BU-304: Why are Protection Circuits Needed?

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. (See BU-304b: Making Lithium-ion Safe.)

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 2015-09-16

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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 [email protected]

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

On April 12, 2015 at 3:58am
geoff wrote:

My 36v 9aH ebike battery’s power indicator shows only 20% after charging. The battery voltage measured at the output socket is exactly 36volts. The battery is only just out of warrenty. Any idea about (i) the cause of the problem and (ii) How to resolve it.

On April 22, 2015 at 1:33am
Edward wrote:

geoff , maybe one cell in th pack is failure, you can find the professional person to find the bad cell and change them

On April 22, 2015 at 2:49am
geoff wrote:

Thanks Edward. I’ve measured the voltage between the battery positive and each of the balancing wires of which there are nine and the potential difference between each is 3.6v .This seems to be correct and wouldn’t suggest a bad cell. I’m reluctant to take the cells apart without any indication of a defective cell and I wonder if there could be a malfunction in the control circuit board. This has a number on it : KLH36S88A and I’d be obliged if anyone has any info on this board.

On April 24, 2015 at 4:15am
Jonathan wrote:

Yes Edward I am well aware of the danger and it is a test which I carry out almost every day. The test is however conducted in side an explosion proof box and contact is made via an external breaker. As Intrinsic Safety is mentioned above, all cells and batteries used in side of IS equipment must be subjected to the short circuit fault condition tests and a maximum temperature and current obtained (sometimes even with PTC device removed) as per the 60079-11 standard for IS equipment protection. This seems bizarre to a lot of people but these are tests which must be conducted to ensure user safety!! regardless of whether it is in a hazardous or normal location.

On May 10, 2015 at 4:36am
Ed wrote:

Most protecion circuits i looked at disconnect the battery when a overcurrent situation occurs, a ptc for instance allows a larger current for a certain amount of time until it is heated so far that it disconnects or limts the current, electronic protection circuits do the same but disconnect faster.
This means that for a short time you can have a large current that is only limited by the total resistance in your circuit. The current of 32 amp you measured could be correct, it could even be higher, looking at the voltage across a resistor in your circuit with a fast digital memory scope can be very interresting (and maybe shocking).
I once designed a IS battery pack under the old EX rules and it first failed the short circuit test done by the certifing agency because it diconnected the current instead of limiting it. By adding extra circuits to limit the current i managed to create a pack that passed the certification test

On May 19, 2015 at 12:23pm
martyn jenkins wrote:

hi I have a Makita 36v li ion battery drill about 2 years old but unused I have 2 new batteries with it and one good second hand which worked fine when bought I went to charge them this week new ones charged ok but second hand one would not take charge and when in drill the led light comes on but no drill I measured between contacts and shows 36v was thinking of buying CADEX 7400ER to test this and do other batteries any help please thanks martyn

On July 7, 2015 at 2:23am
Absolom Mykhal wrote:

i am in the process of building an 18650 battery pack for a small solar system its a 3s 80ah battery pack with a pcb i just want to find out can i make 2 or 3 of these packs and wire them in parallel to increase amperage as is usually done in solar systems with any problem or effect to the battery packs then also is there a big difference with a pcb or bms that has balancing and one that does not

On September 6, 2015 at 8:20pm
Robert wrote:

Does a simple li-ion (actually, lifepo4) battery protective circuit board “eat up” a portion of the voltage in the same manner a voltage regulator would? Or does it somehow not drop any of the charging voltage and use the (3.2v) battery, and some little current, to protect the battery from over/under discharge?
Thanks in advance.

On September 23, 2015 at 5:43am

I need intrinsic safe batery of 2700mAh with 3 pin, please suggest some suitable part number.

On September 29, 2015 at 4:23am
Antony wrote:

Hi I have a Bosch 18v li 4ah battery out of a drill, hardly been used charged 5 to 10 times only, I know I shouldn’t of done this but used it to power my makita site radio and I think it depleted the battery so much when I go to charge it battery charger doesnt recognize it or something, is the battery fooked?  Or is there something I can do to trick the charger to make it charge. Many thanks for any advise antony