want an LED indicator when my battery is charging

[callagga]

Hi,

Any ideas on a design so I could have an LED to shine when my battery is charging?

I have a 36V LiFePO4 battery, and it is charged off a dedicated set of connections to it's BMS. I effectively just want to put something in series with the charging circuit that lights when there is some amount of current flowing. I was going to mount it into my lockable box near the XLR charging connector.

Actually ideally (but don't worry if this complicates things) it would:
* be off when not connected to charger (remembering there will always be voltage on the line from the battery, even before the battery is connected)
* orange or red or something - when charging and current is flowing into the battery
* green - when connected but is charged, i.e. no current flowing

thanks

 

[dogman dan]

Some kind of wattmeter would be the best way to indicate current flowing I'd think. I use a killawatt meter on my chargers sometimes, and can tell by the watts the charger is using whether a charge is flowing, or the battery is topped up. Same sort of thing could be measured on the output side. My old dumb chargers have an ampmeter on them, Maybe a 5 buck automotive ampmeter is all you need on the DC side.

 

[callagga]

thanks, but I'm specifically interested in just a small little LED (kind of interested in hearing how I could do this too simply/cheaply)

 

[dogman dan]

Not sure how to do that. Wouldn't an led simply sense the pack voltage and light up?

I understand where you are coming from, the trouble with the led's on the charger is that full pack turns the led green. So does a broken wire on the plug. I've rode off with a half charge a few times because of this, when I had an intermittent bad connection at the plug. The killawatt, if I use it, would tell me that less than the required watts were used by the charger, letting me know something is fishy about the green led on the charger.

Again, a cheap automotive ampmeter would show you if amps are flowing from the charger to the battery or not. 48v wont kill a 12v one.

 

[fechter]

Voltage will always be present on the battery, so it will be hard to tell when the charger is on based on the voltage at that point. You can place a current monitor in series with the connection that will detect when there is current. It is a somewhat complicated circuit, but they make chips specially designed to do this. One downside of this would be the sensing circuit would always drain the battery a little. Depending on the chip, the drain could be so low that you don't need to worry about it.

How many amps is the charger rated for?

Dogman's ammeter idea is about as simple as it gets, but you need room to mount the meter.

It would be easier to tap into the charger to pick up the voltage. If you have a 3pin XLR, the charger may only be using 2 of the pins, so it may be possible to use the 3rd pin to power the LED.

 

[amberwolf]

There is one very simple way to do it, but it is not efficient.

If you don't mind the huge power wastage, and can up the voltage on your charger by the same amount an LED will need to light up, you could put a very large power resistor directly in either of the charger-to-battery leads, and parallel an LED with it so that the LED anode is at the more positive end of the current path. You may need to also put a current limiting resistor in series with the LED.

Calculate the resistor value so that at the max current flow level the charger will supply, the voltage across the resistor will end up enough to light the LED brightly, but that at the minimum current flow (near end of charging) the LED still gets enough to be lit up a little bit.

May take some experimentation to perfect the values. Will probably require a heatsinked inline resistor.

 

[callagga]

Some kind of wattmeter would be the best way to indicate current flowing I'd think. I use a killawatt meter on my chargers sometimes, and can tell by the watts the charger is using whether a charge is flowing, or the battery is topped up. Same sort of thing could be measured on the output side. My old dumb chargers have an ampmeter on them, Maybe a 5 buck automotive ampmeter is all you need on the DC side.

Any suggestions re a liitle 3A or 5A gauge I could mount in the side of my rear carrier? (trying to find one at the moment)

 

[callagga]

Here's the best one I've found so far, US$25 - not from Australia however:

http://www.surplussales.com/Meters/MtrDCAmps.html

 

[dogman dan]

The automotive 0-50 amp one lower down on the page is very common and cheap. In the US, it's about 5 bucks at harbor freight. It may have a too big scale, but you should still be able to tell 0 amps from 2 amps when the charger turns on. Got anything like harbor freight in AU? The other meter would be real nice though, for telling what amps a charger actually puts out.

 

[dak664]

Here's a suggestion, the idea is the base-emitter current is enough to light the led while the collector-emitter current does the bulk of the charging. Power dissipation in the transistor will be charging amps*(0.6+voltage drop of LED), several watts probably.

                       2N2222
                      C      E
Charger + ----+--------\___/>----------- Battery + 
              |          |
              +---->|----+ B
                   LED
                             
Charger - ------------------------------ Battery -

 

[callagga]

@dak664 - Not a bad idea at all, thanks. The power drain would only be whilst charging so thats ok. Can I ask:
a) do this work for sure? (i.e. just checking it is something you've come up with without testing or knowing for sure)
b) would there need to be a heatsink do you think for the transistor?

thanks again

 

[dak664]

@dak664 - Not a bad idea at all, thanks. The power drain would only be whilst charging so thats ok. Can I ask:
a) do this work for sure? (i.e. just checking it is something you've come up with without testing or knowing for sure)
b) would there need to be a heatsink do you think for the transistor?

I've used it for <500 millamp USB charging with red leds. Heatsink if the transistor is >60C (e.g. you can't touch it for more than three seconds).

 

[amberwolf]

A number of panel meters:
http://www.allelectronics.com/make-a-store/category/385/Meters-Panel-/1.html

 

[heathyoung]

Problem with these ideas (indicator running from battery) is always going to be parasitic current draw.

You could do this with a relatively simple setup though. A 0.01 Ohm shunt resistor in line with the power to the battery - negative (low side shunt) and like others have said, use the third pin of the charger for voltage supply. Use a comparator to sense the current across the resistor, and use this to drive a tri-colour LED. Run the green segment from the voltage supply (also run your comparator from this as well). This will give you an orange light when charging, and a green light when charged. Easy.