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Old Feb 07, 2014, 08:10 AM
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Letchworth, Great Britain (UK)
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Go/no-go battery indicator

Having tried to take off with an uncharged battery a couple of weeks ago, I've thought about installing a go/no-go indicator on my model to switch on a green LED if the battery voltage is at or near its fully-charged value. I know I can buy various kinds of meters, but I'm thinking of something simple and cheap that could be installed on each model, configured for whatever battery the particular model uses.

I've tried out the principle by connecting a zener diode, an LED, and a 20mA LED driver in series, and it can differentiate between my two test batteries which were 5S and 6S A123.

But I'm not sure how to figure the correct zener value -- in my test it took a 6S A123 at 20.2v to trigger the LED, using a 16v zener; whereas a 5S A123 at 17.2v didn't trigger it.

Would I be right in thinking that to determine the correct zener value I need to subtract the LED's forward voltage from the battery voltage that I wish to trigger at? Or is it more complicated than that?
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Old Feb 07, 2014, 01:22 PM
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United States, ID
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Battery Monitor 2-6S
PRODUCT ID: GT-BMON6
PRICE $2.01
http://www.hobbyking.com/hobbyking/s...itor_2_6S.html

It has a three digit display and is about as compact and simple as you're going to get. Unless you have the parts you need on hand, and are willing to settle for a simple led for output... you're not going to beat that price.
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Old Feb 07, 2014, 02:01 PM
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I know voltage checkers are commercially available, but I didn't realise they were that cheap! But postage, even from the UK warehouse, triples the cost

But I do have the parts in my box, like dabbling in electronics, and would prefer a go/no-go LED instead of having to (a) read a numeric display in bright sunlight and (b) remember what voltage is good to go, considering I use LiPos and A123s in different models (in fact, in the same models in one case).

Not counting my time, the components will cost a fraction of even HK's low price (if my circuit is a viable one), so I'm thinking of putting one in the battery bay of each model, configured for the battery that that model uses.
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Old Feb 07, 2014, 03:48 PM
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Orleans, MA
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Here's an approach for making a simple monitor using just 3 discrete components.
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Old Feb 07, 2014, 03:53 PM
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Letchworth, Great Britain (UK)
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That's exactly what I'm thinking of whitecrest. Just that I'm using a 20mA current regulator chip instead of the series resistor.

But that article confirms my thoughts about the voltage value for the zener, so thank you.
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Old Feb 09, 2014, 06:12 AM
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Malaysia, Selangor, Kajang
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I just use a multimeter. If you're the kind of guy who can make a voltage indicator circuit then you obviously already own a multimeter. Since it's something I already own anyway I consider it a zero cost voltage indicator.
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Old Feb 09, 2014, 07:26 AM
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Le Treport, France
Joined Jun 2004
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I think a TL 431 plus some cheapies can do that ... with enough accuracy

just resistors to calculate for required voltage ... here it is for 5 ni cells.

Alain
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Old Feb 09, 2014, 08:20 AM
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You know what would be cool? Use a tri-color LED (red/yellow/green), and make the circuit detect three voltage levels for a NO GO/CAUTION/OK TO GO type display using just a single LED. You could mount it on a models' cockpit dash, and plug into any channel, or wye off the battery. Now that would be useful, especially on some warbirds. I don't like having to mount one of the larger "Voltwatch" type devices on a nice-looking plane.
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Old Feb 09, 2014, 01:53 PM
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Thanks for the link Acetronics. I wasn't aware of those devices.

Slebetman, I don't really want to carry a DVM to the field, and I don't want to have to think whether the voltage I'm seeing is good to go, or not That's why I'm thinking of a simple go/no-go light.

dmccormick001, it has to be Y'd off the battery, not from a receiver channel, because it's the motor battery's voltage I'm interested in.

It's been blowing a gale here this afternoon, so I've been experimenting with this circuit instead of flying. First thing is, I've come to the conclusion that the circuit in the link in post #4 can't work -- if there's no current flowing because the zener hasn't switched, then the series resistor won't reduce the voltage. The result is the circuit will always trigger at the zener voltage because in its "open" state it's seeing the full battery voltage. At least, when I tried it with a 6.2v zener and a 2.5v LED, a 3S A123 (around 10v) was always able to light the LED irrespective of what value resistor was in series, but a 2S A123 (around 6.4v) would not.

But a variation of that design does seem to work: I simply put another resistor in parallel with the zener and LED, so that it and the first resistor form a voltage divider. The "high" resistor's value it chosen so that it limits the current through the LED when the zener triggers; and the "low" resistor's value is calculated so that the voltage at the junction between the two resistors triggers the zener when the battery drops to whatever value you want. Using a variable resistor for the "low" one will allow me to fine-tune the trigger voltage.
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Old Feb 09, 2014, 02:36 PM
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Belgium, Brussels
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I'm trying to figure out a volt test too to integrate into a personal project. I want to test each cell and as soon as one goes under all led will move to red. I'll share my circuit when I have one that I think it works
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Old Feb 09, 2014, 03:14 PM
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Quote:
Originally Posted by Halford View Post
I'm trying to figure out a volt test too to integrate into a personal project. I want to test each cell and as soon as one goes under all led will move to red. I'll share my circuit when I have one that I think it works
For something like that I would use a programmable integrated circuit, for it's easy to program it to do anything you want whenever one of several inputs falls below a certain value.
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Old Feb 09, 2014, 03:17 PM
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Originally Posted by abenn View Post
For something like that I would use a programmable integrated circuit, for it's easy to program it to do anything you want whenever one of several inputs falls below a certain value.
true, but I was going with a simple idea with zener diods and transistors.

I know little of programming and always used ATTiny chips that do not like more then 5v.

I'm very new to the Electronic scene, learning a lot as we go.
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