Li-Ion 18650 battery charging. Here we go again. But - - -

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,554
Have a battery from an oxygen generator. Labeled at 14.4V Nominal 5.8Ah. It consists of 16 batteries in series/parallel of 4 packs of four batteries. They have sat uncharged for a long time and have no power. There's a test button and four indicators that doesn't seem to be working. I've opened the pack and read battery voltages directly from the packs of batteries. They have read less than 1 volt. However, they WILL take a charge if I force them to. By that I mean applying 13.8V to a single pack - or a pack of 8 series/parallel or the whole 16 batteries again in their original series/parallel configuration.

I KNOW there's a limit to the amount of current required AND I KNOW there's a current charging profile that should be followed.

These batteries don't seem to want to charge the normal way via the oxygen generator - and I don't want to hurt the generator either. Since these batteries won't charge the proper way I'm looking to learn more about charging them. I've read a couple threads regarding charging Li-Ion's but haven't learned anything I could find useful.

The batteries are marked 18650 3.6V and no other markings. No Ah ratings, nothing more. Reading from a site called Battery University they state {quote}: "Voltage calculations include measuring the mid-way point from a full-charge of 4.20V/cell to the 3.0V/cell cutoff with a 0.5C load." {end quote} What I don't understand is what they mean by "0.5C load". And I'm not able to find anything more specific about charging current other than the site said {quote} "Full charge occurs when the battery reaches the voltage threshold and the current drops to 3 percent of the rated current." {end quote} That "3% of the rated current" doesn't help if you don't know the capacity of the battery (Ah). Since the battery doesn't state this - I can't (won't) go forward with attempting to charge them until I know how to achieve proper charging.

I have a smart charger that can charge a single cell at a time and do a nice job at that. however, I don't want to start busting these batteries apart. I'd rather keep them in their configuration of four parallel packs of four series batteries. If needed I can draw a picture.

So what would be considered a safe charging current if I were to just push a voltage into them without a smart charger? Yeah, I know that wouldn't' be smart. But the packs are 14.4 volts and I'm able to push 13.8 volts at the most. And at present I have almost no ability to monitor temperature either.
 

Audioguru again

Joined Oct 21, 2019
1,508
A lithium cell is destroyed if its voltage has dropped to below about 2.8V. The capacity will be extremely low and there is a possibility of an explosion/fire if charging is attempted. If a charger IC measures a low voltage then it attempts charging with a low current. If the temperature rises or if the voltage does not rise then it stops and issues an error.

I have the lithium battery from my daughter's first cell phone. It is about 20 or 22 years old and still holds a charge of about 3.7V. It is Name-Brand Japanese, not a cheap Chinese copy.
My electric radio controlled model airplanes sense the battery voltage and pulse the main motor as a warning to me that the voltage per cell is becoming low at 3.5V. When the voltage drops to 3.2V per cell then the main motor stops but there is enough power for steering a glide. The manufacturer says not to frequently let the voltage drop to 3.2V. I store them over the winter at a storage voltage of 3.8V then they last for hundreds of flights for years. Over the winter their 3.8V does not drop.
Lithium cells in series are charged with a "balanced" charger that forces the series cells to have equal voltages so that one does not undercharge causing another to overcharge then explode. Your attempt at charging without balancing is very dangerous.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,554
I think the voltage may have dropped to effectively zero volts because of battery monitoring on the controlling board.

The board has four LED's and a test button to signify the state of the battery. When I press the button I get no lights whatsoever. And that's even when I have the 13.8 volts applied to the whole battery pack.

I have several other batteries that came out of an old laptop that took a charge and have now sat on the bench without being connected to anything. I just tested one of the batteries for voltage and got a reading of 3.15 volts. Are you saying that's a shot battery as well?

Out of curiosity I'm checking them all. Have found 1.5v, 3.15, 3.95, 3.72, and several at 4.17 to 4.18V. They've sat uncharged and unconnected for a few months. These are open voltage readings. No load whatsoever.
 

Audioguru again

Joined Oct 21, 2019
1,508
You said you opened the battery pack and measured the battery cells directly and got only 1V. Now you are measuring various voltages of laptop cells. If the voltage of a cell has dropped to 3.2V then it is damaged a little and will not work like new. Below 3.2V then there is more damage. A fully charged cell (near 4.2V) is also damaged if it stays at full charge for a length of time which is why a storage charge of 3.8V is used when stored.
Note that there is a new kind of lithium battery (LiFeP04) that lasts longer because it charges to only 3.6V and average operation is at only 3.2V. It is not damaged much when stored at a full charge.

If the battery monitoring circuit discharged the battery too low then the manufacturer did that to sell lots of replacement batteries.
 

KeithWalker

Joined Jul 10, 2017
917
Have a battery from an oxygen generator. Labeled at 14.4V Nominal 5.8Ah. It consists of 16 batteries in series/parallel of 4 packs of four batteries. They have sat uncharged for a long time and have no power. There's a test button and four indicators that doesn't seem to be working. I've opened the pack and read battery voltages directly from the packs of batteries. They have read less than 1 volt. However, they WILL take a charge if I force them to. By that I mean applying 13.8V to a single pack - or a pack of 8 series/parallel or the whole 16 batteries again in their original series/parallel configuration.

I KNOW there's a limit to the amount of current required AND I KNOW there's a current charging profile that should be followed.

These batteries don't seem to want to charge the normal way via the oxygen generator - and I don't want to hurt the generator either. Since these batteries won't charge the proper way I'm looking to learn more about charging them. I've read a couple threads regarding charging Li-Ion's but haven't learned anything I could find useful.

The batteries are marked 18650 3.6V and no other markings. No Ah ratings, nothing more. Reading from a site called Battery University they state {quote}: "Voltage calculations include measuring the mid-way point from a full-charge of 4.20V/cell to the 3.0V/cell cutoff with a 0.5C load." {end quote} What I don't understand is what they mean by "0.5C load". And I'm not able to find anything more specific about charging current other than the site said {quote} "Full charge occurs when the battery reaches the voltage threshold and the current drops to 3 percent of the rated current." {end quote} That "3% of the rated current" doesn't help if you don't know the capacity of the battery (Ah). Since the battery doesn't state this - I can't (won't) go forward with attempting to charge them until I know how to achieve proper charging.

I have a smart charger that can charge a single cell at a time and do a nice job at that. however, I don't want to start busting these batteries apart. I'd rather keep them in their configuration of four parallel packs of four series batteries. If needed I can draw a picture.

So what would be considered a safe charging current if I were to just push a voltage into them without a smart charger? Yeah, I know that wouldn't' be smart. But the packs are 14.4 volts and I'm able to push 13.8 volts at the most. And at present I have almost no ability to monitor temperature either.
I have successfully recovered a number of different LiPo batteries in the past. All it takes is a lot of caution and constant monitoring.
The capacity of the pack (C) is 5.8 Ah. That is for four packs in parallel, so one pack will have a capacity (C) of 5.8 / 4 = 1.45 Ah.
C refers to the current capacity of the battery so 0.5C for your batteries is 5.8 / 2 = 2.9A for the full pack and a quarter of that for each 4 cell pack. The normal recommended charging current for them is 0.37 x C.
You may be able to recover the batteries if you are very careful but the capacity will probably be low after sitting discharged for so long.
If you have a bench supply with current control, set it at about an amp with the voltage set at about 16 volts. Connect it across one 4 cell battery pack and check the temperature of each cell. If one starts getting hot, disconnect quickly and throw the pack away.
If the battery takes a charge without over-heating, continually monitor the temperature and voltage of each cell. If one cell reaches 3.6 volts before the rest, disconnect the supply. Reset the voltage to 4 volts and connect it across each cell in turn, continually checking it's temperature and voltage until they all read 3.6 volts when the charger is connected.
If you are successful, you can then finish charging them with the oxygen generator charger.
Good luck,
Keith
 

bassbindevil

Joined Jan 23, 2014
94
I would try charging the pack using a "hobby charger" like an Imax B6, at one amp for 4 parallel cells, 250 mA for individual cells (at least for the first charge). The BMS in the pack might take care of balancing, or I could hook up a balance harness to the B6, or just charge one bank at a time (as I'd do if I only had a 4.2V charger and set of clip leads). Then use the B6 charger to discharge the pack or bank and measure the mAh capacity. If there's a big discrepancy between the measured charge going in and the discharge capacity, that could be a symptom of high internal resistance. If no B6 was available, then see how much cell or pack voltage drops under load, which should be small for a healthy cell or pack. And, another thing to check; after charging, record the voltages, then let it sit for a few days and measure again.

Not completely off-topic... if anyone can suggest a simple cheap off-the-shelf charger module for 4S to 5S (or higher) packs, please PM me. I have one based on the SY6912 chip that can do 1S to 3S at up to 2A, but haven't found a comparable solution for higher voltage packs.
 
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Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,554
@Audioguru again My apologies if I caused confusion. The pack I'm trying to recover is from the O2 generator. It's the one that sat at near zero volts. The laptop batteries I mentioned are not the ones of concern. I only brought those up to mention that I managed to get them to take a charge and hold it. With the exception of the few batteries that had low volts.

I don't have a purpose for a whole bunch of single cell 18650's and don't want to break apart another battery pack. I've tried getting them up to a recognizable charge so the normal charging could take place. However, there must be something wrong with the battery monitor system because even when I have it held at 13.8 volts the battery status lights don't light at all. So I'm thinking the charge control board may be fried. But I haven't even had a look at that because there are 20 connection points that this battery connects to the charger. With the pack open it looks like the two 10 pin connectors are using 3 pins for positive, 4 pins for monitoring and 3 for negative. I THINK! Not sure. Don't want to start pushing power at different points. Don't want to hurt the board if it's actually OK. But because the status lights don't light I'm assuming there may be an electronic problem. For now I'm only wanting to get these batteries into a state where the board will think (if working) that the batteries are in need of charging. I've read somewhere that if the battery voltage is too low the charger might not attempt to recharge them.

I have successfully recovered a number of different LiPo batteries in the past. All it takes is a lot of caution and constant monitoring.
The capacity of the pack (C) is 5.8 Ah. That is for four packs in parallel, so one pack will have a capacity (C) of 5.8 / 4 = 1.45 Ah.
C refers to the current capacity of the battery so 0.5C for your batteries is 5.8 / 2 = 2.9A for the full pack and a quarter of that for each 4 cell pack. The normal recommended charging current for them is 0.37 x C.
You may be able to recover the batteries if you are very careful but the capacity will probably be low after sitting discharged for so long.
If you have a bench supply with current control, set it at about an amp with the voltage set at about 16 volts. Connect it across one 4 cell battery pack and check the temperature of each cell. If one starts getting hot, disconnect quickly and throw the pack away.
If the battery takes a charge without over-heating, continually monitor the temperature and voltage of each cell. If one cell reaches 3.6 volts before the rest, disconnect the supply. Reset the voltage to 4 volts and connect it across each cell in turn, continually checking it's temperature and voltage until they all read 3.6 volts when the charger is connected.
If you are successful, you can then finish charging them with the oxygen generator charger.
Good luck,
Keith
Thank you for explaining the "0.5C" thing. It makes sense to me now.

I'll do some more testing when I have time. For now the packs are sitting at 8.3 volts.
 

Audioguru again

Joined Oct 21, 2019
1,508
I think The Battery University discusses it and datasheets for Lithium battery charger ICs say that when a lithium cell voltage becomes less than about 2.5V then metallic Lithium replaces some of the Lithium ions. The metallic Lithium is a short circuit that can cause a fire when a tiny piece "fuses" during attempted charging and sets the Lithium on fire. Lithium is a very active substance (magnesium and titanium are similar) and can violently explode or catch on fire. Since you measure cell voltages far below the dangerous 2.5V then your battery should be taken to a hazardous substance recycling depot.
 

ThePanMan

Joined Mar 13, 2020
26
I think I have something similar to your batteries. They are two 4S packs consisting of 8 batteries each and I have two packs of them. They are currently sitting at the following voltages: ("Left" and "Right" only refers to location on my bench)
1589911756176.png
How critical is the balance of these batteries? I currently don't have a BMS and am considering getting something for it. Anyone have any recommendations?
 

Audioguru again

Joined Oct 21, 2019
1,508
If you are charging 4 cells in series without a balanced charger then there is nothing to prevent the weakest cell voltage from going higher than 4.20V and becoming a bomb.
 

ThePanMan

Joined Mar 13, 2020
26
I kind of got that from your message to TonyR. I'm not yet trying to charge it. This is just where they are sitting voltage wise. Anyone have any suggestions on BMS's? Chargers for 4S batteries?

Also, what's your (anyone's) opinion on having two cells in parallel, four times? Couldn't one of the two cells be bad and cause both to experience problems?
 

Audioguru again

Joined Oct 21, 2019
1,508
Model airplanes, drones, cars and boats use multi-cell Lithium batteries. Local Hobby stores sell the batteries and reliable chargers for them. I would not buy a no-name-brand charger from ebay or one of the other Chinese online places.
 
For a 4S charger, there's boards using the MAX745 on ebay. Dipswitches to set the number of cells, trimpots to set the current limit, and (I think) tweak the voltage, so if you want to be conservative, you could charge to 4.1V instead of 4.2?
I've been very pleased with my Imax B6 eBay charger; the calibration is very close to a Fluke DMM.
As for cells, I like to charge them individually and then discharge to 3.00 V to test the capacity. With small cells like a phone or camera, I calculate a discharge rate based on what the expected battery life is. For 18650 cells, 1 amp is reasonable. Lower rates may not reveal cells with high internal resistance.
When I built up a pack with 4 cells in parallel, they were matched to within a few millivolts of each other before linking them. But, I made a pack for an impatient friend using cells that were more in the 10s of mV range, and none of the fuse wires went pop as I wired up the pack.
As for BMS, the most basic boards just protect for over-charging and over-discharging and over-loading. Better ones provide balancing, at varying levels of current. Smart BMS have computer (or Bluetooth) interfaces to report voltage levels and configure stuff, for considerably more money. Balancers are available as separate modules, ranging from low-current resistive units to active balancers that use something like a switching converter to move energy from one bank of cells to another. Active ones are more efficient and flexible, and are a no-brainer for powerwalls and EV-sized packs. Unnecessary for small packs with reasonably matched cells.
 
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