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mctrivia
01-18-2009, 06:56 AM
Does anyone have any idea on what would be required to make a battery backup system for the prop and let the prop know when it is on wall power or battery?

I would like to be able to use a standard 9v or 3v button cell battery to keep my system clock running. All other cogs can be suspended to save power but when the power comes back on I want it to remember the last state it was in and still know the correct time.

SRLM
01-18-2009, 07:32 AM
I was just researching that this morning... :) Here's what I found:

http://picnote.blogspot.com/2008/10/battery-backup-for-microcontroller.html

mctrivia
01-18-2009, 07:40 AM
that is so sympel and would work great if the propeller was 5v drive

mctrivia
01-18-2009, 07:49 AM
that gave me a great idea though

use the following schematic

SRLM
01-18-2009, 09:41 AM
The problem with the circuit you posted is that it requires a chip and a few other specialty devices. I like the other one because it only needs two diodes.

T Chap
01-18-2009, 09:46 AM
Look at the DS1307 which has the battery backup switching built in:


www.maxim-ic.com/quick_view2.cfm/qv_pk/2688 (http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2688)

mctrivia
01-18-2009, 09:49 AM
SRLM said...
The problem with the circuit you posted is that it requires a chip and a few other specialty devices. I like the other one because it only needs two diodes.


true but the voltage at the prop would only be 2.3v when on battery using a 3v.


TChapman said...
Look at the DS1307 which has the battery backup switching built in


I will have to remember this. Not sure if I can get away with just keeping the clock running but that is a great option.

Post Edited (mctrivia) : 1/18/2009 1:57:56 AM GMT

Beau Schwabe
01-18-2009, 10:03 AM
mctrivia,

"that is so sympel and would work great if the propeller was 5v drive" --- So why not drive it at 3.3V

from the diagram that SRLM posted... replace D1 with a resistor to trickle charge a 3.3V battery. Keep D2 but keep your wall supply regulated at about 3.9V

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

mctrivia
01-18-2009, 10:59 AM
good idea. This won't cause any trouble when the battery is fully charged? never worked with rechargable batteries.

Beau Schwabe
01-18-2009, 11:04 AM
mctrivia (http://forums.parallax.com/member.php?u=55171),

"This won't cause any trouble when the battery is fully charged?" ... not if you trickle charge it about 1/10th the normal charging rate.· The old style "over-night" 10-14 hour chargers are what I'm referring to.
·

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

hippy
01-18-2009, 11:32 PM
Beau Schwabe (Parallax) said...
from the diagram that SRLM posted... replace D1 with a resistor to trickle charge a 3.3V battery. Keep D2 but keep your wall supply regulated at about 3.9V


Won't D1 replaced by an R ( large enough for trickle charging ) limit the current for the Prop ?

Would it not be better to keep D1 and add an R from battery +Ve to VDD/+5V/Vin ?

I've used the diode-mixing technique and it works well because it's so simple. I have been told that very long term trickle charging could cause problems for the battery but I'm no expert. One solution would be to use two, remove one and recharge as required externally.

It's a shame the Propeller doesn't have zero-extra component on-chip ADC because that makes it easy to add voltage monitoring for the batteries. IMO that's the only real weak spot in the Propeller design compared to alternatives.

Beau Schwabe
01-19-2009, 02:48 AM
Try this design...

When the battery is at 3.0V, the trickle charge current is limited to about 70mA
When the battery is at 3.7V, the trickle charge current is almost 0mA

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

mctrivia
01-19-2009, 04:53 AM
what battery could be used with this? have a 3V Lithium rechargable. Digikey does not sell rechargable lithiums over 3.1V

Beau Schwabe
01-19-2009, 08:42 AM
mctrivia (http://forums.parallax.com/member.php?u=55171),

Typical Lithium-Poly or LiFePO4's·are 3.7 Volts per cell... just make sure that the regulated voltage is at least 0.3V above your battery.

The problem is, that the Schottky diode will drop the voltage about 0.3V, so If you start out with a 3V battery, you won't have much operating overhead for the Propeller.· A 3.7V battery or more is a better choice.

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 1/19/2009 12:50:10 AM GMT