First of all I'm not talking about charging at the maximum C-rating. While you will get some loss over time if you do that from what others have said the loss isn't all that much. What I'm talking about is going way above what the battery is suppose to be charged at.
I have been using reBel batteries for a few weeks now and haven't had any problems with them at all, but, the 6AH LiFePO4 battery has been used in several projects and demonstrations several of have pushed it to its limit and above. One of the things that have really pushed this battery is charging. Most of the time it has been charged with two amps, but, when I first got the battery all I had was a 7 amp charger and used it to charge the battery four or five times assuming the cells were one-C for both charge and discharge.
The battery got warm to the touch but not hot at all. I later found out the max charge was suppose to be 3 amps. On top of that this battery has been cycled close to sixty times, completely discharged over thirty times and has been pushed slightly above its discharge C-rating of 6 amps a time or two by about one amp. That wasn't intentional for testing, it happened when my small inverter compensated for lower voltage. You should never discharge Lithium Iron Phosphate batteries above their C- rating, but, they can be discharged to their max C-rating time after time. Fast charging is what is always stated as shortening their life to some extent. In this case though it was twice the charge rate which this battery was never meant to handle. Even if the cells do have a one-C charge rating seven amps is still way more.
I decided to run a test on my 6AH reBel battery to see how much capacity had been lost after all this particular battery had been through. I wasn't having any issues but was sure there had been some loss. I ran one of the original tests I did that used a 2.7 amp load. It bounced between 2.7 and 2.8 amps and dropped to a little above 2.5 amps when the battery got close to 12V. You can see this test in the review I did on this battery.
For those of you who just want the answer, there was some loss in capacity but not nearly as much as I thought there would be. I reached 12V a few seconds over six minutes sooner than I did in the original test. Keep in mind if this had been a comparable AGM 7AH alarm or UPS battery it would have been destroyed in less than ten cycles being fully discharged not to mention the damage charging at twice what it's supposed to be charged at would have done. After all the Lithium battery had been through it still provided over two amps above 12V for two hours and four minutes and over 5AH of its capacity. No matter how you look at it that's pretty impressive.
I can't say how much capacity would have been lost in 60 cycles if the battery had never been charged above its C-rating. I doubt the loss would have been more than a minute if that though. My 12AH battery has fifteen cycles on it and I haven't seen any noticeable Los in capacity from that battery, the 24AH that has about the same number of cycles, the 50AH battery that has about twenty cycles or the 100AH battery that has been cycled five times have shown no noticeable Los in capacity either. Those batteries have all been completely discharged three or four times, but, most of the time they haven't been discharged below 11.5V. Loads have varied from a few amps to close to their C-rating depending on what I was using them for.
In conclusion, you should always charge at the stated charge rating. Going above the max C-rating will cause faster loss in capacity, but if you do a time or two clearly these batteries are more forgiving than Led-Acid batteries are. I've seen good UPS AGM batteries as low as 15 dollars. The reBel battery will cost you twice that, but, even if you use it extremely hard and get three or four hundred cycles it's still a real deal. I have worked with several products that use a 7AH AGM battery and have never seen one last beyond fifteen or twenty cycles when they were fully discharged and have only seen a hand full reach 30 cycles discharged to fifty percent that still worked but had very little usable capacity left. As for those that sit on standby being charged all the time? The ones I've seen had to be replaced in a year or two never being used more than once or twice. How one of these batteries would stand up to sitting fully charged all the time I don't know, but I think you can be sure they would last more than a couple years because the BMS cuts the charge off when they're full.
The one drawback is the reBel 6AH battery is only rated for a 6 amp discharge. I had no problem discharging it at 7 amps but that was above its rating. My test using it in a UPS showed it worked fine but 6 amps means you shouldn't use the UPS above 60 watts of its capacity which is fine for some applications. Maybe in the future reBel will come out with a model of this battery that can handle a higher discharge amperage. It would be ideal for a small UPS if it could handle fifteen amps.