(I just want to say up front - sorry for the noobish questions!)
I am currently in the process of trying to build an autonomous exploration vehicle based on the Arduino platform. To this end I have decided on using a brushless motor & ESC combo which draws approx. 18A with a burst current of 50A @ 7.4-11.1V (either 2 or 3 LiPo cells). I have a couple of questions about possible battery configurations however, and would appreciate any help.
Can I use the same battery/power source to power both the motor/ESC and the Arduino? My main concern is if I simply connect the two circuits and the battery in parallel, could this possibly lead to current surges as the motor's resistance/speed changes and thereby possibly damage the Arduino? Or is this not a large enough issue to worry about?
If question 1 above is an issue, will simply connecting a capacitor across the two terminals of the battery fix this problem?
I am hesitant to use LiPo cells as my research suggests that they are very complicated/dangerous to charge and discharge (bear in mind this is my first project so I am likely to short out the battery by accident at one point or another. Would simply stringing together 8 rechargeable NiMH AAs work? Is there a better way?
I have been reading alot about mAh ratings of batteries. If I want to drive my robot around for say, 30min at a time, what sort of mAh ratings should I be looking for? (does 25A for 30min mean 12,500mAh per battery? :o)
Following on from question two, I have done a little more research and found a tutorial on decoupling (De-coupling) which seems to suggest that If I were to connect the motor as shown below
and also insert a capacitor across the two wires supplying power to the arduino board it would eliminate the problem. Is this right? Am I missing anything? Thanks.
I am currently in the process of trying to build an autonomous exploration vehicle based on the Arduino platform. To this end I have decided on using a brushless motor & ESC combo which draws approx. 18A with a burst current of 50A @ 7.4-11.1V (either 2 or 3 LiPo cells). I have a couple of questions about possible battery configurations however, and would appreciate any help.
Can I use the same battery/power source to power both the motor/ESC and the Arduino? My main concern is if I simply connect the two circuits and the battery in parallel, could this possibly lead to current surges as the motor's resistance/speed changes and thereby possibly damage the Arduino? Or is this not a large enough issue to worry about?
I don't think it should be a big problem. Brushless motors do not generate as much trash as brushed motors. You may need some added capacitance filtering at the Arduino end. Be sure to run separate ground and power wires from the battery to the Arduino so that motor current is not passing through the Arduino power wires.
If question 1 above is an issue, will simply connecting a capacitor across the two terminals of the battery fix this problem?
Probably more effective if near the Arduino power connector end, but then again you may not need any additional filtering at all, try without first.
I am hesitant to use LiPo cells as my research suggests that they are very complicated/dangerous to charge and discharge (bear in mind this is my first project so I am likely to short out the battery by accident at one point or another. Would simply stringing together 8 rechargeable NiMH AAs work? Is there a better way?
Well both NiMH and Lipo batteries require battery chargers designed for them, so as long as the charger you use is designed for that battery type and you follow the instructions it shouldn't be a problem. An additional concern for Lipo are the dangers of overdischarging them, which can cause permanent damage to them. Some lipo battery packs have internal low voltage cut-off circuitry, but some do not. I higher recommend using ones that have this internal protection in them. If you do plan on causing short circuits on your battery (??? ;)) just get some inline fuses to protect the batteries. Automotive fuses are pretty easy to work with and can be obtained at any auto parts store in many different amp sizes.
4. I have been reading alot about mAh ratings of batteries. If I want to drive my robot around for say, 30min at a time, what sort of mAh ratings should I be looking for? (does 25A for 30min mean 12,500mAh per battery?
That is about right, but mah ratings are total capacity at some specified (usually quite low) fixed discharge rate. Some batteries won't deliver 100% of rated capacity at higher current discharge rates. Specific manufacturers datasheets will give a better picture of how a given battery will perform at different current discharge rates.
While more expensive, Lipo batteries by far have a greater energy per mass value, if weight is important such as aircraft use. If weight is not important then NiMH are less expensive for the same voltage/mah size. 12vdc gel cell lead/acid batteries can be useful and can be pretty cost effective if size and weight are not a big issue.
...An additional concern for Lipo are the dangers of overdischarging them, which can cause permanent damage to them...
Does this mean that if I was to use a lipo cell I would require some extra circutry (e.g. something to cut out when V<2.3ish) in between the battery and the motor/arduino? Alot of the ESC circuts I have been looking at have this functionality built in (e.g. the http://www.dealextreme.com/details.dx/sku.35771) - is there way way to piggy-back off this circutry when drawing power for the arduino so I don't need 2 circuts that do essentially the same thing? Can I just use a BEC output from the ESC to power the Arduino? (6V - 1A output) -> Forgive me if this is way off, i am throughly confused by ESC and BECs
Lipo batteries by far have a greater energy per mass value
That said, a lot of the lipo packs I have been looking at are designed for model cars & planes yet only seem to have, at most, a 2500mah rating. Yet if a motor draws 20A, it would only be able to drive for 7ish minutes (and then charge for 5hrs). Is there any practical way to get around this/draw more power?
Connecting several batteries together in parallel (of the same type/rating) will increase the overall capacity. For example 500mah + 500mah will give give you 1000mah at the same voltage. Connected in series, you add the voltages together, so 3.7v + 3.7v = 7.4v
By combining the two techniques you can build a battery supply of practically any voltage and capacity you desire (subject to available funds).
Lithium polymer batteries are actually easier to charge than NiMH. All you have to do is limit the current to 1C while monitoring the voltage. Around 90% target voltage the current naturally drops off and you switch to voltage control, you can basically float it to full charge. NiMH you can't do that, you have to monitor the charging current history and even the temperature to be sure. Of course it seems the lithium batteries do fail a little more spectacularly when you get it wrong.
Lithium FerroPhosphate (LiFeP04) batteries are a recent development, still pretty new, and maybe not so cheap. They are mostly used in electric bikes and scooters so far. They don't have quite as much energy density as LiPo, and have lower voltage (nominal 3.3 Volts) and needs a specific charger (charge up to 3.6V only, NOT 4.2 V like lithium ion)
If you are worried about safety, it's almost impossible to get LiFePO4 cells to burn up. I have intentionally shorted some cells in testing... you have very low internal resistance on the larger cells; you can get over 100 amps out of the high-current 26650 types, but the solder melts off the wire or the nickel end tabs melt open (red hot metal!) but the battery does not explode. Of course the cardboard or plastic shell may burn or smoke (;-). There are demo videos online of people driving nails and firing bullets into batteries... LiPo will explode, but LiFePO4 will not.
For example, this $100 battery is rated at 12.8V, 87 WH at 20 A current draw. Inside it must have 4 cells in series at 3.2 V each. If you are drawing 20 A at 12.8V, that is 256 watts (about 1/3 horsepower), so the battery would run 20 minutes at full 20 A draw (a little less in practice).
search for "lifepo4 26650" for other batteries of interest. The number is a standard cell size for this type: 26mm diameter and 65mm long.
;)) just get some inline fuses to protect the batteries. Automotive fuses are pretty easy to work with and can be obtained at any auto parts store in many different amp sizes.