Re: [SOLVED] Re: PLEASE HELP!!!!! [SOLVED]
Re: [SOLVED] Re: PLEASE HELP!!!!! [SOLVED]
- Subject: Re: [SOLVED] Re: PLEASE HELP!!!!! [SOLVED]
- From: Rick Mann <email@hidden>
- Date: Mon, 19 Jan 2009 10:33:09 -0800
On Jan 19, 2009, at 02:43:56, Simon Wilson wrote:
On a side note: The new MacBook Pros charge *much* slower than the
old ones. I can only hope this is in an attempt by Apple to extend
battery lifetime and decrease the failure rate.
This could be because of the cell balancing they're doing.
Most rechargeable packs are a series string of cells (because you can
make the cells as large as you want in terms of capacity, the
chemistry restricts a cell to 3.7 V, give or take almost a volt,
depending on chemistry). To charge the pack, you apply a voltage
across the entire series string. All the cells see the same current,
but because of variability in manufacturing (and it's nearly
impossible to engineer this out), the cells charge at slightly
different rates (some lose more energy to heat than others). Over
time, the voltage across individual cells begins to vary.
Then some things happen: because the charger only knows what's
happening to the pack as a whole, some cells begin to get over-charged
which reduces their life. Other cells never get fully charged, and you
waste capacity. While discharging, some cells get dangerously low, and
for some chemistries (lithium polymer), this can result in them
bursting into flame.
A smarter charger monitors the voltage of each cell in the pack, and
can shunt some of the current around a cell, if its voltage gets too
high during charging. Sometimes this requires reducing the overall
charge current, too (although typically at the shunting stage, charge
current is lower anyway). Sometimes it requires shunting earlier and
reducing current more. All this contributes to longer charging times
(waiting for the lower cells in a pack to catch up).
I have no idea if Apple pre-balances the packs before shipping them,
or if this is something they do on your time during the first several
charge cycles.
--
Rick
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