PowerUp

Power Up!

Preamble

Following on from Service please!, after watching a few more RC tutorials, I realised that I was severely lacking in knowledge on the power side of things.

See also Service please! for a video tutorial on Lithium Polymer (LiPo) batteries. You may also find the article on BECs and ESCs useful.

Video Tutorials

Playing with: Lithium LiPo LiFe Battery Checkers, by Julien Ilett

LiPo Internal resistance

From A Guide to Understanding LiPo Batteries – Roger’s Hobby …

…as a general rule, a per cell rating of between 0-6 mΩ is as good as it gets. Between 7 and 12 mΩ is reasonable. 12 to 20 mΩ is where you start to see the signs of aging on a battery, and beyond 20mΩ per cell, you’ll want to start thinking about retiring the battery pack. But this is only a guide – there is no hard rule set here. And if your charger doesn’t give you the per cell measurements, you’ll have to divide your total count by the number of cells in your battery to get an approximate per cell rating.

Note that the battery capacity checker, mentioned in Service please!, does not have the ability to check the internal resistance, and so it is probably better to get an all singing and dancing charger – one that comes with a balancer and balancing board

Balance Board
Balance Board

Also useful is the balance tap protector

LiPo balance tap protectors
LiPo balance tap protectors

.

What battery capacity requirement?

So, the day of my first ever servo’s arrival came, and I had issues using it as suggested in this video, from EEEnthusiat, although I used my servo shield, 99p,  stacked on top of my Uno, rather than connecting it directly to my Uno’s Vcc, GND and Pin 9, as Vlad does in the video.

Servo Shield
Servo Shield

However, with the Arduino powered from the USB the servo didn’t seem to move correctly, just twitching for a bit and then not moving at all. I plugged in an external wall wart type 9V 850mA. This helped a bit, with the servo performing a few just about 180 turns, but again stopped after four or so twists.

I uploaded this sketch,

/* Sweep
 by BARRAGAN <http://barraganstudio.com> 
 This example code is in the public domain.

 modified 8 Nov 2013
 by Scott Fitzgerald
 http://arduino.cc/en/Tutorial/Sweep
*/ 

#include   

const byte servo1Pin = 12;
Servo servo1;  // create servo object to control a servo 
               // twelve servo objects can be created on most boards
int pos = 0;   // variable to store the servo position 
 
void setup() 
{ 
  servo1.attach(servo1Pin);  
  // attaches the servo on pin 9 to the servo object 
  Serial.begin(9600);
} 
 
void loop() 
{ 
  for(pos = 0; pos < 180; pos += 1)      // goes from 0 degrees to 180 degrees, in steps of 1 degree    {       servo1.write(pos);      // tell servo to go to position in variable 'pos'      Serial.println(pos);      delay(15);      // waits 15ms for the servo to reach the position    }    for(pos = 180; pos > 0; pos -= 1)  
    // goes from 180 degrees to 0 degrees, in steps of 1 degree 
  {                                
    servo1.write(pos); 
    // tell servo to go to position in variable 'pos' 
    Serial.println(pos);
    delay(15);        
    // waits 15ms for the servo to reach the position 
  } 
} 

and monitored the value of the pos variable, via the serial port. I had the Arduino powered via USB and the external wall wart. I would power up the Arduino and the stacked shield first, without the servo connected to the shield. Upon opening the serial monitor, one could see the value of pos varying between 0 and 180. However, as soon as the servo was connected then the Arduino’s serial output would hang, leading me to believe that the current draw from the servo was killing the Arduino. In addition, the PC USB pings would be occurring in quick succession, as it the Arduino, or something, was being repeatedly connected to the USB port and then disconnected.

After reading this article, RCArduino: Servo Problems With Arduino – Part 1, I realised that power must be applied to the servo circuit/shield separately, via the pale blue power blocks in the bottom left of the above photo of the servo shield, as the Arduino’s on-board regulator can not supply sufficient current.

Now, I have some nice, dual 0-30V 3A, bench power supplies arriving tomorrow, see Bench Hardware. These will help, in the meantime. However, long term, some LiPo battery packs will be required, and how big, how many cells, what voltage, will they need to be?

In the article above, the writer alludes to a second part which will cover such a topic,

Large projects will require hobby batteries or fixed power supplies, part 2 will look at some of these options and the power demands of the test servos under a variety of loads.

Unfortunately, however, this alluded-to second part appears to have not been written. What is useful though, is the table, below, giving some examples of servo current draw

Current Requirement

Servo models used and current drawn at no load –

Servo Current (mA)
Futaba S3003 55
HITEC HS-322HD 75
ACE S1903 90
Bluebird BMS410 250

MG996R: Running Current 500 mA-900mA, from MG996R Tower-Pro – Electronicos Caldas

Guestimating MG996 current draw to be 500 mA ,

Current draw = 18 x Current per servo = 18 x 0.5 = 9 A

Guestimating MG996 current draw to be 900 mA ,

Current draw = 18 x Current per servo = 18 x 0.9 = 16.2 A

The most favourite capacity battery is the 5000mAh battery. which would provide around 33 minutes for low power (9A) and high power (16.2A) for 18 minutes.

Voltage Requirement

Noting that one LiPo cell, or 1S, provides 3.7V and most motors run at a max of about 6V, we, therefore, require 2S batteries, that would provide 7.4V is required.

Capacity

From A Guide to Understanding LiPo Batteries

For most applications, a 20C or 25C battery should be fine. But if you’re driving a heavy truck, or you’re geared up for racing, or you have a large motor for 3D flying applications, you should probably start around a 40C battery pack.

A 20C battery, of 5000 mAh, will give 100 A max, which is way more than we require. However, a 40C could be considered.

Typical Costs

The prices of batteries seem to range, for the 1S, from 2 x 3.7 V 300-600 mAh for £0.99 for the pair, up to £1.50 each.

The 2S 7.4 V 900-1200 mAh batteries seem to go for £4-6. However, one pair of 2S 6800 mAh for £0.99 in an auction with one day to go,seem very good, well cheap and cheerful (=Fire?!?).

Battery Testers

The best meter reads all of the cells’ voltages simultaneously, the 3 in 1 LCD Discharger Balancer Meter, for £5.90

3 in 1 2-6S cell meter
3 in 1 2-6S cell meter

There is the simpler battery capacity checker, for £2.60 or so, which can read one cell at a time, as well as giving the overall voltage, and percentage charge.

Cell Meter
Cell Meter

Note that some of these [clone] checkers can balance, whereas others can not. It seems as if the circuitry is there, see Julian Ilett’s video Playing with: Lithium LiPo LiFe Battery Checkers @ 3:45, but it is not always enabled.

Battery Checker matrix
Battery Checker matrix

According to this post, version 2 can balance, whereas version 1 can not. SO, knowing eBay, it will probably be down to the luck of the draw, or how much you pay in an attempt to get an original, as to whether you get one that can balance or not.

Another battery tester, 1-8S Lipo/Li-ion/Fe Battery Voltage 2IN1 Tester Low Voltage Buzzer Alarm BT, for £1.03

LiPo/Li-ion/LiMn/Li-Fe Battery Tester
LiPo/Li-ion/LiMn/Li-Fe Battery Tester

Chargers

A great article is, Multi-Chargers 101, which explains multi-chargers, charging NiCd and LiPo batteries and their differences, and the care of LiPo batteries.

B6 based chargers

The SkyRC/iMax B6, B6AC, B6 mini, Bantam BC6 and Turnigy Accucel-6 chargers, amongst others are all based on very similar hardware, and are very popular chargers.

See the blog B6 based chargers for much more information.

Other chargers

Multiport chargers

If you need to charge multiple batteries simultaneously, then use multi-port chargers, such as

HiTec X4 Micro
HiTec X4 Micro
Photovoltaic Charger

Arduino R3 Solar Charger shield module -LiPo rider with USB Interface is a Solar Power battery charger. For £6.59. See Lipo Rider Pro – Wiki – Seeed Studio for more information. Note that, despite the name, this is not actually an Arduino Shield.

LiPo Rider
LiPo Rider

Balance connector adapter boards

Balance boards are used in conjunction with chargers that only offer one balance connector socket (for example a 6S socket). In order to plug in a 2S, 3S or 4S plug a balance board is required.

JST XH Balance Board
JST XH Balance Board

However, if your charger comes with a variety of balance connectors, as the iMax B6 does, then a balance board is not required.

Read All about lipo balance connectors, which explains the balance connectors, but not the balance boards. There are three main types:

  1. Hyperion (Polyquest)
  2. JST-XH
  3. Thunder Power (TP)
  4. Kokam
  5. FMA Cellpro

Balance boards are unnecessary. You can shave off the positive lug on the JST plug and use just one 6S socket:

Balance charger boards

Connection Warning Tip!

One very important caution when you hook all your packs up in parallel is, you MUST plug the main power plugs from the packs into the para-harness, or para-board first. [source]

How to Charge Multiple LiPo Batteries With Just One Charger.

Note that the balancing connectors are in opposing directions, either side of the board.

eBay examples:

Charging a single cell (1S) battery with a bench power supply

How to charge a LiPo battery with a bench power supply. Note that you should charge at half the C, so 120mAh, NOT 240mAh as stated in the video.

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