current of the motor.
- You need some kind of battery management. When a LiPo goes below 3.2V, you will ruin the battery, and if you load it so it gets too hot, it may even burst into flames and possibly explode! Heat sensors, voltage sensors, and fail-safe cut-offs are important. Either a large number of very low-Rdson MOSFETs, or a relay with very high-amperage rated contacts, can do the job.
- You also need to think about motor control. What controllers are you using that won't fry out at > 130A current draw?

Hi thanks for your valuable comment.
I am gonna use 1 of this gearbox
http://banebots.com/pc/P60K-S7/P60K-44-0007

and 7 of this gearbox
http://banebots.com/pc/P60K-S7/P60K-444-0007


my motor controller can give 60A continuous current and 120A peak per channel.