# switching dual battery packs between series and parallel wiring



## dougingraham (Jul 26, 2011)

itchyback said:


> Then reading through other threads everyone seems to be using really small ah (less than 80ish) and the fastest cars had big volts and very small ah. i just couldn't imagine how that could be useful (except in drag racing i guess) and i wondered if there was some sort of switching between amps and volts.


A motor controller combined with the inductance in the motor is a switching power supply which reduces the voltage in order to control the motor speed. Your motor can see 1000 amps at stall and the battery will only see a few amps because of this. Here is an example. If you have a 300 volt battery and you are cruising down the highway at 60 mph the battery might be seeing a 60 amp load (18kw). The motor will also be seeing 18kw but it will be seeing different voltage and current. Depending on RPM the motor might be seeing 150 amps at 120 volts. Because of the ability of the motor controller to convert a higher voltage to lower at more current it is unlikely you would ever need to do a split pack and parallel/series arrangement of the pack. At one time there was a small advantage to doing this with lead acid on the drag strip. This appears to be true only in the situation where your battery was not capable of putting out as much power as the motor could accept. So you paralleled to allow the batteries to deliver full motor current at low RPM and then switched to series to increase voltage at higher RPM. Completely useless if the batteries can handle the max motor power.

Lets assume you have a DC system where the motor has a 170 volt max capability. Lets assume you have a 50 ah battery with a 340 volts. Lets assume the controller has a 340 volt limit at 1000 amps. The motor limit is 170 volts times 1000 amps or 170 kw. At this max power point of 170kw the battery will see a 500 amp load while the motor will see 170 volts at 1000 amps. This battery current is half the motor current and is the maximum the battery will ever see. This sounds like a 10C load on the battery but this only happens briefly in normal operation because you shift or the motor controller limits the voltage the motor sees which causes it to demand less current as the RPM continues to increase. From a standstill the motor current could reach 1000 amps but the battery current will ramp up as the motor rpm increases only reaching the 500 amp load at the maximum power point of the motor. When the RPM goes beyond that point the battery current will go down because the current the motor can demand goes down. This is where you would shift up. So if you are running down the drag strip you will see a rise in battery current up to the point where you shift and then battery current will drop off because the motor RPM is reduced after the shift. You will get one of these ramps for every shift.

It is a little more complicated than this because of efficiency losses in the motor controller and battery sag but those are usually minor factors when trying to understand.


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## arklan (Dec 10, 2012)

what doug is trying to say is that it would be a waste of time switching like that, it wouldnt make a difference, except that the higher voltage version would be a bit better
144v x 200 = 28.8kwh
288v x 100 = 28.8kwh

cars use kwh's to do distance
not ah to do distance

because the controller takes amps and volts in 1 end and puts out different amps and volts at the other end

edit to add
drag racers use series parallel switching with 2 motors so u could use that in ur pack if u wanted to pursue this​


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## itchyback (May 28, 2014)

Ok, so i think i've read this ten times. Thanks heaps. 
volts and amps seem interchangeable like dollars and cents. The controller will switch some for the other as required. thats a terrible analogy. 

so if i had a 96v battery at 50ah it would have 4.8kwh. If i had a motor that could handle 72v and max 250 amps and a controller that could handle 100v and 500 amps.
I guess at top speed the battery could see 200amps (batteries limited to 4c) and 96volts and the controller would switch some of those extra 24 volts into amps to max out the motor at 72volts and 200amps (plus 24volts worth of amps lol) (is that two amps? i think im mixing terms here). (minus some for losses like heat etc).
assuming the car needed the max amps because it was going up a hill.
if it was just chilling on a flat road at 50km/h the volts and amps would be much lower and the controller will use black magic to decide how much of each is needed, switch some around depending on load, rpm and the Coriolis effect

Something like that? (minus the coriolis effect)

So does that mean, my 1700kg car which i thought needed 45 x 3.2v x 200ah batteries (= 28.8kwh) to go roughly 100km. Instead i could make up any combination of volts and ah to reach 28.8kw/h and it would still go roughly 100km? so like 180 x 3.2v x 50ah batteries or 1695 x 3.2v x 17ah batteries?
or is that something else?


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## arklan (Dec 10, 2012)

itchyback said:


> So does that mean, my 1700kg car which i thought needed 45 x 3.2v x 200ah batteries (= 28.8kwh) to go roughly 100km. Instead i could make up any combination of volts and ah to reach 28.8kw/h and it would still go roughly 100km? so like 180 x 3.2v x 50ah batteries or 1695 x 3.2v x 17ah batteries?
> or is that something else?


basically yes

though higher volts means higher top speed, so i wouldnt make it 1v and 28800ah 
rule of thumb is to use the highest voltage your controller can handle


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## onegreenev (May 18, 2012)

My controller will handle 192 volts nominal and 900 amps. My motor will never see above 156 volts. I limit my motor voltage to protect the motor. I limit the battery amps to like 5C but allow up to 900 motor amps when needed. That way I never over tax the batteries but allow the motor to see a good voltage and amperage for some fun. So my pack consists of 60 100Ah cells. So at 192 volts and 100ah cells, my pack is 19.2 kWh. A fine size for my little ride.


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## dougingraham (Jul 26, 2011)

arklan said:


> rule of thumb is to use the highest voltage your controller can handle


I know I have said this myself in the past but running the controller up to the voltage limits has some downsides so you don't want to push it right to the edge unless you are drag racing. Efficiency starts to suffer at the limits. The Soliton 1 can do 340 volts. This would be 100 fully charged cells after resting. You would be better served limiting to 94 cells which is a 300 volt nominal pack with this controller. I believe both the Soliton 1 and the Zilla limit the current a little at really high voltages.

Moderation in all things even voltage.


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## itchyback (May 28, 2014)

Thank you, things are slowly making more sense.


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## miev 1 (Jan 22, 2015)

Yes, I was wondering that myself. Most OEM ev cars seem to have higher v. Is that so they have good speed to keep up with Ice and range is secondary?



> Instead i could make up any combination of volts and ah to reach 28.8kw/h and it would still go roughly 100km? so like 180 x 3.2v x 50ah batteries or 1695 x 3.2v x 17ah batteries?
> or is that something else?


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## major (Apr 4, 2008)

miev 1 said:


> Yes, I was wondering that myself. Most OEM ev cars seem to have higher v. Is that so they have good speed to keep up with Ice and range is secondary?


Please indicate the source of your quotes. After some searching, I see it was from itchyback: 


itchyback said:


> Instead i could make up any combination of volts and ah to reach 28.8kw/h and it would still go roughly 100km? so like 180 x 3.2v x 50ah batteries or 1695 x 3.2v x 17ah batteries?


His 2 examples are hardly equal in energy. 

3.2V/cell * 50Ah = 160Wh/cell. 180 cells * 160Wh/cell = 28.8kWh total.

3.2V/cell * 17Ah = 54.4Wh/cell. 1695 cells * 54.4Wh/cell = 92.2kWh total.

Bad math on his part I guess. Energy always adds, so it does not matter if those cells are in series or parallel with regards to the total energy stored.

It is the energy (measured in Watt hours or Wh or kWh) which determines the distance or range of the EV. It is the power (measured in Watts or W or kW) which determines the speed at which an EV can travel. That power is the Voltage (Volts or V) times the current (Amperes or Amps or A). The proportion of voltage to current really does not affect the speed or range as long as the power and energy is adequate for the task, AND the system is designed appropriately for that voltage and current.

Manufacturers of EVs design the systems to achieve their objectives for cost, performance, reliability, durability, marketability, regulations, safety, etc and arrive at a system voltage. There is no underlying fundamental principal about voltage and speed governing their choice.


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## Sunking (Aug 10, 2009)

itchyback said:


> short version: can i switch two battery packs between series and parrallel wiring to double my volts = extra speed or double my ah to give me extra range? is that how that works?


No it is not as you stated. RPM is directly related to voltage for a DC motor, not for an AC motor as frequency determines speed in an AC motor assuming there is enough voltage to push enough current for the motor to spin up.

Amp Hours is a meaningless number without a voltage assigned to it. Range is determined by the battery Watt Hour Capacity, not Amp Hour Capacity which is a common mistake even engineers make. A battery WH = Amp Hours x Nominal Voltage. Example a 144 volt battery @ 100 AH = 14400 wh or 14.4 Kwh. 

So lets say you have 90-100 AH LFP cells. You can make either 45S2P battery of 72 volts @ 200AH or use a switch to make 90S1P of 144 volts @ 100 AH. Either way you still have a 14.4 Kwh battery. So if your vehicle has an efficiency of say 144 wh/mile, you get the same 100 mile range either way.


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