Battery Backup for the Propeller
william chan
Posts: 1,326
Hi,
How do I connect 2 AA rechargeable alkaline batteries in such a way that
the batteries would automatically take over and supply power to the Propeller when there is a power failure?
How do I charge the batteries when power supply is restored?
Thanks.
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How do I connect 2 AA rechargeable alkaline batteries in such a way that
the batteries would automatically take over and supply power to the Propeller when there is a power failure?
How do I charge the batteries when power supply is restored?
Thanks.
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www.fd.com.my
www.mercedes.com.my
Comments
I have a "Accessory Board" for my PSM module that does what you describe. But, recharging the right way takes a lot of circuitry. I've used a circuit very much like that of the STM32 Circle. You can find those circuit diagrams on the web. It trickle charges the batteries in a way that is very slow (maybe 20-40 hours to fully charge) and does not maximize battery life.
Basically, you use 3 Shottky diodes. One goes from supply to battery, one from supply to output, and one from battery to output...
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My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm
For example, the supply voltage is 12v but a sealed lead acid battery needs to be float charged at 13.5v
In such a case, where the battery voltage is higher than the output voltage, how do we implement the auto takeover when power failure occurs?
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www.fd.com.my
www.mercedes.com.my
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My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm
But I need to float the lead acid battery at 13.5v How do I do that?
The output is also 12v, but it would be a problem b'cos the battery could start to supply current to the output
in normal operation b'cos it's voltage is higher!
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www.fd.com.my
www.mercedes.com.my
Use a 13.5V supply (Or 14.1V - 13.5V + One Diode drop). Job done.
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Missed it by ->" "<- that much!
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My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm
It does not, however require "a lot of circuitry"--usually just a few external components. Take a look at the LTC1325 if you want something that you can use with NiMH batteries and hook up to a microcontroller with 3 or 4 wire SPI.
As far as a backup supply providing "exactly 12V"--that just isn't possible if you intend to draw directly off of the backup battery. When specifying a battery-based supply you must always remember that "rated voltage" is not the same thing as "actual voltage"--ever. All batteries have a discharge curve, and all batteries follow a predictable swing of voltage through each charge/discharge cycle.
I'd follow the paradigm used by laptop manufacturers: bulk supply feeding redundant power element feeding final regulation. It sure seems to work for them.
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--RvnPhnx
If you do that, then the battery will get depleted very soon because the charging current is much less than the output current.
If you increase the (trickle) charging current, the battery may be badly overcharged when the output takes too little current.
So, how now?
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www.fd.com.my
www.mercedes.com.my
You float charge a lead acid battery with a constant voltage charge, not a constant current. Put 13.8V at the battery terminals and use that to power your devices.
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Missed it by ->" "<- that much!
Or use a cog to keep track of V and do the switch over?
No. All the brown out detector can do for you is hold the chip in reset until it no longer detects a brown out.
Sounds overly complex to me. This form of battery backup is in every type of alarm panel out there, plus my 5 dollar alarm clock. It's seriously not hard to do.
As for charging nimh batteries, you can safely float charge them (according to duracell anyway) at C/30 - C/50 from now until eternity. Why not use three nimh batteries and float charge them using a simple constant current source with diodes to separate them from the propeller and the PSU. If you want to get really complex, monitor the battery voltage and have the propeller switch a C/5 load on the batteries from time to time to ensure they are not going soft (A resistor and logic level MOSFET will do that). That way your propeller can notify you if the batteries need replacing when it does a battery test.
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Missed it by ->" "<- that much!
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My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm