At 09:18 AM 26/03/2010, Skip wrote:
>Is it true that you need to be more careful charging LiFePO4
>batteries, that poor charging can wreck them?
G'day Skip, All
Words from the wise on the Electric Vehicle Discussion List and
EVtech discussion list (i.e. those who've been working with various
lithium chemistries for a while) is don not exceed 4.(something,
varies with manufacturer) volts on charge, do not go below
2.(something, again, varies with manufacturer) volts on discharge,
unless you are happy to throw the cells away.
==========
Typically, quoting Jack Rickard
Subject: Re: [Evtech] Evtech Digest, Vol 63, Issue 34
It's not quite that much of a mystery. The fully charged terminal
voltage of virtually all the LiFePo4 cells I have seen is 3.40 volts.
Your CHARGE voltage, the voltage at which you charge to in a CC/CV
charge cycle will vary depending on the cell used. By spec, the
Thundersky cells are charged to a level of 4.2v and held there until
current reduces to 0.05C. We actually charge series strings to 3.65
v. Sky Energy (now CALB) cells are spec'd at 3.6v and we charge
series strings to an average 3.5v.
When you terminate charging, the cells will fall to their static
voltage over the course of about 3 hours. If the cells were truly
FULLY charged, they would read right at 3.4v. Charging to the lower
string average voltages we usually see 3.35 v or thereabouts.
During the charge cycle, as you climb the sharp wall at the end, you
will notice quite some variation from cell to cell. In charging TS
cells, for example, although we are charging to an average of 3.65v,
we might see anywhere from 3.55 to 3.85 or even 3.9 on any one cell.
On settling to their static voltage, they will all be plus or minus
0.02 I would say and often 0.01 v. At 100%SOC, they would by
definition be at 3.40v.
I think undercharging them slightly this way is kind of a key to long
cell life, particularly when dealing with long series strings.
==========
So when buying lithium cells, expect to spend up to half as much
again on battery charging and management.
Regards
[Technik] James
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