[maemo-users] phone charger for N800 (was: kind of off-topic, but not completely: some questions about the N800)

From: Paul Klapperich maemo.org at bobpaul.org
Date: Wed Apr 4 01:28:25 EEST 2007
On 4/3/07, Eric Cooper <ecc at cmu.edu> wrote:
> On Tue, Apr 03, 2007 at 01:33:10PM -0400, andrei raevsky wrote:
> >  Just lemme know if I can use the same charger on my phone & N800
> >  and I promise to get off your collective backs - at least for a
> >  while
> Since the phone charger is rated for less current than the N800
> requires, you would risk damaging the charger -- possibly overheating
> or catching fire -- but probably not the tablet (unless it's consumed
> in the same fire :-).  I'd recommend against it.

Normally I would agree with this statement, but I'm not sure if I would in
this case. Based on previous mailing list posts, it seems the N800
essentially connects the charger port directly to the battery. Charging is
basically controlled using pulse-width modulation--that is, the port is
switched on and off with the percent on time and percent off time varied
depending on amount of charge. This necessitates a current limited supply,
as nothing is done to limit current in the device--equivalent resistance is
equivalent to the battery internal resistance, which is going to be very

Because the proposed power charger is of lower current, you're correct that
the N800 won't be damaged by it. The battery will surely charge slower, but
there shouldn't be any damage. I would be surprised to see any damage to the
charger as well, though. It's true, the charger will be running at 100%
current draw for much of the charge cycle, but this is true for any battery
charger supply, and is definitely true for the charger built for the N800
for the reasons explained above. It is possible that a PSU has inadequate
cooling to hold it's max rated current draw indefinitely, but I find that
highly unlikely for the charger built for the N800--and one would assume
Nokia probably uses the same technique in their mobile phones, so I'd expect
that to be unlikely of any Nokia charger.

If you can find a charger that provides greater than the amount of
> current required, it would be fine.

No, this could be worse. Once the current is too high, the PWM will not be
able to react fast enough and damage could occur. Generally, you're thought
is a good rule of thumb, but not based on how this device is constructed.

Ideally, an equivalent charger to the N800 would be best, but a lower
current equal voltage supply should also be adequate.

An unscientific test would be to charge your phone and your N800 both from a
dead battery. If the charger gets noticeably hotter charging the N800, you
probably don't want to do that again.

On 4/3/07, andrei raevsky <raevsky.andrei at gmail.com> wrote:well, how abouyt
> using the N800 charger on the phone, the other way around?

If the phone uses the same method as the N800, charging the phone with the
N800 charger might not be a good idea as it could damage the phone.

I'd recommend you stick with the adapters meant for the units.

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