Pre charge resistor Wattage

[icecube57]

Ive had people lose controllers from trying to power the ignition line. I didnt tell them to do this or even hint at it.... it look like that was the correct place to plug it in and they paid dearly for it. I even tried doing something similar in my early days of ebiking so i dont blame them. Trying to run 72v but not upgrade the input resistors so I ran 36v to the ignition line... it made it out the driveway before the controller died.

 

[Alan B]

There are two problems with a 1K precharge resistor. One is that it makes the precharge period somewhat long, and the other is that small fixed loads may prevent the precharge from reaching the supply voltage. Nicobie mentioned the final charge value as being only 60V of 100V full battery voltage, so naturally there was still a lot of capacitor charging to do when the main line was plugged in, and so there was still a large arc since there was still 40V to go. This would only happen if one of two things were going on. Either it was taking too long so the precharge was not complete, or the fixed loads were drawing 60/1000 = 60 mA and preventing the precharge from completing. The precharge cycle should reach 95% of the supply voltage, if not the precharge resistor is too large (or the precharge wait time is too short). If the only fixed load is the 10K bleeder resistor in the controller then a 1K precharge resistor will only charge to 90% and at 100V this still leaves 10V to charge when the main connection is made. If there are other fixed loads like the controller being turned on and perhaps DC-DC converters or high voltage lighting loads the shortfall in precharging voltage may be much more significant.

I agree with Jeremy that a solid state switch is a good way of improving the usability of the ebike and solve the precharging problem, however be careful of the design as the peak currents can reach tens of thousands of amps if the circuit is not correctly designed to turn on slowly and limit the current. This can be enough to destroy the switch. I made one design that uses feedback to slow the turn-on. I modelled the design and built a pc board but have not tested it yet. Using a simple RC network from the switch to the gate doesn't help much since the turn-on transition of the FET is over such a small voltage range, and parallel FETs may not turn on simultaneously so they may not share the precharge load. However it is done the current must be limited to a value safe for the devices.

 

[Jeremy Harris]

There are two problems with a 1K precharge resistor. One is that it makes the precharge period somewhat long, and the other is that small fixed loads may prevent the precharge from reaching the supply voltage. Nicobie mentioned the final charge value as being only 60V of 100V full battery voltage, so naturally there was still a lot of capacitor charging to do when the main line was plugged in, and so there was still a large arc since there was still 40V to go. This would only happen if one of two things were going on. Either it was taking too long so the precharge was not complete, or the fixed loads were drawing 60/1000 = 60 mA and preventing the precharge from completing. The precharge cycle should reach 95% of the supply voltage, if not the precharge resistor is too large (or the precharge wait time is too short). If the only fixed load is the 10K bleeder resistor in the controller then a 1K precharge resistor will only charge to 90% and at 100V this still leaves 10V to charge when the main connection is made. If there are other fixed loads like the controller being turned on and perhaps DC-DC converters or high voltage lighting loads the shortfall in precharging voltage may be much more significant.

I agree with Jeremy that a solid state switch is a good way of improving the usability of the ebike and solve the precharging problem, however be careful of the design as the peak currents can reach tens of thousands of amps if the circuit is not correctly designed to turn on slowly and limit the current. This can be enough to destroy the switch. I made one design that uses feedback to slow the turn-on. I modelled the design and built a pc board but have not tested it yet. Using a simple RC network from the switch to the gate doesn't help much since the turn-on transition of the FET is over such a small voltage range, and parallel FETs may not turn on simultaneously so they may not share the precharge load. However it is done the current must be limited to a value safe for the devices.

The current is never going to be "tens of thousands of amps", thank goodness, most probably it'll be a few hundred amps peak for a very short period of time. The capacitors in these controllers have a typical ESR of a few mohms at the very best best, a few tens of mohms more typically (low ESR capacitors are expensive). Similarly the battery will have an internal resistance of a few tens of mohms or more, and the wiring and connectors will add another few mohms. The net result is that the total circuit resistance at the instant of connecting the power to the capacitors is very unlikely to be less than 50mohm, and most probably will be well over 100mohms.

If you took the worst case, of around 50mohms total circuit resistance (capacitor ESR + wiring resistance + battery internal resistance) with a 100V battery then the instantaneous peak current could be around 2000 A at most, comfortably within the peak current rating of 4 off paralleled 4110s if switched on instantly (which isn't really possible). Add in that Rdson will be high at initial turn on and you will find, if you model it, that a single 4110 would handle the max current that could occur in practice. If you then take a more typical scenario, say a 72V battery and a total circuit resistance of 100mohms, then the peak current could never exceed about 720A anyway, and even with a nasty slow drift up through transition a single 4110 would comfortably deal with the few tens of amps peak it would see. I modelled the worst case for turn on before publishing that circuit here, and found that at no time could you exceed the device ratings, or get even close to exceeding them.

When it comes to pre-charge time, then I agree, using too large a resistor gives an inconveniently long charge time. The pre-charge resistor only needs to have enough resistance to keep the peak charge current down to less than about an amp, so even 100 ohm would be just about OK, 220 or 330 ohm might be about optimum. 330 ohm is going to give a pre-charge time of around 8 seconds to 99V for a controller with around 5000µF (bigger than most), which is a bit too slow in my view. 100 ohm would give around 2.5 seconds, which would seem to be OK. Peak current would be less than 1 amp with a 100V battery.

 

[wesnewell]

I don't use an ignition switch. Seems pointless to me since I disconnect the battery when not riding anyway. So the ignition wire is tied in directly with the + battery cable one the controller side. And there's no reason this should cause a problem. If you blew a controller doing this, you did something wrong or the controller was defective to start with. Alan B. hit the nail on the head about the resistor size. Too high, and it won't let the caps charge high enough to eliminate the spark when you connect the main line. I don't use a switch on my resistor either. It's hard wired across the main power connection. Works great on everyone I've done, with at least 2000 connects by now. Use 150 ohm for any size pack to 100V and you can't go wrong.

 

[Jeremy Harris]

I don't use an ignition switch. Seems pointless to me since I disconnect the battery when not riding anyway. So the ignition wire is tied in directly with the + battery cable one the controller side. And there's no reason this should cause a problem. If you blew a controller doing this, you did something wrong or the controller was defective to start with. Alan B. hit the nail on the head about the resistor size. Too high, and it won't let the caps charge high enough to eliminate the spark when you connect the main line. I don't use a switch on my resistor either. It's hard wired across the main power connection. Works great on everyone I've done, with at least 2000 connects by now. Use 150 ohm for any size pack to 100V and you can't go wrong.

You've got lucky, as you're not pre-charging to battery voltage, are you?.

The controller "ignition" feed draws typically 50 to 60mA.

A 150 ohm resistor connected in series with the "ignition" feed will drop between 7.5 and 9V, so the capacitors will still draw a high current when you make the connection as they charge from this lower voltage to the battery voltage, which isn't great, but workable.

If you ran this arrangement on a 36V battery with a 36 to 60V capable controller then you'd probably cause too much additional voltage drop for the first stage regulator and risk damage, just as described above.

 

[Alan B]

Well, I just don't agree with Jeremy on these design assumptions. I take a more conservative approach.

I did model a number of designs (perhaps a year ago), and it seemed prudent not take credit for unknown impedances from the battery, wiring and capacitors, and it was easy to generate many thousands of amps peak current. Those unknown circuit impedance values can vary a lot (and we are working to lower them all the time), and in a high powered ebike I believe they will be a lot lower than Jeremy's estimates. Good bypassing in the controller can lower the capacitive ESR of the controller (eg a Methods monster controller), and paralleled packs and nano lipo lower the effective battery resistance. I don't even think in my moderate power ebike that I see anything like 100 milliohms, at 50 amps this would be a 5 volt drop and I don't see anything like that in normal operations. Since the 4110's aren't turning on equally one will have to take the whole surge for a conservative design. The parallel paths will be used for current capacity once the switch is fully on, but during turn-on only one FET can be taken credit for in a conservative design.

For Example,

If a 90C rated 66V Nano Lipo pack 18S 4P 20AH was run at 90C which is 1800 amps and had 50 milliohms of internal resistance it would drop 90 volts! 50 milliohms is just not reasonable for a high capacity paralleled ebike Lipo pack. Even at 25C the pack would drop 50 volts and we know that is not happening. If it were then Luke's super-ebike would not work very well. Of course we don't run our packs at these current values, but these impedance estimates are higher than what a fresh fully charged Lipo pack is capable of especially for a very short term discharge. The impedances quoted for batteries are generally not for very short-term duration loads, they include chemical processes that slow the delivery of current for longer term discharges. But in a short term capacitor charging event the impedance may be even lower than the specifications.

I don't think the negative feedback design even takes any more parts, but it is not dependent on taking credit for external circuit features to survive. I have not tested my design yet, so perhaps it has some other flaw, but it certainly should solve the current inrush problem, and the model shows that works nicely.

 

[wesnewell]

You've got lucky, as you're not pre-charging to battery voltage, are you?.

The controller "ignition" feed draws typically 50 to 60mA.

A 150 ohm resistor connected in series with the "ignition" feed will drop between 7.5 and 9V, so the capacitors will still draw a high current when you make the connection as they charge from this lower voltage to the battery voltage, which isn't great, but workable.

If you ran this arrangement on a 36V battery with a 36 to 60V capable controller then you'd probably cause too much additional voltage drop for the first stage regulator and risk damage, just as described above.

Lucky? I have no idea what you're talking about. There's no luck involved anywhere. This is what I use.

 

[Jeremy Harris]

AlanB,

I'm just a realist, all the parts of an ebike circuit will have resistance, and in the case of the battery (even a really good one) and the controller capacitor ESR that resistance will be quite high enough to significantly limit the maximum inrush current to values that cause no damage to the FET switch. If you wish to be even more conservative and adopt a different view, then that's fine, but for those who just want to make something that works reliably then using a FET power switch with a slow turn on has been proven to work well, with no failures reported here, AFAIK.

wesnewell,

You've missed out the extra series resistor that is already inside the controller in your diagram. The controller already has a series resistor internally in the "ignition" wire. This is typically between 180 and 220 ohm, sometimes it may be as high as 330 ohm. It's function is to drop the voltage to the FET drive 12V voltage regulator, to keep it within limits. If you add another resistor in series with this, as you have, then there is a risk for those operating at low voltages (say 36V) that the FET drive regulator will drop out of regulation during pre-charge. Your 150 ohm resistor will drop around 9V during pre-charge, the internal series resistor can drop another 13V to 18V or so, the regulator is typically set for 12V and needs at least 3 or 4 V across it to remain in stable regulation. This means that with the added 150 ohm in series with the "ignition" wire the minimum safe battery voltage is maybe 40V or maybe more before the regulator starts to drop out of regulation. Use this arrangement with a 36V battery and the chances are that odd things may happen. With luck the controller LVC may just trip, if you're not lucky then the FET drives may get into an odd state and cause some other problem. Simply putting the "ignition" connection on the other side removes this risk and is easy and simple to do.

 

[wesnewell]

wesnewell,

You've missed out the extra series resistor that is already inside the controller in your diagram. The controller already has a series resistor internally in the "ignition" wire. This is typically between 180 and 220 ohm, sometimes it may be as high as 330 ohm. It's function is to drop the voltage to the FET drive 12V voltage regulator, to keep it within limits. If you add another resistor in series with this, as you have, then there is a risk for those operating at low voltages (say 36V) that the FET drive regulator will drop out of regulation during pre-charge. Your 150 ohm resistor will drop around 9V during pre-charge, the internal series resistor can drop another 13V to 18V or so, the regulator is typically set for 12V and needs at least 3 or 4 V across it to remain in stable regulation. This means that with the added 150 ohm in series with the "ignition" wire the minimum safe battery voltage is maybe 40V or maybe more before the regulator starts to drop out of regulation. Use this arrangement with a 36V battery and the chances are that odd things may happen. With luck the controller LVC may just trip, if you're not lucky then the FET drives may get into an odd state and cause some other problem. Simply putting the "ignition" connection on the other side removes this risk and is easy and simple to do.

Any controller that would be damaged by low voltage on the ignition wire is defective by design. Even without a precharge resistor, battery voltage can drop all the way down to 0V. To think hooking up a low voltage to the controller either via a resistor or an under charged battery could damage it is just ridiculous. I've never heard of a component that will fail if it has lower than max voltage applied to it. I don't think it could even be designed without a higher voltage to pass through the circuit, which isn't the case here.

 

[Jeremy Harris]

Any controller that would be damaged by low voltage on the ignition wire is defective by design. Even without a precharge resistor, battery voltage can drop all the way down to 0V. To think hooking up a low voltage to the controller either via a resistor or an under charged battery could damage it is just ridiculous. I've never heard of a component that will fail if it has lower than max voltage applied to it. I don't think it could even be designed without a higher voltage to pass through the circuit, which isn't the case here.

I don't disagree with you at all. It just happens to be a scenario that fits with some known cases (one mentioned earlier in this thread) of failure. My other point, that doing it as you've done doesn't ever charge the capacitors to battery voltage seems to have been missed, but is probably more significant. After all, why not pre-charge to full battery voltage, rather than some arbitrary voltage that's maybe up to around 10V less?

 

[whatever]

just going back to start of thread where using a globe was talked about, I used an led in series with precharge resistor on a 90v system,
has worked well for many years, cant remember what value resistors I used but it takes maybe 30sec for the led to go out, a bit longer than
I wanted but it works. The resistors do get quite warm.

 

[wesnewell]

The best reason not to precharge to battery voltage would be that if you then hit the throttle without the main connection think of what would happen. Ever seen a resistor explode. Better carry a bunch of spares. By charging to 10V under, lvc, along with sag, should help keep you from damaging the resistor.

 

[Alan B]

Better to put a switch on the controller on/off line and separate the precharge activity from the on/off for running. Then the system is only precharging and not powering the controller and accessories, and the voltages will come up to the full battery value and save the connectors from arc pitting. Having a handlebar mounted kill switch that feeds the controller is a recommended design practice and standard in motorcycles. Anytime you want the ebike to be safe just hit the kill switch. I have found this feature to be extremely valuable on the bike and I use it frequently.

 

[migueralliart]

I got mine right next to the regent (green) button. simple controller on/off FTMFW

 

[pendragon8000]

cool

a 60watt bulb at 220v is 806 ohms resistance
806 ohms resistance at 100v (same bulb) is 12watts

it'll work fine, and it might just glow for you if you're lucky

test it with an old bulb first before you buy one... that way if it sucks you can always forget about it

the higher wattage the bulb, the higher the resistance, and the better it is for what you want because more resistance = less spark

you could always try something like this....
http://www.ebay.co.uk/itm/LIG0230-LED-Pilot-Light-Indicating-Warning-Lamp-22MM-230V-Panel-Mounting-2PK-/190690684219?pt=UK_Home_Garden_Wall_Lights&hash=item2c660ced3b
not sure how many watts it is... but I think it should be enough

Just to clarify wattage is inversely proportional to resistance. Higher wattage globes have less resistance. Ohms law confirms this.

 

[pendragon8000]

Just set up a 100 ohm 10w resistor in parallel with a 560 ohm in series with a 2.7 v led. All in series with a momentary push button switch.
Should get a bit of a glow as it charges. It sits tidily next to big plug between battery box and controller.
Vid later. Need batteries to demo.

 

[shorza]

I am hoping to run a USB charger (for a mini camera) from my controller's ignition wire AFTER the pre-charge resistor.

The problem is that pre-charge resistor is small at 0.5w 300ohm. It handles the short rush of current from the controller at start up fine, but will it handle the constant draw of the USB charger without overheating?

Battery supply is 96v.
USB charger draws about 300mA max.

 

[pendragon8000]

0.3amp x 5v =1.5wats, assume the usb thing is 75% efficient (thats not generous or conservative IMO) you'll need the resistor to be (1/.75) x 1.5=2watt.
but anyway whynot just run it to main pack voltage? you could fuse it near battery if thats your consern..

 

[shorza]

Wouldn't it be drawing 0.3amp x 96v?

I want it to turn on and off with the controller.

 

[pendragon8000]

Nah out put determins usage x efficiency 1/x
Your consoming 5v at 300ma not 96v at 300ma
Anyway. Connect it to the switch on the controller.(open controller)

 

[shorza]

My infineon controller doesn't have a switch, it's my own switch the connects the battery to the controller's ignition cable.

Thanks for your help.

 

[Super Slim]

I am Looking a buying a 36 V 15 A.hr Ping LiFePO4 battery with a BMS of 35 amps max current, and a Bafang BBS-001 350W (18 amp controller) mid drive motor, with a handle mounted on/off push button switch, for a Greenspeed Magnum trike.
Ping does not want any mods to the BMS to allow a 2 amp rated ignition switch to cut the power from the battery.

I like the impulse switch + 100 Ohm and 560 Ohm+led in parallel, so the led is lit while the capacitors of the motor controller are charging.

I am looking for a keyed 42V 30 amp rated 3 position switch spring loaded to position 3 or 1, so
position 1 is OFF, and the key is removable,
Position 2 is Pre-charge resistors and LED to the motor controller to raise the controller to 42 V.
Position 3 is 30 amp rated for power to the motor controller with the motor running.

Is this a practical switch as leaving it in position 2 would not be good!!

 

[pendragon8000]

I like it.

 

[markz]

What are you suppose to do with the ignition wire on the controllers?
What kind of fuse is good to use on the power cables?
Is a 330ohm 10% wirewound good to go for 72V 65A, or should I go lower for "quicker" what-ya-maCall-its (controller cap charge?)
I normally plug in, and triple check everything, so takes longer then 2 seconds! But after what John in CR says, I am double thinking the rational behind unplugging everytime.

don't use an ignition switch. Seems pointless to me since I disconnect the battery when not riding anyway. So the ignition wire is tied in directly with the + battery cable one the controller side. And there's no reason this should cause a problem.

Ive had people lose controllers from trying to power the ignition line. I didnt tell them to do this or even hint at it.... it look like that was the correct place to plug it in and they paid dearly for it. I even tried doing something similar in my early days of ebiking so i dont blame them. Trying to run 72v but not upgrade the input resistors so I ran 36v to the ignition line... it made it out the driveway before the controller died.

 

[Alan B]

I've killed 2 batteries by forgetting to unplug over an extended time. That can be very expensive. I generally don't disconnect unless the bike is going to be idle for awhile. However if you don't disconnect the battery then a kill switch and/or keyswitch becomes more important. This powers the small wire to the controller and keeps the bike off when there is still power to the controller. This is important for safety (accidental activation) and there is also more power associated with keeping the controller active. This same switch can power high voltage accessories.

Most any several watt wirewound resistor, from 10-1000 ohms is fine. The larger reistance they are the longer they take to complete the precharge. Even a 20 ohm value limits initial current to under 4 amps. 72 volts divided by 330 ohms will limit current to about 0.2 amps.

The operating current doesn't enter into it because the resistor is shorted during operation. If you power up the motor during precharge you will cook the resistor, and possibly damage the controller as well.