Charge Complete LED?

[bgarner]

I am looking for a way to switch on (or maybe off) a simple LED when my battery is fully charged.
The battery is fully-sealed 10Ah, 24V LiFePO4 with a built-in BMS. I am charging it at 4A with a power supply set to 29.2V.
The charger is on-board and 110VAC is applied to it when the power cord is plugged into the system.

Thanks!!

 

[dmwahl]

If you are looking for something simple that just turns on when the voltage gets to a certain point, read up on the TL431 shunt regulator. TI will even send you samples if you want. You could set one up to turn on a LED at whatever voltage you want with just a resistor divider, the 431, and LED/current limit resistor. Super simple, although it wouldn't tell you if the pack is balanced or not, just whether or not the voltage is above a given point.

 

[bgarner]

Thanks dmwahl!
I am a mechanical engineer, and I struggle sometimes when I bump up against these electrical things.
I will check out the TL431.

 

[bgarner]

OK...I hate to admit my ignorance, but the schematics I see for that TL431 shunt regulator make my head hurt.
So how would I have an LED light up only when the power supply has reached the set 29.2V?

I was also thinking that the current basically drops to zero once the battery pack has reached full charge 29.2V.
Can I use that somehow?

 

[geeeyejo1]

You can also wire in a simple LED voltmeter (about $6 on Ebay) that will show the voltage of your battery when charging and during use as well - Even though I use a wattmeter to track wattage/voltage/amp hours when riding, I like the LED voltmeter on my pack for a quick glance at it's status at any time...

 

[999zip999]

The lifepo4 likes to be left on the charger for a some time after the light on the charger lights up so the bms can bleed down the high cells and let the rest of the cells fill up.

 

[dogman dan]

I would highly recommend just adding a voltmeter to the bike.

It could be a crazy cheap led voltmeter (under $10), a very affordable wattmeter (under $20), or a cycleananlyst which though expensive is worth every penny.

Then you know your voltage, and know the charger worked, or did not. You will have a fuel guage of sorts, but not that great since lifepo4 has a flatter curve. But you will still know when it's getting low, even if guessing halfway is still difficult.

Some kind of switch on the leads to the meter, if you don't have a switch that turns off the battery itself. Don't leave it drawing a watt or two all night.

 

[bgarner]

Thanks for the ideas.
I am going to start with a watt meter for the prototype.
The end product will likely require a much more basic 'idiot-light' so I am still looking at stuff like the TL431 shunt regulator etc.
The final user of this device will not typically be battery-savvy at all so watching and interpreting voltages etc. will probably not be well received.

Some sort of battery fuel gauging will also be needed for the final product. This is also a head-scratcher for me...

 

[dogman dan]

That's your watt meter. Once you know your capacity, or the capacity to the point you wish to discharge to, you simply look at the watt meter. So if you have 400 available wh to 80% dod for example, you'd be at half way when you see 200wh used.

 

[dmwahl]

Thanks dmwahl!
I am a mechanical engineer, and I struggle sometimes when I bump up against these electrical things.
I will check out the TL431.

I'm an ME too, so if I can figure it out so can you

If you decide you want to go the TL431 route let me know and I'll draw up a schematic you can breadboard, otherwise I think a watt meter as others have suggested will be a more accurate way to go.

 

[bgarner]

Thanks dmwahl!
That is very generous of you, and I will take you up on that offer to sketch up a schematic.
Please let me know what specifics you need to know.

 

[dmwahl]

Here you go, I made 2 versions, one using fixed resistors and the other with a potentiometer so you could vary the set point. The pot could also be a 10k, what's really important is the ratio R1/R2.

Note that the TL431 is only rated up to 36V, any higher could damage it. I prefer to stay well away from maximum voltages, so I would suggest no higher than 30V.

 

[dnmun]

why not just use a regular charger instead? it has the led built in.

 

[bgarner]

Wow dmwahl, that was fast. Most appreciated.
So please bear with me here, the LED (D1) will light up once the battery voltage reaches 29.2V?
What specs does D1 need to have? 24V?

dnmun: thanks for the input. We do that on other products, but this design is not real friendly for that.

 

[dmwahl]

R3 sets the current through the LED to about 3mA. D1 can be pretty much any LED, no specific rating as long as it can handle 3mA. If there's any chance of reverse biasing the LED then you would need to consider the reverse breakdown voltage, but for what you described it won't come into play.

What are you designing?

 

[bgarner]

dmwahl, I just noticed you are in Madison. I am in Neenah.

The device is a load-moving rig designed to latch onto a 1000 lb cart (with casters) and traction drive it. The operator docks it onto the cart then can throttle and steer the load as needed. The battery and charger will be permanently mounted on-board and an AC power cord is plugged into the unit for charging.

 

[dmwahl]

dmwahl, I just noticed you are in Madison. I am in Neenah.

The device is a load-moving rig designed to latch onto a 1000 lb cart (with casters) and traction drive it. The operator docks it onto the cart then can throttle and steer the load as needed. The battery and charger will be permanently mounted on-board and an AC power cord is plugged into the unit for charging.

Got it. If you need a BMS check out my modular bms thread, might be helpful since I assume you're using 8S LiFePO4 at 29.2V. Each cell has a "charged" LED on it too.

 

[bgarner]

The battery has a built-in BMS.
148890 spec sheet.pdf[/attachment]

The charger is just a power supply set to 29.2V aand 4A.

 

[dmwahl]

The battery has a built-in BMS.
148890 spec sheet.pdf[/attachment]

The charger is just a power supply set to 29.2V aand 4A.

Sounds good, let me know if you have any other questions about that circuit.

It may be wise to lower the current output unless you need a fast charge. It will give the internal BMS time to balance the cells properly. Shouldn't be a problem, but I usually err on the side of the slowest charge that meets my needs.