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what kind of batteries?

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yoyo_yk

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hello everybody...I'm new to robotics

Plz if any one knows, what kind of batteries is more suitable for a robot?
NiMH or NiCd??

the robot uses 3 DC motors (may the stall current increase above 0.5A?if yeah plz give me the worst condition)

the overall robot weights about 13 Kg..


thnx in advance :)
 
NiMH- non-toxic and charges faster with higher capacities (but higher internal resistance than NiCds, but usually an extra cell is added on to compensate).
The battery you use is often limited by how much money you have to get the battery (and the charger). Don't even worry about battery brand- changes are you have zero choice.

The current the motor will use in normal operation is dictated by voltage, wheel size, weight, maximum incline encountered, motor torque, motor speed, and gear reduction.

But to get worst case, you must find the stall current of the motor which has nothing to do with the robot. Take your motor and clamp the shaft. Momentarily apply power to it and measure the current. That is your worst case current for that motor. THis will not tell you if the motor is strong enough to drive your robot though- just the worst case current the motor will draw.
 
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NiMH's contain more energy per volume/mass, but at a lower current sourcing ability. Basically, you're likely to want to use two power sources. NiMH or Lithium for the control electronics and NiCad's for the motor electronics. Manufacturer is really irrelevant.
Basically you can use the general rule that
Given equal capacity (1000mah's say)
A NiCD will be able to source 10 times that much current (10 amps)
A NiMH will be able to source 5 times that much current (5 amps)
A LiPO will be able to source 2 times that much current (2 amps)

These are general guidelines and typical ratios for a given chemical cell structure of a standard chemistry. Recent innovations in NiMH and LiPO technologies increase the NiMH power storing capacity and LiPO's current sourcing ability with various caveats and requirements over 'standard' cells.
Namely, size weight and power dissipating limiting elements which prevent cells from bursting into flames.
 
thnx alot guys :)

but when searching for a battery on the web I found the 7.2 SANYO 3700 mAh battery ....the site tells it can give up to 50 Amps!!


so...what about these (typical) ratios? :confused:


thnx alot again:)
 
dknguyen said:
But to get worst case, you must find the stall current of the motor which has nothing to do with the robot. Take your motor and clamp the shaft. Momentarily apply power to it and measure the current. That is your worst case current for that motor. THis will not tell you if the motor is strong enough to drive your robot though- just the worst case current the motor will draw.


I got it ..thnx alot
 
3700mAh means that the battery if 3700mA is drawn from the battery at a constant rate, it will last for one hour (this number can actually be much worse because manufacturers might draw the current out of the battery at a rate so the battery actually lasts 10 hours and then they scale the number mathematically down to 1 hour). Drawing the current from the battery so slowly makes the the internal resistance of the battery negligible, and when you are drawing current out of the battery fast it plays a large role. THis makes the battery life exponentially worse than the given rating as you pull more and more current out of the battery.

The 50A means that the battery can provide 50A maximum *momentarily*. Any longer and it would overheat or explode.
 
yoyo_yk said:
the robot uses 3 DC motors (may the stall current increase above 0.5A?if yeah plz give me the worst condition)

the overall robot weights about 13 Kg..

You need more power, if the motors have a stall current of only 0.5A they are far to wimpy to move a 13kg robot at any speed.
 
NASA seems to be quite happy with their Mars ROvers that move at 2cm/s though lol so it can be done! Probably not what you want though. I guess they used really efficient gearing so that all the torque wouldn't be eaten up through the transmission.
 
Sanyo 3700mah what batteries? NiCad or Nimh? Those ratio's weren't meant for peak current, they're just good general guidelines for rateing a batteries capacity to it's current handling ability, go past those and you're over stressing your packs, which will reduce the MAh rateing and number of charges you'll get out of it. The lower under that general ratio you go the longer the packs will last, both per use, and number of charges.
 
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