Pre-Charge Resistor Value / Question

[Storapa]

Hi Folks,

I have built a 20s 20Ah pack for my latest bike. Hot off the charger this will be at around 83v (4.15v per cell). I have split the pack up into two 10s modules. When I leave the bike I will remove a plug that will separate them.
My question is, how do I calculate the value of the resistor to use for the pre-charge circuit?

Any help much appreciated.

Cheers,

Darren.

 

[trex]

switch off the electrics, put the plug back in, you get your 20S.
why do you need to precharge?

 

[Storapa]

Can I just switch off the motor controller ignition (Infineon 12fet IRFB4110) and plug the packs together and not get a spark?

 

[Deleted member 4366]

No. The capacitor/s is/are directly on the battery wires. Once you're connected, you can use the ignition wire to switch the controller on and off. Some controllers have a drain resistor to drain down the capacitor/s when the battery is disconnected, so you might get a constant loss of a few milliamps while connected You need to check that if you want to leave it connected and uncharged for a long time.

There's threads on Endless-sphere about precharge resistors.

 

[Storapa]

Hi Folks,

So I do need a pre-charge resistor when I reconnect the two packs if I want to prevent sparking at the connector?

I've spent ages looking at the Endless-sphere threads about pre-charge resistors until my head spun. I'm a mechanical engineer, so all of this electrical stuff is tricky to get my head around.

Looking at the Endless sphere threads some time back I came up with a resistor value of 5w 300ohm. Does this sound reasonable?

Thanks for your replies so far.

 

[Deleted member 4366]

The time to fill a capacitor to 62% = resistance x capacitance. Assuming a 1000 mfd capacitor, the time will be 1 x 300 seconds, so about 5 minutes.

The initial power through the resistor = volts x volts ÷ resistance = 83 x 83 ÷ 300, which is 23w, but it falls rapidly as the capicitor fills up, so 5w might be OK. Double the resistance will halve the power, but double the time. I guess it all comes down to how long you want to wait

 

[trex]

depending on the actual controller you use, the capacitance can vary between 0.1 Farad to as much as 1F.
There is a tool to calculate the appropriate value of the precharge resistor here:

http://scriptasylum.com/rc_speed/nospark.html

Assuming 0.1F capacitance and 20S, I think the appropriate value is about 10 Ohm 5W quality wirewound ceramic resistor.

 

[Storapa]

Thanks for the replies gents.

I'll build up the pre-charge circuit using the info above and see if we can eliminate the spark.

 

[Deleted member 4366]

I thought that time I calculated above was a bit high. 1000mfd = 0.001 fd, so the precharge time is 0.3 secs. You could therefore go much higher on the resistance to cut the power down.

 

[Storapa]

Thanks for the update.

I've purchased a 200 Ohm, 5 W resistor.

 

[Storapa]

I built the system using a 200 Ohm 5 W resistor and it works perfectly.

Flick the switch before making the connection. Not a spark in sight.

Thanks for all of the help and advice.

 

[tpesl]

I built the system using a 200 Ohm 5 W resistor and it works perfectly.

Flick the switch before making the connection. Not a spark in sight.

Thanks for all of the help and advice.

Hi anyway you would have a simple schematic of the conecting. Thanks Tomas pesl@shaw.ca