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[BBarnes1006]

What are the pro's and con's of running your batteries parallel compared with series? I am looking at doing 8 12V batterires minimum.

Thanks for the advise.

[Bogan]

essentially all it comes down to is what voltage does ur motor require? if in parralel the voltages dont add up, but in series they do.

[BBarnes1006]

The motor i am looking at runs 72V to 144V. I am wondering if there is a difference in top end speed between the tow configurations. i know that if you go series you add the Volts together and if your go parallel you add the Amps or Ah.

[Bogan]

well if you put the batterys in paralel, you get 12 volt total, and in series 96V total, as speed is voltage dependant, in series you would get approximately 8 times the top speed. The other thing to consider however is the max current, both of the motor, and the batteries.

[frodus]

as speed is voltage dependant, in series you would get approximately 8 times the top speed.

well, not really....Speed is voltage dependant on permanent magnet and its linear... so its volts per rpm... and it would be 8 times the top speed at 96V than at 12V. Other DC motors, like sepex and series wound, are NOT linear, there is a relationship, but its not volts per RPM. AC and BLDC motors are frequency dependant and are limited by both frequency and voltage.



Back to the Original poster's question...

Power is power. If you have a controller that can do 144V and a motor that can do 144V then do that (8s1p)... because the cables you can run can be much smaller since they're carrying less current and higher voltage. It'l be lighter and more simple to wire everything in series. If you go more in parallel and less in series (4s2p), you get more battery current, but also less voltage... so you go slower, but you still have the same available power (volts x amps = power)....

The widely accepted way to go is to go as HIGH of a voltage as you can, let the controller convert the power. PWM and the caps inside take care of higher motor amps at low RPM.... you'll see the same torque/current at low RPM with either configuration.


What motor are you using? Controller?

[Bogan]

ah yes, i should have mentioned that. I had just assumed brushed dc as almost all eb conversions seem to be running this system.

[djbakke]

I have a related question about battery configuration. 
My assumptions:
- Current draw on an electric motor is dicated by load condition. Typically on an EV, maximum current draw is during acceleration from a stopped position.  On a flat road, current demand will taper off as rpm's increase.
- Voltage dictates the speed of the motor.  With a direct drive EV, voltage dictates the vehicle theoretical top speed.
Has anyone developed or thought of a battery management system that electrically reconfigures the batteries on an EV to optimize for current, voltage and an equal compromise between the two?  Basically an electronic transmission by rearranging the battteies from parallel to series and combinations in between.   The objective is to maintain good acceleration and have good top speed that operates the motor ideal efficency range.  Is this a possibility or am I dreaming again?  I'm in the planning phase of converting a Suzuki GS500 and this is one idea that came to mind.

[frodus]

there's no need to, thats what a controller does.

Its a power converter. It will still put out high amps at low RPM whether its connected to a High V, low A pack... or a High A low V pack. The packs are the same power right? It all comes down to the controller. There's really no need to convert on the fly. The higher voltage lets you go higher RPM with a series/sepex/perm brushed motor.

The caps and the freewheeling diodes in the controller help regulate the current. Also, if you measure current from bats and current into motor at LOW RPM, you'll see higher motor amps than battery amps, but the voltage on the motor is lower. Same power, different volts and amps on input/output.

[djbakke]

I was under the impression a motor controller is just a Pulse Width Modulator(PWM) and switches the power off/on very fast to control motor speed.  I thought a DC/DC converter does what you are describing and is usually used to step down the voltage to 12V to run lights and other accessories on the EV. 

Frodus - Now if what you are saying is correct I could wire six 12V 50Ah batteries in parallel and the motor controller can make a DC motor perform like it were on 72V.  What motor controller can do this?  I can stop thinking about this if there is already one commercially available.  Thanks

[frodus]

thats not what I said.... you're still limited to top speed of 100% throttle... which would be 12v. But you're going to have the same amount of power. The max battery current will be higher with one, of course, but the power you get out of the controller into the motor would technically be the same. At very low RPM's you might very well see the max amps of the controller with either battery setup.

I've tested this directly with my Synkromotive controller, 150A in, 400+ Amps output with a 72V pack at a complete stop with my brakes fully on.

It's using PWM, but its NOT just a switch, there's caps inside the controller (in parallel with batteries, hence the precharge) and freewheeling diodes that keep the current flowing when the FET's are off.

[Bogan]

I've tested this directly with my Synkromotive controller, 150A in, 400+ Amps output with a 72V pack at a complete stop with my brakes fully on.

It's using PWM, but its NOT just a switch, there's caps inside the controller (in parallel with batteries, hence the precharge) and freewheeling diodes that keep the current flowing when the FET's are off.

jeepers! 400amp stall test, things must have gotten pretty hot! Do you know how large the caps inside the motor controler should be? I could probably work it out for various voltage drops, but the question of how much voltage drop is acceptable would still remain....

[frodus]

The motor heated up a little bit, but the controller stayed cool. I can't tell you the Caps inside the controller, I signed an NDA.... sorry.


One more thing:

If you want to do series parallel switching, the controller must be able to have that wide voltage swing. Another thing to consider is that most controllers have a low voltage cutoff. If you set it too high, it'l cut off when you're in parallel. The controller must be built to control the series parallel switch, otherwise you get some large arcing across the contactors when you switch under load.

[Bogan]

no worries, will probly just put some big ones in then scope the terminals and increase if necessary.