Battery Safety 101: Anatomy - PTC vs PCB vs CID February 18 2015, 3 Comments

The different kinds of protection inside and outside your 18650 batteries.

Figure 1. A close-up look at the anatomy of an 18650. Take a look at the different protection devices. By NASA.

Internal protective devices:

PTC (Pressure, Temperature, Current) Switch.

  • Built-in to almost all 18650's
  • Inhibits high current surges
  • Protects against high-pressure, over temperature
  • Resets and does not permanently disable the battery when triggered. However it's best not to trip them often as it irreversibly increases their electrical resistance by up to a factor of two and makes them more likely to have catastrophic failure.
  • May not work when module included multi-cell series and/or parallel configurations

CID (Current Interrupt Device)

  • Built-in to almost all 18650's
  • Not visible by just looking at your battery
  • Collocated (placed side by side) with the PTC
  • Is a pressure valve, which will disable the cell permanently of pressure in the cell is too high. (For example, if you battery over-charges and reaches over 145psi.)
  • Works by releasing the connection of the positive terminal, rendering the plus pole useless.
  • Does not always reset, does not always open completely when needed
  • May not work when module included multi-cell series and/or parallel configurations

Tab/lead meltdown (fusible link type)

Fuses and tabs that connect batteries joined together are designed to break the circuit under high voltage.

Figure 2. External short circuiting under Vacuum conditions. By NASA.

Bimetallic disconnects

Figure 3. How a bimetallic disconnect on 18650 batteries work from HVAC.

Temperature changes will let the metals expand or contract. When bimetallic blade is 'closed' or 'down' it provides contact and forms a circuit. Because the current provides heat, the metal begins to expand. This prevents the temperature from ever getting too hot or too cold.

 

External protective devices:

Diodes

You've probably heard of LEDs (light emitting diodes), but what is a diode? It's like a valve, and only let's current flow one way. For a better understanding, check out this video: 

Vents

  • Basically little holes at the top of your battery
  • Will spew toxic chemicals like ether instead of exploding

Thermal fuses (hard blow or resettable)

  • Sometimes called PTC resistors
  • Often hidden just beneath the positive cap

PCB - Circuit boards with specialized wire traces

  • Highly recommended for older lithium ion batteries.
  • Not necessary in newer, safer chemistries like INR
  • Mainly used in flash-lights, NOT in vaporizers or other high-drain applications
  • Limits amp discharge to 6A or lower
  • Protects against, over-charge, over-discharge, short-circuiting, and potentially other things.

Let's take a look at a popular protection board circuit in use on 18650 batteries, the Tenergy 23002 PCB with a 6A cut-off

Figure 4. A close-up of an 18650 PCB protection board

 

This board has the following features:

  1. Over-charge protection
  2. Charge protection
  3. Over-discharge protection
  4. Over-current protection
  5. Short protection

This is what an 18650 battery looks like when it is connected to a PCB:

Figure 5. The anatomy of a protected 18650 battery by Lygte Info

Does your battery have protection circuitry?

18650 batteries sold in the US are required to have CID and PTC protection. However most cells for vaporizers are sold without PCB's. This is because the PCB will limit the amp discharge of your battery to 6A, when vaporizers need 10A - 30A. 

To know whether your battery has PCB protection, there are a few signs:

  • Your battery is longer than the unprotected version (use Best 18650 Battery to look-up the size).
  • The bottom of your battery is not steel (the color is copper, or some other color different from your top cap).
  • You can feel the wire running from the negative pole to the positive pole on the side of your battery.

What kind of battery does TESLA use?

Tesla uses 18650 batteries but has modified them. They have taken out the PTC and CID protection circuitry and made them truly bare-bones. Instead of relying on these protection devices, TESLA has made their own out of a type of foam that floods the battery module and prevents fire.