Charging a LiPo Battery with an LED driver

[TmaxElectronics]

I am currently planning to build a portable LED studio light, that runs from a pair of 6 cell lipos, and would like to charge them inside the lamp.
But finding a charger circuit, with enough power and that still fits on the PCB might be tricky, so I was wondering if maybe it might be possible to repurpose (or perhaps abuse ) the LED driver for this purpose.
I'd use mosfets on the input and output of the converter to disconnect the batteries from the input and take power from a mains adapter, and then connect the output of the converter to the batteries.

I am using a dual channel TPS92682 (https://www.ti.com/lit/ds/symlink/tps92682-q1.pdf?ts=1597172678809) boost led driver,
that drives two strings in a color temperature adjustable LED.
It actually has a constant voltage mode, but the issue is that the CC/CV modes are mutually exclusive, so it can only run either CC with overvoltage shutdown, or CV with overcurrent shutdown. However the modes can be changed via SPI.

So my idea to get around this was to use the micro-controller in the lamp and sense amps. for each cell to monitor the voltage of the battery while charging.
Basically I would start off running the converter in CC mode with overvoltage lockout set at 25.5V(4.25V/cell), but continuously read the battery voltage and switch the converter into CV mode once 4.2V/cell is reached.
Then I would monitor the current and stop once it has dropped to 10% of the nominal.
Balancing would not be included, but should not be a problem, as each cell would be monitored for voltage (so even with imbalance, they wouldn't over-discharge) and the batteries will have matched cells. If this did become an issue after a while, it would always be possible to remove the batteries and balance them in a dedicated charger

The specs are:
150W total LED power limit, with a maximum of 100W per color temperature (a 55V cob)
two 70wh, six cell lipo packs, designed for model-flying
2.5A @ up to 60V, per channel of the converter
12V @ 13A mains adapter, for charging and powering the LED if mains power is available.

Has anybody tried something like this before? Would this even work, and more importantly be safe?
I'd like to hear your thoughts on it

[bin_liu]

This chip can be done.
By setting FB1 or FB2, ensure that the output port is not greater than 4.2V/CELL when the battery is not connected.
By setting CSP and CSN, the current does not exceed the charging current of the battery.
The whole machine works in CC+CV mode.

[TmaxElectronics]

are you sure?
the way I understand the datasheet, is that in CC mode the chip will shut off if the set voltage is reached, and in CV it will shut off if a certain current is exceeded.
So when using the CC mode, the battery would charge until it reaches 4.2V/cell, but then stop, which is no good when charging a lipo cell, as a CV charging phase is required for the last ~30%.

Or did I understand the datasheet wrong?

[vad]

Does the 6S battery pack include BMS with protection against overvoltage, overcurrent and overtemperature?

If not, you need a reliable BMS with primary protection for at least OV/OC/OT, and preferably with balancing. TI has plenty of BMS chips that support 6S packs. Ideally, you should also add secondary protection that can permanently disable the pack in the event if the primary protection fails. However most secondary protection circuits rely on chemical fuses, and sourcing those is virtually impossible.

And then you would need CC/CV charger. When using TI’s parametric search, I can find 5 chips that support 6S packs. For example, BQ24610 seems to be Ok.

PS. Before embarking on such a project, I highly recommend searching YouTube videos that demonstrate what could happen to mishandled lithium battery, and then rethink reliability of your own MCU-based OV/OC protection solution, weigh in risks of software bugs, consider all possible fault scenarios, create and execute test plan for each potential fault. Here is one example of such a video that clearly demonstrates amount of energy stored in lithium battery,  how fast the energy can be released:

https://youtu.be/CUgbmCSmSNY

[tooki]

It would not be safe. Just use a real lithium charger IC plus a BMS chip, just like vad said.

[NiHaoMike]

Actually limit the voltage to 4.1V/cell or less, you won't lose much usable capacity but will gain a significant amount of service life.

[TmaxElectronics]

searching YouTube videos that demonstrate what could happen to mishandled lithium battery

I sure know those videos, that's why I'm not 100% sure about this I witnessed a few going nuclear first hand too... not something I'd want to happen in my basement.

Does the 6S battery pack include BMS with protection against overvoltage, overcurrent and overtemperature?

overcurrent would be handled with a melting fuse, as the maximum continuous current of the pack is 60A. So short time overloads wouldn't cause it to go boom (the thing that would is the PCB )
Undervoltage is handled in hardware with an undervoltage lockout, and the uC constantly monitoring individual cell voltages, and a watchdog monitoring it (and those flight lipos usually get quite abused in terms of discharge without exploding anyway, so that would only be a lifetime-concern).
overvoltage is the riskier part, if I mess this up I will have a very spicy pillow in my lamp... I did think about including one of the TI BMS chips, but if I can, I would still like to only use the uC. The converter chip has overvoltage lockout for the CC mode, so that could take the part of the redundant limiter, switching the chip off until reset externally if the set voltage is reached, so if the uC crashed, it would just stop charging...

I can find 5 chips that support 6S packs.

Yeah I have used some of them before, but was just hoping to get around having to increase the pcb size

Actually limit the voltage to 4.1V/cell or less

I'll look into it thanks

[vad]

I did think about including one of the TI BMS chips, but if I can, I would still like to only use the uC. The converter chip has overvoltage lockout for the CC mode, so that could take the part of the redundant limiter, switching the chip off until reset externally if the set voltage is reached, so if the uC crashed, it would just stop charging...

Any protection when the converter is off and battery pack is discharging? For example, are you including thermal shutdown in case the pack’s surface temperature goes above 60-65C for any reason?

[TmaxElectronics]

Any protection when the converter is off and battery pack is discharging?

How exactly do you mean? Idle power draw? That I would protect against, by having the uC disconnect the battery once it is discharged, in a way that it can not reverse, unless another power source was connected (like the mains power brick).

are you including thermal shutdown

Definitely, since the converter will go up to 200W. I already had a battery swell up because I didn't pay attention once (though that was because of a bad battery).
I'll but a thermal sensor inside the holder for the battery (it is only a mechanical one thought). I might even use one of those digital thermal switches, and use some logic to make it turn off the converter and disconnect the battery...

[tooki]

Why go to the ridiculous effort to do that with the wrong product, when you could much more easily just get a chip designed to do everything you need??

[TmaxElectronics]

because of pcb space. Currently there is practically no room for such a converter. And if the LED driver can do it, that would be a nice workaround.
Having two multi-hundred watt converters is more effort that one...

Sure if it was impossible I'd just have to do something else, but i'm not here to find a solution to "how to charge a large lipo battery in a product". I just want to know if it was possible to charge the battery with the same converter that drives the LEDs.