Simple battery Status LED

[crvogt]

For over a year and a half I have been using simple car (cigarette lighter) chargers to charge my li-ion 2S1P(18650) packs. These have been working very well and I have been sharing them with my friends. My supply of Rado Shack chargers ran out so , a while back picked up some (G6668) 8.5V chargers from Goldmine Electronics for 4/$1.00 and they are really nice but did not have a LED indicator and some were set to 8.5-8.68 Volts. I found that a 10k pot in series with R6 allowed adjustment of the output of the TL494 IC to precise 8.4v.
http://sales.goldmine-elec.com/prodinfo.asp?prodid=9195

Because these chargers did not have an LED showing the charging, I started looking for a way to make a LED light when the voltage reached a level of 8.15 - 8.3V showing when the pack was charged. I found a nice circuit used for the low voltage screamer using a TL431 shunt. The only problem was a regular LED would come on low and increase in brightness. I wanted more of an "Go No Go" indicator.. I replaced the LED with a Blinking LED and the voltage span of turn on was much tighter... while playing with the breadboard I found the blinking LED would work fine without the TL431 using only a couple of resistors.. This seems to work great on the breadboard, but seems too simple!!! The LED starts blinking at the setpoint set by adjusting the R2 POT. This simple circuit could also be used to detect a low battery the LED could be set to stop blinking as the battery drops below a preset voltage..

OK Electronic Wiz Kids, what am I missing??? or is this a valid approach??
I even think the 330 ohm resistor is redundent.. and using only the pot would work... Circuits Below Original With TL431 and With Only LED

[Happy Hobit]

Interesting. At what voltage does the LED start blinking at? (LED off) / (LED on)

What you’re describing should work fine if it’s stable.

Now if someone else bought blinking LED’s from another source it might not work as well, but it seems works with these.


Does anyone have a schematic of the electronics in the ‘Blinking LED’s’?


Don’t remove the 330 ohm resister. If the pot got dialed too far, the LED would blink only ONCE.

Jay

[Mr.RC-CAM]

If this is for field use, how stable is the voltage threshold over a wide temperature range ? Unless the LED has temp compensation, drift may occur. If it drifts a couple hundred mV's then your LiPO's could be harmed.

For reliable performance, test from 50F (cool day) to 135F (hot car). Those are practical temperatures.

RC-CAM

[crvogt]

The preliminary results are in, I used an ice water bath to cool the circuit board to check for temperature dirft... Temperature with the bliking Led only had a lot of drift, While it was set to go on at 8.18V at room temp (71deg F) in the ice water bath (32 deg) would come on at 9.55v. As Mr, RC-CAM was thinking the led alone has a lot of temperature instablility...
I have gone back to the original curcuit with the TL431 and it looks like with a set point of 8.16V at room temp only shifts to 7.8V in ice water.. The TL431 must have some built in temperature compensation, because it seems to be much more stable.
Will try some more temp studies around more normal room temps, using water bath. Have to wait till the circuit board dries out, the plastic bag sprung a leak in my last test... OOOOOOOPPSSSS!!
Carl

[Mr.RC-CAM]

Too bad it drifts that much. Don't forget the household fridge is wet-free source for cooling things down. But use a plastic bag or condensation will get in the way.


The TL431 must have some built in temperature compensation, because it seems to be much more stable.
Yes, that is why they are such good V-sources.

RC-CAM