Advice seeked to find a LiPo cell with short length (< 7 cm)

Hello,

I am currently using 3 AA 2000 mAH NiMH batteries to feed 3 power LEDs. I want to have an higher voltage to feed an Arduino Pro Mini so I plan to replace the AA cells by a 7.4 LiPo battery pack.

However most LiPo with middle to high capacities are quite extended. Most of them are longer than 10cm which is a too long to be put on the back of a hand.

Anyone knows about a shorter 7.4V (dual-cell) LiPo pack, with the biggest dimension being below 7 cm ?

Also, are there recommended brands or brands to avoid ?

What about the plug (and charger) to choose ?

Thank you in advance.

I have never used 2s (2 cell) lipo batteries, so I can't advise you there. I know you're not supposed to make your own packs out of individual cells, since the lipos need "balancing" to keep them happy.

I just wanted to point out that you could use a single lipo cell to power a 3.3v Pro Mini, instead of a 5v Pro Mini https://www.sparkfun.com/products/11114. It only runs at 8mhz (instead of 16), so I can't say whether or not it'll work for your project, but it's an option. You could also also use the exact same setup you have, just with the 3.3v Pro Mini (which accepts 3.3-12v).

This is about the closest I could find.
http://hobbyking.com.au/hobbyking/store/__23820__Turnigy_nano_tech_2000mAh_2S1P_20_40C_Lipo_Receiver_Pack.html
To go smaller, you would have to reduce the battery capacity.
Note that you have to use a LIPO balance charger to charge these batteries if used over a long time frame.

amundsen:
Hello,

I am currently using 3 AA 2000 mAH NiMH batteries to feed 3 power LEDs. I want to have an higher voltage to feed an Arduino Pro Mini so I plan to replace the AA cells by a 7.4 LiPo battery pack.

However most LiPo with middle to high capacities are quite extended. Most of them are longer than 10cm which is a too long to be put on the back of a hand.

Anyone knows about a shorter 7.4V (dual-cell) LiPo pack, with the biggest dimension being below 7 cm ?

Also, are there recommended brands or brands to avoid ?

What about the plug (and charger) to choose ?

Thank you in advance.

first, you do not need dual-cell, since you have 3*AA NiMH batteries, the voltage is 3.6 V. single cell of LiPo should be fine.

second, total W·h of 3AA NiMH batteries=1.23*2=7.2 W·h

your request is < 7 cm, then 6 cm cubed = 666= 216 cm cubed=0.216 L

total W·h of 1LiPo cell = 3000.216= 64.8 W·h

You can use one 6 cm cubed of 1LiPo cell to replace 9 of 3AA NiMH batteries !

LiPo has one benefit to make any shape as you like. but it only stay true if you are working for apple or somebody like it.

@sonnyyu: as indicated in my first message, my reason to go to 7.4V is that I do also want to provide power to a 5V Arduino Pro Mini (I know there are 3.3V models but I want the 16MHz version).

However your calculations make me feel I do not understand correctly how to compare a power source made of NiMH cells with a source made of LiPo cells. I'll read the article you provided.

Thank you everyone for the answers.

Okay, if you're set on 16mhz, you can still keep things very small by using a power booster with a single lipo cell. I have this one from Sparkfun, https://www.sparkfun.com/products/11231, which will boost the output of a lipo cell to 5v with an a available current of 600ma (which is well above your maximum current draw through an Arduino). Then you power the Arduino directly through the Vin pin, and bypass the regulator (since your power is already regulated).

This booster is also a charger, which you can use via USB or your own external supply, and it's only about 2.5cm, square. If you just want the booster, and you will get a separate charger, that's LiPower - Boost Converter - PRT-10255 - SparkFun Electronics.

Sonnyyu's comment didn't take into account the 5v Arduino (as you noticed) or the size of usability of the final design. Based on what I know from your limited description, this charger/booster combo will give you all the voltage and current you need in a very small size, with charger attached.

If you want to match the capacity of your current system, Sparkfun has a single cell lipo battery with 2000mah capacity for $16.95 that is 5.8x54x60mm Lithium Ion Battery - 2Ah - PRT-13855 - SparkFun Electronics or several smaller sizes if you prefer portability over battery life.

If you want to stick with the two cell idea, they have a 1500mah battery pack at almost exactly 7cm (71.6mm x 34.5mm x 22mm) Polymer Lithium Ion Battery - 1500mAh 11.1v - PRT-10470 - SparkFun Electronics and a compatible charger SkyRC IMAX B6 V2 Professional Balance Charger / Discharger - PRT-16793 - SparkFun Electronics. But that is much more expensive, heavier, larger, and provides less power overall.

II didn't know there were combined boost converter/chargers for LiPo. Great. However is 5V really enough as it is the strict minimum input voltage required for a 5V Pro Mini ?

The booster and the pro-mini are essentially designed to work together, so you won't have any problems. The pro-mini has two voltage inputs, either RAW or Vcc. You want the Vcc since RAW would run it through the on-board regulator, which you don't need because your power is coming from an off-board regulator. Either way, you're running your power through a regulator and the rest of the board is only getting 5v.

Also, I said to use the Vin pin earlier, forgetting you were using the mini. Standard Arduinos use the label Vin, but Sparkfun's pro-mini is labeled Vcc for this type of input.

PS-If I have been helpful to you, please click the green plus to the left, which gives "karma," and lets others know I'm here to help. Thanks!

Sure. Done. I wished one got karma points for questions too. I have always more questions than answers... XD

amundsen:
@sonnyyu: as indicated in my first message, my reason to go to 7.4V is that I do also want to provide power to a 5V Arduino Pro Mini (I know there are 3.3V models but I want the 16MHz version).

Use 3.7 v to 5 v dc-dc converter. It is very standard these day and it have good efficiency ( >95%) and cheap. Anyway all we care is W·h, use one or two cell doesn't matter for final number. Two cell might increase complication of battery management circuit ( balancing, individual cell protection...) .

Apple use 3.7 V battery to power Ipad, You can not power Arduino Pro Mini with 3.7v ?

iPad 2 Battery Capacity: 6583mAh ~3.7v ~24WattHours
iPad (3rd Generation): 11560mAh ~3.7v ~42WattHours

sonnyyu:

Mini DC 2.5-6V to 4-12V 5V Step Up Converter Voltage Regulator Charging Module

Step Up Converter, Ebay, $4.90

Take look at edge of PCB, It is guaranty no QA for production. But it is sold dirty cheap at Shenzhen market. I guess "you get what you pay for"