Electronics > Projects, Designs, and Technical Stuff
Float Charging Lithium-Ion Batteries at Lower Voltages
<< < (3/4) > >>
KL27x:
People are idiots.
"Trickle charge" is a term derived from lead acid batteries.

If the float charge of a li ion cell reaches 4.2V (a good cell), and you charge it from 4.2V source, there is no appreciable current. No current, no trickle. That said, after some damage, a cell might no longer hold a float voltage of 4.2V. It could be lower. But battery university has a mish mash of information, much of which is nothing more than myth and completely incorrect. This is maybe halfway correct if you squint at it the right way.

When you trickle charge a lead acid cell it's because you are using a voltage that is higher than the float voltage to begin with. So after it is fully charged, there is still a small current that continues flowing into the battery which is greater than the self-discharge rate. Again, this is only possible because the psu/charger outputs a higher voltage than the float voltage of the battery. This is not typically the case with a li ion charger.

So when you trickle charge a lead acid battery, instead of pushing a high current into it and disconnecting it when it's done charging, you use a higher voltage than the float voltage and have some simple current limiting like a series resistor, so that the charge current is relatively low. And you don't disconnect it when it's done charging, because the current flow is relatively small, and with lead acid batteries there is no harm no foul.

It is completely true that you shouldn't trickle charge a li ion cell. To trickle charge a cell you'd need to use a voltage source that is higher than 4.2V. We already know that you can't do that. This is redundant information that has been twisted into something that is pretty much meaningless. By idiots.
Siwastaja:

--- Quote from: David Hess on April 26, 2019, 02:08:35 am ---
--- Quote from: MadScientist on April 24, 2019, 11:54:49 pm ---The major factor in Li ion degradation is largely calendar  life time , ( they die just sitting there ) due to the constant parasitic action. After that high charge rates have a factor
--- End quote ---

I agree.  But float charging them to the highest rated voltage for maximum capacity increases the degradation by some non-linear factor.

--- End quote ---

If negligible cycling happens, and floating causes damage mostly by calendar aging, then the function is indeed non-linear, and interesting. From what I have read and measured myself, it's clearly a piecewise function of two slopes:

Below about 75-80%, you have strongly increasing life expectancy, the lower you store it at. You don't need to go arbitrarily low; at around 50%, it's already so freaking good it's hard to say if there's anything more to gain.

But, contrary to intuition and common belief, over about 80%, strange things happen. The cells I have tested show very little difference at all (i.e., 80% and 100% are equally bad), and I have read a paper in which a Panasonic cell (IIRC) actually showed better calendar life at 100% compared to at 80%. I really don't know the mechanism behind this (haven't looked at it). Do note that this is based on actual production chemistries available now (visible on both LCO and NCA), but is not a fundamental physical law, so if you read this post after years, things may be different.

Now, between 80-100%, charging current does more damage (than at, say, between 60-80%), so if a lot of cycling happens, then limiting yourself to 80% will be a benefit. But for storage, or very low-ripple, low-cycling floating, 80% is not any better than 100%. So you can as well use the near-full capacity; or go even lower to really increase the life.

You need to make the distinction between calendar and cycle life anyway: for calendar aging, lower temperature helps (the lower the better), but for charge current induced damage (cycling), higher temperature helps (optimum can be near the rated maximum charge temperature, often 45 degC). (This is unsurprising; as you know, charging tends to be completely forbidden below 0degC. In reality, it's not a step function, and as there is no water anywhere to freeze, 0 degC is nothing magical, just a nice number where charging already produces "too much" damage.)

So when you combine both calendar and cycling damage (as you do in a practical float system), you'll have a combination of opposing constraints, and need to know which damage mode dominates. Even a guesstimate is much better than nothing.

For example, in a highly cycling system, I have measured that a heated battery (at 50 degC) does over 1500 cycles just fine, but a cooled (at 10 degC) battery dies in around 100 cycles. The heated use case was outside the manufacturer's specification; the cooled one was inside the specification, yet died early.
edpalmer42:
How much does the chemistry of the cell affect this behaviour?  I've got a stash of LiFePO4 cells that I thought would work great for a 24V backup system.

For those who aren't familiar with them, LiFePO4 cells are still Lithium ion cells, but they have a lower voltage and lower power density than typical Lithium cells.  Their big advantage is that they're much more stable than their more volatile cousins.  It's almost impossible to get them to burst into flames.  They also seem to be more forgiving regarding charging and discharging.

Ed
Siwastaja:
LFP cells are easy to get burst into flames, and tend to contain fewer safety mechanisms due to the "false sense of security" phenomenon caused by the safer cathode chemistry, and substandard manufacturers. Lack of shutdown separators, current interruption devices, and PTC fuses is alarming, and it will enter thermal runaway somewhere around 350degC anyway, although with less energy, still shooting the exact same flammable electrolyte out. So in some cases, they are more dangerous, sometimes less dangerous.


The voltage curve of the LFP chemistry is almost a straight line when compared to the classical LCO or modern NCA. This means, better regulation is needed to float it at any particular SoC, and it's still difficult to say what the exact SoC is, based on voltage. If you try to keep it, say, 70%, you are easily accidentally at 60% or 80%. Does this matter? Maybe not, just float it at 100%, it'll do fine.

LFP is quite good even at 100%, if you think about this the same way you think LCO/NCA - through the open-circuit voltage. What's "4.20V" on a modern NCA cell? The same for LFP is not 3.65V or 3.60V, the typical "charge cutoff voltage". If you keep an LFP indefinitely at 3.65V by floating, you'll overcharge it slightly, without getting much more capacity out of it. I can't offhand remember the exact 100% open-circuit voltage, but you can easily measure it by fully charging the cell, disconnecting it, then measuring the voltage after a few minutes (or hours, to be sure). For an NCA cell, it's almost 4.20V, but for LFP, it isn't even close to 3.65V, even if you charged it to 3.65V. So, I'd float an LFP cell at 100%, no problem, but just make sure you know what voltage 100% is at. It's somewhere around 3.40-3.45V IIRC, but I haven't done anything with LFP cells in years.

The best thing in LFP is that, by luck, the voltage range matches so that's it's fairly easy to replace lead acid packs: 2s for 6V, 4s for 12V, 8s for 24V. With LCO/NCA, can't have 3.5s to replace 12V lead-acid! The curve shape is also closer to lead-acid.
digsys:
I would add an extra comment to Swijas Li charge / trickle comments above, which I very much agree with. We often get "experimental" or "new generation"
Li chemistry to trial, and everything changes !! In coming years, with all the new chemistry "tweaks" they are doing, these % figures will flap around all over
the place. Sometimes, you can't even be sure what batch chemistry you buy. Generally need to run general tests to check.
They are "guidelines" for now :-)
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod