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theoB610
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Battery Power

Sat Apr 27, 2013 6:35 am

So for my robot project, which batteries should I use?
I've seen people using LiPo, but then a friend said that they had disadvantages. I don't know what.
Also, what voltage, power etc should I get? Can you split power between the Pi and motors?

edwardij
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Re: Battery Power

Sat Apr 27, 2013 11:30 am

This site ( http://www.stefanv.com/electronics/qf200312.html ) says that
Lithium-Ion and Lithium-Polymer batteries have the special requirement that they not be discharged too low (3 Volts per cell under load). You must use an ESC that will cut-off the power before that level is reached, or cell damage will occur.
and so he uses "four-cell 600mAh nickel cadmium (NiCd)" which it is implied does not have the same problem.
Edward

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theoB610
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Re: Battery Power

Sun Apr 28, 2013 3:38 pm

edwardij wrote:This site ( http://www.stefanv.com/electronics/qf200312.html ) says that
Lithium-Ion and Lithium-Polymer batteries have the special requirement that they not be discharged too low (3 Volts per cell under load). You must use an ESC that will cut-off the power before that level is reached, or cell damage will occur.
and so he uses "four-cell 600mAh nickel cadmium (NiCd)" which it is implied does not have the same problem.
Edward
lol cheers ed :P
NiCad have some disadvantages too, though. I think that they recharge less and less as you go through.

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rurwin
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Re: Battery Power

Sun Apr 28, 2013 4:10 pm

NiCd batteries have what is called the "memory effect". If you discharge them part-way and then charge them, on their next cycle they will run out of power at the same point. You should always fully discharge them before recharging them. Some battery chargers are able to do that discharge-recharge automatically. NiCds also self-discharge over a few weeks, whereas Li-ion don't. However Li-ion have a limited shelf-life -- after 12 months they have about half the capacity they started with. It varies depending on the charge level and temperature, the worst case being fully charged and hot -- like a laptop battery. To keep a Li-ion battery at maximum capacity, discharge it to 40% and put it in the fridge. (If you do that, leave it at room temperature for a few hours to dry out condensation before you use it.)

There may be a faster charge available for Li-ion cells too -- charging stuff seems to have got faster since they came out.

edwardij
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Re: Battery Power

Sun Apr 28, 2013 7:20 pm

Cheer that's very useful :) would you recommend LiPo then??

mikerr
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Re: Battery Power

Sun Apr 28, 2013 10:12 pm

Nimh have no memory effect, and are easy to use.

Lion or lipo have to be carefully charged/discharged and can explode if mistreated !
Not to sound too hysterical, but it's more a consideration when you're tinkering with self made circuits
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coyotebush
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Re: Battery Power

Mon Apr 29, 2013 12:56 am

Lithium batteries pack more power for the weight than other technologies, but they get expensive for large batteries. Although it has never happened to me, lithium batteries are notorious for catching fire if damaged or improperly charged.

You can pick up everything you need from stores that sell R/C airplane supplies. I would also check out on-line stores like Hobby King. Besides batteries, you'll be looking for chargers, motors, controllers and servos. Just about any of the motor controllers (ESC's) have a 5V circuit which can be used for powering the PI.

Lithium batteries for RC airplanes are categorized with 3 numbers -
  • the capacity (in milliamp hours),
  • the voltage (how many cells are stacked in series), and
  • the max discharge rate (a ratio).
A battery described as "2200 3S 40" would provide 2200 milliamp-hours of power at roughly 11.1 V (3x3.7V) with a max discharge rate of 88 amps (40x2200).

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Re: Battery Power

Thu May 23, 2013 3:05 pm

OpenStick wrote:Hi,
SNIP
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Please stop spamming...
How To ask Questions :- http://www.catb.org/esr/faqs/smart-questions.html
WARNING - some parts of this post may be erroneous YMMV

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Tage
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Re: Battery Power

Fri May 24, 2013 2:14 pm

Try to google LiFePO4 and you will find a number of battery packs for sale and also single cells that you can combine into packs of your own design. Lithium Iron Phosphate cells have 3.2V per cell which is a bit lower than the standard "lithium" cells used in cellphones and laptops, but they are much safer. The problem with the standard cells is that if you abuse them you have a terrible fire hazard. If the cell catches fire there is no way you can put it out, it will burn until the material is gone. Kind of like gun powder.
For a small robot it makes sense to use two cells in series for 6.4V nominal battery voltage. You can then create 5V for your Pi using a 5V series regulator and run your motors directly from the 6.4V if they can take the voltage.
LiFePO4 cells will be immediately destroyed if discharged below 2V, so you must have a way of measuring cell voltage and stopping discharge. You could for example have a comparator that disconnects the battery if the voltage drops below 5V (with two cells in series). When charging it is important to limit cell voltage to 3.65V. Overcharging will cause the cell to went flammable fumes. If you are using individual cells without electronics to balance the battery you should use a separate charger for each cell. This allows you to fully charge all cells and keep your battery balanced. Each charger must have voltage limit set at 3.65V. You could use two lab bench power supplies with adjustable voltage.
If you buy individual cells you must get the type that has tabs welded to the ends, so you can solder wires without overheating the cells. Do not even think of touching the cell with a soldering iron. If you have to use cells without solder tabs then you must get some type of battery holder where you do not need to solder.

edwardij
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Re: Battery Power

Fri May 24, 2013 6:35 pm

Awesome thanks for the detailed information it really helps and seeing as up to date information about usage or anything is scarce than you for the info! :)

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Tage
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Re: Battery Power

Wed May 29, 2013 2:44 am

I noticed that it is possible to upload attachments to this forum -neat! I have to test that right now..
Here is a picture of a typical 18650 cell with solder tabs. This type of cell is used extensively even in large batteries for electrical cars.
Tage
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18650.jpg
LiFePO cell
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EffieT
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Re: Battery Power

Wed May 29, 2013 6:56 am

You can use 3.7V one LiPo battery; this is close enough to 3.6V and is the minimum to run certain microcontrollers and 7.4V two LiPo cells can often power a microcontroller and works great for 7.2V DC gear motors but Unfortunately 7.4V two LiPo too high for most hobby servo motors.


coral springs auto
Last edited by EffieT on Thu May 30, 2013 6:40 am, edited 1 time in total.

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Tage
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Re: Battery Power

Wed May 29, 2013 1:39 pm

LiPO batteries with built-in electronics are relatively safe to use. But I would strongly advice against using single cells without the mandatory protections circuitry that is included in battery packs. Especially when using home made chargers. It only takes that you crank up the charge voltage a tiny bit too high, or forget to turn off the charger. The resulting fire is very difficult to handle, as you cannot put it out and the flames are very hot.
As an example, the battery fires that grounded the Dreamliner. Those batteries caught fire despite protection circuitry. I believe the final fix was to build a fireproof enclosure around the battery, with a small chimney to the outside so that next time a battery catches fire it will burn without creating a cloud of smoke inside the plane... If there had been a way of extinguishing the flames I am sure that would have been a more suitable option. I bet those engineers wish they would have used the safer LiFePO4 cells...
Lithium battery packs must have complex electronics that balance the state of charge in each cell, measure temperature and individual cell voltages. The also have MOSFETs that disconnect the cells from the battery terminals if the cell voltage drops too low, or if cell voltage becomes too high due to charger malfunction. So my advice is that if you must mess with LiPO batteries, never try to build you own battery, and always use a ready made battery pack with built in electronics for safety! And put a timer on your charger.

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