2
\$\begingroup\$

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?

\$\endgroup\$
  • \$\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
1
\$\begingroup\$

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.

\$\endgroup\$
  • 1
    \$\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
1
\$\begingroup\$

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]

\$\endgroup\$
1
\$\begingroup\$

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.

\$\endgroup\$
0
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.