BU-304: 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 devices open permanently and render the battery useless; others are more forgiving and reset. The Polyswitch™ is such a re-settable device. It creates a 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 the current and disconnects on excessive load conditions. All switching devices have a residual resistance during normal operation, 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 the formation of high currents, which could lead to excess heat, sparks and explosion. Authorities mandate IS batteries for two-way radios, gas detectors and other electronic instruments operating in hazardous areas such as oil refineries, chemical plants and grain elevators. There are several levels of intrinsic safety, each serving a specific hazard level, and the requirements vary from country to country. The provisions are in addition to the protection circuit for lithium-ion, and the approval standards are rigorous. This results in a high price for the battery.

Making Lithium-ion Safe

Battery packs for laptops and other portable devices contain many levels of protection to assure safety under (almost) all circumstances when in the hands of the public. The safety begins with the battery cell, which includes: [1] a built-in temperature switch called PTC that protects against high current surges, [2] a circuit interrupt device (CID) that opens the electrical path if an over-charge raises the internal cell pressure to 1000 kPa (145psi), and [3]a safety vent that releases gas in the event of a rapid increase in cell pressure.

In addition to these internal safeguards, an external electronic protection circuit prevents the charge voltage of any cell from exceeding 4.30V. Furthermore, a fuse cuts the current if the skin temperature of any cell approaches 90°C (194°F). To prevent the battery from over-discharging, a control circuit cuts off the current path at about 2.20V/cell.

Each cell in a string needs independent voltage monitoring. The higher the cell count, the more complex the protection circuit becomes. Four cells in series had been the practical limit for consumer applications. Today, new chips accommodate 5–7, 7–10 and 13 cells in series. For specialty applications, such as the hybrid or electric vehicle delivering several hundred volts, specialty protection circuits are made, which sharply increases the overall cost of the battery. Monitoring two or more cells in parallel to get higher current is less critical than controlling voltages in a string configuration.

Protection circuits can only shield abuse from the outside, such as an electrical short or faulty charger. If, however, a defect occurs within the cell, such as contamination caused by microscopic metal particles, the external protection circuit has little effect and cannot arrest the reaction. Reinforced and self-healing separators are being developed for cells used in electric powertrains, but this makes the batteries large and expensive. While a Li-ion for a laptop provides a capacity of 170–200Wh/kg, the EV Li-ion has only 100–110Wh/kg.

The gas released by venting of a Li-ion cell as part of pressure buildup is mainly carbon dioxide (CO2). Other gases that form through abusive heating are vaporized electrolyte consisting of ethylene and/or propylene. Burning gases include combustion products of the organic solvents.

Li-ion commonly discharges to 3.0V/cell. This is the threshold at which most portable equipment stops working. The lowest “low-voltage” power cut-off is 2.5V/cell, and during prolonged storage, the self-discharge causes the voltage to drop further. This causes the protection circuit to turn off and the battery goes to sleep as if dead. Most chargers ignore Li-ion packs that have gone to sleep and a charge is no longer possible.

While in the ON position, the internal protection circuit has a resistance of 50 to 100mOhm. The circuit typically consists of two switches connected in series; one is responsible for the high cut-off, and the other for the low cut-off. The protection circuit of some smaller cellular batteries can be relaxed, and some get away with only the cell’s intrinsic protection and/or an external fuse. The absence of a full protection circuit saves money, but a new problem arises. Here is what can happen.

Some low-cost chargers rely solely on the battery’s protection circuit to terminate charge current. Without a functioning voltage termination switch in the battery, the cell voltage can rise too high and overcharge the battery. Heat buildup and bulging are early indications of pending failures before potential disintegration occurs. Figure 1 shows a battery that has fragmented while charging in a car.

Exploded cellular phone


Figure 1:
Exploded cellular phone

Generic cell phone disintegrated while charging in the back of a car.Combination of unsafe battery and charger can have a lethal effect. Manufacturers advise only to use approved batteries and chargers.

By owner’s permission


A concern also arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. This can fuse the solid-state switches into a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but fail to provide the required safety.

Low price makes generic replacement batteries from Asia popular with cell phone users. While the quality and performance of these batteries is improving, some do not provide the same high safety as the original branded version. A wise shopper spends a little more and replaces the battery with an approved model.

I receive many questions on www.BatteryUniversity.com from visitors wanting to know why the aftermarket does not provide low-cost laptop batteries as readily as cellular batteries. This is mainly due to safety. While a 1,400mAh cellular battery stores only 4Wh of energy, a laptop battery holds about 60Wh, 15 times more. Many manufacturers of consumer batteries protect the batteries with a security inscription that very few can break. Although aftermarket batteries are available, many do not offer all the functions of the branded version. Typical problems are fuel-gauge errors and not being able to charge correctly.

Manufacturers of lithium-ion batteries do not mention the word “explosion” and refer to “venting with flame” or “rapid disassembly.” Although seen as a slower and more controlled process than explosion, venting with flame, or rapid disassembly, can nevertheless be violent and inflict injury to those in close proximity. The court hears many legal cases involving laptops and other batteries that are said to have caused property damage, fire and personal injury. This is also a large concern in the aviation industry. Most of the batteries for consumer products are shipped by air just in time for improved inventory control.

Simple Guidelines for Using Lithium-ion Batteries


*   IATA (International Air Transport Association) works with airlines and air transport industry to promote safe, reliable, secure and economical air travel.


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?