BETTER KNOW YOUR BATTERY: Part One December 20 2014, 20 Comments

Why is it important to know how your batteries work? Because the world of lithium-ion batteries is rife with bullsh%t, that’s why. That’s the main reason we started Battery Bro - to deliver high-quality, wholesale li-ion batteries without the BS.

So we thought we’d give you a short write-up of all the tech terms you’ll see in battery tests and spec sheets. Comment away if there’s something we’ve missed.

The basics:

The most important measures of any electrical circuit are voltage, resistance, current, and power. There’s a very simple analogy to understand these. Think of a battery’s circuit as a pipe we’re trying to pump water through, like in the picture below (credit to kevinboone.net):

  • Voltage is the water pressure that is pushing the water down through the pipe. Voltage is measured in volts.
  • Resistance is the size of the lower pipe. If the pipe is wider, more water will flow through. When the pipe is smaller, there is more resistance. Resistance is measured in ohms or Ω.
  • Current is how much water comes out of the pipe. Current is measured in amperes, amps or A. Sometimes it’s called I, and this is France’s fault.

If you know any two of these numbers, you can calculate the third. The math is super simple:

  • Voltage = current x resistance
  • Current = voltage / resistance
  • Resistance = voltage / current

Another important concept is electrical power. This is simply voltage x current. Think back to the water analogy: you’re taking the voltage, or the water pressure, and multiplying it by the current, or how much water is actually coming out. This is a measure of how ‘hard’ the water is coming out of the pipe.

Why is this important? It tells us how much work we can actually do with the electricity. 

Imagine you're trying to put out a fire with a firehose.

  • If a lot of water comes out of the pipe but comes out slowly (no pressure), then you can't put out a fire.
  • If the water comes out quickly but there's very little water, then you can't put out a fire.
  • If the water comes out quickly (high voltage) AND there's a lot of water (high current), then you can put out a fire.

Electrical power is like the ability of your firehose to put out a fire.

Power is measured in Watts (W).

How to read a battery spec sheet

Let’s dive in to a real li-ion battery spec sheet:

First, let’s look at capacity. This is just a measure of how long you can use your battery. The number is in milliamp hours, or mAh. (There are 1,000 milliamps to one amp.) Amps is a measure of current, so we know we’re dealing with how much ‘water’ is coming out of the pipe. If your battery has 1 amp hour, that means it can discharge 1 ampere of current over one hour, or half an ampere an hour for two hours, etc.

This battery is rated at 3,200 mAh, meaning that in one hour, it can discharge 3,200 milliamps (3.2 amps) and then it’s done. That also means that if you want to do half that (1.6 amps) in an hour, then you would get two hours of power until the battery’s charge is depleted.

Batteries operate under a rule known as Peukert's Law. Simply put, the more current you draw from your battery, the less mAh you'll be able to get out of it. 

That's why you should always look at the discharge current they quote in the Notes section. They obtained this number by discharging the battery at 0.65A discharge current. This means to get the figure of 3.2 amp hours, they discharged the battery at 0.65A for 3.2 / 0.65 = 4.92 hours. In reality, batteries can have different mAh ratings depending on how big the discharge current is. The mAh capacities are never exact.

Don't worry too much about the difference between rated and nominal figures. Much more important is the discharge current and temperature they used to get those figures.

Next up is nominal voltage. This is an important number. You don’t want your device to exceed that voltage when discharging. Most devices will have their voltage ratings printed prominently on the label or in the instruction manual.

Batteries are usually charged at a higher voltage than they’re discharged, as you can see in the charging voltage figure. Check your battery charger to make sure you don’t exceed this figure.

Continuous discharging current Max is an important figure for anybody who pushes their devices to the max, especially vapers. As the wording implies, this is the maximum discharge current in amps… to a point. Even if it says the maximum current can be sustained continuously, in practice it’s better to stay on the safe side with a slightly lower discharge current.

Finally, we have the internal resistance of the battery. This will always be expressed in Ω, or ohms. A battery’s internal resistance increases over its lifetime, so that by the time a battery is dead it will have a very high internal resistance. A battery with a lower internal resistance will, generally, be able to sustain higher Amp (current) loads.

 

Coming soon, Part Two… battery testing, alternative chemistries, and more battery geekery!