# [EVDL] Effects of UNDERcharging LiFe4Po batteries?



## EVDL List (Jul 27, 2007)

> Richard Acuti wrote:
> > I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time.
> it's not hard data but it is indicative that GM has made the preliminary
> decision to do just that for the Chevrolet Volt. I think it was
> ...


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## EVDL List (Jul 27, 2007)

As far as I know, undercharging them just reduces capacity for that
cycle; it won't harm the cells.

I'm basing this on "regular" LiCoO2 cells; for them, storage at high
SOC ages cells faster; the lower the SOC, the slower they age, down to
an optimal storage SOC of 40%.

If this holds over to LiFePO4, your cells may actually be better off
being undercharged (at a penalty to range).

-Morgan LaMoore



> Richard Acuti <[email protected]> wrote:
> >
> > I am curious if anyone has any data on the effect or consequence of slightly UNDERcharging Lithium batteries. I am accepting wild theories, anecdotal evidence and tribal knowledge at this time. At the moment, I have no printed literature from any vendors to refer to. When I get closer to an actual purchase, I will dig deeper and make contact with the vendors.
> >
> ...


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

I hope I am not "stating the obvious"

I think "State of charge" and "balance" are separate issues; even though
balance is relative state of charge.

The failure mode for most lithium ion chemistries is that high voltage
disassociates the electrolyte and allows the chemicals to fowl up the
pores on the electrode that accept the intercalated ions during charge.
LIfepo4 is more tolerant than other lithium chemistry as it is actually
lithium starved at end of charge and its resistance shoots up to limit
current.(as opposed to thermal runaway)

For any single cell, charging to max allowed voltage, gets you the most
charge. It has a bit of the destructive part associated with it.
For any single cell, charging to a lower voltage, lessens this mechanism
at the expense of capacity.

The real problem is that if you start off with cells in series that are
all at 80% SOC and run them all down to 30% SOC for the pack, some will
be at 29% and others will be at 31%. This is caused by different
temperatures or different internal resistances. Those same conditions
probably exist on the next charge so when you get back to "pack is at
80% SOC" the ones at 29% have come up to 78% and the ones at 31% are now
at 82%.

The balance issue is true of any string of series batteries charged as a
pack, regardless of chemistry.

I think keeping an entire pack between 30% and 80% state of charge will
actually be more complicated than 30% to 100%. You will have to keep
track of the columbs in and out of each cell and measure during
operation the internal resistance for each cell. Then actively balance
in advance of hitting the 80% SOC mark. If you are allowed to go to
100% then it is easier to bypass some and let others continue to get
them all at the same point.

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## EVDL List (Jul 27, 2007)

> Jeff Shanab wrote:
> > I think keeping an entire pack between 30% and 80% state of charge will
> > actually be more complicated than 30% to 100%. You will have to keep
> > track of the columbs in and out of each cell and measure during
> ...


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## EVDL List (Jul 27, 2007)

my understanding is that there should be no problem with undercharging, as a matter of fact leaving a little space might be a good thing.....especially if you live at the top of a big hill and use regen.....hybrids are designed for it....
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