Actually have been looking at using a large capacitor as a battery
instead, as charging requirements are not really an issue.. but am not
sure what size capacitor would do the job.
I have low ESR caps on the board to cope with the digital GSM peak
current requirements so it should be ok...

Alternatively, I like your idea of a dual battery setup.. I think I
can implement that with minimal effort :)

Was using SLA batteries in another solar powered product with no
problems, it is just the issue of size (isn't it always ;)

Cheers
Peter Mcalpine

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Kyrre Aalerud
Sent: Wednesday, 27 November 2002 12:34 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: NiMh and solar charging


Or, you can use a very large capacitor battery :-)

Alternatively, you can do a dual-supply solution with two set's of
batteries.
That way, you can fully discharge one, while charging the other, then
switch.
Would actually be kinda cool :-)

    KreAture

----- Original Message -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, November 26, 2002 1:38 PM
Subject: Re: [EE]: NiMh and solar charging


> > pondering away on my latest project here which is a
> > remote GSM based telemetry device. Got my current consumption
> > under control with an average of about 5ma.
> >
> > So, now to how to power the thing and keep it self sufficient.
> >
> > My idea at this stage is 600mAh NiMh 3.6V battery pack (as
> > the circuit runs at 2.7-4v) and charge this via a 6V solar
> > cell. (about 3-4days with no sun I hope..)
> >
> > I intend to use 2 fixed resistors as a trickle / fast charge
> > cct, with the fast charge one switched in when battery falls
> > below 3v, and switch to trickle when fully charged. (fast charge
> > at about 60ma) also given that max charge will only be when
> > the solar cell gets the good sunshine (plenty of that in Aus at
> > the moment..)
>
> NiMH batteries don't like long term trickle charging.  You also want to
> discharge them almost fully before recharging them.  I think a lead-acid
> battery is more approriate here.  Look into "sealed lead acid" or SLA
> batteries.
>
>
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