ShDesigns Li-Po Charger Troubleshooting.

[MiseryQ]

I started building a charger awhile ago but never finished it.
My charger died so I decided to finish it.

Built the LED version according to the PDF and charged a 800 2s just as it should've.
Most of my batteries are 3s so I tried to convert it.
Changed R2 to 3900 ohms and R1 to three parallel 2200 ohm resistors.
I also changed the pot to a 15 turn version.

Now the LED stays on without battery connected, measuring across R1 shows 650mA. Output voltage is only 6.5v.
I tried using a 12 wall transformer that puts out 14.8v with the charger connected as well as connecting to a car battery. Same results.

I've tripled checked everything.

-edit- WOW! Incredibly stupid mistake. What was I thinking?!? (:

[MiseryQ]

Not sure of the problem.
I changed the parallel resistor back to the single 1 ohm, after changing pretty much everything else, and it works properly, just limited to 640mA charge rate ):
Output voltage is adjusted to 12.6v.
I also had to rig a computer supply since the adapter I planned on using was only putting out 220mA.

[Robert_A_S]

Did you mean to say that you changed R1 to three parallel 2.2ohm resistors which should increase the charge current to approx 880ma?

[MiseryQ]

Hence the stupid mistake \= I'll just say I need to get some 2.2Ω resistors Monday.
But yeah I wanted to up the charge rate since my smallest 3s pack at the moment is 1320mA.

Since I now know the circuit very well I'd like to try a version that charges at 1.4A for my 2100 3s. I have most of the spare parts to do it.

[Dan Baldwin]

The SHdesign charger uses an LM317, which can have a dropout voltage of as much as 2.5 volts at 1.5 amp, and there will be a voltage drop of about .7 volts across R1 at full current, so you will need a power supply capable of putting out at least 15.8 volts to be sure you can charge a 3S pack at 1.4 amps.

Dan

[MiseryQ]

I went to the electronic shop today instead of RS.
They didn't have anything that would get me close to the .7ohm I figured for close to 1amp charging.
I picked up a .68ohm 2W flameproof resistor.

Strange thing is that it's only charging a little more than before, measured across R1, at 680mA.

On the computer power supply I'm using the +5v and -12v rails to get close to 17v.
The +5v is rated at 12amps but the -12v is only rated at .8amp. I'm wondering if that's my bottle neck.

[GeneSS]

Most certainly is. When you series the voltage sources, you are limited by the one with the least output; 0.8 amps in this case.

Gene

[MiseryQ]

Thanks.

Looks like I'm boned for now.
I did see a 18vdc wallformer at Radio Shack though. I don't know the output.

Trying to feed the charger from +12 and -5 would limit it to .5amp ):

[Dan Baldwin]

I went to the electronic shop today instead of RS.
They didn't have anything that would get me close to the .7ohm I figured for close to 1amp charging.
I picked up a .68ohm 2W flameproof resistor.

Strange thing is that it's only charging a little more than before, measured across R1, at 680mA.

On the computer power supply I'm using the +5v and -12v rails to get close to 17v.
The +5v is rated at 12amps but the -12v is only rated at .8amp. I'm wondering if that's my bottle neck.

Have you measured the actual voltage at the charger while charging?

Dan

[MiseryQ]

If you mean the output to the battery yes 12.60v at full.
It's lower depending on how discharged the battery is.

If you mean at the chargers input, No.
I'll check that next time I charge a battery. The supplies output under no load is 16.65v.

-edit- I'm charging a pack that I flew to cutoff: Power Supply Voltage, No Load: 16.75v Charger Output, No Load: 12.6v Power Supply Voltage, Load: 16.25v Charger Output, Load: 610mA@11.8v (640mA max)

For now I'm using a hard drive to load the supply.