Voltage Failure Modes

Question from a reader: "Hi Dan, What is the failure mode for Lithium Polymer batteries? I understand that Nickel Cadmium batteries fail as a short circuit usually. Therefore, a pack of NiCads will continue to operate with a shorted cell but with a voltage that is one cell lower. Do LiPo's fail...resulting in the loss of the entire pack? Thanks

Answer:

First of all the reference to a "failure mode" is not necessarily a singular event. It is in effect a catch-all phrase representing potential problems. I wrote a 3-part article series on the concept of Battery Failure Mode and Effects Analysis so for more detail please see my blog. But at the core of the concept failure modes in general are not new but a phenomenon in manufacturing quality assurance analysis (this phenomenon could also be applied to other industries).

Specific to the readers questions on lithium polymer batteries there could be a number of "failure mode(s)" and sub-failure modes related to the manufacturing and personal usage of a battery including Li-Po chemistry based cells. For example:

Batteries can have faulty cell design

Batteries can be manufactured under uncontrolled processes

Batteries can be operated in uncontrolled conditions

Batteries can be abused

Batteries can degrade and lose power

Heat and water for example are not good for any battery. But it is surprising to me how many people use their battery and device in both and of course that represents several potential failure modes. An example is when one of our customers several years ago always took his iPod mini into the sauna at his gym. Well that is not the best place for a battery or device. Eventually his battery "died" and so did the device (batteries technically do not die, but, they do degrade to the point where they will not transfer electrons from an electrical source to the internal chemical, for the required electrochemical energy change necessary recreate electricity for a device). That is a bad failure mode brought about be a user.

Another example is when there are metallic electrode shavings evident in the batteries electrolyte. When current is running from one electrode to another through the electrolyte the shavings cause any battery to short circuit. The short circuiting could result in the battery smoking, bubbling or stopping. That is an example of bad manufacturing and of course a failure effect. Another example of a failure mode is when you open the battery casing. If you open the casing once it is sealed hermeneutically then you run the big risk of causing a failure mode to occur.

In any event - any failure mode will cause a battery to either operate at a lower performance than originally designed or stop working period. One interesting side note is that devices can operate with a battery that is at a lower voltage (but not higher voltage) as the device originally manufactured. The reason is because a battery's voltage is not necessarily a static measurement.

Volts - or V - are an electrical measure of energy potential. Voltage can also be thought of as the amount of "pressure" of electrons that pass from a negative connector to a positive connector. Or V can be defined as the measure of the strength of an electrical source of power for a given current level.

Voltage can also be defined as the Electrical Potential difference - a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. In the fields of electronics the electrical potential difference is the amount of work per charge needed to move electric charge from the second point to the first, or equivalently, the amount of work that unit charge flowing from the first point to the second can perform.

Mathematically voltage is commonly measured by V= I x R; where V=Voltage, I=Current, R=Resistance.

Beyond the definition what challenges many is the confusion that a battery contains four unique types of voltage measurements.

Each of the following voltage measurements can be taken from a battery:

Float Voltage - is battery voltage at zero current (with battery disconnected).
Nominal Voltage - is battery voltage range 3.7V, 5.2V, 10.2V, 12V etc that says that a voltage range exists depending on the number of cells in the battery. For example a 12 Volt battery is made of 6 cells and has a Float voltage of about 12V.

Charge Voltage - The voltage of a battery while charging.

Discharge Voltage - The voltage of a battery while discharging. Again, this voltage is determined by the charge state and the current flowing in the battery.

So yes a battery with a drop in voltage will continue to operate but not necessarily at a level that will power the intended device.

? Dan Hagopian
You have permission to publish this article electronically or in print, free of charge, as long as the bylines are included with link. Dan Hagopian of http://www.batteryship.com has been writing about consumer electronics since 1996. Dan has a personal passion for understanding how electronic devices can be powered, for learning about various types of battery devices and their internal systems, as well as the interaction of batteries with interfacing systems, and finally about the internal control mechanisms of batteries. Dan writes on battery power related devices for mobile computing and portable power. Dan writes about this technology extensively and his work can be found at http://www.batteryeducation.com

Process Failure Modes and Effects Analysis (PFMEA) From the Process Flow to Production

PFMEA is a tried and tested approach to critique risk. Its purpose is simple, to identify potential failures, their consequences, the seriousness of the consequences and the reasons why they happen. Control methods are considered in two ways:

The first being activities in place which reduce the likelihood of the reasons for failure happening. The second being activities which tell you when a failure has occurred and thus enabling you to do something about it.

The overall purpose is to establish where the high risk areas are based on the current situation and then take action to reduce the risks. The approach taken and the timing of the task are instrumental to achieve maximum payback. When used correctly it is one of the singly most powerful quality tools available to companies. Process FMEA used in isolation like this is very thorough, but a missed opportunity to reap the full intended benefits of the document.

PFMEA strategy

Process Flow Diagram

The starting point is not the PFMEA but defining the process in the process flow diagram. Care must be taken here to select the correct wording and level of resolution. The output of process flow diagram must then be synchronised with the input to the PFMEA so that you critique the correct aspects of the process for risk.

PFMEA

You now compile your PFMEA in terms of failure modes, effects, severity of effects, causes of failure, prevention controls, occurrence, detection controls and their ranking followed by priority. At this stage it is still important to have recommended actions to reduce risk. However you now need to bring the PFMEA to life through the practical deployment of the content of the PFMEA to production. Primary data being the prevention controls to avoid failure and the detection controls to detect failure at the various stages of the process as defined in the process flow.

Production documents

It is essential that the primary data contained within the PFMEA is displayed at the point of use in plain view to the person who needs to know and act on the data. Failure to do this simple but necessary task will result in a company being exposed to the latent risk of a loss of process control, often resulting in customer complaints. Deployment typically is through the use of visual instructions located where the task takes place, sometimes the data flow is via the control plan (automotive sector), although this does depend of sector specific requirements.

Key points for consideration are:

Sufficient resolution of the process steps in the process flow diagram. The data contained in the process flow diagram is synchronised to the functional requirements column on the PFMEA. PFMEA is a true reflection of the current situation, as a living document. The prevention and detection controls defined in the PFMEA are communicated to the person carrying out that particular step of the process at the location where the task is undertaken. Appropriate reaction planning is identified at the process step location to clearly identify what to do if a failure is identified.
Summary

This article is intended to provide an overview of the necessary linkage of the PFMEA to achieve maximum benefit to a business who are considering the utilisation of PFMEA for the first time or are existing practitioners but don't believe they are reaping the true benefits that PFMEA can deliver.

Roger Thorpe has been an FMEA facilitator and consultant for 13 years. Many of his customers being tier 1 automotive manufacturers. For more information on PFMEA construction from an expert visit his website: http://www.fmea.org.uk