charging a lion battery through a diode at 5V? - Page 1
Forum section: electronics / repair

author: Simon
author_rank: Unknown
timestamp: April 11, 2025, 08:21:48 pm
content: Not the first time I have seen this but now it's in the family. My sister has asked me to change the colour of my nieces night light. Easy enough, chop out the multi colour changing led for a single colour one. But I see that is has a lion battery charged via a diode from a USB socket.Hm, is this really a good idea? So the battery will sit on permanent charge, if the current should drop enough the voltage drop across the diode goes down and the voltage into the battery goes up.Should I just disconnect the battery and have them use it on a permanent USB supply?

author: Gyro
author_rank: Super Contributor
timestamp: April 11, 2025, 09:20:44 pm
content: Better to fit a little charger PCB, eg.https://www.ebay.co.uk/itm/255992633627(there are many). They have input pads so you don't have to use the included USB connector. The TP4056 will perform a proper charging profile, including shut-off when charged. The max charge current can be set by a single resistor change. Best to play safe with these things.P.S. You can get them with discharge limiters too, eg.https://www.ebay.co.uk/itm/286101115663

author: thm_w
author_rank: Super Contributor
timestamp: April 11, 2025, 10:09:31 pm
content: Big Clive has shown this a few times, they are relying on the protection circuitry inside the battery to cutoff at 4.2V, which is not a good idea.I would just disconnect it, assuming you don't have regular power outages.

author: tszaboo
author_rank: Super Contributor
timestamp: April 11, 2025, 10:46:53 pm
content: you cannot safely CCCV charge a li-ion battery, especially not with the wrong voltage.

author: amyk
author_rank: Super Contributor
timestamp: April 11, 2025, 11:03:25 pm
content: It's widely done and works "well enough" with low currents and capacities. As mentioned above, the cell itself has protection circuitry to stop the charge from going too high.Others have posted here over the years, in shock and awe, when they discover this. I remember one was even a highly well-respected brand's product.

author: Simon
author_rank: Unknown
timestamp: April 12, 2025, 06:24:28 am
content: If the battery has it's own management circuitry then yes I expect that will deal with it well enough. I just had visions of a battery trickle charging to over 4.25V and boom.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 07:45:34 am
content: Quote from: tszaboo on April 11, 2025, 10:46:53 pmBecause of a hazard or because it will damage the battery?What charging method is required?

author: Psi
author_rank: Super Contributor
timestamp: April 12, 2025, 08:24:30 am
content: Quote from: David Hess on April 12, 2025, 07:45:34 amI assume he was referring to the lack of any temperature sensing to reduce the currentThe safe amount of CC to charge Lion/Lipo at is temp dependent.It is also dependent on how charged the cell is. (Like how EV's ramp down the charge current when the car reaches ~80%)So just using CC+CV without anything else isn't safe.That being said, a lot of engineers might charge a lithium cell using CC+CV on the work bench and use some common sense to use a lower current and not try and charge at -10C, or just keep an eye on it.But you would never want to use purely CC+CV on anything being sold to the public

author: Gyro
author_rank: Super Contributor
timestamp: April 12, 2025, 08:43:06 am
content: Quote from: Simon on April 12, 2025, 06:24:28 amThat's a big "If" for a child's room. Have you checked? Cells are available with or without an internal protection circuit - guess which is cheaper. If present, the protection circuit is there for fire protection, not 'proper care and feeding'.If you're unsure, use a proper charger circuit or disconnect it.

author: Psi
author_rank: Super Contributor
timestamp: April 12, 2025, 08:57:06 am
content: The PCM is meant to be a last resort, if you rely on it to work you take that away.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 09:10:31 am
content: Quote from: Psi on April 12, 2025, 08:24:30 amThat does not make any sense. Current decreases once CV is reached. Nothing further needs to be done to "ramp down the current".Do you mean temperature sensing to lower the current because the temperature is too cold or too hot?QuoteSo the only other requirement is temperature monitoring?

author: Simon
author_rank: Unknown
timestamp: April 12, 2025, 10:19:14 am
content: I think people are confusing the natural behavior of a battery to not charge as fast as it approaches 100% with requirements that that be the imposed charge rate.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 10:31:21 am
content: Quote from: Simon on April 12, 2025, 10:19:14 amI asked because there are conflicting recommendations. Does the charge current need to be reduced further? That would mean that as the battery approaches the charging voltage, the charging voltage needs to be reduced. How does that make sense?The only consistent information I found online is to limit the charging current up to the charge voltage limit, and then once the charging current falls to some fraction of the capacity, stop charging. It is unclear how this applies when the charging voltage is lower than the peak charging voltage, like if you were charging to 80% capacity to increase operating life.

author: tszaboo
author_rank: Super Contributor
timestamp: April 12, 2025, 12:41:12 pm
content: Quote from: David Hess on April 12, 2025, 10:31:21 amYou don't want to charge at freezing conditions.You typically terminate charge when the charge current reaches 1/10th of the nominal charge current.Quote from: Psi on April 12, 2025, 08:57:06 amExactly. For the same reason you wouldn't drive into a railing on the side of a cliff intentionally, just because it's there to stop cars.Quote from: amyk on April 11, 2025, 11:03:25 pmIt wouldn't pass any basic regulation, if it is done by competent people.There just was some discussion of some medical company doing this with (as I remember) glucose meters, and they had to recall all of them. I don't remember if they went out of business in the end.

author: Peabody
author_rank: Super Contributor
timestamp: April 12, 2025, 02:08:31 pm
content: Quote from: David Hess on April 12, 2025, 10:31:21 amThere was a question somewhere recently as to which charging IC would let you adjust the CV voltage to something lower than 4.2V to maximize battery life. Nobody knew of one, but I did a test using a TP4056 to charge an 18650, with the input voltage set at 4V instead of 5V. The TP4056 charged the cell up to 3.96V, but never stopped charging (never terminated), and the cell continued to draw a few milliamps. When I removed power altogether, the cell measured 4.93V.I think that means that if you do not terminate charging, the cell will never quite reach the CV voltage, and will continue to draw a little current. Is that bad for the cell, or even dangerous? I don't know. But I've never seen lithium charger IC that did not terminate charging. I suspect there's a reason for that.But I have to say that Big Clive has a video claiming there's no problem continuing to apply 4.2V to a fully charged cell. He says the need to terminate is "a myth". His credentials are better than mine, but I suspect he is wrong about that.

author: Simon
author_rank: Unknown
timestamp: April 12, 2025, 07:39:03 pm
content: Well i cut the battery out, they don't use it anyway as after only two months the light goes too dim to read with on the battery so it's plugged in.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 09:23:27 pm
content: Quote from: Peabody on April 12, 2025, 02:08:31 pmIt will depend on the design. I know of older lithium charger ASICs where the external reference or voltage divider sets the charging voltage.QuoteThere are several chemical reactions and physical mechanisms which damage the cell. The chemical reactions have different voltage thresholds and exponential relationships to the voltage. Holding the cell at 4.2 volts encourages the most damaging and irreversible one so time spent at 4.2 volts shortens cell life independently of charge and discharge cycles.Shown below is me trying to replace a 12 volt 7 amp-hour lead-acid battery with an 18650 based lithium battery from an ebike, hence my interest. I added some diodes in series to reduce the 13.8 volt charging voltage to a little more than 12 volts, or 4 volts per cell.

author: Simon
author_rank: Unknown
timestamp: April 13, 2025, 06:01:11 am
content: The problem with diodes is that the voltage drop reduces with lower current. A typical silicone diode does not have a 0.7V drop but a 1.1V drop at rated current. It will have 0.7V drop at a lower current and will have less than 0.7V drop at very low currents. This is why I am concerned about charging a lion battery through a diode from 5V.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2025, 08:33:05 am
content: Quote from: Simon on April 13, 2025, 06:01:11 amIn my current test, the three silicon diodes in series are dropping 0.8 volts each once the 3 series cell lithium battery is fully charged and the voltage becomes stable.

author: Simon
author_rank: Unknown
timestamp: April 13, 2025, 10:42:50 am
content: so not a lot of current is flowing. When I was a teenager I wanted to reduce a 9V battery to 5V, so I put diodes in series and measured with a meter, I still got 9V.

author: unseenninja
author_rank: Regular Contributor
timestamp: April 13, 2025, 11:06:24 am
content: Quote from: Simon on April 13, 2025, 06:01:11 amSilicon != siliconeSiliconis a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre.Siliconeor polysiloxane is a polymer composed of repeating units of siloxane. They are typically colorless oils or rubber-like substances.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2025, 10:02:52 pm
content: I ended up using 1 silicon rectifier and 2 schottky rectifiers in series to drop 13.8 volts to slightly less than 12 volts and 4 volts per cell with 3 LMO cells in series. I will run some tests to see if that is still too high. The normal charge termination is 4.2 volts per cell and less then C/50.

author: Peabody
author_rank: Super Contributor
timestamp: April 15, 2025, 01:33:03 pm
content: As @Simon pointed out, as the charge current drops, the diodes will drop less voltage. So your charge voltage will get higher and higher. With the batteries disconnected, what does your meter say is the voltage at the end of the diode string when it's connected to 13.8V? Perhaps you have an LM317 in your junque box that would give you a more stable voltage.And what about balancing?Well, based on my experiment, I would predict that your cells never stop drawing current. I just don't know if that's bad for them.

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2025, 08:26:03 pm
content: The diode series string drops 0.4 volts to 13.4 volts with a 10 megohm multimeter load. The diode string will reach equilibrium at some point with the leakage current into the batteries producing the desired terminal voltage.A 317 cannot drop 13.8 volts to 11.85 volts, which is where equilibrium was reached, or 3.9 volts per cell. I could arrange something more sophisticated of course.This is a battery backup application, so I have a continuous 13.8 float voltage available. For cell balancing I put a matched 71.5 ohm resistor across each cell, so about 50 milliamps is always being drawn through the diodes. This wastes about 0.75 watts so I may change this to use a couple of micropower operational amplifiers which will also provide more current for cell balancing.After reviewing various documents about lithium-ion charging, I suspect the C/50 cutoff only applies when charging to 4.2 volts per cell, with the threshold falling as the charge voltage is decreased.

author: metrologist
author_rank: Super Contributor
timestamp: April 15, 2025, 11:37:41 pm
content: Quote from: unseenninja on April 13, 2025, 11:06:24 amLuster != Lustre.Lustre is a French word translated to English as a chandelier.Luster is a gentle sheen or soft glow, especially that of a partly reflective surface, or a thin coating containing unoxidized metal which gives an iridescent glaze to, i.e., ceramics.

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2025, 09:37:48 pm
content: I found a replacement MOCA bridge which accepts 12 volts instead of 5 volts, simplifying my battery backup hack. So far it works fine.