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I can't find any reliable info on float charging LiFePo4 cells. It's quite clear that it's not possible to float charge Li-ion and Lipo cells, but LiFePo4 looks like quite different technology.

I'm interested in building a simple charger using a constant voltage with a very limited current (0.1C). I don't care about a charging speed and a maximal capacity, but I want to maximize a battery lifetime.

I looked into few datasheets but they deal only with classic CC/CV charging. I can't find any info about float charging. Is it OK to charge LiFePo4 with constant voltage (3.5V? 0.1C)? Is it necessary to remove voltage when charging is finished?

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  • \$\begingroup\$ I think the question is really how to know when to stop charging. Will voltage always rise to 3.6V, even if charged with 0.1C or less? In that case, it's easy. You could interrupt float charging when it reaches 3.6V and resume a few hours later, thus constantly hitting the 3.6V. The problem is that in my experience, the datasheets don't have that information, manufacturers aren't reachable for comment, and dealers don't know. \$\endgroup\$ – user95482301 Apr 8 '17 at 8:26
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The only reason to float the battery is to buffer the charge/discharge requirements.

The float voltage chosen will depend on how fast the recharge should occur and what SOC needs to be maintained. There was a study of calendar ageing vs SOC (Journal of The Electrochemical Society, 163 (9) A1872-A1880 (2016)) which showed a 6% loss of capacity over 9 months for SOC between 80 and 100%, 3% loss for SOC from 40 to 70% , and no loss at 0% SOC . If the 40 to 70% range was used, there would be no worry of overcharging and not much worry over recovery rate.

In this case a float voltage of 3.315V might work well but I would have no confidence that I was working up against the 70% SOC desirable for energy storage. In my case, I want to keep as much energy on hand as I can and since there is no difference in ageing from 80 to 100% SOC, I want to push the 100% and overcharging becomes a worry. I use the variable source solar PV and the load is various things in my house. I looked at some fractional C charging rates which I posted here. For floating, the green curve in fig. 2 is most useful (You will see that a higher voltage is required for charging, then you can drop back to float). For my purposes a float of 3.40V seems to be a good compromise between responsiveness and risk of overcharge. One of the A123 cells specifies a float of 3.45V which is higher than needed for my QH cells (https://www.lifepo4-batteries.com).

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LiFePO4 batteries have a very low internal impedance so constant voltage charging does not work well. Essentially you provide the battery with current and it controls the voltage.

You do not say much about your application but these batteries will give maximum life when they are kept in the 50-80% charged range so float charging is a bad idea. Given that they show very small changes in voltage with charge level it will not be easy to do this simply.

If you look at your datasheets there should be a charge curve. This will show that at around 90% charge the voltage starts to rise quickly. If you design your charger to pick up the voltage at this point and then to shut off it will give the easiest approximation to this.

The big question is when to turn the charger back on. This will depend on your application but my best suggestion without getting into Amp Hour measurement is to use a delay which you can set up to suit your application and the battery capacity.

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    \$\begingroup\$ I'm well aware of a low internal resistance. Voltage source would have quite low maximal current anyway. My question is about a possibility to select such voltage that it can be connected to the battery all the time (even for a price of not using full capacity). I think there is no such voltage for Li-ion battery (it has to be disconnected) but is it possible for LiFePo4? \$\endgroup\$ – Glubin Dec 19 '16 at 22:42
  • \$\begingroup\$ No there is not, you do not float charge LiFePO4. \$\endgroup\$ – RoyC Apr 8 '17 at 8:36
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The LiFePO4 technology is not that sensible against overcharging like normal Lithium Ion batteries, which tend to destroy themselves. The Problem about float charging is

  1. your charger produces power dissipation,
  2. a full cell tends to convert the applied power into heat, which accelerates aging,
  3. the electrode is likely to produce pure Lithium out of the Lithium ions which lowers the capacity.

To sum this up, float charging LiFePO4s is not as dangerous as overcharging normal Lithium Ion batteries, but has an bad influence on the aging of the cells. The best practice is to charge the cells with CC, then CV. If the current drops below a certain limit (e.g. 5% of C) turn the charging off.

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The LFP battery will have a no-load voltage that varies a bit with SOC. Let's say your LFP sits idle at 13.30 volts when it's 80% charged and you want to keep it there. You could float it at 13.30V and no charging would occur beyond compensating for the slight self-discharge that would otherwise cause voltage to drop slightly (over some months). Ditto for other SOC levels. This approach would be suitable for a battery connected to some load but you want your charger to carry that load, not the battery. Again, you would set the charger to float the battery at that voltage (13.30 in the above example) and the charger will carry the load without charging the battery. But, if there will be no load on the battery, there is no point to floating it because the self-discharge rate is very small. Even a six month idle period will not drop the charge much; maybe a percent or two. An LFP SOC monitor will usually be a coulomb or AH counter and will not record the self-discharge.

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  • \$\begingroup\$ Additional thoughts .... the charger voltage should be set while the load is applied; the voltage setting needs to be quite precise; LFP life is longest if it sits at SOC~50% \$\endgroup\$ – hclarkx Mar 29 '18 at 3:47
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I'm a beginner researching this as well.

From what I understand, once the chemistry is fully charged, additional current will cause damage to the battery.

This article suggests that LiFePO4 are so different in chemistry, that float charging is simply not a relevant methodology. Which might be why no one has bothered to answer this question yet.

Is it possible? Maybe. But the only legitimate reason is being too lazy to implement the correct solution.

Floating LFP is a complex subject and I will touch on this later. Bottom line is to avoid floating LFP banks if you can. Some have argued that a float voltage of 3.35VPC or lower (13.4V for a 12V bank) is not badly damaging. Remember this type of charging keeps you in the upper SOC range for long periods of time and these batteries prefer to sit at 50-60% SOC when not being used not 90%+ SOC.. Can you float at 3.35VPC or lower? Sure you can do what ever you want to, but we don't really know the long term affects other than to say it is likely going to shorten the life. Of the 80 or so white papers I have on LFP batteries not a single one of them has dealt with fractional "C" use and floating at 3.35V or lower, not one. [emphasis added]

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Float charging at less than full charge is not that big of a deal since you will normally discharge the batteries according to need and usually this occurs many times during a 24 hour period. The important thing to remember is to keep your float voltage around 3.5 volts or less per cell. You will find that the current will approach 0 amps once the SOC reaches your float voltage. LiFePo4 is not a woo woo magical chemical reaction. Our physical laws still apply such as ohms law. I would say that “float” voltage is somewhat of a misnomer once the dynamics are looked at.

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  • \$\begingroup\$ We manufacture a battery management system for this technology and it is very important to keep series strings of batteries such that each cell is close to the others with respect to SOC. Gross mis balance will cause damage to both cells that have higher and lower SOC than the average. \$\endgroup\$ – STANLEY HARRISON Nov 19 '19 at 22:26

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