Why does a matchbook sized cell phone battery require 3 hours to charge

[Don Klipstein]

I haven't bothered to look into the numerous consumer alert-type stories
that have been on the TV news, but there have been quite a few regarding
laptop and cell phone batteries starting fires, sometimes in pockets,
briefcases or luggage.

All the stories I ever heard had to do with lithium ion, which is
fussier (extreme intolerance of overcharging) and involves more reactive
chemicals. In that past year, I have been hearing a lot fewer horror
stories.

It's been suggested/discussed here in RCM before, that in general, the best
battery charging is slow charging.. both in terms of quality/level of
battery output and long battery life.

NiCd and NiMH is easily overcharged by slow charging. NiCd usually
tolerates slow-charge overcharge well, with a slight voltage depression
that can generally be fixed by full discharge followed by full charge
without overcharging. NiMH appears to me less tolerant of overcharging,
and prefers fast chargers because faster charging enables better detection
of full charge. Smart chargers for NiCd and NiMH tend to have charge
times 4 hours or less.

The instant gratification or demand for instantaneous results (commercial
uses) of power tool, or critical use batteries have contributed to the
availability of rapid-charging battery chemistry and construction design
technology.

Forcing rapid chemical changes can't be good for long battery life, and
probably reduces the useful charge output, in most cases (for most types of
batteries).

- Don Klipstein ([email protected])

 

[Don Klipstein]

My guess is it's lithium ion. Typically those can be recharged most of
the way at a rate that can charge them in an hour or so, but "topping off"
requires strict adherence to a cell voltage limit (or else a cell safety
feature breaks the internal cell connection, otherwise KABLOOEY). At that
strict voltage limit, it appears to me to easily take a couple hours to
get the cell to accept the last 20-30% or whatever of a fulll charge.

Lead-acid is similar, except much less serious damage if the cell
voltage limit is exceeded from fully charging with a fast charge. (as
long as the resulting hydrogen and oxygen do not get ignited) Much
less still if the cell has provisions to replace the electrolyzed water.

It's probably NiCd or NiMH. With usual modern "smart charging"
technology, plenty probably most of those can be reasonably safely
recharged in maybe an hour, even less if designed for faster charging.

Most cell phones I've owned lately, have had optional "travel-
chargers" available from the manufacturer, which charges the battery
in about 1 hour.

I suspect with not quite a full charge. Keep in mind that the "fuel
gauge" battery charge indicator appears to me to be full when the battery
is above maybe 60-70% or so.

Also the phone controls the charging so that when the
battery is fully charged, it will stop charging. So, I don't think
that the long charge times are unavoidable due to the battery type
itself. There has to be another explanation... maybe it's a marketing
thing... include the "slow" charger with the phone and sell the faster
one as an "option" for about $40 more.

It appears to be mainly a Li ion often taking a lot more than an hour to
get really fully charged in a safe manner.

- Don Klipstein ([email protected])