Battery dilemma - Voltage

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careca   10 mW

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Battery dilemma - Voltage

Post by careca » Aug 24 2019 11:48am

Hi, i have a dual motor DD hub bike that runs on 72v but at top speed it's amp consumption drops to mid 30's.
Now, i intend to build a battery, but it occured to me that i could increase it's voltage, by option is between:

20S8P - 160 Cells - 72v - 16.8Ah - 240A Max Continuous Discharge
or
24S7P - 168 Cells - 84v - 14.7Ah - 210A

Sony VTC4 2100mAh 30A

Is it better to give up on some Ah on favor of more voltage or the other way around?
Im affraid that the more voltage, even if i limit the current a bit to compensate, will drain the battery faster than keeping the 72v.

john61ct   100 MW

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Re: Battery dilemma - Voltage

Post by john61ct » Aug 24 2019 2:52pm


careca wrote:Hi, i have a dual motor DD hub bike that runs on 72v but at top speed it's amp consumption drops to mid 30's.
That is inherent in the basic laws like Ohm's.

For a given power, (Watts) as speed (Volts) increases then current (Amps) must drop.

At low speed/volts, the power is going into torque (weight, hills, initial overcoming startup inertia).

Increasing pack / system voltage will only gain top speed.

And **lose** torque at lower speeds, for that **lower** voltage is more efficient.

Or if you stick with 72V, increase Ah capacity to get higher Amps output at a lower C-rate, and increase range and longevity as fringe benefits.

Going to a **higher** C-rate cell instead will keep the weight down, may help with short term boosts, but not help longevity nor range,

flat tire   1 MW

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Re: Battery dilemma - Voltage

Post by flat tire » Aug 24 2019 9:34pm

If you want to spin the wheel faster, and have enough power to break thru the air, more voltage or a controller with flux weakening can both do that.

If you ride faster, you will use more power.

Get a bigger battery than you can afford if you have any doubts. You will be very pleased.

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careca   10 mW

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Re: Battery dilemma - Voltage

Post by careca » Aug 25 2019 9:57am

Thank you for your inputs. Very appreciated.

Ohbse   10 kW

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Re: Battery dilemma - Voltage

Post by Ohbse » Aug 25 2019 11:17pm

Unless you have a controller with >100v rating, then moving to 24s will most likely end in disappointment with a fried controller. There are many tales of people getting away with it for a period of time in the pursuit of peak performance, but that's not really relevant any more in this age of more advanced controller designs.

More pack voltage will allow for more top speed, but it will be worse in almost every other regard.

j bjork   1 kW

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Re: Battery dilemma - Voltage

Post by j bjork » Aug 26 2019 10:39am

john61ct wrote:
Aug 24 2019 2:52pm


And **lose** torque at lower speeds, for that **lower** voltage is more efficient.
This does not make any sense to me, can you explain how this can happen?

j bjork   1 kW

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Re: Battery dilemma - Voltage

Post by j bjork » Aug 26 2019 10:47am

Here is an example:
https://www.ebikes.ca/tools/simulator.h ... b=B4823_AC

I just took whatever was in the calculator and compared to the same with higher voltage.
They look the same to me at low speeds (limited by phase amps) then the higher voltage gives more power and speed.

markz   100 GW

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Re: Battery dilemma - Voltage

Post by markz » Aug 28 2019 6:19pm

72V x 16.8Ah = 1209.6Wh
You'd go slower, and giving up 25.2Wh which is 1, 1.5 or 2km, which is about a mile (1.6km in a mile)

84V x 14.7Ah = 1234.8Wh
You'd go faster, having 25.2Wh more!

Remember that when you are going fast, wind resistance is a factor and you start using up more and more Wh the faster you go.

You'd also need a controller capable of handling 72V or 84V. They'd both have 100V capacitors in there.
Remember that 72V fully charged is ~80V and 84V is ~97V. Not sure what the Kelly controllers have for caps when running 84V.
Your batteries can handle the amp discharge, but your surely not using all of that in the controller.
Better to build a battery that has more Wh then you need.

20S8P - 160 Cells - 72v - 16.8Ah - 240A Max Continuous Discharge
or
24S7P - 168 Cells - 84v - 14.7Ah - 210A

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