The alkalines don't give particularly high currents either, but the current
limiting is gradual
whereas the electronic current shutoff is like dropping off a cliff or even may
foldback where the
current actual decreases from its peak value as the voltage starts to drop in
limit. The foldback
characteristic helps prevent overheating of the electronics, which then does
not need a heatsink,
to prevent it being burnt out in a short circuit condition. An alkaline AA can
produce something
like 2-3A at about 1~1.1V for a short time, while a NiMH, like say an Eneloop
might produce 4 to 5
times more at the same voltage, say ~11A but sustain that voltage/current much
longer.
Tim Hughes
--- Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> You're right (as always) as I had forgotten about the current limiting
> devices... at least on those that are made as they're supposed to be :-)
> I hadn't thought about it before but I guess a battery that it expected
> to be driving a camera or other similar device might not be built to
> handle the current for a flash. Still I'm very surprised that the
> current limit is less than alkalines let alone NiMH.
>
> Chuck Norcut
>
> Tim Hughes wrote:
> > Chuck,
> > Actually most common Li ion batteries are not "rated" to give
> > particularly high current,
> > many are only rated 2C. There are some types rated for higher currents
> > typically using a
> > different electrode material, but they are not so common except for special
> > uses radio control
> > helicopter users and power drills. Most common Li-ion cells have built in
> > internal curent
> > limiting electronics, so you cannot draw too high a current, to prevent
> > them self igniting!
> The
> > NiMH are actually frequently rated for higher current and more importantly,
> > don't have any
> built
> > in protection electronics.
> >
> > In this particular case, of the so called "3V " lithium batteries these
> > are frequently not
> > Li-ion, but some other Li chemistry like Li-FePO4 (~3.3) with electronics
> > or a diode to reduce
> the
> > voltage a bit more. They reqiuire a special lower voltage charger too. I
> > would not be
> surprised
> > if their rated current was quite modest, like <2A.
> > But this is a guess based on not seeing the batteries so, I can't say
> > definitively, except the
> > symptoms fit well with them having limited current capability.
> >
> > Tim Hughes
> >
> >
> > --- Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> >
> >> But remember that these are supposedly rechargeable lithium ion
> >> batteries and not elemental lithium. The thing that makes lithium ion
> >> batteries potentially dangerous is that they are capable of delivering
> >> far higher currents than alkaline or even NiMH batteries.
> >>
> >> So, I don't think your analysis can be correct if these are truly
> >> lithium ion batteries. But maybe the problem is that they're not
> >> lithium ion.
> >>
> >> Chuck Norcutt
> >>
> >>
> >> Tim Hughes wrote:
> >>> John,
> >>> I am Making some big assumptions here, since I don't have enough
> >>> info from what you
> have
> >>> said, but I think this is what happens:
> >>>
> >>> When the flash capacitor is completely discharged at a cold start then
> >>> initially the current
> >> draw
> >>> is greatest. As the capacitor charges up a little the current is somewhat
> >>> reduced and then
> >>> eventually drops off rapidly near full charge.
> >>>
> >>> It may be the particular Li batteries you have, can't supply the very
> >>> high initial current
> of
> >> a
> >>> completely discharged flash capacitor and the internal electronics then
> >>> stops the charging,
> >> when
> >>> the battery voltage drops too much at cold start. Once the capacitor is
> >>> slightly charged
> >> using
> >>> the regular batteries, then the Li batteries can supply the lower current
> >>> to continue
> charging
> >> it
> >>> up. The flash capacitor does not completely discharge even after a full
> >>> dump, so once you
> have
> >>> charged up the capacitor a little, with the regular batteries then it
> >>> will operate fine
> >>> thereafter, off the Li, until you switch it off and leave it to discharge
> >>> completely.
> >>>
> >>> Tim Hughes
> >>>
> >>>
> >>>
> >>>
> >>> --- om4t@xxxxxxxxxxx wrote:
> >>>
> >>>> I normally use two rechargable 3 volt lithium ion battery packs in an OM
> >>>> flash gun with my
> >> E510.
> >>>> The flash is a dedicated gun for the E series.
> >>>>
> >>>> When the flash has not been used for several days the battery packs do
> >>>> not bring the flash
> >> gun
> >>>> to life. If I change over to four Ultra Duracell 1.5 volt batteries the
> >>>> gun springs to
> life.
> >> If
> >>>> I then change yet again to the LI battery packs the flash springs to
> >>>> life immediately. It
> is
> >>>> almost as if the flash needs to be initially energized with the Duracell
> >>>> batteries before
> the
> >> LI
> >>>> packs become operative.
> >>>>
> >>>> The LI packs are only a matter of a few weeks old, are fully charged and
> >>>> the connector
> points
> >> on
> >>>> both packs and in the camera are clean.
> >>>>
> >>>> When I start off from "cold" with the LI packs installed in the flash
> >>>> gun, the gun still
> >> remains
> >>>> inoperative even if I jiggle the packs slightly or apply a little
> >>>> pressure to the slide
> mount
> >>>> cover on the flash gun casing so as to effect a "better" contact between
> >>>> the terminals on
> the
> >>>> packs and in the camera.
> >>>>
> >>>> Does anyone have any idea about what might be ailing my flash gun and /
> >>>> or LI battery
> packs?
> >>>>
> >>>> John Hudson
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
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