Boron overcharging battery

I was just looking through my database where I log a few device health statistics for all our devices and noticed one of my three prototype Boron devices climbed up to ~ 4.5v over the weekend.

The devices use 18650 batteries (which I’m guessing are about to vent…if they haven’t already) and charge via a small solar panel. They are in a system.sleep() state most of the time, setChargeVoltage is at 4208 (mV) and setInputVoltageLimit is 4840 (which I hope isn’t where it will go to before stopping the charge).

Device is in manual mode, runs very efficiently to get weeks of charge when out of the sun, and the other two devices are doing ok.

Any ideas? Maybe a PMIC controller burned up? Anything I might have done in software?

Hopefully the 4.5V was an incorrect reading.

If not, that implies that you’ve used an Un-Protected 18650 Cell, which you should always Avoid for these applications.

As you would expect, the PMIC would never attempt to charge higher than your setChargeVoltage of 4.208V. (I assume that you are using the Boron to perform the charging, and not another board to handle the Solar Input and charging).

Also as you already expected, setInputVoltageLimit won’t influence the termination voltage of the charge cycle. That’s only to help keep your Solar Panel voltage from collapsing due to current demands.

Being in Sleep mode shouldn’t be your problem either, but I haven’t specifically checked with the recent firmware versions. In the past, the PMIC would still obey the settings when the Boron went to Sleep.

This may be a long shot, but elevated temperatures will cause an increase in the Cell Voltage.
Is there any chance that the 18650 was charged to the terminal voltage of 4.208V by the Boron, then the enclosure/battery temperature was increased significantly by the Sun ?

If you are charging to 4.208V, you really must include temperature monitoring for the battery.
And Always only use batteries that contain their own protection circuit.

If elevated temperature (after the charge cycle) isn’t the culprit, I’m also very interested in how/why a Boron charged above 4.208V.

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The PMIC will not charge that high and no matter what I have tried could never get it to charge past 4.2v for any decent period of time.

Just for yours and other peoples safety do try and use Li-Ion cells with BMS chips on board. sells them if you are looking.

The PMIC has tons of self protection features built in so if it was damaged I doubt charging or any of the PMIC charging features would work at all.

Keep any eye on it and if you see that high of a voltage again check the cell voltage with a multi-meter to see what it really is, I’m assuming you have access to that device your talking about.

I do see elevated rates during the day that I consider normal- but in this case it dropped back down to 4.51 from 4.58v once the night (colder than usual even) set in. Right now I’m thinking there’s something messing with its readings rather than the battery actually happily sitting at such a high voltage because otherwise I would have expected the battery to have an event by now. Looks like I’ll have the opportunity to retrieve the device later today and get some more information, hopefully be able to put it back out in the field.

BTW: the batteries do supposedly have the protection circuit in the cap, which I can confirm there is at least something in there, though if it’s actually at that charge then it’s clearly not doing its job. These are the batteries:

Looks like I’ll get access later today, so I’ll report back with what I find. Batteries are these:

which at least seem to have a BMS under the positive side cap…if they’re really running that high it must not be working…really hoping it’s something wrong with the PMIC reading and not the battery.

I looked at the datasheet and didn’t see any Over-voltage Protection (OV). But maybe I missed it.

@Rftop, I looked at the datasheet and the Sparkfun site as well and I don’t see ANY protection hardware whatsoever. Another battery on Digikey from Adafruit seems to have the protection:

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@peekay123 @Rftop
Page 16 at least makes a statement about an overcharge test, but it’s pretty vague (“no explosion, no fire.”) I may be taking a battery apart today too, depending on what the numbers look like on inspection.

“Seem” in this case, I meant there’s definitely a separate segment inside the battery between the cell and the positive cap where a circuit should be. It could just be a spacer… If that’s the case I’ll be in for replacing a bunch of batteries in the near future :frowning:

but so far at least that still doesn’t explain any mysteries on either overcharging the cell itself or misreading the cell voltage.

FYI reporting is via PMIC, where getVCell is returning 4.5+v and getSOC is returning ~150%. So the system at least thinks it’s observing this behavior…

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Indeed, we are all still interested in your findings.
It would be great to see a picture of your setup and a brief explanation to the Power Circuitry, when you get a chance.

Adding to @RWB’s comments above, I’ve tried to find any situation to get the Boron’s PMIC to operate a 3.7V Li-On/Li-Po in an unsafe condition. It’s performed extremely well even when I purposely abused it for testing.

If your Multi-Meter confirms the 18650 cell is at 4.5V, please don’t try and use that cell.
Venting may happen during a recharge cycle way into the future. Consider that Cell a loss.

And yes, I believe everyone would agree you should swap all your other devices to Protected Batteries, regardless.

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New data: batteries are indeed overcharged: measuring 4.53v. @Rftop unfortunately might not be able to share that publicly since it’s a prototype device, but I’m having a good look over the circuit to see if there’s another route to the battery that could explain power bypassing the charge circuit…

Short description of that side of the circuit, at this point I’ve traced the tracks on my board and measured everything to make sure there’s nothing else connected:

Two 18650 batteries in parallel, positive side connects only to Li+ and to a common connection of a relay. Relay occasionally connects that to and inductor an then to “SW” of a 5v boost converter (TPS613222ADBVT).

Negative side of the batteries of course connect to a lot of things. Battery side is pretty simple, so the mystery continues…

This can be very problematic.

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@Rftop Can be- safety wise fusing is built in to my PCB tracks as a fuse-able link. If a battery starts pulling a bunch of current from the other one it will burn that through and I’ll lose capacity- I think this is how they do that in some of the big battery boxes too.

Otherwise I wouldn’t think this would give PMIC troubles would it?

Well… remember the PMIC was designed to operate a Single Cell 3.7V Li-On type.
You are operating a 1S2P pack constructed from unprotected cells which resulted in an OV condition of 4.5V.

We know that larger capacity Li-Po packs are indeed paralleled cells, but they also contain a protection and balancing circuit inside.

But I still cant think of a good reason for the 18650’s to reach 4.5V .
Statistically speaking, I’d lean towards putting my money on something external to the PMIC, if I were a betting man.

Please let us know either way.

It may be the relay / boost converter combo causing a voltage boost that is leaking back to the batteries causing the voltage increase.

Relays are known to have high voltage kickbacks and your boost converter is a 5v device so both could be sources of the higher voltage on the Li-ion cells which is dangerous. Imagine one of those venting or exploding unexpectedly while your around it.

Here is what the 18650 protection circuits do when they go over 4.25v.

The PMIC does not provide the battery voltage and SOC readings, the Fuel Gauge chip does that.

The PMIC will not charge the cells up to 4.5v so some other Circuit is causing it. My bet is the 5v boost converter feeding back or not setup properly.

The high voltage relay coil generates high voltages but only briefly so I doubt that’s it.

Just my $0.02 here: Your 18650’s are not protected. They are “naked” cells with solder tabs welded to them for easy soldering. I would strongly recommend using protected 18650’s. The protection circuit usually goes on the bottom (negative) end of the cell can, not under the top button, although a connection from the protection chip does go to the top button so the protection circuit can sense voltage on the cell.

I also have to second others here and say that using even 2 lithium cells in parallel without a balance circuit can be troublesome indeed.

I agree that it’s difficult to see how the PMIC is supplying upwards of 5V to the cells. But, hey, there’s a first time for everything, right?

Best of luck getting to the root cause.

Just wanted to follow up for future explorers or those interested:

I never found a satisfactory answer for the device. After much testing, time on the bench feeding it pseudo-solar power, with and without protected cells, the device didn’t repeat the overcharging issue. I have three identical prototypes, only the one overcharged its batteries, and only the once.

Wish I had a more substantial answer.

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