BU-304: Protection Circuits

Learn how to make batteries safe with built-in protection circuits.

Batteries can release high power, and most packs include protection to safeguard against malfunction. The most basic safety device in a battery is a fuse that opens on high current. Some fuses open permanently and render the battery useless; others are more forgiving and reset. The positive thermal coefficient (PTC) is such a re-settable device that creates high resistance on excess current and reverts back to the low ON position when the condition normalizes. A third method is a solid-state switch that measures current and voltage and disconnects the circuit if either value is too high. The protection circuits of Li-ion work on this basis.

All switching devices have a residual resistance, which causes a slight increase in overall battery resistance and a subsequent voltage drop.

Intrinsically Safe Batteries

Intrinsically safe (IS) batteries contain protection circuits that prevent excessive currents that could lead to excess heat, sparks and explosion. Agencies mandate intrinsically safe batteries for two-way radios, gas detectors and other electronic instruments that operate in hazardous areas such as oil refineries, chemical plants and grain elevators.

Intrinsic safety requirements are divided into a specific hazard level and the requirements vary from country to country. North America has the Factory Mutual Research Corporation, Underwriters Laboratories (UL) and Canadian Standards Association (CSA). Europe has the ATEX directive; while in other countries follow the IECEx standards. To facilitate world trade, agencies engage in harmonization to eventually agree on a mutually approved standard.

Last Updated 2/20/2015

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On April 25, 2011 at 1:40pm
Rudy Gerritsen wrote:

I am charging two 3.2 volt 1500 mAH batteries in parallel wiith a 4.5 volt solar panel. I use a SPDT relay to switch the batteries in seies after the panel goes dark or no sun light,
and in parallel for charging when the panel sees sun light. The device I am powering requires 6.4 volts
I would like to do this without using a simple relay. I tried using a PFet for switching the batteries in series, without success. Any ideas?
Thank you,

On May 18, 2011 at 5:58pm
VWFringe wrote:

Need directions for retrofitting CPM to unprotected 18650’s.  Four pads are given: B+,B-,P+,P-....  Can I leave P+ and P- open, and can I use cat-5 solid conductor wire?

On November 4, 2011 at 12:16pm
Ernest Danso wrote:

I have a 7.4V battery that output about 13500mAH but doesn’t have any protective circuit. Can someone recommend one for me or know any thing out there that i can design. Thanks

On February 23, 2012 at 5:17am
Spring Grove Battery LLC wrote:

I rebuild power tool batteries and have been working with the lithium batteries but have found that they have a time out protection device in them.  Is it possible to reset this device or replace it.  Do you know if the factory scan tool would be able to reset or detect the time out device.  Thanks Joel

On April 15, 2012 at 1:33pm
Smac wrote:

How do you wake up a battery that has gone to sleep? I have about 12 bosch 10.8v Li batteries that no longer charge.

On June 7, 2012 at 10:29am
kurt anderson wrote:

we have a hazardous environment that requires intrinsically safe instrument.  Are any low voltage bateries (watch or AAA) acceptable.  2 devices we would liketo approve are a laser pointer for training and a simple disc camera (no flash). Both use 2 AAA batteries.  These seem very low risk just trying to determine if any exceptions exist for low voltage devices. 

On June 13, 2012 at 1:08pm
Jason Long wrote:

Are the safety circuit components for Lithium Ion batteries (internal PTC and the safety board) mandatory per any sort of standard, or does industry just do this because it’s the right thing to do for consumer safety? 

Further to that, are the NTC output to chargers and/or temperature monitoring ICs on the safety boards required?  Would you deem a battery pack without at least one of these protection mechanisms as unsafe?  Would it violate any standards?


On November 6, 2012 at 3:22am
M.Rajashekar wrote:

battery over charge protection circuits

On August 7, 2014 at 7:56am
Patrick McEnnerney wrote:

How do you wake up ly-ion battery’s

On August 12, 2014 at 8:15am
Jonathan Samuels wrote:

If I have a Li-Ion battery which contains a safety cut off device with a cut off current of 2A is it possible that a current well in excess of the ‘2A’ can be measured and present for a very small amount of time? I have simulated a short circuit fault condition to measure current (using a fluke 287 DMM and fluke i30 current clamp which gives a voltage output) and measuring close to 32A? I was wondering if there is something I am missing or doing completely wrong?

On August 18, 2014 at 7:26pm
Edward wrote:

Jonathan , you are crazy to do the test? it is dangerous? please email to me for more detail information zzrm316@163.com

On August 19, 2014 at 1:22am
Paul wrote:

I have a Li-ion laptop battery, it doesnt charge and it only discharges when the laptop is first connected to ac and removed, when u open the BIOS, the laptop shows that the battery isnt present even if it is operating on it. What might be wrong with it? B’se a few days ago it was functioning normally.

On August 19, 2014 at 7:58pm
Edward wrote:

is it the original battery??

On August 19, 2014 at 10:34pm
Paul wrote:

Yes edward, some days ago it was functioning normally but woke up one day and started malfunctioning.. I think it is original

On August 19, 2014 at 11:45pm
Edward wrote:

i think there is something wrong with the battery or laptop , you would better find another laptop or battery to check whose wrong?

On March 25, 2015 at 4:53am
veeravalli saibabu wrote:

basics of battery