Charging a Li Ion battery: 3.7V, 4400mAh - Page 1
Forum section: electronics / general

author: fourfathom
author_rank: Super Contributor
timestamp: September 20, 2022, 05:31:06 pm
content: I'm using a 18650 3.7v 4400mAh 1S2P Li ion Rechargeable Battery Pack, and was planning to use a STC4054 charge controller chip between my small 2W @ 5.5V solar panel and the battery. But when I look at the battery specs, it appears to have a controller already built in, and isn't just a couple of bare Li cells. The specs show that the pack can handle an overcharge at 14.4V, 3C without leakage or explosion.I haven't yet peeled off the shrink-wrap cover to see what's inside, but do I actually need a Li charge-controller or can I just feed the battery from the solar panel? I would actually put a pre-regulator between the panel and the battery to keep any voltage > 5V from hitting my board's 3.3V regulators.

author: tunk
author_rank: Super Contributor
timestamp: September 20, 2022, 05:39:39 pm
content: I think some cheap solar power banks have this setup:solar cell -> (schottky) diode -> BMS -> cell/batteryA bit puzzled by the 14.4V, there's no way two cellsin series or parallel can use 14.4V directly.

author: fourfathom
author_rank: Super Contributor
timestamp: September 20, 2022, 05:46:20 pm
content: Quote from: tunk on September 20, 2022, 05:39:39 pmI was puzzled too. I'm going to peel off the cover and see what's up. The spec doesn't mention it, but I did see Polyswitch mentioned in some of the documentation (similar batteries), so perhaps that's all it is (and that would explain how a two-wire connection would work like this). If so, I still need to properly limit the charge voltage.

author: TimNJ
author_rank: Super Contributor
timestamp: September 20, 2022, 07:35:19 pm
content: Packs like this often have a protector chip which controls two back-to-back MOSFETs which can be turned off in the case of a fault condition. Sometimes an all-in-one chip is used which integrates the controller and MOSFETs in a single package. Almost all packs will have over-voltage protection (and usually under-voltage protection). Some packs will have over-current protection.An onboard charger is not a given. Probably easiest to just peel back the shrink wrap and check if you see some sort of buck-looking circuit on there. If not, you can discharge the battery to ~50% or so, and then check if it draws a constant current when you a apply a voltage to it (I guess do that through some sort of current limiting resistor). Li-ion charges charge by CC for the bulk of the charge cycle.

author: Faringdon
author_rank: Super Contributor
timestamp: September 21, 2022, 10:29:33 pm
content: A purist would say put a switch mode battery charger between the solar panel and the battery.But you solar panel is 2w, 5.5v and is pretty weak.So therefore, i bet knowing the impedance of a solar panel...you can probably connect it directly to the battery, and just put in a comparator based overvoltage charge shut-off.Add some hysteresis with the comparator.Otherwise you need a buckboost SMPS with output current control...or in fact, input current control would be fine.....to charge the battery.I dont think you can abuse that battery with a 2w, 5v5 solar panel though....but best keep battery voltage below 4.2v.....mind you as you say, it may already have an OV shutdown fet cct in itself.

author: fourfathom
author_rank: Super Contributor
timestamp: September 21, 2022, 11:43:32 pm
content: So I opened up the battery package and there was s small circuit board under the wrapper. It had an 8205A dual MOSFET, and a DW01A Battery Protection IC, plus two resistors and one capacitor. So it looks like I could hook up the solar panel directly to the battery pack, but for two factors:The solar panel can put out up to 10V unloaded. The DW10A has a max voltage spec of 10V, and once the battery is charged the panel will see essentially no load.Once the battery is charged, the terminal voltage will equal the solar panel voltage. My system regulator has a Vmax of 5.5V and I don't know if the system minimum current draw of 0.25mA will drag down the panel voltage enough.So I could put a voltage clamp at the solar panel output, or use a low dropout pre-regulator between the panel and the battery. I will probably go with the simple pre-regulator. I'm not using MPPT for such a small solar panel, so throwing away excess voltage doesn't hurt charging efficiency much (I lose the regulator drop-out voltage). It does look like I can rely on the built-in DW01A to manage the charging voltage.

author: Peabody
author_rank: Super Contributor
timestamp: September 22, 2022, 03:01:56 pm
content: It appears you are missing a charger - one designed for lithium batteries and which provides a constant current stage, followed by a constant voltage stage, followed by charge termination. See the datasheet for the TP4056. You can't just connect the panel to the batteries, with or without a pre-regulator. The parts you found are just the standard protection circuit. They have nothing to do with charging.

author: fourfathom
author_rank: Super Contributor
timestamp: September 22, 2022, 03:31:36 pm
content: Quote from: Peabody on September 22, 2022, 03:01:56 pmTell me I'm wrong: The battery is 3.7V, 4.4Ah Li ion (two cells in parallel). My charge current is limited to 320mA by the solar panels -- effectively a constant-current, voltage-limited source. The DW01A + dual MOSFET stops the charge at 4.3V +/- 50 mV, and restarts charge at 4.1V. It also provides overcurrent protection, but that's not going to happen unless something is broken. To me this sounds like a workable charging circuit.I looked at the TP4056, and other than the current limit (which won't be approached in my system), I don't see a significant functional difference. The TP4056 has a voltage cutoff at 4.2V, and the DW01A cuts off at 4.3V.What am I missing?

author: tunk
author_rank: Super Contributor
timestamp: September 22, 2022, 04:31:00 pm
content: It should work fine, but it's not constant current,it will vary depending on solar intensity. The onlyproblems may be the 4.3V BMS cut-off if the cellsare 4.2V, or if the BMS fails.Edit: You may do some research into if the cellsmay discharge through the panel during the night,i.e. if you need a diode in series.

author: fourfathom
author_rank: Super Contributor
timestamp: September 22, 2022, 05:57:43 pm
content: Quote from: tunk on September 22, 2022, 04:31:00 pmYeah, but that's what we get with solar panels.Quote from: tunk on September 22, 2022, 04:31:00 pmYes, the panel will conduct more reverse current that I would care to waste, I measure 1.5mA @ 5V (per panel). So a series diode will be necessary.I'm probably going to use my own charge controller anyway, since I'm already laying out a board for a simple boost regulator that pumps the battery 4.2V up to 12V @ 100mA load. The battery controller can go on the same board. I've got some STC4054 charge controllers that ought to work for me (this has a 4.2V cutoff), and as a bonus, there is a charge-current monitor pin available.

author: Peabody
author_rank: Super Contributor
timestamp: September 23, 2022, 03:31:36 am
content: I just don't think it's safe to use the DW01 to terminate charging. I've never seen that done in any circuit. And of course it doesn't terminate at all if the voltage doesn't quite get to 4.3V. You can't continue to apply 4.2V or higher to a fully charged lithium battery. I would just urge you to use your charger chip, which will handle this correctly. You might also look at the J5019 module, which is a TP4056 charger plus an adjustable MT3608 boost converter on one tiny board. It has no protection, but you already have that.

author: fourfathom
author_rank: Super Contributor
timestamp: September 23, 2022, 06:19:33 am
content: Quote from: Peabody on September 23, 2022, 03:31:36 amThanks for the heads-up on that J5019 -- pretty much exactly what I need and stupidly cheap from China. I need an enable for the boost, and I didn't notice one on that board. I just sent out my little charger / boost board for fab (JLCPCB, $8 for five bare boards, delivered) and I already have the parts, but I also ordered a couple of the J5019 boards from ebay to try them out.

author: Peabody
author_rank: Super Contributor
timestamp: September 23, 2022, 02:45:45 pm
content: The boost chip on the J5019 does have an ENable pin, but it's not brought out on the module. But it looks like there's a way to wire into the ENable:https://www.andersknelson.com/blog/?p=765Not a solution for production stuff, but should work ok for hobby projects.Edit: With the boost converter disabled, unboosted current will still flow through the coil and the schottky diode, and it looks like the chip will still draw about 65uA. So for a real shutdown, it may be better to use separate charger and boost modules, with a switch or mosfet in the middle.

author: Faringdon
author_rank: Super Contributor
timestamp: September 23, 2022, 07:08:38 pm
content: Since your solar is so weak, 5v and 2w.....it just seems overkill to do it with a charger.I mean...you coudl just shovel the solar into the battery.....and have a voltage clamp circuit in there....which only kicks in if the batt voltage starts straying above say 4.3v..........this voltage clamp cct woudl be pretty well out-of-cct when the batt voltage was low....so it wouldnt incur losses.....because with a small weak solar like you have , you dont want big losses as you appear to be well aware

author: fourfathom
author_rank: Super Contributor
timestamp: September 23, 2022, 07:21:16 pm
content: I considered a clamp but it would have been dissipating about 2W once the battery was charged. That power would have been wasted anyway, but I didn't want to deal with the thermal on the shunt transistor.