Battery isolation switch with pre charge anti spark question

[Ham]

Hi All,

I am trying to work out a basic on off power situation for my bike (running 3220 and CC HV120 esc so pulling about 120 amps give or take).

Now forgive me as I am more a cut and weld kind of guy than an electron flowing guru.

As I understand it, using something like a boat or car battery kill switch will work in principle as an on/off switch but without a resistor to pre charge the esc capacitors will still cause an arc, prematurely eroding the connectors.

I think I understand that a 5ohm or so resistor across the terminals will prevent the spark by pre charging the esc but if it is just a permanent connection will see a small power drain on the batteries over time.

So my question becomes:

Can I use the key ignition switch I have as a first stage switch to connect the resistor circuit and then turn the big bad boy isolator switch to release the Kraken? A two stage starting process if you will...1) key switch for pre charge circuit with the resistor and then a second or two later 2) Big red isolator on?

Thanks in advance,

Ham

 

[amberwolf]

I am trying to work out a basic on off power situation for my bike (running 3220 and CC HV120 esc so pulling about 120 amps give or take).

Depending on the rest of the system, and voltage, 120A continuous is going to generate a lot of heat; it must be a much bigger bike than even my SB Cruiser pulling a piano for that (unless you have long steep hills). I can only get SBC to pull that kind of power for a few seconds at most, until I'm at 20MPH, then it's about a sixth of that or less.

If you're pulling that kind of current all the time and it's not a heavy load, maybe you could regear the system to let the motor spin in it's efficient (fast) speed zone. Would give you more torque from startup too (though it lowers the top speed).

As I understand it, using something like a boat or car battery kill switch will work in principle as an on/off switch but without a resistor to pre charge the esc capacitors will still cause an arc, prematurely eroding the connectors.

I've been using a Harbor Freight red-t-handle battery cutoff on SBC for at least a couple of years or more, switched at least twice a day, sometimes several, no precharge, twin controllers (so doulbe the caps to charge), without any switch problems. 14s NMC, so almsot 58v fully charged, several times the voltage the switch was designed for.

I think I understand that a 5ohm or so resistor across the terminals

Not across, but in series with.

If you put it across them, you're just draining power thru it, and would need a very large and well-heatsinked resistor, or it can get hot enough to glow.

Precharge goes in series with just one side or the other (positive or negative) to slow down the current flow.

So my question becomes:

Can I use the key ignition switch I have as a first stage switch to connect the resistor circuit and then turn the big bad boy isolator switch to release the Kraken? A two stage starting process if you will...1) key switch for pre charge circuit with the resistor and then a second or two later 2) Big red isolator on?

Yes. The isolator just shorts across the precharge resistor.

 

[Ham]

Hi Amberwolf,

Thanks for the reply. The 120amp is just the maximun the talon esc will draw under load, I imagine the average is probably somewhere nearer 50amps.

Great to know your standard isolator is working well!.

Apologies for the drawing. I was imaging something like this for the pre charge circuit. ..

 

[teklektik]

I think I understand that a 5ohm or so resistor across the terminals will prevent the spark by pre charging the esc but if it is just a permanent connection will see a small power drain on the batteries over time.
...
Can I use the key ignition switch I have as a first stage switch to connect the resistor circuit

Yes to the switch, no to the resistor value.

Realize that when you close the pre-ignition switch the caps provide a dead short and the entire battery voltage is applied to the precharge resistor. For a 52V system and 5ohm resistor, this is 10A which in spite its short duration (as the caps almost instantly charge) will pretty much arc the dickens out of your key switch. More common values are in the realm of 300ohms. Here you are looking at less that a quarter amp. Setups with a fixed non-switchable pre-charge resistor tend to have larger values (1000ohms) since the charge delay is unimportant - the caps are always charged. Which brings us to your underlying concern of power drain....

I'm not sure about the CC HV120, but some controllers have drain-down resistors across the caps - others don't. If your controller has none, then the current draw to charge the caps is transient and there is only a tiny leakage current from the caps self-discharging (assuming the controller has a separate 'ignition' or 'power' lead to turn on the electronics). This tends to strongly diminish the value of a pre-charge switch and eliminates the whole charge-time vs charge current balancing question. If you do have a drain-down resistor, you can either disconnect the resistor or go the keyswitch route.

Easiest thing to do is drag out the DMM and a 1K resistor and just hook the thing up. Look at the quiescent current drain and you can see what the discharge time/impact will be for storage...

 

[Ham]

Thanks,

I wrote the 5ohm value as I thought the xt90 anti spark connectors had 5.6 ohm resistors in them?!