jan_de_jong(at)casema.nl
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 Posted: Tue Feb 08, 2011 4:21 pm Post subject: {Spam?} Re: Shorai LiFePO4 |
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Actually, the battery being held at 14.5V is supposed to be the way the
cells get balanced. The fully charged cells pass enough current to allow
the laggards to become fully charged as well. While this is happening
the fully charged cells have a somewhat elevated voltage across them. In
the end all cells are fully charged and at 1/4 of the 14.5V applied and
so they remain.
A battery gets unbalanced cells when it is never allowed to sit at 14.5V
long enough (so 8-hour trips should be perfect....).
We keep our lead-acid batteries balanced the same way. But lead-acid
cells conduct charging current much better when fully charged than LFP
cells and balancing is much quicker.
Some pictures of LFP cell characteristics:
http://mvp090-1.104web.com.tw/cetacean/front/bin/ptdetail.phtml?Part=e40138f1
At 90% charge (60% of charging time) the constant volts charging current
drops from 1C and is down to 0.02C when 100% of charge and time is reached.
In the following report, among other tests, a cell is overcharged to
destruction (page 2 :
http://www.lifebatt.com/sandiareport.pdf
They force a 1C charging current into the fully charged cell. After 34
minutes it reaches 11V, loses continuity and vents at 110 'C. Maximum
temperature at 160 'C is reached 20 minutes later.
For this to happen in a real life battery charging situation the
imbalance would have to be very extreme indeed.
Still, I might want an alarm if any cell moved beyond 4 volts while the
battery was kept at 14.5V.
And maybe a location good for 200 'C with venting to the outside.
In the meantime we can wait for the motorcyclists.
Jan de Jong
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