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Gomez
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7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry PI

Tue Dec 16, 2014 1:23 am

Greetings,

I am working on a project for my final exam of school, its a vocation course of eletronic, automation and computers which has a final exam known as professional aptitude test.

What I want to do has already been done with the Raspberry PI, there is a popular video from Ben Heck that shows it, the mobile raspberry pi retro emulator.

I have a 3d printer done from scratch with Arduino so I can make the case with no problems..
I got RetroPie ready to install on the sd card.
And now I am working on the list of items that I have to order from ebay but I am faced with a problem that I have no knowledge about..

What does the 20C means? I have done some research but didn't understand much, I have to send 7.4V and like 0,50 A to the 4.3 inches LCD and 5V with around 1,5A to the raspberry itself.. This is possible with this battery?

I have seen some LM-series regulators like the LM317 but I am still not sure which one to choose or if they are viable to my issue..

The main question is, I am looking for a cheap and effective regulator to downgrade 7.4V to 5V in ebay and I also want to know if 1800mAh 20C causes any issue on the LCD or Raspberry PI ?

I would appreciate any information related to this issues that you guys could provide.

Thanks very much :)

Mortar Art
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 2:09 am

Basically what you're looking for is a BEC. I have one of them in a camera unit, although I'm not completely sure that they're necessary with the Pi. Many devices can handle a fairly wide range of voltage without failing. There should be information out there about this.

W. H. Heydt
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 2:40 am

The spec for the Pi is 5v+/-5%. Even if it could take 7.4v, I would drop the grade of anyone set it up that way. If the Pi were designed to handle higher voltages, it would be in the specs.

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Tage
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 3:28 am

Gomez wrote:Greetings,

I am working on a project for my final exam of school, its a vocation course of eletronic, automation and computers which has a final exam known as professional aptitude test.

What I want to do has already been done with the Raspberry PI, there is a popular video from Ben Heck that shows it, the mobile raspberry pi retro emulator.

I have a 3d printer done from scratch with Arduino so I can make the case with no problems..
I got RetroPie ready to install on the sd card.
And now I am working on the list of items that I have to order from ebay but I am faced with a problem that I have no knowledge about..

What does the 20C means? I have done some research but didn't understand much, I have to send 7.4V and like 0,50 A to the 4.3 inches LCD and 5V with around 1,5A to the raspberry itself.. This is possible with this battery?

I have seen some LM-series regulators like the LM317 but I am still not sure which one to choose or if they are viable to my issue..

The main question is, I am looking for a cheap and effective regulator to downgrade 7.4V to 5V in ebay and I also want to know if 1800mAh 20C causes any issue on the LCD or Raspberry PI ?

I would appreciate any information related to this issues that you guys could provide.

Thanks very much :)
20C usually means that the battery can withstand a discharge current that is 20 times the C rate. in this case, the C is 1.8A. you don't really need to worry about the 20C figure. it just tells that the battery is able to deliver 36A current. the discharge current will be much lower in your case.

2S means there are two 3.7V cells in series. the battery must have built in electronics for protections. don't mess with bare cells without protection against overcharge and deep discharge. you could easily have a fire.
the battery is a 7.4V 1.8Ah battery, so you have 13.3Watt hour of energy storage.
your load is 5V * 1.5A + 7.4V * 0.5A = 11.2W, so your battery will last for about an hour.

if the LCD can take 7.4V you can connect it to the battery.
the Pi certainly needs a switching regulator to drop the voltage to 5V. the correct name for this regulator is a buck regulator or a step down regulator. it does not make sense to use a linear regulator because it would dissipate another 3.6W. you could try to get a USB charger of the type that plugs into a cigarette lighter in the car and see if it runs at 7.4V it probably does.

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Gomez
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 12:17 pm

Tage wrote: 20C usually means that the battery can withstand a discharge current that is 20 times the C rate. in this case, the C is 1.8A. you don't really need to worry about the 20C figure. it just tells that the battery is able to deliver 36A current. the discharge current will be much lower in your case.

2S means there are two 3.7V cells in series. the battery must have built in electronics for protections. don't mess with bare cells without protection against overcharge and deep discharge. you could easily have a fire.
the battery is a 7.4V 1.8Ah battery, so you have 13.3Watt hour of energy storage.
your load is 5V * 1.5A + 7.4V * 0.5A = 11.2W, so your battery will last for about an hour.

if the LCD can take 7.4V you can connect it to the battery.
the Pi certainly needs a switching regulator to drop the voltage to 5V. the correct name for this regulator is a buck regulator or a step down regulator. it does not make sense to use a linear regulator because it would dissipate another 3.6W. you could try to get a USB charger of the type that plugs into a cigarette lighter in the car and see if it runs at 7.4V it probably does.
that was exacly the information I was looking for :)

I am gonna use a LiPo battery, which are batteries for RC , does it have built-in protection? it looks like it has but I don't understand much about batteries.

http://www.ebay.com/itm/261650402876
or
http://www.ebay.com/itm/7-4-V-30C-5200m ... 3a90660f68

I only have to care about regulating the 7.4v to 5v because about the amperage, the eletronics (lcd, raspberry) drains the amps it needs of the battery right? I don't have to do anything related to the amperage, is this correct?

Thanks for your time and information dude :)

edit:
does this do the job for regulating?
http://www.ebay.com/itm/Boost-Buck-Volt ... 2c91827871

Also, how do I set the output voltage that I want to regulate to?

The specs are :
Input: DC 4.75-24V
Output: DC 0.92-15V (Max 3A)

This means I can input 7.4V but how can I set the 5V for output?

klricks
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 1:50 pm

Connect your battery to the inputs. Connect a voltmeter on the output. Carefully turn the dial on the potentiometer with a screwdriver until the voltage is 5.00V
Last edited by klricks on Tue Dec 16, 2014 3:18 pm, edited 1 time in total.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Stretch w/ Desktop OS.

drgeoff
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 3:03 pm

klricks wrote:Connect your battery to the inputs. Connect a voltmeter on the output. Carefully turn the dial on the potentiometer with as screwdriver until the voltage is 5.00V
Be sure to do that BEFORE connecting it to the RPi.

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Gomez
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 6:25 pm

Alright, thats all information I need for now, I'l create a new topic if any problem that I can't solve comes my way, and I will for sure recommend this forum to my friends that are working in the final exam with raspberry pi, it has a very helpful community if they have any issues.

Thanks for all the information dudes :)

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Tage
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 8:06 pm

the voltage converter should work. adjust the output voltage before connecting it to the Pi, as people already mentioned..

regarding the battery there is not enough information to know what kind of protection circuitry is included. if any.
the danger with this type of battery is that it will burst into flames if overcharged, and it will burn until there is only ashes left. so it is safer to use LiFePO4 type of batteries. it has 3.2V per cell, so the voltage would be 6.4V during discharge (7.2V max charge voltage). the 7.4V battery has max 8.2V charge voltage.

both types of battery will be destroyed if deep discharged below 2.5V per cell, and cannot be recovered afterwards. that is one reason why both battery types require a small circuit board for protection. the circuit board will disconnect the cells from the wires so avoid overcharge or deep discharge. the fact that your battery has 20C rating is already telling me that it cannot have deep discharge disconnect function, but it may have overcharge protection as there seems to be a separate wire for charging.

the LiFePO4 battery can also burst into flames if overcharged, but the flame is typically very brief. if a cell vents there is a release of flammable steam from the electrolyte inside that will ignite if there is a spark. usually there is a spark when the lid of the cell is ripped off by the high pressure inside. but the battery will not keep burning, as the more dangerous type will.

you do not need to worry about regulating the current to the load, only the voltage.

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Gomez
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 8:38 pm

Tage wrote:the voltage converter should work. adjust the output voltage before connecting it to the Pi, as people already mentioned..

regarding the battery there is not enough information to know what kind of protection circuitry is included. if any.
the danger with this type of battery is that it will burst into flames if overcharged, and it will burn until there is only ashes left. so it is safer to use LiFePO4 type of batteries. it has 3.2V per cell, so the voltage would be 6.4V during discharge (7.2V max charge voltage). the 7.4V battery has max 8.2V charge voltage.

both types of battery will be destroyed if deep discharged below 2.5V per cell, and cannot be recovered afterwards. that is one reason why both battery types require a small circuit board for protection. the circuit board will disconnect the cells from the wires so avoid overcharge or deep discharge. the fact that your battery has 20C rating is already telling me that it cannot have deep discharge disconnect function, but it may have overcharge protection as there seems to be a separate wire for charging.

the LiFePO4 battery can also burst into flames if overcharged, but the flame is typically very brief. if a cell vents there is a release of flammable steam from the electrolyte inside that will ignite if there is a spark. usually there is a spark when the lid of the cell is ripped off by the high pressure inside. but the battery will not keep burning, as the more dangerous type will.

you do not need to worry about regulating the current to the load, only the voltage.
The problem with the battery over charging and getting into flames is that in the final design, the battery is located vertically on the bottom of the layout, making all the width of the box, if it gets into flames then other important components may also be damaged, the battery will not be removable on my 3d model of the final design, it has an entrance to the HDMI, headphones and a sliding panel with the wires to connect to the charger.

The charger is http://www.ebay.com/itm/281490732231 and it seems to have some kind of protection that stops charging when the FULL status comes in, if it doesn't have that protection, well, at least it gives the indication that is full so I can remove the charger to avoid problems..

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rurwin
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Re: 7.4V/2S 1800mAh 20C LiPo battery for LCD and Raspberry P

Tue Dec 16, 2014 9:08 pm

You should also be aware that 3.7V is only the nominal voltage. A fully charged cell will have a higher voltage (4.2V) and a discharged cell will be lower (3V or less.) So if you are using the output voltage directly you need to be sure the display can work off anything from 6V up to 8.4V.

Personally, I'd use a single 18650 cell and charge it externally. If you already have a trustworthy battery charger circuit then you could use a 1S LiPo. Then I'd use two boost-mode conversion modules to get the voltages I needed. You still need a circuit to cut power when the voltage drops to 3V.

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