Advice designing a minimal but functional LiPo charger

[VinzC]

Hi everyone.

I have two SD 602048 LiPo batteries of 600mAh each which were given to me by my father a while ago after he bought a batch of ten (on Amazon, if I'm not mistaken). They came with bare wires (there are only 2 of them) and no connectors, the end of the red wire being wrapped with tape (a bit dodgy I admit). I have a few Mini-Spox connectors so I crimped them. As I didn't have individual cells before and therefore have no charger around, I decided to build my own and see how hard it would be. I'm not quite familiar with these kinds of batteries, I only have partial theoretical knowledge so far.

I tried to find a datasheet for these batteries but it appeared to be not that straightforward. I found some information at globalsource.com: https://www.globalsources.com/Lithium-polymer/Rechargeable-lithium-battery-1211915674p.htm

However information sources can be somehow contradictory, especially about the safe charge and discharge currents.

Here are the properties I used, from that page:

EPT 3.7V 602048 600mAh Lithium Polymer  rechargeable battery  for GPS personal tracker
Specification
1. EPT 3.7V 602048 600mAh Lithium Polymer  rechargeable battery
2. Demension:6*20*48mm
3. Nominal capacity: 600mAh
4. Nominal voltage: 3.7V
5. Long cycle life: >500 times
6. Constant discharge current: 0.2C (standard)
7. Peak discharge current: 1C
8. Maximum charging current: 0.2C
9. Protection functions: overcharge, over-discharge, overload, short-circuit
10.Working temperatures: 10 to 45°C (charge)/-10 to 55°C (discharge)
11. UL, CE, RoHS, KC, BIS, KC, PSE, MSDS, UN38.3 approved

I used Dave's videos #176 and #919 for reference. So I designed a precision voltage source with current limiting. Then I added an auto-off switch, The charging current is set to 100mA instead of the recommended 120mA. I essentially wanted to use the discrete parts I have in my drawers (otherwise I'd have used a dedicated circuit, of course). So this project is only meant for experiment and education purposes, I won't implement it in further designs, I only wanted a crude, convenient but functional charger for these batteries.

The schematic is attached.

Do you see any inherent flaw in that design? If you have any recommendation, please don't hesitate.

[westfw]

it appeared to be not that straightforward

It depends on how safe you want to be.  For example, the circuit you posted doesn't seem to include the "timeout" function that most "real" charging circuits seem to include (ie "if this cell doesn't charge in N hours, there's something wrong with it and I should give up!")

OTOH, I've seen much simpler circuits, like: https://shdesigns.org/lionchg.shtml
And I charge my random cells using a "lab" CCCV power supply (keeping a careful eye on them while they sit in a fireproof container.)

You can buy cheap charger modules, like: https://www.amazon.com/ENHANA-TP4056-Charging-Protection-Functions/dp/B0FVFKMMKK  (But what's the fun in that?)

[VinzC]

It depends on how safe you want to be.  For example, the circuit you posted doesn't seem to include the "timeout" function that most "real" charging circuits seem to include (ie "if this cell doesn't charge in N hours, there's something wrong with it and I should give up!")

Thanks! That's an excellent point. I have added a connector for monitoring/logging, e.g. so I can very well use that to control charging time. It'll require a slight change, e.g. an "enable" input. Or even a simple buzzer. That's quite feasible.

OTOH, I've seen much simpler circuits, like: https://shdesigns.org/lionchg.shtml
And I charge my random cells using a "lab" CCCV power supply (keeping a careful eye on them while they sit in a fireproof container.)

Well that's interesting indeed. I don't have a bench PSU (except for the salvaged computer PSU, of course, crude but useful) so I wanted something minimalist and dedicated.

You can buy cheap charger modules, like: https://www.amazon.com/ENHANA-TP4056-Charging-Protection-Functions/dp/B0FVFKMMKK  (But what's the fun in that?)

You nailed it, perfectly

[The TP4056 is a classic; BigClive has produced a few analysis of that module, along with a debunk.]

[Peabody]

[The TP4056 is a classic; BigClive has produced a few analysis of that module, along with a debunk.]

Yes, in his "Myth" video, Clive says you can continue to apply 4.2V to a lithium cell even after it is fully charged. I think he is wrong about that.  Current will continue to flow into the cell, and eventually it will lose its smoke.  But Clive has more experience and knowledge than I do.

[Kilrah]

Yes, in his "Myth" video, Clive says you can continue to apply 4.2V to a lithium cell even after it is fully charged. I think he is wrong about that.  Current will continue to flow into the cell, and eventually it will lose its smoke.

No. Once both the source and the battery are at 4.2V or the final voltage appropriate for the particular cell chemistry then no current will flow for obvious reasons (well other than maybe a few microamps that compensate cell self-discharge, but whatever).
What's important is that the source's voltage doesn't creep up with low/no current.

[Johnny B Good]

 Oh, he's well wrong about that!

 Although you can float charge a Lipo cell provided you limit the float voltage to the half charged voltage state recommended for maximum shelf storage life (3.8v afair, I set my gpsdo backup Lipo float voltage to 3.85v and I believe the military use 3.9v for their ICBM battery pack cells).

 The obvious trade off in this case being that you're prepared to accept the reduced autonomy in exchange for an extended service life with a less complicated (in the case of a single cell backup at least) charging circuit (cheaper and less to go wrong).

 Float charging a lithium cell at 4.2v results in the lithium plating out and reducing the capacity in rather short order, hence the cut off once the charging current has dropped to 10% of its recommended (typically 1C) charging rate .

[Peabody]

Yes, in his "Myth" video, Clive says you can continue to apply 4.2V to a lithium cell even after it is fully charged. I think he is wrong about that.  Current will continue to flow into the cell, and eventually it will lose its smoke.

No. Once both the source and the battery are at 4.2V or the final voltage appropriate for the particular cell chemistry then no current will flow for obvious reasons (well other than maybe a few microamps that compensate cell self-discharge, but whatever).
What's important is that the source's voltage doesn't creep up with low/no current.

I went to the trouble of setting up a test, and found that at a continuous 4.2V, current never stops flowing into the cell.  Apparently the cell never quite gets to 4.2V, so there is always a differential that leads to current flowing.  I don't remember the actual value, but it wasn't just microamps.  I think Clive just has this wrong.

The interesting question is what would happen if a lower voltage is used.  If you apply a continuous 3.9V to the cell, does current ever stop flowing?  If it doesn't, does this harm the battery?  I think this would make an interesing EEVBLOG video, complete with microCurrent and such.

[Johnny B Good]

Was that Lipo cell a bare cell or a protected one? This might account for the continuous current flow.

 BTW, I agree that it would make an interesting EEVBlog topic thread.

 In my case, I'd deliberately chosen the float charging method since it was the simplest way to avoid even the briefest of interruptions to the OCXO 12v rail to prevent its micro controller resetting and acting as if it were dealing with a stone cold start sequence with the consequential temperature overshoot which then required just as long to recover from as if it had actually been powered up from cold - a process that took a frustratingly bad 15 minutes to stabilise the frequency.

 I really only needed a 10 or 15 minute autonomy to allow me to untangle the dc cable to reroute it or allow a seamless change over to another dc psu able to supply a peak startup power demand of 8 or 9 watts (1.8W warmed up and Lipo fully recharged) from a 5 to 24 volt supply. First test to depletion ran 110 minutes off the 1500mAh cell recovered from a damaged in transit Mustool microscope.

 That was some 3 or 4 years ago and I haven't repeated this test, other than for a few brief minutes once or twice a year (if that) to test that the Lipo still has some reserve capacity left. I think it's high time I repeated a full discharge test to check how much capacity remains.

[thm_w]

Oh, he's well wrong about that!

 Although you can float charge a Lipo cell provided you limit the float voltage to the half charged voltage state recommended for maximum shelf storage life (3.8v afair, I set my gpsdo backup Lipo float voltage to 3.85v and I believe the military use 3.9v for their ICBM battery pack cells).

 The obvious trade off in this case being that you're prepared to accept the reduced autonomy in exchange for an extended service life with a less complicated (in the case of a single cell backup at least) charging circuit (cheaper and less to go wrong).

 Float charging a lithium cell at 4.2v results in the lithium plating out and reducing the capacity in rather short order, hence the cut off once the charging current has dropped to 10% of its recommended (typically 1C) charging rate .

He's not wrong, as noted above people think that float charging *itself* is doing immediate damage and will destroy the cell. Degradation at 4.2V is obviously higher than at 4.0V or 3.8V, so yes the degradation will be accelerated. But its calender aging at a specific voltage/soc.

If you care about reducing aging, then as you said, you float charge to 3.9V instead of 4.2. Which can be done with one or two diodes, or a different charging IC.

I went to the trouble of setting up a test, and found that at a continuous 4.2V, current never stops flowing into the cell.  Apparently the cell never quite gets to 4.2V, so there is always a differential that leads to current flowing.  I don't remember the actual value, but it wasn't just microamps.  I think Clive just has this wrong.

The interesting question is what would happen if a lower voltage is used.  If you apply a continuous 3.9V to the cell, does current ever stop flowing?  If it doesn't, does this harm the battery?  I think this would make an interesing EEVBLOG video, complete with microCurrent and such.

The current flowing into the battery is going to be equivalent to its self discharge current, its never going to damage the battery unless the voltage is excessive. We've had this discussion before.

https://www.sciencedirect.com/science/article/abs/pii/S2352152X25011259?fr=rjs&ref=cra_js_challenge#1


edit: I will say I'm incorrect as this paper notes higher failure rates. But I still would say that float charging itself is most likely not the problem, its float charging at 4.2V which is the problem.

With the global installed capacity of energy storage LIBs exceeding 300 GWh, over 80% of these systems operate continuously under float-charging condition.  In this mode, once the battery is fully charged, the charger supplies a small continuous current to compensate for self-discharge and maintain a stable terminal voltage.

https://www.sciencedirect.com/science/article/abs/pii/S2352152X2600945X#1

[Johnny B Good]

I decided to run the autonomy test some twenty minutes ago (the MK II gpsdo - M8T G3RUH based design). I watched its blinken power led with stopwatch in hand for the first 16 minutes before observing its ultimate demise indirectly through its effect on the MK III ZED9FT G3RUH due to it being the one supplying some 45mA to the Multiband GNSS/RTK antenna that feeds both GPSDOs via a splitter.

 It's now been 25 minutes since I started the test. I'm curious to see just how long it will run for. I know I won't be able to see another 110 minute run on account of the 20mA or so greater antenna current compared to the original active magnetic puck antenna I' been using in the original test setup.

 I'll post an update on the result sometime in the next hour - it's just completed a 31 minute run so far.

[Johnny B Good]

Well, I'm happy to be able to report a run time of just under 15 seconds shy of a 105 minutes  A lot better than I'd anticipated considering the extra 22mA load from the antenna upgrade. I think the very first test run had been 121 minutes and I now suspect that I may not have allowed enough recharge time (like another 24 hours' worth?) for the second run, hence the 110 minute result.

 I'm fairly certain I'd added the Lipo backup to the MK II at least 2 1/2 to 3 years ago (I'd have to go through my ebay/aliexpress purchase histories to get a more accurate idea).

 All in all, it looks like being on a relentless 3.85v float charge over the past two or three years hasn't done the Lipo cell any noticeable harm, quite possibly only as much as simply being left in storage at the recommended 50% state of charge for that length of time.

[Smokey]

Putting on my "that guy" hat....

If you just need to charge two random batteries you are playing with, use your bench supply set to 4.2V and current limited to like 100mA (or whatever your pack supports)...

If you really need a lithium battery charger in circuit, maybe look at dedicated ICs?

At the cheapest we are talking a $0.10 part.. in 1s, from digikey!!
https://www.digikey.com/en/products/detail/shenzhen-slkormicro-semicon-co-ltd/SL4056E/29287008

And only $1.01 in 1s for a part with a charge timeout.. from TI even..
https://www.digikey.com/en/products/detail/texas-instruments/BQ25170JDSGR/15856833

[westfw]

Are there still 4.1V Li-Ion cells out there?

In my experience, charging at 4.2V, the current does go WAY down, although I'm not confident that it goes to 0.
I'd feel pretty confident in float-charging at 4.0V...

Some of the cheap charge chips apparently count on something "external" limiting the maximum current.  Like assuming that a USB port will only provide 500mA (hah!)  This somewhat implies that if you don't care much about charge time, you could get away with a 4.2V constant-voltage power suppy and a resistor...

[Kilrah]

I went to the trouble of setting up a test, and found that at a continuous 4.2V, current never stops flowing into the cell.

Since you already posted that you wouldn't have run it for long enough, as voltage gets close the current will follow an expenential-ish decay, so of course it will take an "infinite" time to for it to fall to exactly zero/self discharge current.

[Peabody]

I went to the trouble of setting up a test, and found that at a continuous 4.2V, current never stops flowing into the cell.

Since you already posted that you wouldn't have run it for long enough, as voltage gets close the current will follow an expenential-ish decay, so of course it will take an "infinite" time to for it to fall to exactly zero/self discharge current.

It's been too long since I did the test, but my memory is that the current stopped going down, but leveled off at some significant value.  It was an 18650, and could have been protected or not,  but the protection circuit only draws a couple microamps.  Well, maybe I can do the test again, or somebody else is welcome to try it.

But I would just say that I've never seen a lithium charger chip that did not terminate charging when current drops below a certain level.  It seems there must be a reason why all that engineering effort has gone into providing that function.  Or is it that they just didn't check with Clive before doing their designs?

[Johnny B Good]

It seems to me that your final statement lacks a smiley 

I understand exactly where you're coming from when recalling the details of a test done several years back. After looking through my ebay/aliexpress purchase histories, I've been able to ascertain that the protected Lipos removed from the broken Mustool microscopes are now just over seven years old. Also, it seems my battery backup upgrade to the MK II gpsdo must have predated my antenna upgrade completed May 2022.

 Judging from my recent backup reserve autonomy endurance test, it seems my strategy of floating the Lipo cell at 3.85v to provide a "No Break" integrated UPS to eliminate the annoyance of the OCXO needlessly cycling its cold start algorithm, was absolutely the optimum choice in my specific case.

 Although not everyone would accept the sacrificed run time to the altar of long Lipo cell life and the simplicity of ensuring a "no break" ups feature, my experience might provide a useful insight to anyone with a similar requirement (or the space to "over-provision" the battery capacity to compensate for the 40 to 50% reduction in run time).

 Obviously, this will largely only be of academic interest to the RC hobbyist .

[VinzC]

Was that Lipo cell a bare cell or a protected one? This might account for the continuous current flow.

This is indeed a single LiPo pouch cell. It seems to have some form of protection although I haven't dissected it to inspect what it is as I don't have the required gears. The form factor is similar to this (https://ampul.eu/22987-large_default/li-pol-battery-600mah-37v-602248.jpg) but the only markings are "SD 602048 600mAh" and that's it. There are two IC's protected by the kapton tape as well. There are only two wires like in the picture.

I shall also add that have not given 100% of my trust to these batteries as my father was alone to find a (trusted?) supplier. I believe he had difficulties finding the appropriate replacement battery model and he turned to Amazon, if I'm not mistaken, in spite, sort of... My increased skepticism is also linked to the discrepancy I noticed while looking for the datasheet. I noticed the storage voltage is as expected (around 3.7V). It's about all I'm able to assess with my limited knowledge in the field.

If you really need a lithium battery charger in circuit, maybe look at dedicated ICs?

Then that makes another set of chips to pick up dust in my drawers ;-)

Yes, I know there are components ready-for-the-job, that's not the point of my initial post, it's about reviewing the validity of my schematic and optionally adding features that would be (although not necessarily limited to that) dangerous if absent or overlooked, for example.

[thm_w]

But I would just say that I've never seen a lithium charger chip that did not terminate charging when current drops below a certain level.  It seems there must be a reason why all that engineering effort has gone into providing that function.  Or is it that they just didn't check with Clive before doing their designs?

I doubt Clive ever said float charging was better, he just said its not inherently unsafe, which is generally true.

Charge current termination gives you an arbitrary "full" point you can use to toggle a LED, which is important for consumer products. Also does increase the life of the cell, though its hard to tell how much manufacturers actually care about that.

As I quoted above, 80% of large storage lithium batteries use float type charging. In those scenarios you don't care about being done charging, you want the system to have as much capacity ready as it can at all times.