Is it safe to hold a LiFePO4 at or near charge voltage permanently? - Page 1
Forum section: electronics / projects

author: Whales
author_rank: Super Contributor
timestamp: January 29, 2024, 04:57:23 am
content: I am thinking of using an LiFePO4 18650 in an alarm clock to keep it running when wired power fails. This project is a one-off for fun, not for production.The simplest implementation I can think of is to use the battery as a giant decoupling capacitor (almost) directly attached to the main voltage rail:* 5V USB -> diode -> LDO with current limiting (~0.5A) -> 3.6V VCC* Battery connects to 3.6V VCC via a fuse and some back-to-back mosfets (for OVP and UVP, controlled by aTPS3700 or similar)Question:Is it safe to supply a LiFePO4 cell with a constant 3.6V (current limited) for very long periods of time (years)?I have been reading random LiFePO4 datasheets off the web and I notice two interesting things:(1) Most charging occurs between 3.2V and 3.5V, then 3.6V suddenly appears like a brick wall right at the end. Eg:https://cebabattery.com/wp-content/uploads/2018/01/LiFePO4-1868130-3.2V-10Ah-Battery_DATASHEET.pdfWhen you start discharging you immediately drop back down a few hundred millivolts (perhaps more so then just cell resistance would make you do?). That would suggest some interesting chemistry is going on near 3.5-3.6V and holding the cell there constantly _might_ be a bad idea?(2) Recommendation to terminate charging at 0.05C. I suspect this is an arbitrary number but it dodges the question of whether or not you can hold it at charging voltage forever (days, weeks, months).I am flexible to hold the battery at lower voltages (eg 3.4V) but I would be guessing at what value to use.(Aside: I believe mobile phones do it the way I'm describing, but for standard Li-ion cells. Some also charge to 4.3V or so and many are known to swell their cells if left plugged in all of the time. I was considering using an Li-ion 18650 held at 4V constant, but I have some LiFePO4 18650's lying around with no use and they'resupposedto be less likely to catch fire. Perhaps I'm wrong.)

author: mikerj
author_rank: Super Contributor
timestamp: January 29, 2024, 12:00:48 pm
content: Maintaining the cell at it's maximum voltage for "float charging" will result in rapid cell aging, do not do this. Float charging Lithium Ion cells is typically not required as their self discharge is very low, but LiFePo4 can be float charged at around 3.37V, though this will not yield the full rated cell capacity (around 70% ish).

author: Whales
author_rank: Super Contributor
timestamp: January 29, 2024, 12:04:48 pm
content: 70% or so is still perfectly fine for what I want.Do you know what the degradation process is? I'd like to read up more on it.EDIT: Ah the search terms "LiFePO4 float charging" seem to be yielding some results, thankyou.EDIT2: Hmm, they're all secondary and tertiary sources. It would be nice to find primary source (manufacturer) or research papers on this, I'll keep digging.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 29, 2024, 12:05:27 pm
content: LFP chemistry is a special case because it is usually charged to a higher voltage (3.60 or 3.65V) than the long-time open-circuit voltage equivalent to 100% (3.45V or so, IIRC). Using this higher voltage helps charging fully faster, and quite obviously cannot be very harmful, otherwise it would not be used. Then again, it is a very bad idea to keep floating the cell at this increased voltage for prolonged times. But, there is not need - just float the cell at 3.40V or so. You can try to do a full discharge after holding for days at 3.40V to find how much capacity you exactly got. If insufficient and far from 100%, you can go a little bit higher, say 3.45V as the next step.

author: Psi
author_rank: Super Contributor
timestamp: January 29, 2024, 12:10:56 pm
content: As above, you can float charge them fine, just make sure the voltage is low enough that you are holding them at between 30-70% capacity left in the cell.The damage occurs from sustained time in a highly charged or highly discharged state. If you're in the middle range you're fine.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 29, 2024, 01:46:23 pm
content: Quote from: Siwastaja on January 29, 2024, 12:05:27 pmAn even quicker way to find the maximum recommended float voltage is to fully charge the cell the normal way, then leave it sitting with no load for at least a day and measure the voltage. (Or just measure the voltage at 1 hour intervals until you see it no longer changing much.)Have you considered a supercap? A modern RTC chip only uses microamps.

author: shapirus
author_rank: Super Contributor
timestamp: January 29, 2024, 02:10:00 pm
content: Well, from my personal anecdotal evidence I can tell that the cell charged with CC/CV and kept floating for some hours (e.g., over night) at what is a typical open circuit voltage for LiFePO4, which is around 3.35-3.40V, will give back 100% of its rated capacity on a discharge test, or very near that.I have a couple of batteries that are used as buffer batteries: one is in the computer UPS, where each cell is kept at about 3.42V, another is for the DIY router UPS with the cells floating at ~3.35V. The former has been working in this configuration for almost 3 years now, and I don't see any sign of degradation yet, but that's based on the run time under load, which may vary -- I haven't done proper measurements of remaining capacity. The latter has worked less than that, and it's just too big for the load it powers to make any judgements about its capacity change.But let's put it this way: what difference does it make for the cell if it just sits on the shelf unconnected at its 3.35V "settled-down" voltage, or is connected to an always-on CC/CV power supply with max voltage limited to the same 3.35V? Right, there's no difference, at least, none that I can imagine. In either case there will be zero current flowing to or from the battery, save for the negligible amount that compensates the self-discharge current in the second scenario.

author: Whales
author_rank: Super Contributor
timestamp: January 29, 2024, 09:36:34 pm
content: Thankyou all.Good to know that the voltage spiking to 3.6V during charging does have some special meaning and that the actual final cell voltage is lower than that.It's interesting how precise of a voltage I'll need to hold the cell at. The difference between 3.3V and 3.4V is almost all of the cell's capacity. EDIT: No this seems to be datasheet/product/chargerate specific. Some other sources show a wider range than this. Hmm.Quote from: NiHaoMike on January 29, 2024, 01:46:23 pmOriginally I was using just some AA batteries as the clock backup, but I want to go one further and have the clock fully operational when running off batteries. With the display on it pulls up to 150mA, so I'll keep that off unless the alarm has tripped or the timeset button is held.

author: bson
author_rank: Unknown
timestamp: January 29, 2024, 09:44:31 pm
content: If you don't need maximum capacity from the batteries, downrate their capacity to 50%. This means charge them to and keep them at 50% of capacity. And, yes, it's a pretty narrow voltage range.

author: shapirus
author_rank: Super Contributor
timestamp: January 29, 2024, 10:20:46 pm
content: Quote from: Whales on January 29, 2024, 09:36:34 pmIt may differ for different cells, but from what I've seen so far, the flattest, if there's such a word, region of the discharge curve is at about 3.30-3.32V (unless sagged by high current), so if you want to charge the cells to something that won't be definitely too low, try maybe 3.33-3.34 or thereabouts.It's best to verify this yourself: CC/CV charge the cell(s) to a given target voltage, keep it connected to the charger maintaining that target voltage for at least a few hours, and then measure capacity on discharge (you surely do have an electronic load that can measure it, don't you?). Then do a textbook charge (CC/CV to 3.60, then disconnect), measure what you get on discharge, and then you can calculate the percentage of what you had with the float charging method at each particular voltage. Use a good multimeter to set that target voltage: you need a solid millivolt resolution there. Accuracy is welcome, but isn't so important, unless it's unreasonably off: you only need repeatable results relative to your multimeter's readings as a reference.