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'[EE]: NiMh and solar charging'
2002\11\26@032301 by Peter Mcalpine

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Hi All,
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..)

Does this sound a feasibly plan?
Or is there a much better solution not using something complicated
like NiMh batteries?

Any thoughts much appreciated :)

Regards
Peter Mcalpine

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2002\11\26@040633 by Russell McMahon

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> 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..)
>
> Does this sound a feasibly plan?
> Or is there a much better solution not using something complicated
> like NiMh batteries?

The following is MORE complicated than NiMH batteries :-) - but may be felt
to be worth the effort.

If solar cell optimisation is of interest (max run time for a given panel or
minimum panel for a given run time) then running a switching regulator
between panels and battery can very significantly improve your delivered
power. A solar panel has a very very wide swing in available power and
voltage output and if you want to make use of the available energy at either
end of the day or during partial cloud you need an "adaptable load". For
very best results you need an optimising controller * to hold the panel at
its current peak power point and this is not as trivial as it may seem, but
a "dumb" switcher is an excellent start.

Similarly, a switcher between battery and load may well do the same but
probably not as spectacularly due to the relatively flat voltage curve of
the NiMH cell.

At these power levels there are numerous small off the shelf switcher ICs
that allow a design with minimal effort and good probability of a result
better than just hooking it up.

* - a cell will deliver peak power at different voltages depending on the
level of insolation. Cell voltage depends on both insolation and load. If
you load the cell to the same voltage under all conditions the output will
not be optimal in most cases. Working out where the optimum point lies can
be achieved either by making small excursions either side of the current
load point and seeing which way the power output goes or by use of a
separate monitoring cell with a fixed load whose insolation/voltage
characteristics are predetermined. Ability to measure both current and
voltage is indicated as effective load resistance is, of course, dynamic.

Now all you have to do is work out an optimising controller for your
switcher(s) that draws far less than 5mA equivalent at Vout :-)



       RM

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2002\11\26@073945 by Olin Lathrop

face picon face
> 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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2002\11\26@092926 by Kyrre Aalerud

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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 removed}

2002\11\27@192808 by Peter Mcalpine

picon face
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 removed}

2002\11\27@211724 by Kyrre Aalerud

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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" <.....petermcaKILLspamspam.....OZEMAIL.COM.AU>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Thursday, November 28, 2002 2:27 AM
Subject: Re: [EE]: NiMh and solar charging


{Quote hidden}

> {Original Message removed}

2002\11\27@213221 by William Chops Westfield

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   Find your load, then your needed operating time.
   Then you have your size from the formulae t/R = C.
           :
   Fairly basic stuff, but it's nice to have it on the list.

It's not quite so simple.  The RC time constant is how long it takes the
capacitor voltage to decay to 1/e of it's original value, but the
interesting number is how long it takes to decay to a voltage where your
circuit stops working (which is a somewhat ambiguous number in itself.)

I thought cooper electric had a little excel spreadsheet you could download
that would figure out the needed capacitance based on assorted "normal"
circuit criteria, ie:

http://www.cooperet.com/documents/PowerStor_Aerogel_Capacitor_Calculator.xls

Enjoy
BillW

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2002\11\27@215756 by Kyrre Aalerud

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I was waiting for someone to correct me on that one :-)
Actally it all boils down to 63%.  The magic value of charge / discharge.
The formulae t = R*C gives how long it will take to discharge the cap from a
full load, down to 100%-63%= 37% of the charge.  That may be far too low for
your concern.  Here is where the step-up voltage converters come in handy.
Find one that will give a nice voltage for you rapp.  Then see it's specs
for max/min.
Now have fun calculating.

   KreAture


{Original Message removed}

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