Battery C rate: do I need a lot of it?

One of the values that describe LiPo batteries used in RC crafts is so called C rating. For example: 3S 1300mAh 45-90C. Or 3S 5000mAh 25-30C. Or 4S 1300mAh 65-95C. S is simple, it tells the voltage. mAh or Ah tell capacity.

But what about C rate? According to Wikipedia is can be described as:

The C-rate is a measure of the rate at which a battery is being discharged. It is defined as the discharge current divided by the theoretical current draw under which the battery would deliver its nominal rated capacity in one hour. A 1C discharge rate would deliver the battery’s rated capacity in 1 hour. A 2C discharge rate means it will discharge twice as fast (30 minutes). A 1C discharge rate on a 1.6 Ah battery means a discharge current of 1.6 A. A 2C rate would mean a discharge current of 3.2 A.

So, if we take a look at Turnigy Nano-tech 3S 1300mAh 45-90C we will know that:

  • Constant discharge current on 45C is 58.5A
  • Burst discharge current (10 seconds) on 90C is 117A

Neat, yeah? Almost 60A constant current! That gives 15A per motor. And that a lot on 250 or smaller quad. So, I should be fine! Continue reading Battery C rate: do I need a lot of it?

Hands-on: Turnigy Graphene LiPo batteries

LiPo battery life is hard. Charged, discharged, beaten in a crash, charged, discharged and so on. Because last week I had to utilize few of my old Turnigy Nano-tech LiPos, I’ve decided to try something “better” this time. Turnigy Graphene 1300mAh 3S 65C. Package came today and this is what I got:

Turnigy Graphene LiPo in box

Turnigy Graphene LiPo there is even a bag inside

Turnigy Graphene LiPo Battery

Turnigy Graphene LiPo

  • Box is huge. Really, it would fit 3 LiPos, not just one. Kind of waste. But box is nice enough to reuse for something else
  • Black is the color of choice here. Black and red. I like this combination
  • They are noticeably heavier than 1300mAh 3S Nano-tech LiPos. But they have 20C more, so that was predictable

More after the weekend when I should be able to give them a first try.

Magical LiPo tree

Magical LiPo tree

Since LiPo batteries are potentially dangerous, they can not be just put into garbage. They have to be properly handled, utilized, before they can be safely thrown out. I will write more on this topic later. Here is only a picture of my magical LiPo tree. Knife, electrical connector, 12V halogen lightbulb, some sand, a flowerpot and LiPo discharger ready.

How to measure battery capacity with Arduino

Battery capacity measurement can be useful in many situations. And it is not hard, only requires enough time to discharge battery completely with know resistance and a way to measure voltage in the circuit. Ohm’s law will to the rest: I = U / R

Let’s say, we want to measure standard AA 1.5V alkaline battery capacity. Why 1.5V? They are common, made by many manufacturers and sold for different prices. And not always more expensive is better. To do this, we will need:

  • AA 1.5V battery
  • resistor to discharge it. We need high current to discharge battery in reasonable time, so low resistance is suggested. On the other hand, high current means o lot of heat, so we need a resistor that can survive this. I suggest using 2.2Ohm 5W ceramic resistors.
  • Arduino to measure voltage in circuit. Any Arduino or plain ATmega or ATtiny with A/D converter will do.

So, first a simple electrical circuit:

how to measure battery capacity with arduino

And some code that will be run every second:

voltage = 5.0 * ((float) analogRead(V_METER)) / 1024.0;

float current = voltage / R_LOAD;
joules += voltage * current;
float wattHours = (joules / 3600.0) * 1000.0;

And here how it work:

  1. We need to measure voltage in circuit. This is why, in first step, we read 10bit A/D converter and scale output to 5V. Why 5V? Arduino Uno works on 5V, and it is the reference voltage here,
  2. Next, lets compute current using Ohm’s law I = U/R,
  3. With know current current we can compute work using P = U * I and store it in joules variable,
  4. Last step is to change joules to Watt hours.

If instead of Watt hours we want Ampere hours, there is no need to count joules. Instead of that, sum current and final value divide by 3600 (there are 3600 seconds in one hour). Like this:

voltage = 5.0 * ((float) analogRead(V_METER)) / 1024.0;

float current = voltage / R_LOAD;
ampereSeconds += current;
float ampereHours = ampereSeconds / 3600.0;

Full code is available here

Notes

  • this circuit allows to measure batteries with voltage up to 5V. Anything above it will damage A/D converter
  • to measure higher voltages, voltage divider will be required
  • with higher voltage, power loss on resistor will increase. It will get very hot and might burn

LiPo on fire

It is true what internet writes: LiPo batteries can be dangerous. This is how ended one of my buddy’s batteries during indoor FPV session. Small crash, 2 seconds later cloud of white smoke and flames.

Reason was pretty simple. During crash battery got loose and still spinning propeller has cut 2 cells. Good thing, besides dead LiPo, no damage. We were lucky enough to get that thing outside before it damaged quadcopter or got anything else on fire.

LiPo battery on fire