Detecting a power source

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Sambuchi
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Detecting a power source

Post by Sambuchi » Thu May 06, 2010 1:13 pm

Hello all,

I had an idea to simplify my battery charging around my house with a battery charger that could "sense" the needed charging voltage. I'm not sure if something like this is out there already or not. The idea seems nice to have one jack on a box. Plug in any device and it will charge it..... someone wake me up.

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MrAl
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Re: Detecting a power source

Post by MrAl » Thu May 06, 2010 6:14 pm

Hi there,


If i understand you correctly you want to build a charger that can sense what it is going to have to charge and automatically switch to a mode that will charge that device correctly without a problem and without having to flip any switches.

What you are talking about can be done quite easily if you dont mind providing at least one extra pin on the connector.
That extra pin would be the charge voltage, current, and method pin. Note that i am stating three things here:
1. Voltage
2. Current
3. Method

The reason we need to consider these three things at least is because different batteries take different max currents and different voltages and even different charge methods altogether. It's not possible to specify only the voltage or only the current and not even only the method, but all three need to be known by the charger to be effective in switching to the right mode.

For example, we can have a 4.2v Li-ion cell that takes 300ma max, or 700ma max, or even 1 amp max, and the charger has to know this or else damage can occur to the cell or at the least the cell may take an exceptionally long time to charge or maybe wont charge up all the way. Add to that, we can have two Li-ion cells in serires for 8.4v (although i dont really recommend this anyway but some people do it) and the charger would have to know that too.
Alternately, we may plug in a set of three NiMH cells in series that total up to about 4.5v at up to 5 amps max perhaps, and the charger would have to know all that PLUS a new algorithm (method) for charging NiMH instead of Li-ion.
The list goes on forever though, that's just a brief couple of examples.

As i said in the beginning however, it is possible to do this with ONE extra pin on the connector. This pin would supply a voltage to the charger that tells it specifically what voltage and current and method to use. For example, 0.5v could mean an Li-ion cell that is 4.2v and takes 300ma, and 0.6v could mean 4.2v and 700ma. It depends on how many different cells or devices you need to charge at your home or wherever you have to.

You could get more sophisticated with two pins:
One pin for voltage and one for current and charge method,
or three pins:
One for voltage, one for current, and one for the charge method, and this would give you a ton of possible configurations.

Of course a digital code would be nice too, but that would take more pins unless you were willing to build a uC into each device to tell what spec's should be applied.

Alternately, a somewhat precision resistor built into each device and read by the charger would tell the charger what spec's to apply to that device. A small resistor could be taped right onto a cell perhaps.

Just some ideas...
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Sambuchi
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Re: Detecting a power source

Post by Sambuchi » Fri May 07, 2010 5:59 am

Hello Mr Al.. I was hoping that you were going to chime in! and yes you understood me correctly. Over the years I have made numerous rechargeable gadgets and it getting hard to remember which needs which charger!

I have a wide variety of battery packs and types... 3V lithium, AA NiMH, wide range of battery packs, etc
MrAl wrote:As i said in the beginning however, it is possible to do this with ONE extra pin on the connector. This pin would supply a voltage to the charger that tells it specifically what voltage and current and method to use. For example, 0.5v could mean an Li-ion cell that is 4.2v and takes 300ma, and 0.6v could mean 4.2v and 700ma. It depends on how many different cells or devices you need to charge at your home or wherever you have to.
The method you explain above with the third pin. Are you requiring me to also have a third pin on all of my devices that need charging? outputting a certain voltage so when hooked up to my "charger box" I can verify what it is. This would be great if I started this practice from the beginning with all my projects. I will keep this in mind for future projects.

I was hoping there was a way to use the device that needs charging; the load resistance, current voltage of the cell(s) and some way of pulsing the battery with current to determine what is on the other end.

Am I dreaming again?

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MrAl
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Re: Detecting a power source

Post by MrAl » Fri May 07, 2010 7:12 am

Hi again,


Well, it is extremely difficult to detect what kind of cell(s) you are dealing with unless maybe you have a very limited number of types and voltages, etc. I guess you have already built a bunch of things but cant you modify them just a little by adding one little resistor? The charger would measure the resistance and that would tell it what way to charge the device.

One of the problems with trying to measure the type of cell(s) with test signals is that the failure mode of one type of cell pack could make it look like a completely different cell pack. I guess a test resistor could go bad too though, but that would be somewhat rare i think, not that it couldnt ever happen though i guess.

I dont know if this helps, but could you maybe mark the packs very clearly with some kind of label that tells you how to set the charger? Maybe that would be good enough? Maybe make some very neat looking labels on the computer printer using some decent graphics, or maybe even color coding like resistors.

Of course one really nice way to do this would be to use say a 16 pin connector, where the charger connects to the pack via only two pins out of those 16 for any given device. That would mean you always plug in a 16 pin connector, but the pack is wired to the right pins to get charged the right way. One pack connects to say pins 1 and 2, and another to 1 and 3, etc., so that each pack gets it's own pin(s). I know this would require a lot of connector pins though.

[The following was added later]
If you do decide that you can use precision resistors, you could stick to very set values like 1k, 2k, 3k, etc., and make the charger detect circuit recognize a bad resistor value (anything that is not a perfect value very close to the nearest k ohm).
For example, a 1k, one-percent resistor might be anywhere from 990 ohms to 1010 ohms...so if you read 1020 ohms then you know it's a bad resistor or something else is wrong, as the next valid resistor value would be possibly 1980 ohms (low 2k resistor) and that is much different.

Another idea is if you only use AA and AAA cells that are NiMH (these are most common) you could simply use a constant current circuit of say 1 amp and that would automatically adjust to the right voltage. 1 amp can charge these kinds of cells pretty well, or you could go higher. Point is, once you identify NiMH, switch to constant current charging at some predetermined rate and that should take care of every pack you own if the charger is designed right. Of course you still look for a minus delta V or whatever.
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haklesup
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Re: Detecting a power source

Post by haklesup » Fri May 07, 2010 4:03 pm

FWIW About half way down this web page you'll find a heading called Battery Management. Several of these are very generic and appearingly informative about battery charcing and other related subjects

AVR450, AVR354 look particularly interesting
http://www.atmel.com/dyn/products/app_n ... ily_id=607

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