Author Topic: Charging a LiPo Battery with an LED driver  (Read 1593 times)

Offline TMaxElectronics

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Charging a LiPo Battery with an LED driver
« on: August 11, 2020, 11:24:18 PM »
I am currently planning to build a portable LED studio light, that runs from a pair of 6 cell lipos, and would somehow need to charge them...
Using an internal charger would be ideal, but finding a charger circuit, with enough power and that still fits in the case might be tricky, so I was wondering if maybe it might be possible to repurpose (or perhaps abuse :P) the LED driver for this purpose.

It is a dual channel TPS92682 (https://www.ti.com/lit/ds/symlink/tps92682-q1.pdf?ts=1597172678809),
that drives two strings in a color temperature adjustable LED, and actually has a CV 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.

My driver will have diff. amplifiers for sensing the individual cell voltages (and thus battery voltage), and it is possible to dynamically switch between the CC & CV modes via SPI.
So my idea is to use the MCU to run the chip in CC mode with OV lockout set to just over 4.2V/cell (incase the uC crashes while charging or something) monitoring the battery voltage, and switch to CV mode once 25.2V (4.2V/cell) is reached, and operate in that mode until the current drops below 10% Ichg.
The batteries would be connected to the boost input and output with p-mosfets, switched by some logic, so all the uC can output is a select signal to enable battery charging, so any bad configurations are prevented in hardware.

This is how the output would be wired:


only using one mosfet on the in- and output should be ok, as reverse conduction is not an issue, as the battery voltage is always above the charger (which would have a diode in series with it) and the led is a 55V one, so that can't be damaged by reverse current through the output select p-mosfet (Q10 in the picture).

Has anybody tried something like this before? Would this even work, and more importantly be safe or did I overlook something simple once again ;) ?
I'd like to hear your thoughts on it :D
« Last Edit: August 11, 2020, 11:25:55 PM by TMaxElectronics »

Offline Twospoons

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Re: Charging a LiPo Battery with an LED driver
« Reply #1 on: August 12, 2020, 12:56:02 AM »
Well , I've read your post twice and still can't see what you're trying to do or how you're doing it.  You haven't said what your battery capacity is so we can't gauge the charging requirements.
If it were me I'd make the battery pack removable, and have two. That way you can charge one while using the other, and have a purpose built external charger.  Nothing is more frustrating than having the battery die then having stop everything for an hour while it recharges.

Offline TMaxElectronics

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Re: Charging a LiPo Battery with an LED driver
« Reply #2 on: August 12, 2020, 01:58:47 AM »
Ok I'll try to explain myself a little better :)

Basically I am trying to use a boost converter intended for LEDs to charge a Lipo battery pack.
To do this I basically disconnect the input of the LED driver from the battery packs, and use a mains power brick instead, and then connect the output of the driver to the battery. (of course all with transistors)
But because the driver supports only constant voltage or constant current at one time, i would need to have the uC switch between those modes while the batteries are charging.

The LED has a maximum power of 150W (two separate halves with 100W each and a 150W package limit), and i will use two 70wh batteries with six cells. The converter can output up to 2.5A on each of its two channels, and each battery would be connected to one.

Making them replaceable is not all that practical, as these batteries require quite powerful external chargers that can only charge one at a time. In addition to that, the lamps will be used in situations where mains power is unavailable (such as on-location shoots), and even if there was, it would be possible to run the lamp while plugged in too.

Offline davekni

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Re: Charging a LiPo Battery with an LED driver
« Reply #3 on: August 12, 2020, 04:06:09 AM »
If your LED lights are color-temperature-adjustable, it seems likely they have internal drivers, so may expect CV power input.  Do you have spec's on the LEDs?

To get reasonable life, lithium battery packs need some form of individual cell balancing.  Early lithium batteries just monitored cell voltage and shut down when too far out-of-balance.  That results in premature failure, as even slight cell leakage current accumulates eventually into large imbalance.

Newer batteries balance cells.  The simplest and most common version is a 4.2V (or 3.6V for LiFePO4) shunt-regulator across each cell.  This balances only in the fully-charged state, bleeding charge from low-leakage-current cells so that remaining cells can finish charging.  You'll find instructions for electric vehicles and other battery-powered devices saying that they need to be at-least-occasionally charged completely to maintain battery life.  That's the reason.  The charger needs to taper from CC to CV near full charge to avoid excessive power dissipation in the shunt regulators.  (Or, have some communication from the shunt regulators to the charger to back-off current near full charge, but I haven't heard of any such design.)

The other option is active balancing, where charge-pumps or switching-regulators take current from strong cells to charge weak cells.  TI makes chips for that.  You can also buy both versions of balance boards from Chinese companies on EBay etc.  Active balancing is becoming more common for small to medium battery packs.  I haven't yet heard of an EV using active balancing, but some may be by now.
David Knierim

Offline Twospoons

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Re: Charging a LiPo Battery with an LED driver
« Reply #4 on: August 12, 2020, 12:23:58 PM »

To do this I basically disconnect the input of the LED driver from the battery packs, and use a mains power brick instead, and then connect the output of the driver to the battery.

So why not just make the mains brick a proper lipo charger?

Offline TMaxElectronics

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Re: Charging a LiPo Battery with an LED driver
« Reply #5 on: August 12, 2020, 12:42:50 PM »
Quote
So why not just make the mains brick a proper lipo charger?
1. Because it also has to power the LEDs
2. Because I don't want to design one. ;)
That's why I'd use the LED driver if possible.

Quote
Do you have spec's on the LEDs?
It is basically two white LEDs in one, with seperate strings. There is no control inside it.
bridgelux.com/sites/default/files/resource_media/Bridgelux%20DS355%20Vesta%20Series%20Tunable%20White%2029mm%20Gen%202%20Array%2020190614%20Rev%20A.pdf
And the ones that do have internal control actually require CC drive too (at least those I have used) ;)

Cell balancing shouldn't be an issue, as the overdischarge protection will measure all cells, and the cells are matched. They don't really seem to become unbalanced at all after a few uses and when they eventually do become that after some time, they can be removed and balanced externally.

Offline davekni

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Re: Charging a LiPo Battery with an LED driver
« Reply #6 on: August 12, 2020, 11:08:35 PM »
OK, I see.  Color temperature is adjusted by changing the current through the two internal LED strings, one cool-white and the other warm-white.

If you buy a sufficiently-high-power battery charger, you could wire the charger to battery, and battery to TPS92682 LED controller.  As long as the charger can provide enough power to run the LEDs and perhaps slightly more, then you could rapid-charge when the light is off and slow-charge when the light is on.

Yes, new battery packs generally maintain balance.  It will need rebalancing more often as the battery packs age.  Also, make sure your circuit shuts down charging on excess cell voltage.  That's much more important than shutting down discharge on low cell voltage.  Excess charging voltage is the primary cause of fires.
David Knierim

Offline TMaxElectronics

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Re: Charging a LiPo Battery with an LED driver
« Reply #7 on: August 14, 2020, 06:38:45 PM »
damn it seems I forgot to hit post last time...

Quote
If you buy a sufficiently-high-power battery charger,
Yes that might work, but then I'd have the issue of needing a dedicated battery charger again...

Quote
Also, make sure your circuit shuts down charging on excess cell voltage.
That should be doable, as the output overvoltage lockout of the chip could act as redundant protection, if the uC crashes. And otherwise I'd just have the controller keep track of the highest cell voltage. The converters output voltage is adjustable (at least a little bit), so if one cell happened to get to a higher voltage, the uC should be able to adjust the CV phase accordingly.

I think I'll just make a prototype circuit with plenty of probe points, and see how well the idea works, by now I really want to know what would happen.
Guess I better start digging a trench in the garden and putting 112 on speed dial :P

Offline petespaco

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Re: Charging a LiPo Battery with an LED driver
« Reply #8 on: August 14, 2020, 06:56:46 PM »
That 112 thing seems to be false:
https://www.fox6now.com/news/alert-in-case-of-emergency-dial-911-and-not-112
---at least in the USA and Canada.

Pete Stanaitis
---------------

Offline davekni

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Re: Charging a LiPo Battery with an LED driver
« Reply #9 on: August 14, 2020, 09:16:38 PM »
I'm still a bit confused.  You need some power supply that takes line voltage in and outputs DC, for LEDs and/or battery charging.  Why is it worse if that one line-voltage-fed supply is designed for battery charging rather than for LED driving?
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Offline TMaxElectronics

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Re: Charging a LiPo Battery with an LED driver
« Reply #10 on: August 14, 2020, 09:46:53 PM »
Quote
at least in the USA and Canada.
I live in germany so I should be fine ;)
And I believe if I was to call 112 from my german sim card while in north america it should automatically forward the call to 911, but not sure.

Quote
Why is it worse if that one line-voltage-fed supply is designed for battery charging rather than for LED driving?
The chargers that are designed to charge lipos with 6 cells at sufficient power are tabletop units (like this one https://www.amazon.de/ISDT-Netzteil-Batterie-Ladeger%C3%A4t-Entlader/dp/B07NJ29LKW) that would be impractical to connect to a large lamp. Of course if this is the only option I'd have to find a way around it (like using an external XT60 on the lamp with an extension cable or something like that), but if all that was needed to charge it was some dumb laptop charger, it would be a lot better.


Offline davekni

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Re: Charging a LiPo Battery with an LED driver
« Reply #11 on: August 14, 2020, 11:01:58 PM »
I'm still not understanding the top-level system.  Are the LiPo batteries in series and driving the TI TPS92682-Q1 chip, with the two chip outputs driving the two separate LED strings (cool and warm white)?  Is there a separate line-powered dual-LED-driver that runs the LEDs when not on battery power?  In other words, I'm missing the context in which your initial diagram lives.  In that diagram, I see only one LED connector.  If that drives both LEDs, then there's no color-temperature adjustment as the two currents can't be independently adjusted.

If the above isn't correct, where does line power come into your system normally when not running on battery power?
David Knierim

Offline TMaxElectronics

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Re: Charging a LiPo Battery with an LED driver
« Reply #12 on: August 15, 2020, 12:01:20 AM »
Yeah I admit my original post was quite confusing... sorry for that.

I made a diagram in glorious ms paint  :D


The Power "destination" selectors would be logically linked to the source selector, so the batteries can only be charged, when the mains charger is selected as the power source.
And the source would switch from one battery to the other, once one is discharged.

The decision where to take power from would be done by the uC, as well as all of the DC-DC adjustments. The only issue I see is if somehow the thing managed to switch the destination to the batteries, but the DC-DC was still in LED mode, which would allow up to 65V on the output. To prevent this I would make the FB divider ratio adjustable with a mosfet (that enables the higher voltage range by bridging a resistor on the lower half of the divider) that would be connected to the LED OUT enable, so the HV feedback ratio is only active, when the LED is actually selected.

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Re: Charging a LiPo Battery with an LED driver
« Reply #13 on: August 15, 2020, 04:47:06 AM »
Thank you for the top-level view, which does look at least theoretically possible to implement and function.

Hobbyist LiPo battery packs are an exception to regulations requiring internal pack protection.  All packs for internal charging applications such as laptops and cell phones have two layers of protection.  A circuit internal to the battery pack disconnects the pack's power terminals if any cell gets above 4.25-4.35V (slight margin above 4.2V per cell charging cut-off voltage).  The external charging circuit limits total voltage to 4.2V * cell_count.  (There are some adjustments for fast-charge modes where known internal series resistance is compensated for by using slightly higher charging voltages at higher current adjusted to keep internal cell voltage < 4.2V.)  I don't know if software is allowed to be in the loop for the external charge voltage limit.  (For most safety-critical circuits, UL doesn't allow software in the loop.  I don't know about this case, nor about other regulatory requirements.)

Of course, for a home-made appliance, you aren't bound by any of the regulatory requirements.  However, I strongly recommend at least one passive voltage limiting element.  An amp or two of charging current that doesn't get shut down due to some SW glitch is an almost-certain way to start a fire.  (I haven't personally tested fire hazards in a long time, but did when initially playing with cells scavenged from "dead" laptop packs.  These were 18650 LiIon cells, not LiPo.  Tried shorts and over-charging.  Over-charging at 100mA caused small ruptures and electrolyte leakage.  Overcharging at 1A caused abrupt rupture and flames and huge amounts of acrid black smoke.  Short circuits caused very-hot cells with some fluid leakage, but no fires.  I gather that newer higher-energy-density cells are more prone to fires caused by abuse, which makes sense.  Only LiFePO4 cells are reasonably-safe w/o protection circuits, but have lower energy density than other lithium chemistries.)

The binary choice of CC or CV for the TI TPS92682-Q1 is a major limitation for your application idea.  If you do try charging with this chip, I'd suggest running CV with a hack for current limiting: Add external clamps to the COMPx pins to limit how high each chop-cycle's current can reach.  I haven't read the spec in enough detail, but it looks like those pins run from 0.1V up to ~4V.  Adding a clamp (ie. TLV431 or just a PNP emitter with base to some voltage source or ...) that limits COMPx to something below it's max should limit current without causing over-current shut-down.  Or, just pick some simpler boost-regulator chip and implement the CC/CV modes externally.

Why switch between battery packs rather than running the two in parallel?  (If paralleling, I'd suggest fuses so that a fault in one pack can't borrow energy from the other pack.)  Paralleling will half the current drain of each pack.  Batteries last longer (more cycle life) when used at lower currents.  Paralleling will half both discharge and charge current.
David Knierim

Offline TMaxElectronics

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Re: Charging a LiPo Battery with an LED driver
« Reply #14 on: August 15, 2020, 02:04:42 PM »
Quote
However, I strongly recommend at least one passive voltage limiting element
For that the overvoltage lockout in CC mode should be fine right? If the voltage goes slightly too high, the converter would be disabled until the uC reset it.
Though there are some TI BMS solutions that can do up to six cells, but those would of course take up PCB space again... I think I'll look into some of them again.
And that "Hack" in the CV mode sound interesting, never thought about that ;D

Quote
For most safety-critical circuits, UL doesn't allow software in the loop.
Nice to hear that there is regulation on this, and also very comforting ;) I kind of assumed that everybody just followed the safety guidelines because of the risks involved.
And just because I don't plan to commercialize the thing right now, doesn't mean I won't ever :D

Quote
Why switch between battery packs rather than running the two in parallel?
I want to avoid the issue of somebody removing the batteries for some reason, and then connecting two that have dramatically different charge states, though I admit that is very unlikely with internal batteries... But being able to split them would allow me to use the converter with both channels in parallel.

Quote
I gather that newer higher-energy-density cells are more prone to fires caused by abuse
interestingly enough it seems like this might not be the case... I have seen a few cases where somebody has shorted out cells (either with nails through them or just clamping them to a copper pipe), which in the past were always guaranteed fireballs, but more recently the cells fizzle a bit and then stop (or one out of the pack swells up, and increases in resistance massively), so maybe they have figured out a way to make them safer?
Not that that means I will just charge it without any protection of course ;)
when 75Wh of energy get dumped into a plastic bag filled with flammable electrolyte in a second or to, it will always be bad news...

Offline davekni

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Re: Charging a LiPo Battery with an LED driver
« Reply #15 on: August 15, 2020, 06:57:59 PM »
"For that the overvoltage lockout in CC mode should be fine right?"  Yes, presuming it can be set accurately enough.  Haven't studied that TI chip in detail.  Over-voltage protection features are often to catch gross errors to avoid frying chips, such as old 5V logic chips that had absolute-maximum voltage limit of 7V.  If you can set it precisely enough to catch 4.3V/cell, then it's good for a balanced pack.

Yes, I've seen pictures of nails through LiPo cells.  I've also seen pictures of cell-phone fires etc.  (Cell phones use LiPo cells.)  I didn't manage to make any fires from shorting, only from rapid over-charging.  Perhaps I should do a few more tests, both with newer higher-capacity 18650 laptop cells and LiPo.  I suspect LiPo behavior will depend heavily on what external mechanical constraints limit cell expansion.
David Knierim

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Re: Charging a LiPo Battery with an LED driver
« Reply #15 on: August 15, 2020, 06:57:59 PM »

 


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