High Voltage Battery Packs

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Hello guys,

Trying to understand battery pack design of different electrical vehicles. I am new to this electric stuff and trying to learn new things so please forgive if questions are naive.

So I was taking look at different battery pack specifications and found out that they range between 300-400 V while there is a lot of variations in their Ah. For eg Tesla has discharge capacity of about 232 Ah, Ford focus electric has 75Ah while that of BMW i3 94Ah.

What is the reason behind this?
Why are they keeping voltages in same range as that of eachother but changing their Ah so drastically?
How will it affect the performance of electric vehicle?
How is Ah capacity determined by OEMs?

Why in case of conversion we cannot use high voltage pack as that of EVs with low Ah?

Hope you guys clear my confusion
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Bmmoffat

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Ah or amp hours is the ammount of energy a battery has stored. For instance if the bmw battery is giving all it has it can give 94 amps for 1 hour straight before being completely drained. Or 188amps for a half an hour, or 376 amps for 15 mins ect. So the higher the amp hour the longer the driving distance.

Emyr

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Suppose you have 24 lithium ion cells rated for 2Ah at 3.7v. Each cell holds 7.4Wh, so the whole pack, however it's arranged, holds 177.6Wh.

If those 24 cells were all in series (24S1P), you'd have an 88.8v 2Ah battery holding 177.6Wh.

If the 24 cells were all in parallel (1S24P), you'd have a 3.7v 48Ah battery holding 177.6Wh.

Comparing the Ah capacity of two packs is meaningless if they have different voltage. Multiple the nominal (not peak) voltage by the Ah to get Wh.

brian_

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Why are they keeping voltages in same range as that of eachother but changing their Ah so drastically?
How will it affect the performance of electric vehicle?
How is Ah capacity determined by OEMs?
They choose the voltage primarily to get the desired motor performance. Since they're all using similar motors (all AC, almost all permanent magnet), they choose similar voltage.

Amp-hour capacity is just energy capacity (watt-hours or kilowatt-hours) divided by voltage. Energy capacity is a trade-off of range versus weight, cost, and bulk of the battery pack. The different choices of energy capacity lead to different amp-hour capacities.

Why in case of conversion we cannot use high voltage pack as that of EVs with low Ah?
You can. Builders using salvaged production EV motors, inverters, and battery packs run up to the stock production voltage, and so do a few builders using expensive aftermarket components capable of running those voltages.

Readily available components are mostly for lower voltage, and the old (or newly built but old-design) brushed DC motors can only reliably handle lower voltage.

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