Find your load, then your needed operating time.
Then you have your size from the formulae t/R = C.
You have to recalculate your current with regard to voltage in order to find
the implied R of your circuit.
It will be a lot lower during your GTM-activity slots, so average it.

Fairly basic stuff, but it's nice to have it on the list.

    KreAture

----- Original Message -----
From: "Peter Mcalpine" <petermca@OZEMAIL.COM.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, November 28, 2002 2:27 AM
Subject: Re: [EE]: NiMh and solar charging


> 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.
> >
> >
> > *****************************************************************
> > Embed Inc, embedded system specialists in Littleton Massachusetts
> > (978) 742-9014, http://www.embedinc.com
> >
> > --
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