# What battery to use?

Hi im a newbie when it comes to electronics, I want to design a robot similar to this:

http://www.me.berkeley.edu/ME102B/Past_Proj/f09/1%20BearClaw%20Tennis%20Ball%20Collector/design.php

The thing is, the link doesn't specify what power supply was used. The above robot uses two of this DC motor:

https://www.pololu.com/product/1103

If i am not mistaken, this motor operates at 12V and 5A.
So 2 motors will require 10A at 12V?
So to operate for one hour would I need at least a 12v 10Ah battery?

If this question deserves to be closed please allow it to be answered first.

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i have to power and Aruidno UNO R3 board as well, will powering it directly with a 12V 5Ah SLA battery work?

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i have just discovered that the Bear Claw robot (linked to above) uses only 6 AA batteries (from the looks of it in the Media section of the site). How can that be sufficient to run two of those motors in the link above? Doesn't high torque mean higher current requirements and so a NiMH battery's capacity won't be enough?

• It may be a good idea to have a separate power supply for the Arduino even if it would work with the 12V, as this avoids stability and noise problems. Tie the grounds together though. – pjc50 Feb 6 '15 at 19:24

The motors draw 5A at 12V "at stall" - ie power applied and motor mechanically locked) so they will draw no more than 5A during normal operation.

So, if you work on a 5A continuous draw, then in reality you should require less or even much less. Discharging a battery to 100% capacity is in most cases not good for its cycle lifetime, so designing for less or much less than 100% discharge is usually wise. For first calculations the safety factor built into the "continuous 5A" assumption above gives you some safety margin.

As you note 5A x 1 hour = 5Ah, and 2 motors = 10 AH.
I 12V x 10 Ah battery would allow 1 hour of operation at stall level currents for 1 hour at 100% discharge.

For 12V you could use a 12V SLA (sealed Lead acid) battery, or LiIon or LiFePO4 or NimH or. Initially best cost-performance will probably be obtained using an SLA battery. These will not have the lifetime or peak current capability of some alternatives but are a (relatively) low cost starting point.

So - as a starting point a lead acid 12V, 7Ah "brick" battery as used in many alarm systems and similar, would be a usable starting point. Using eg LiIon or LiFePO4 would give better results but initial cost is much higher.

Once you have 'played with a 12V 7Ah SLA battery you will have a better feel for what works well for you.

Note that though that webpage is mainly about the 30:1 gear ratio motor. Higher gearing = lower top speed but more torque so better operation in rough conditions and when the battery is getting low (although you should never run the battery so low that it matters). The 5):1 version MAY be a better choice - but at the expense of top speed.

THESE may be their wheels. If so they are 90mm in diameter. On a 50:1 motor you'd get about 180 RPM "wide open" or a speed of about
180 rpm / 60s x 90 mm x 3.14 = 850mm/second ~= 3 km/h.
The 30:1 motor would give about 5 km/h as long as the overall load was not excessive.
At these speeds air drag is minimal and friction and any slope are the main power consumers.

• sir if you could enlighten me what "mechanically locked" means. btw great info – Mywiki Witwiki Feb 6 '15 at 12:58
• last q (sorry for being dumb), i have to power an arduino uno r3 board as well, can the board be powered directly using your suggested 12v 7Ah SLA battery? – Mywiki Witwiki Feb 6 '15 at 13:02
• @MywikiWitwiki Mechanically locked = wheels prevented from turning - robot may be against wall OR wheel may be prevented from rotation for testing. – Russell McMahon Feb 6 '15 at 13:30
• The Arduino Vin pin will notionally accept 12V input directly BUT I would not trust a processor with direct 12V SLA battery feed - and motor noise may on the processor power supply may be an issue. I'd use a regulator between 12V and Vin so Vin "sees" only slightly more than the regulator needs (maybe 6V with the LDO regulators used) and I'd add filtering to remove motor noise - maybe just large and small capacitors at battery and intermediate regulator input and output. – Russell McMahon Feb 6 '15 at 13:37
• @MywikiWitwiki The Arduino R3 current is much less than the motor current and will not have a major effect on battery life if the robot moves often. – Russell McMahon Feb 6 '15 at 13:38

Well, 10Ah Battery is more than enough. You may use lesser beacause 5A is the stall current of the motor. Actual current is less. How less? That depends on the load. If your load is very high, then its better to keep the stall current in mind and calculate the battery capacity. And, always keep some margin because you never know if a battery is charged full or not.