# Battery Packs

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## Battery pack power density and energy density

Energy Density and Power Density are two ways to measure the speed and range you can get with a given vehicle. Each vehicle has a maximum weight it can carry, and a maximum volume or size it can carry. Together those form a budget into which you fit the people, cargo, drive train, electronics, and battery pack.

volume = the size of the area for batteries (in liters)
weight = the carrying capacity of the vehicle (in kilograms)

Energy density is the kilowatt-hours stored by volume or by weight. Power density is the energy (in watts) that can be delivered by volume or by weight.

power-density = max-watts / liter or kilogram
energy-density = kilowatt-hours / liter or kilogram

These are usually measured as

volume energy density = kilowatt-hours / liter = kwh / l
weight energy density = kilowatt-hours / kilogram = kwh / kg

Remember that

1 kilowatt-hour = 1 kilowatt used over 1 hour = kwh
1 kilowatt = 1,000 watts

And remember that, as an electric vehicle moves down the road, it consumes electricity. Say the vehicle has a 120 volt electrical system, and uses 30 amps to cruise, therefore the vehicle cruises at 3.6 kilowatts. If the vehicle is run for an hour, it consumes 3.6 kilowatt hours of electricity.

The main measurement controlling the range capability of a given battery pack in a given vehicle is, how many kilowatt-hours can you carry in the vehicle. Each vehicle has a designed carrying capacity in both volume and weight. Based on the energy density of a given battery pack determines how many kilowatt-hours can be fit into the vehicle. Obviously the batteries have to fit within the physical dimensions and carrying capacity of the vehicle.

Adapted from: Power Density in Batteries and Electric Vehicles

## Basic battery pack wiring to achieve specific voltage and amp-hour capacities

Refer to Electrical basic measurements and values for basic electric vehicle for basic electrical values. More details about batteries are in: EV Batteries

Batteries come with voltage values which depend on the battery chemistry, and on the arrangement of battery cells to form the battery pack. The amp-hour capacity is based on the battery chemistry, and essentially how heavy the battery is because the more material in the battery the more electrical charge it can store.

Typical per-battery-cell voltages are:-

• Lead acid: 12 volts (technically lead acid batteries are delivered as multiple cells packaged as one battery)
• NiMH: 1.2 volts
• Lithium ION: 3.7 volts
• LiFePO4: 3.2 volts

How do we get a 36 volt, 48 volt, 120 volt (or more) battery pack? It's simply done by wiring the batteries in parallel or serial fashion to get the desired numbers.

In a series-connected battery pack the measurements come out as follows:

voltage = volts-per-cell * number-of-cells
amp-hours = amps-hours-per-cell

In other words, for a series connected pack the voltages add based on the number of cells. However the amp-hour capacity does not increase.

A big assumption in this is that each battery in the pack has the same rating. It's best to have matched batteries in a battery pack. It's been observed that in a mismatched pack cells with less capacity will cause the rest of the pack to work harder, and the pack becomes damaged more quickly.

In a parallel-connected battery pack the measurements are the reverse of the series-connected ones:

voltage = volts-per-cell
amp-hours = amp-hours-per-cell * number-of-cells

You can also combine techniques if necessary, to create a series-parallel battery pack

You can wire a series-parallel pack as shown (wire the batteries in parallel "strings", and connect each string in series) or the other way around (wire them in series strings, connecting each string in parallel). The effect will be the same, namely:

voltage = volts-per-string * number-of-strings
amp-hours = amp-hours-per-string * number-of-strings

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Commute and get around town on the G3 Volt Electric Scooter from E-Moto.

• ayon69