Battery voltage switching

kisshu

100 mW
Joined
Jun 6, 2007
Messages
47
Location
CT, USA
does anyone currently use variable voltage switching?

i would like to use a switch to change my battery configuration while traveling to take advantage of higher voltage for increased speed and power for hills; then switch down to a lower voltage for distance.
 
I know someone on the Power-Assist groups talked of a setup that would swap Serial/Paralell and between 2 speeds on a crystalyte motor depending on speed etc and i think it might have used GPS as well for the logic on the switching.. i'm not 100 % sure... might be a member here ?
 
You can get almost the same effect if you simply limit the throttle. A resistor between the throttle and controller with a switch would do it.
Running half voltage does have a slight efficiency advantage for going slow.
 
are you sure? wouldn't the voltage be the same? would there be any advantage to switching add-on batteries from serries to parallel?

does a higher voltage allow you to back off on the throttle and get the same performance with less current? and longer run time?
 
kisshu said:
are you sure? wouldn't the voltage be the same? would there be any advantage to switching add-on batteries from serries to parallel?

does a higher voltage allow you to back off on the throttle and get the same performance with less current? and longer run time?

There's a thread about just this issue somewhere else here -- I'll leave the searching to you if you want to. In short, a higher voltage does allow you to back off the throttle. Your ebike takes X watts of power to travel Y speed. Let's say 400 watts to hold 20mph on the level, no wind, no pedaling. At 36 volts, that's just over 10 amps of current. At 72 volts, that's just over 5 amps of current. So additional batteries strung in series will increase distance and run-time just as if they were added in parallel. There's no significant advantage to choosing a parallel configuration beyond the point that paralleling is required to provide sufficient amperage. On the other side, we can't just keep adding batteries in series forever either since the motor and controller both have a maximum voltage beyond which they'll burn-out. So my general theory with pack design is to add just enough in parallel so that the amp requirement can be met, then add any more batteries in series up to the point the controller and motor can handle no more voltage (giving maximal power on tap at full-throttle); then if you still want a bigger pack, go back and add more batteries in parallel again, or beef up the voltage rating of the controller and motor.

You'll note this is basically the concept with pack design for larger EVs -- one recently discussed example is the killacycle with its 380+ volt, 19.8 ah 110s9p pack of A123 cells.
 
kisshu said:
does a higher voltage allow you to back off on the throttle and get the same performance with less current? and longer run time?

Correct.


At a lower pack voltage, if the controller is running 100% (full throttle), you could eliminate 'switching losses', but switching losses are typically very low, like under 2%.

Running a higher pack voltage allows you to get the same power with lower current, reducing the resistance losses in the wiring, so the two modes end up being nearly the same efficiency.
 
This guy has a "controllerless" contactor circuit. It doesn't use a controller, so you have no controller losses. Its based on contactors. I've heard of people who are running cars in a similar way.

http://www.aaroncake.net/circuits/cntctcon.asp
 
Beagle123 said:
This guy has a "controllerless" contactor circuit. It doesn't use a controller, so you have no controller losses. Its based on contactors. I've heard of people who are running cars in a similar way.

http://www.aaroncake.net/circuits/cntctcon.asp

The major problems with this approach compared to PWM is the power lost when going slow, and speed modulation. Say I want to go slow up a steep hill and 12 volts won't get me up the hill, but 24 volts will. But 24 volts accelerates me up the hill to a speed faster than I want to go. To get up the hill, I have to choose the 24 volt setting. But in so doing I waste a lot of energy through unwanted acceleration and higher top speed. In such situations the contactor/relay solution is much less efficient than ~95% efficient PWM.
 
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