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Understanding Battery Capability/Capacity

Published by sonicbomb in the blog sonicbomb's blog. Views: 139

mAh (milliamp-hour) - The run-time capacity of a battery.
One mAh is equivalent to 1/1000th of an amp hour, 1000 milliamps is equal to one amp. So for example a 3000 mAh battery has 3 amp hours, meaning that it can provide 3 amps for 1 hour, 6 amps for half an hour etc. A real world example might be more in the region of 15 amps being drawn from the battery, giving approximately 10 minutes of use, equivalent to about 120 puffs.

Voltage (volts) - The charge state of the battery.
A fully charged 18650 will have 4.2 volts, and will usually be considered discharged at around 3.2 volts. 18650s should not be discharged below 2.5 volts as this distresses the battery. Below 2.0 volts the internal copper electrodes start to dissolve. Then when the battery is recharged this free copper will 'plate' to the internal components damaging the battery and creating the possibility of a short-circuit. Battery's discharged below 2.5 volts should be considered potentially unsafe, with recycling being a cheap and safe option.

18650s often display a value of 3.7 volts on the wrapper. This is a 'nominal' value, meaning that over the battery discharge curve the majority of the charge life is in the 3.7 volt range.

CDR (Continuous Discharge Rate) - The number of amps that a battery can safely deliver, continuously.
As well as creating a ‘green’ safe usage zone and an effective amp limit, it is highly relevant in the event that your mod auto fires or the button on the device is actuated when it is unattended. If your devices power demands are within the CDR of your battery, it will discharge until it is flat without being damaged or reaching unsafe internal temperatures. If your setups power demands grossly exceed CDR, your battery will be damaged and in extreme cases vent or explode. Beware of 'pulse' ratings.

There is no need to exceed CDR. Should your atomizer require more than one battery can provide, then the amp load should be split/shared using a multi-battery mod.
Mooch's test data should be consulted for a reliable CDR value for any given battery.
List of Battery Tests | E-Cigarette Forum

Pulse Rating - Amp capability of a battery over an undefined short period of time.
There are no standards or clear definitions for this value, and it should be discounted as a factor when choosing a battery. Some vapers choose to exceed the CDR of a battery, but at the low end this will reduce the lifespan of the battery, and at the higher end this can result in a venting or exploding battery.

Voltage Sag (voltage under load) - The transient drop in available voltage under load caused by the battery's internal resistance.
Some batteries have less voltage sag than others often leading them to be described as 'hard hitting'. On a regulated device voltage sag can be responsible for your mod displaying a 'weak/low battery' message even though there appears to be charge remaining.
What is battery internal resistance? | E-Cigarette Forum

The relationship between capacity (mAh) and the amp limit (CDR)

This can be thought of as akin to a seesaw. The higher the amp limit is the lower the mAh value will be, that's just how battery chemistry works.

So if your vaping style requires a battery with a high amp limit, a battery capable of delivering this will have a relatively low mAh value.
eg. The LG HB6 - 30 amp CDR, 1500 mAh

Conversely if your amp requirements are lower, then you could use a battery with an accordingly higher mAh value.
eg. The Samsung 30Q - 20 amp CDR, 3000 mAh

If your setup requires less than 10 amps it might seem attractive to use a high capacity battery in the 3100 mAh to 3600 mAh range. Unfortunately these batteries have very high internal resistances meaning that you will actually get longer run times using a good quality 3000 mAh battery.

As a rule of thumb there is no 18650 on the market that has:
  • A genuine CDR rating of over 30 amps
  • A genuine CDR of 20 amps capable of more than 3000 mAh
  • A genuine CDR of 10 amps capable of more than 3100mAh-3600mAh
  • There are no 18650s capable of over 3600mAh

Lies, more lies and re-wraps

It's worth mentioning that just because a battery has 50 amps written on the wrapper, it does not mean it is capable of it. I've been lied to to, How is this possible?!

There are only a few companies in the world with sufficient manufacturing power to actually produce batteries, those companies being Sony, LG, Samsung and Panasonic. All other batteries you see on the market are B or C grade castoffs from these companies that have been bought in bulk and 're-wrapped'. A new re-branded plastic wrap is placed on the battery before being resold at an inflated price. Inevitably the specifications shown on these re-wrapped batteries will also be inflated, usually with pulse ratings being passed off as CDR.

In my opinion re-wraps should be completely avoided, and only genuine battery's from Sony LG Samsung and Panasonic should be used purchased from reputable vendors.
List of Battery Tests | E-Cigarette Forum
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