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Batteries

Li-Ion/LiPo Batteries
Overview A very basic overview of terms and concepts encountered while managing the battery packs on our prototype LEVs.
Battery Management System ("BMS") Connection diagrams, etc.
Balancing Cells Notes on cell balancing.
Building Packs Building and repairing (re-celling) battery packs.
Testing Cells Finding a cell's true energy storage capacity and its general state of health.
Resources Online content to get you up to speed with lithium chemistry cell-based packs.
Batteries we use This describes our current preference for the vehicles "traction battery".

 

 

 

 

 

 

Battery Overview

Cells, Batteries, and Packs

Our primary purpose for this battery overview is to reduce the amount of confusion that exists when we discuss the virtually unavoidable topic of using stored energy with our LEVs. Unless you park your vehicle at the top of a hill and only ever ride it downhill, you will need to have an onboard device to store electrical energy. While there's no consensus on what to call that device, you will likely encounter the following words to describe it: cell, battery, and pack.

Cell

In the following text, and on this site, we'll use "cell" to mean the most basic building block of any energy storage system. It will always be a single, discrete physical container of some sort having a two "poles" - one positive and one negative. The terms "anode" and "cathode" are sometimes used in place of positive and negative, but that distinction isn't critical to understanding and using electrical storage devices. One commonly used container for cells is the "18650" cylindrical format, and it gets its name from the fact that it's 18mm wide and 65.0mm long. If a seller was advertising a "2170" cylindrical cell, you'd correctly guess that it was 21mm wide and 70mm long. If you took apart the battery pack in your new Tesla Model 3 automobile, you could cover your floor with several thousand of these "2170" cells. We don't suggest you do this. Cells can also take the shape of a flat rectangular pouch, making it easier to fit into your smartphone, for instance.

The following photo shows the unfortunate consequences of a drug-inebriated driver of a Tesla Model 3 who ignored our advice. He drove the car at an estimated 100MPH, lost control, and sheared off a telephone pole and two trees before coming to a stop 300 feet later. The force of the several impacts disassembled his battery pack into its component cells, and scattered them over a wide swath of a Corvallis, OR, neighborhood in 2020. It's up to the first responders to clean up the somewhat hazardous mess left behind. The impaired driver, suffering only minor injuries, tried to flee the scene on foot. [photo courtesy of the Corvallis Police Department]
Tesla-Model-3-accident-Corvallis-Police-Department-6_photo

*S*P: Series and Parallel connection of cells

Almost all modern (made within the last 5-10 years) LEVs use traction batteries made up of multiple cells connected either, or both, "in series" and/or "in parallel." Think of old-fashioned flashlights where you unscrewed the rear cap and two "batteries" slid out. These two (or more) batteries were actually "cells", and when inserted into the flashlight end-to-end ("in series"), would provide the voltage needed to make the light shine brightly.

Battery packs will be described by the number of cells "in series" using the capital letter "S", and the number of cells "in parallel" using the capital letter "P". Multiplying the "S" and "P" numbers together gives you the total number of cells in the battery. For instance, a "14S3P" battery pack contains a total of 42 cells, consisting of three 'strings' of 14 cells in series. Each of these strings is then also connected in parallel. In a typical ad for an ebike however, this pack might simply be described as a "52V lithium ion battery."

To gain more "voltage," a battery will use more cells connected in series. To gain more "current" (measured in "amps") and energy (measured in either amp hours or watt hours) a battery will use more cells connected "in parallel." Any cell in any common battery is manufactured using a particular "chemistry."

Cell chemistries

Those "old-fashioned" cells mentioned above were typically made of carbon-zinc and were rated at 1.5 volts each. They were not rechargeable and would be discarded when the light became too dim. As batteries have improved over time (not nearly as much as we've come to expect), flashlights and other devices now use batteries made up of cells that are rechargeable, but they still are made up of one or more "cells". This is true even if the cells are completely hidden from view, as they frequently are.

The currently (2020's) favored chemistries are in either the lithium-ion ("li-ion") or lithium-polymer ("lipo") families, or other variations such as lithium iron phosphate (LiFePo). It is way beyond the scope of this brief Overview to delve into battery chemistries other than to suggest that picking one over another involves making compromises. Each user needs to decide between such factors as the amount of stored energy a battery has and its ability to deliver a specific amount of power. Other considerations include the cells' life expectancy, weight, safety, and cost.

Power vs Energy

Power and energy are not the same, and different batteries will be designed to favor one or the other. While this is *wildly* over-simplified, the following illustrates the difference in a basic way. Think of getting a high "power" battery if you want to accelerate quickly and go fast, and a high "energy" battery if you want to go a long distance. If you want to do both, your battery pack will need to be designed with cells capable of doing both.

Battery

We'll use the word "battery" to mean any collection of one or more connected cells.

We'll also use battery to describe a single container that appears to be usable to power an LEV. It might have an ON/OFF switch, a locking device - often with a key, a gage of some kind to indicate whether it's full or empty, a port for recharging, and certainly a connector or cables which supply electrical power to the vehicle. We rely on the seller's description as to what is actually inside the container. Many low-voltage batteries contain only a single cell, which contributes greatly to the confusion.

Pack

We'll use the word "pack" to mean any collection of connected batteries.

We'll also use pack to describe a rather BIG container used to store electricity where we have no idea of what's inside that container. As far as the average user is concerned, the terms packs and batteries could be used interchangeably.

The word "traction", if used to describe either a battery or pack, simply means that it's used to primarily power the motor(s), as opposed to lights and other accessories on the LEV. A traction battery may, of course, also be used to power the vehicle's accessories, and frequently is. When using accessories powered by the traction battery, provisions need to be made to match the operating voltage of the accessories to the operating voltage of the traction battery. Search the internet for "buck converters" if the voltage needs to be reduced (common), and "boost converters" if it needs to be increased (uncommon).

Storage Capacity: Volts, Amps, and Watts

Volts (Cells in Series)

The "nominal voltage" of a pack is determined by multiplying each cell's nominal voltage times the "S" number. Assuming this pack used lithium-ion cells with a nominal voltage of 3.7 volts each, the pack would have a nominal voltage of 3.7 times 14, or 51.8 volts. This is typically rounded up to the nearest volt, so the seller might describe this as a "52 Volt" battery. Of course the seller might *also* describe this as a "59 Volt" battery, since each cell could be charged up to 4.2 volts, so when fully charged, this battery's voltage would be 58.8 volts. We recommend that rather than relying on the seller's voltage claim, you find out what the "S" number is and use that to make voltage decisions and comparisons.

Amps (Cells in Parallel)

Watts: Volts times Amps

When Batteries Go Bad

Batteries don't last forever (not yet). How do you know a battery has reached its end-of-life ("EOL")? The best answer is "when the battery no longer does its intended job". If a car battery no longer starts the car, or a flashlight battery no longer shines brightly, or your bike battery no longer has the range of miles that gets you where you want to go. Another reason to stop using a battery is when it's no longer safe to do so.

Additional clues pointing to batteries having reached their EOL: