How do I select a battery pack for an application?
Migs
Posts: 95
Friends:
How do I select a battery pack for an application? What are the criteria and margins of error I should observe in order for my little robot not to reset the microprocessor accidentally? What thought process do you all (In the know) employ to select what to use? Do you attempt to make the battery voltage close to the microprocessor required voltage, or do you shoot for something higher? Why? How much? Should I include capacitors in the circuit to protect for this? Should I use separate power packs for the microprocessor circuit vs. the drive? What is common practice? Where is there more information on this subject? What brands do you all recommend? What type of battery is better in the long run? Are there any things to watch out for? (Common errors)
Thanks amigos!-Migs
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"Sometimes we forget that the world itself is paradise." Douglas Coupland, 'Microserfs'
"If the doors of perception were cleansed every thing would appear to man as it is, infinite" - William Blake
"We either make ourselves miserable, or we make ourselves strong. The amount of work is the same." Carlos Castaneda
"One single grateful thought raised to heaven is the most perfect prayer. " G. E. Lessing
“How much of human life is lost in waiting.” Ralph Waldo Emerson
"Men often mistake notoriety for fame, and would rather be remembered for their vices and follies than not be noticed at all.” Harry Truman
My website: www.intoku.net my e-mail:mreznicek@pretensa.com me:Miguel Reznicek
How do I select a battery pack for an application? What are the criteria and margins of error I should observe in order for my little robot not to reset the microprocessor accidentally? What thought process do you all (In the know) employ to select what to use? Do you attempt to make the battery voltage close to the microprocessor required voltage, or do you shoot for something higher? Why? How much? Should I include capacitors in the circuit to protect for this? Should I use separate power packs for the microprocessor circuit vs. the drive? What is common practice? Where is there more information on this subject? What brands do you all recommend? What type of battery is better in the long run? Are there any things to watch out for? (Common errors)
Thanks amigos!-Migs
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"Sometimes we forget that the world itself is paradise." Douglas Coupland, 'Microserfs'
"If the doors of perception were cleansed every thing would appear to man as it is, infinite" - William Blake
"We either make ourselves miserable, or we make ourselves strong. The amount of work is the same." Carlos Castaneda
"One single grateful thought raised to heaven is the most perfect prayer. " G. E. Lessing
“How much of human life is lost in waiting.” Ralph Waldo Emerson
"Men often mistake notoriety for fame, and would rather be remembered for their vices and follies than not be noticed at all.” Harry Truman
My website: www.intoku.net my e-mail:mreznicek@pretensa.com me:Miguel Reznicek
Comments
You select a battery pack based on the obvious: How much current do you need? How long do you want your device to work before recharging? How much weight and volume do you want to haul around?
Ideally, you want to waste as little power as possible in the process of voltage regulation. Servos work best over a voltage range of 4.8-7.2V, but less than 7.2V is better ... the brushes arc a little at the higher voltages and that shortens their life a little. Servos draw an ampere or more at peak, usually if there's a lot of mechanical load, but their average current is lower, perhaps 1/4 to 1/3 A while operating. Idle current is much lower ... on the order of 10ma.
You need 0.5 to 1V minimum "headroom" on a low-dropout voltage regulator to get proper regulation. For a 5V regulator, that's a supply voltage of at least 5.5V (which is marginal at best). Plan for at least 6V normally. If you're using a 7805 (older/cheaper) regulator, you have to allow at least 2.5V headroom. If you're using stepper or DC motors, you may need a 12V supply. Use a switching regulator to drop the 12V to the 5V or 3.3V you'll need for the logic ... it's much more efficient, has less heat dissipation issues, and may help isolate the logic from the electrical noise of the motors. Do plan your power supply around the motors. Most logic circuitry these days draws very little current compared to the motors.
Others may have more advice and experience.
I have a Propeller-based BoeBot with some additional sensors, a PING, and a total of 3 servos. I use 5 NiMH AA batteries with a 2300 maH rating in series for a 6V supply voltage (5 x 1.2V). The servos run directly off the batteries and the Propeller Protoboard's 5V low-dropout regulator runs off the battery supply. I've not tried running the batteries down, but I suspect this setup will go for 5-8 hours minimum.
NiMH batteries are the easiest to use. Lithium batteries require special chargers (or charging ICs) and, if abused, have been known to burst into flame. Lead-acid gel cells or sealed lead-acid batteries are also easy to use, particularly for 12V motors. Again, consider using switching regulators for the logic if you're running off 12V.
Post Edited (Mike Green) : 7/24/2007 2:31:19 PM GMT
It's one of those things that depends on many factors, like how much weight in batteries you can carry, how long you want the 'bot to run between recharges, what you have and what you can afford.
On the 'bot I'm working on currently, I need 5v and 3V. I decided to use 6XAA batteries in series. It is a small bot, and this battery pack gives me a few hours of operation between charges. I bought Monster brand, 2200mAh. I also bought a smart charger: http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3005 . I have been quite happy with it so far.
For my 5V regulation, I use a low drop out type reg, the LM2940. It needs at least a volt higher input voltage, so a min of 6V to work properly. Better is 6.5V. So I decided to go with the 7.6V pack to have a decent amount of overhead. You do not need to us speperate battery packs for the drive and processor. In my 'bot, the proccesor is a Propeller, so I also have a 3.3V regualtor, but they run from the same battery pack.
In terms of the power supply, there are a lot of little things that are important. Proper sized caps, proper sized wires and traces, back EMF diodes for motors and servos and so on. To learn about these things, look at peoples projects and the data sheets.
Jonathan
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
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D Faust
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"Sometimes we forget that the world itself is paradise." Douglas Coupland, 'Microserfs'
"If the doors of perception were cleansed every thing would appear to man as it is, infinite" - William Blake
"We either make ourselves miserable, or we make ourselves strong. The amount of work is the same." Carlos Castaneda
"One single grateful thought raised to heaven is the most perfect prayer. " G. E. Lessing
“How much of human life is lost in waiting.” Ralph Waldo Emerson
"Men often mistake notoriety for fame, and would rather be remembered for their vices and follies than not be noticed at all.” Harry Truman
My website: www.intoku.net my e-mail:mreznicek@pretensa.com me:Miguel Reznicek