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We are using a Li-ion battery with built-in safety PCM (Enix MGL9070), a gas gauge to monitor the battery voltage (STC3115A from ST) and a battery charger (bq24133 from TI).

To protect the battery, the current strategy is:

  • when the gas gauge detects that the battery voltage is below 3.2 V, a warning is shown to the user (low battery) when the board is powered up. The system will go to sleep and the power consumption will then be quite low (100 to 500 uA).

  • the board then relies on the battery's built-in safety PCM to cut-off the power if the voltage goes below 2.75 V.

We initially had also a voltage supervisor at 3.2 V which could pull down the enable pin of the main regulator, so the board power consumption could drop down to almost nothing. This voltage supervisor gave us troubles (bad filtering, high voltage drop when powering other peripherals which triggers it, etc.)

The problem that we would like to solve is this: A user empties the battery down to the low battery warning and then leaves the system unattended. A year or two later, the user tries to charge the system again to use it.

Questions:

  • is it safe to rely on the built-in safety PCM of a Li-ion battery?
  • does it damage the battery to actually reach the built-in safety PCM and can it be then safely charged again?
  • would it be good design to do everything possible (using a voltage supervisor, for example) not to reach the built-in safety PCM of the battery?
  • does a better strategy exist?

EDIT 1: It is a general consumer product.

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  • \$\begingroup\$ Is it safe is a bit of a context-specific question. If the internal PCM has good reputability, for a general consumer product I would imagine it is fine. For a medical device, however... \$\endgroup\$ – Joren Vaes May 27 '20 at 9:39
  • \$\begingroup\$ Link to the battery in question's data sheet please. \$\endgroup\$ – Andy aka May 27 '20 at 9:53
  • \$\begingroup\$ @Andyaka the link is already in the post :) I copy it here: enix-power-solutions.fr/wp-content/uploads/2020/04/… \$\endgroup\$ – Marmoz May 27 '20 at 9:55
  • \$\begingroup\$ @JorenVaes that's a tricky question for the reputability. I guess Enix is a well-known brand. And it is actually a general consumer product (I made an edit). \$\endgroup\$ – Marmoz May 27 '20 at 9:56
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The only proper answer to this is really how much confidence do you need to have in your solution. If this is going to be a consumer product like you say, you will probably want to go after UL listing. I tend to always over-spec li-ion circuits, since failure with them is such a big failure.

For UL testing, the product will need to pass "single-fault" testing. This basically means that if any one component/solution fails, the battery shall not present a hazard. So, they would bypass the onboard battery PCM, and then overload/over-discharge/etc the system, and if you have no other protection, the battery would fail.

The "over-kill" approach that I have used is to have the onboard PCM, a PTC fuse, a battery manager (AP9101), and sampling the voltage of the battery with the MCU/gas-gauge like you suggest.

Note: the AP9101 is just an example, currently using it in a few designs

  • The PCM is treated as the last-ditch safety, and the goal is to never need it.
  • The AP9101 is spec'd to activate before the PCM on under/overvoltage. Potentially also on current.
  • Since the AP9101 typically will have a wide range of current when it will activate, I rely on that only for really fast events, and near dead-shorts. For slight over-current events that are out of spec, the PTC fuse is relied on.
  • The MCU sampling the voltage range of the battery allows you to stop charging at a lower voltage than the AP9101 is spec'd at, allowing for longer life of the battery. (This really isn't always needed, but it's a free addition)

Using these three solutions allow for any one to be bypassed or fail, and will still protect the battery. Of course, this adds some extra cost which may not be acceptable, but if properly implemented, it will pass UL.

...and can it be then safely charged again?

Depends on the charger IC. Some won't allow a deeply discharged battery to be charged again, others have a slow ramp up that allows it.

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  • \$\begingroup\$ +1 for sharing a design example and although I don't believe that the additional circuitry is necessary(explained in my answer), it doesn't hurt (except for the budget) and provides better control and extends the application to be more battery supplier independent. What you add on in costs in your own monitoring and safety control could reduce costs in selection of battery for the design. \$\endgroup\$ – Pau Coma Ramirez Jun 14 '20 at 11:51
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is it safe to rely on the built-in safety PCM of a Li-ion battery?

A better question might be, "is it safe to rely on the PCM in a UN 38.3 approved lithium ion cell or a battery constructed of such cells?".

And that answer is definitely yes. UN 38.3 approval requires a fairly extensive set of safety tests that a battery cell must pass. Furthermore, even if a battery is constructed from UN 38.3 approved cells, the battery as a whole still must be retested to receive its own UN 38.3 approval.

Among these tests are overcharge and overdischarge tests. They involve charging or discharging a battery/cell with a charger that, with no protections within the battery or cell, will charge or discharge the cell at twice the manufacturers maximum charge or discharge rate for 24 hours. The cell is then monitored for 7 days after this abuse to check and see if it does anything dangerous, like catch fire or explode.

In the case of Lithium Ion cells, the only way to ensure that this test doesn't result in a fairly spectacular test failure is a working PCM that reliably cuts off the cell from external loads or power to prevent damage.

UN 38.3's primary purpose is to ensure that batteries can be safely transported on air or ground freight. For example, in the US, it is illegal to ship lithium ion batteries that don't meet UN 38.3 approval except by ground, and even then, this must be declared and usually an additional hazard fee is charged by the carrier.

Simply put, you shouldn't ever use a lithium ion battery that doesn't meet UN 38.3 unless you have no other choice and are very aware of the risks and additional regulations that will apply.

If a cell or battery meets UN 38.3, then you can expect to rely on the PCM. If it is good enough for the United Nations Transportation Department, it is probably good enough for any commercial usage case. That's kind of the point.

The battery you linked is UN 38.3 approved, so that is a good choice and you can expect the PCM to do its job.

does it damage the battery to actually reach the built-in safety PCM and can it be then safely charged again?

Think about this for a second. If the battery falls below the cut-off voltage, the PCM allows the gates of two back-to-back (common drain) configured MOSFETs to discharge. Any stray charge has a conduction path through a comparator-actuated transistor inside the PCM IC, meaning that those MOSFETs are not coming on again unless the battery voltage exceeds the cutoff voltage plus hysteresis.

But such an increase will never occur, because there is no longer a working current path into or out of the battery. You can't discharge it. You can't charge it. There is no way to get any energy into the cells to raise their voltage. At least not without disassembling the pack and cells and removing the PCM. And you are not liable for the results of such activities. If someone really wants to circumvent your protection, they can, but if they do so, it is also their responsibility, not yours.

So let's be clear: if the battery gets discharged low enough to trigger the PCM, either it was a transient drop due to a heavy load, and the terminal voltage recovers enough to be higher than the cutoff and the hysteresis, then it can be charged again. If it recovers to a higher terminal voltage, it was not really discharged below the cutoff, but rather ESR voltage drop made it seem like it was. Hysteresis ensures that only when this is really the case, can the cell return from cutoff.

However, if the cell is really over discharged, or like in your example, the user leaves it uncharged for a year and it self-discharges below the cutoff, it's gone. The PCM will have cut off all current paths to the cell, so there is no way any charger can even charge the cells to raise the voltage again. The voltage cutoff is set at a point that the manufacturer of the cell has chosen as the lowest safe voltage to charge from. If a cell discharges below this voltage, it means the manufacturer considers it unsafe to charge it again and the cell is forfeit. The PCM ensures it cannot be charged again, and this is the intended outcome. The user is forced to replace the battery, which is the correct action to take.

Whether or not it really damages the battery or not is not really important. What matters is the manufacturer believes it could damage their cell, and it isn't worth taking chances. And the answer is no. It cannot be safely charged again. Best practice is to simply consider an overdischarged Lithium Ion cell as destroyed.

If this sounds weird, it isn't. I have personally experienced the self-discharge death of a laptop battery pack because I forgot to leave any charge in it, let it sit unused for a year, and low and behold, the terminals had gone high-z (mosfets turned off). It couldn't be charged. It couldn't be discharged. It was gone.

This is a functionality that exists in most protected lithium ion cells, and letting a device sit unused without charge for a year or two will destroy the batteries in most devices.

would it be good design to do everything possible (using a voltage supervisor, for example) not to reach the built-in safety PCM of the battery?

Yes. Because the battery will effectively be destroyed from the viewpoint of the user, and from the safety stand point of the company that made the cell the moment it falls below the PCM's cutoff voltage. The PCM cutoff is only ever going to happen once. There are no second chances. It is the final and ultimate safety measure and when it engages, it is permanent.

You don't need to avoid relying on it for safety reasons though, it is quite safe and will work. You don't want to rely on it because you only get to rely on it one time. And after that, the battery is toast. Safely toast, but toast all the same. So I guess it depends on your priorities.

does a better strategy exist?

Not really. At the end of the day, you're at the mercy of the user and the battery's own self-discharge, and there isn't really anything you can do about it. Lithium Ion batteries cannot really be expected to survive more extended periods of time in a mostly-discharged state.

Your only options are really more or less what you're already doing: try to warn or otherwise prod the user into preventing the untimely death of their battery when it starts getting really low. Then, you can increase the chances of survival a bit by having a second voltage cutoff where the gas gauge and whatever else you have attached go into deep sleep mode and stop working/warning the user. This reduces the drain to just the self-discharge and whatever the drain of the PCM is (but, typically, this will be very small in comparison to the battery's self-discharge).

After that, you've really done all you can. It is up to the user now, and beyond maximizing the time they have to start charging the cell again by reducing the discharge to the minimum (which is, for all intents and purposes, the self-discharge rate of the cell itself), it is out of your hands.

It isn't ideal, but it also mostly works. It is far better that one's forgetfulness destroys $100 of lithium ion batteries than that same forgetfulness (combined with allowing overdischarged cells to be recharged) instead causing a fire and many thousands of dollars of damage, or worse. And I say this as someone who actually managed to destroy a pretty expensive laptop battery. I'm glad it didn't let me charge it again.

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is it safe to rely on the built-in safety PCM of a Li-ion battery?

If its from a reputable manufacturer and for regular applications, yes. It can give you more confidence if you have schematics of it and can analyze its reliability.

does it damage the battery to actually reach the built-in safety PCM

There is some damage to the battery if it reaches PCM. It will definitely loose some capacity and longevity. If the battery is expected to drop below 2.75V multiple times during its lifespan, then I'd advise to cut power at a higher Voltage to limit damage. The factory PCM is just a safety net so the battery wont be rendered useless in such a situation.

can it be safely charged again?

Some charging IC have certain protections built-in that might disable charging or only at a much slower rate than normal. (I haven't checked yours yet)

would it be good design to do everything possible (using a voltage supervisor, for example) not to reach the built-in safety PCM of the battery?

If the battery is meant to last and expected to drop frequently into low voltages, yes.

does a better strategy exist?

Depends, I suggest to look into having an option to replace batteries if a worst case scenario occurs.

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Answers:

is it safe to rely on the built-in safety PCM of a Li-ion battery?

Of course nothing is 100% safe but we do the best we can. 2.75V as cut-off gives a bit of margin over the "typical" minimum discharge of 2.5V.

Even discharging further, dependind on the discharge characteristics (fast/slow/pulsed/...) , according to this study Dischargin further should not be much of an issue, and detectable durring charging if it is a problem.

If you are still in doubt, I would get a certificate from the company that is providing you the battery describing and certifying this feature and in case there is a problem you are covered.

does it damage the battery to actually reach the built-in safety PCM ...

The purpose of the PCM (Protection Circuit Module) is to protect the battery from damage. If it is from the manufacturer of the battery, they will have fine tuned it to their battery characteristics. If a third party adds a PCM to a battery, packages it and then sells it. You should contact with that party to confirm.

...and can it be then safely charged again?

During charging is where you can detect if it is safe to continue charging. Control and Monitor how the battery is being charged and keep it within your defined safe charging parameters/profile.

would it be good design to do everything possible (using a voltage supervisor, for example) not to reach the built-in safety PCM of the battery?

At the end of the day, no matter how many things you do to prevent further discharge, if a device is not used for a while, a battery will self discharge, so eventually it will discharge beyond whatever point you have defined.

Most of the danger in a Lithium cell is when there is energy stored in it, so discharging it you are getting rid of the stored energy which could cause internal overheating when dissipated in an internally generated short circuit and cause a run-away thermal event.

does a better strategy exist?

You have control over the charging and extensive monitoring of the battery with the chips you have in your design, so whenever the user connects the device for charging, monitor it, regulate the charge current and decide wether you consider it safe or not to continue charging. If the behaviour observed does not fit within your safe/normal operational profile, inhibit the charging (and perhaps inform the user with an error code)

Resources:

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  • \$\begingroup\$ Eh. It really depends on what is defined as "overkill". For example, UL listing requires lithium-ion batteries to not have a "single fault" failure. They (UL) will bypass the built-in PCM, and see if something bad happens. In your example, the battery would fail, and as a result, fail UL testing. This is to ensure that the failure of any one component doesn't cause the battery failure. \$\endgroup\$ – Kyle Hunter Jun 13 '20 at 23:17
  • \$\begingroup\$ Depends on what you consider failure, even without the PCM, according the study explained in the paper I added in resources, overdischarging will not likely cause failure. In any case, no matter how many redundant/additional systems you have after/behind the PCM, if the PCM were to fail in short circuit, you probably will have a problem. I do like your approach though of "how much confidence do you need to have in your solution", so I will modify my answer to remove my assertions of yes/no and just provide my feedback. \$\endgroup\$ – Pau Coma Ramirez Jun 14 '20 at 7:46

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