Voltage versus current pack

Batteries, Chargers, and Battery Management Systems.
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flippy   10 kW

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Voltage versus current pack

Post by flippy » May 24 2018 1:55am

Just wondering what the general consensus is.

If you can build a 16S25P battery or a 20S20P battery what would be better?

My view is that you can get the the same power draw with less amps on the 20S pack. And have to limit top speed so it can drive at the same full (limited) speed the whole voltage range of the pack.

What is the exact advantage to have a lower or higher voltage pack when keeping the cell count the same?
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jonescg   1 GW

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Re: Voltage versus current pack

Post by jonescg » May 24 2018 2:18am

It depends on what your motor/controller combination works best at. If it's got a pole count and turns count such that it spins up to a useful top speed while delivering good torque throughout, then pick the voltage to deliver accordingly.

The key benefit to a lower voltage pack is less cells in series to manage.
The key benefit to a higher voltage pack is the smaller conductor size for an equivalent power level.

I'd say for an e-bike, go for the 16s. Vmax is under 69 volts so most controllers are okay with it.

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flippy   10 kW

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Re: Voltage versus current pack

Post by flippy » May 24 2018 2:59am

the controllers i usually work with have a 90V limit.
speed wise a 16S would barely suffice but does lose speed when the battery drains.
for the BMS it does not matter if i use 16 or 21S even, i buy programmable 24S versions anyway.

i am more interested in what the real world (dis)advantages are to having a high or low voltage pack. what if you would compare a 60V with a 110V pack for example.
Last edited by flippy on May 24 2018 3:05am, edited 1 time in total.
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jonescg   1 GW

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Re: Voltage versus current pack

Post by jonescg » May 24 2018 3:01am

If your motor and controller is set up to run more efficiently at a higher voltage, then build the 20s pack.

Voltron   1 MW

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Re: Voltage versus current pack

Post by Voltron » May 24 2018 1:18pm

It's a hard question to answer, as either could work depending on controller or motor winding, but very generally there's been less need for high voltage as higher amp rated components have become available. Lower voltage has lots of safety advantages, so how safe the design needs to be for a specific user that understands the risks could affect your choice too.

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dogman dan   100 GW

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Re: Voltage versus current pack

Post by dogman dan » May 25 2018 6:35am

If you don't need the excess speed, then I'd say go with the 16s. This is not so much for the sake of the cells, If you are pulling identical watts, the cells would have the same individual c rate.

But likely you don't turn down amps if you up voltage, so maybe you do hit each cell harder.

More important to me personally, I did not prefer the poor low speed performance with 20s. 20s was great when you only want full throttle or none, like off road riding. But in town, maybe wanting to ride slow for better range, then the twitchy throttle at 72v was a pain.

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Alan B   100 GW

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Re: Voltage versus current pack

Post by Alan B » May 25 2018 6:57am

Higher voltage poses more of a shock hazard. Shorts are more catastrophic. Muscle reactions are stronger and more dangerous with increased voltage.

In the electrical safety world, voltages above 50 or 60V require more training and work planning, and additional safety procedures must be in place. Energized work is more restricted. Above 100V the hazards become more important and the consequences of mistakes increase.

Higher voltage FETs are more costly and have higher losses than lower voltage FETs. Higher voltage controllers get more complicated which increases engineering and manufacturing costs.

You want enough voltage to make adequate speed with the motor, and adequate acceleration. More voltage than that increases losses in the controller (or increase costs as more quantity and higher cost FETs are required to drive down the losses).

With a given motor and a given speed/acceleration the motor voltage and current required is fixed. Raising pack voltage above that value lowers battery current but does not lower motor current. The controller converts battery current to motor current. Extra battery voltage is converted to increased motor current.

Regulating low speeds with high voltage is more difficult. Controllers that do this well are more complicated hence more costly (in general).

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