General > General Technical Chat
12V DC UPS
Siwastaja:
--- Quote from: tom66 on July 25, 2022, 01:50:38 pm ---Float charging Li-ion batteries is simply not possible
--- End quote ---
Of course it is, I regularly do it.
The suggestion was using a constant-voltage supply plus a current limiter resistor. This provides both current and voltage limiting.
But you need to understand Ohm's law to be able to dimension the resistor. If you need help with that, you can ask about it in the Beginners section. Thank you.
Mike's suggestion of having a BMS - practically this would be most easily satisfied by using "protected cells" with built-in single-cell protection PCB - would take care of adding another layer of security in case the upstream 12V supply fails in a way making it output a higher voltage - unlikely but possible. Resistors basically never fail short, so current limiting will be very robust. This basically just leaves the low-voltage cutoff functionality on the shoulders of the protection boards. BMS boards would also open the circuit if gross imbalance develops, but this is again unlikely. Or one can use a balancing 3S BMS.
If you use decent quality non-bullshit cells, this is one of the least risky mods one can do. But as always, there is some risk to anything. I just find it extremely funny people jump on this particular li-ion usage tip, because we see orders of magnitude worse every day on this forum and no one gives a shit.
Probably because they don't understand what they are talking about, and get fixated on some single words like "float charging", after reading from Battery University or some other BS site that "float charging a lithium ion cell is impossible".
tom66:
Siwastaja, what happens to your float charging circuit and the battery, if the battery runs to <2.8V and you connect it to the 4V supply with the current limiting resistor?
This is not as simple as Ohm's law: below about 2.5-2.8V, Li-ion cell current must be around C/25 or lower. Otherwise, you risk dendritic build up in the cell in the recovery phase which can lead to future runaway.
Now what happens if the cell is totally dead (0V terminal) and you power the charger circuit up? A good Li-ion charger circuit will not charge the cell at all in that condition. But, your circuit will attempt to charge the cell -- and it will charge it with considerable current given the volt-drop across the resistor -- which could result in the cell releasing its magic smoke. (Note that relying on the battery protection board to do this is not necessarily OK. The protection boards often allow charging in this condition, just do not allow discharging.)
I don't believe you can ever build a safe Li-ion charging circuit with just a CV source and a resistor, and there is no manufacturer of Li-ion cells that will recommend it. For other lithum rechargeable chemistries, for instance lithium manganese silicon (example Seiko ML414) then this is tolerable, but this is only because the parasitic reactions in the cell are so high as to keep the cell in a safe condition at all times. This definitely does not apply to Li-ion cells.
Siwastaja:
--- Quote from: tom66 on July 25, 2022, 03:54:24 pm ---Siwastaja, what happens to your float charging circuit and the battery, if the battery runs to <2.8V and you connect it to the 4V supply with the current limiting resistor?
--- End quote ---
That was already covered. I won't repeat it because it is futile to do so with people who lack the skill to concentrate on the text. Instead, please read, re-read, and re-re-read until understood.
It was already present in NiHaoMike's original post, which was extremely short and to the point. I just elaborated on it.
I'll give you a hint, though: a three-letter acronym was discussed.
tom66:
Read and re-read yet I'm still baffled; if you think a BMS, which I'll assume is the acronym in question, is going to prevent the runaway condition, you are sorely mistaken. But thank you for questioning my reading comprehension; I'll question your battery charging comprehension instead, since this is a technical discussion.
The vast vast majority of 3S packs (e.g. hobby RC packs) have no in built BMS and so the advice suggested (4V constant voltage with resistor per cell) is very dangerous for inexperienced users. If they DO have a proper balancing BMS this may work, however it will need to be close to the protection circuitry that for example a laptop battery offers.
You will still need to provide protection in the case of 2.8V/cell or less. Many battery protection circuits DO NOT isolate the battery from charging currents when the battery is under-voltage; the reason for this is to allow the battery to be recovered by an appropriate charger, which is usually possible down to about 1.5-1.8V/cell with some deterioration in the capacity if the charge is replaced very slowly. The exceptions are things like power tool batteries, because users are idiots and will try anything to recover their 100 euro battery that they left discharged, these tend to prohibit almost all zero volt charging as the terminals are too tempting to hook up to some external source. But I can confirm that the majority of BMS circuits will not block charging below 2.8V; for instance, the BQ20z70 device used in a great deal of laptop batteries allows zero-volt charging, and it applies no special current limit in this case; neither do common single-cell Li-Ion isolators like AP9221 or the venerable DW01 which is seen in pretty much every cheap single cell protected Li-Po. There are a few laptop batteries with pyrofuse devices like 12AH3 which can semi-permanently isolate the pack if it's gone below a certain voltage. Again, quite rare, and usually only seen in very high volume consumer kit.
Therefore, I continue to insist that you should not recommend charging any Li-Ion battery pack with only a voltage source and resistor. It really is very little extra cost to use a proper Li-Ion charger chip and protection circuit, and offers a significant improvement to the safety of the system. The danger, as ever, is that advice is copied by those who have no appreciation for the danger that Li-Ion batteries pose, and then implement chargers like this thinking it's perfectly sufficient.
beanflying:
--- Quote from: Siwastaja on July 25, 2022, 05:17:35 pm ---
--- Quote from: tom66 on July 25, 2022, 03:54:24 pm ---Siwastaja, what happens to your float charging circuit and the battery, if the battery runs to <2.8V and you connect it to the 4V supply with the current limiting resistor?
--- End quote ---
That was already covered. I won't repeat it because it is futile to do so with people who lack the skill to concentrate on the text. Instead, please read, re-read, and re-re-read until understood.
It was already present in NiHaoMike's original post, which was extremely short and to the point. I just elaborated on it.
I'll give you a hint, though: a three-letter acronym was discussed.
--- End quote ---
Speaking of 'attitude' over something you chose to step into :-DD
Mike's suggestion was WRONG on multiple levels.
Your own LACK of knowledge on charging multicell Lithium packs is showing. I have been charging LiPo's for well over a decade so I have more than a little idea about multistage charging of them. Resistors, Diodes and other simplistic current limiters are also NOT the appropriate way to 'limit current' lower voltage or 'float' charge a Pack of Lithium batteries at X volts. Cells are not matched for impedance, capacity and in most cases not even from the same batch run.
A BMS is a safety cutout no more and no less is is not now and never will be an appropriate method to terminate a multicell Lithium Charge process. This LIE and a potentially dangerous one is spread all over the web and as this is an Engineering forum this needs to be stamped out here at least. Stack Exchange, Reddit and the countless YouTube videos are complete :bullshit:
--- Quote ---Or one can use a balancing 3S BMS
--- End quote ---
This is called a Balancing CHARGER congratulations you finally mentioned the appropriate thing to use to charge Lithium multicell packs. The only thing you added INCORRECTLY (in 95% of cases) that is not part of the charging circuit is the BMS which is a battery protection device :palm:
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version