LiFePO4 charging and protection circuit

[russ]

jon_r is making all sorts of wild claims - sounds about like electrothingy.

[PNjunction]

A lot to digest. I guess the question is, "what conditions do lead to plating?"

The condition is when there is basically no current flowing when the absorb cycle is complete. This can actually happen anywhere from around 14.0 to 14.6v. It is merely a matter of time. Once absorb is finished, constantly holding a voltage without any current flowing causes the problem. A few hours may not be harmful to a large battery. More than 8 hours, definitely so. What you are doing at that point is cooking the electrolyte such as it is. To make THAT go haywire requires total failure / abuse of the charger up to about 30v or so. But then again, taking your Pb battery up to 30v or so will also produce some drama.

Since there is no sulfation to deal with, you do not need to charge to 100% all the time. Even if you wanted to, it is basically pointless as there is very very little power available in the steep charge knee. As soon as you place a load on it, you'll be at 90% or less nearly immediately anyway.

Pressing an agm charger into service that charges up to say 14.4v, and then falls back to 13.5v float when absorb is finished when a very low current point is reached is ok, as the 13.5v float does basically nothing for a lifepo4 at that point anyway. Ideally one should use a lifepo4-specific charger, but of course we can fake it with common charge controllers.

Note that I have not brought up any sort of bms considerations, as I only use my lifepo4's in the flat part of the knee, ie only from 20 to 80% DOD and no further. Typically less. So far so good. EV, RC and other mobile users may need a bms to eek out the very last drop of balanced capacity in order to get their vehicle home.

[billvon]

billvon seems to suggest holding them at 3.6v is the problem, so how about holding them at a lower voltage, say 3.4v?

That will definitely help, and that feature has been slowly making its way into chargers for other lithium chemistries (like lithium-cobalt and lithium-manganese, both of which have even bigger problems with this.)