nicd pack charger(finished)

[dirty_d]

well i finished the cahrger and after 2 days of frustration i finally got it to work, there was some kind of oscillation that was making the op-amps just not work right at all it was like they were broken or something, the inputs were like 50mV different, but the output was only about 3V. i put a 0.1uF capacitor across the gate to source of each fet and it cured the problem, now each channel charges the batteries all at 250mA +- 2mA pretty good!

it sure could use a better construction though look at this! im gonna have to build some kind of enclosure for it id also like to make another heatsink for the transformer as it gets about 165F.

 

[dirty_d]

heres the schematic, i used 0.1uF caps instead across the gate and source and it worked fine. the top output is the positive out and below all the negative outputs. all the series string positive go to the positive out, and each of their negatives to go one of the negative outputs. on mine i have 5 negative outs.

 

[dirty_d]

someone told me that i should use a grounding resistor on the gate instead of a capacitor to eliminate the problem i was having, anyone have any opinion on this? i dont really understand why it would reduce oscillations.

 

[dirty_d]

i took the pack off the charger this morning after about 10 hours, the batteries were really hot, i checked the charger and it was still charging each string at 250mA. the batteries specs say the standard charge is 300mA for 10 hours, so i wasnt exceeding anything. i didnt think the batteries would get so hot at a C/10 charge rate maybe its normal i dunno. when i went to sleep the batteries were at 43.0V, this morning they are at 39V so they were overcharged a little. i might add a temp sensor or somethign to stop the charge when the temperature rise happens.

 

[fechter]

someone told me that i should use a grounding resistor on the gate instead of a capacitor to eliminate the problem i was having, anyone have any opinion on this? i dont really understand why it would reduce oscillations.

Hey, if it works, don't fix it...
Looks good.

I'd suggest putting the capacitor between the output of the op amp and the (-) input to slow it down. If placed there, it could be a pretty small cap. The op amp was running at full gain, so oscillation is not surprising. You could also put a resistor from the output to the (-) input to reduce the gain, but just a cap would probably do it.

 

[dirty_d]

ok thanks ill try that instead, i ran out of 0.1uF caps so i used a 0.01uF cap on one of the channels, i think that one still has some oscillation because the current is about 10mA higher than the others, maybe this will fix that untill i get the right caps for all.

 

[z_slo]

Nice individual, fan-cooled heatsinks!
I have a feeling that these tenergy batterys have a lot less capacity than the label says. I tested my 2.4 ah sub-c pack and only got 1.7 ah at a 3.5a rate but hopefully they just need some breaking in.
So if the "C" is less, then C/10 might be cooking them.

 

[dirty_d]

they say the standard charge current is 300mA for 10 hours though. i think they usually do need breaking in, i went on a ride today, about 15 miles before they rapidly started to die.

fechter, i placed a 0.1uF cap from the op-amp output to the -input on two channels, for those two channels the current was 100mA too high, i put them back across gate-source and everything went back to where it should be +/- 1mA. any idea whats going on here?

 

[dirty_d]

i noticed as the battery voltage got close to the supply voltage, within about 4V the circuit started acting funky again, the current was dropping below 250mA and showing a false negative delta-V when i checked the battery voltages, i had two spare 1000uF capacitors, i soldered them to two different fets from drain to ground, after that every channel was perfect, i thik what was happening was with the higher ripple on the bus with the battery voltage getting closer to it, when the op-amps are changing the voltage to the gate it changes the drain current, and that puts a ripple in the bus voltage which affects all the other op-amps, so there is a feedback deal going on, so those capacitors sort of caught the ripple from those two channels and it was enough to make everything work correctly, its not a permanent fix though, ive been told that if i use 0.1uF caps next to the main rectifier 1000uF caps that might fix the whole problem. i dont have any high voltage 0.1uF caps though, so ill have to try it some other time.