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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.
Resources Online content to get you up to speed with lithium chemistry cell-based packs.







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.


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.

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, connected in parallel.

Those "cells", typically made of carbon-zinc and rated at 1.5 volts each, 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".

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

The old-fashioned non-rechargeable cells mentioned above used a carbon-zinc chemistry, while the currently favored chemistries are in either the lithium-ion ("li-ion") or lithium-polymer ("lipo") families. 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, its ability to deliver a specific amount of power, its life expectancy, its safety, and its cost.

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, think of getting a high "power" battery if you want to to 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 lots of cells capable of doing both.


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.


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, 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 this 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

Batteries and Beyond

If any of the above still leaves you wanting to know more, simply 'google' the relevant terms and start following links.





Battery Management Systems ("BMS")

BMS: Should you rely on it?

Almost all batteries designed for ebikes and other light electrical vehicles (skateboards, etc.) will include a BMS. For that matter, even small appliances using lipo battery packs, such as laptops and lawnmowers, will also have batteries with an internal BMS. Typically they will be mounted inside the battery case and invisible to the user. They are meant to be an integral part of the battery pack and if they -- or any of the battery cells -- fail, the entire pack is considered to have failed. If the user is conscientious, the pack will be recycled responsibly.

Speculation abounds as to how many "perfectly good" battery cells end up in the world's landfills each day. A few minutes on youtube reveals a large community of hackers who take apart these failed battery packs and extract usable cells for a second life of storing electrons.

Another common definition for "BMS" is "battery murdering system." This definition is based on the common occurence of a BMS itself failing, while all the individual cells in the battery are still perfectly fine. Either eventually or immediately, depending on the battery's design and the nature of the BMS failure, the battery's cells will also fail and the entire pack becomes useless. The device using the battery will report this failed condition and the user is forced to replace the battery pack. Frequently, the expense of a replacement battery causes the user to either buy a new device containing a new battery, or discard the entire device (in disgust).

Watch this 18 minute youtube video by Shawn McCarty, recorded on June 17, 2016, for a well-reasoned argument in favor of NOT using a BMS in your lipo battery pack, as well as how to do this in a safe and effective manner. Unfortunately, Mr. McCarty's method, while sound in practice, requires more effort than many users will want to invest in their use of larger battery packs. His method involves electrically splitting larger packs in half and using balancing ("hobby") chargers to charge/balance them when necessary. Battery packs using high quality lipo cells and balanced when they left the factory have a tendency to remain *reasonably* well balanced over time. This property can be taken advantage of by doing most recharging using a "bulk" charger, which means simply applying the desired voltage and current to the pack's discharge leads. Periodically the user recharges the pack using the balance charger mentioned above to bring all of the cells back into balance.

To split the battery electrically requires opening up the battery pack itself, which frequently be quite a challenge. Then you must figure out where the two places are that the electrical connections must be interrupted to split the pack. Assuming that we're splitting a 14S/52V battery pack, we need to figure out where to break the positive and negative rails to essentially make two 7S/26V batteries. We also need to convert the single 14S balance plug into two 7S balance plugs to use with the balance charger. (See * elsewhere on this page for details on how this is done.)

How they work

Assuming, for whatever reason, you decide to use battery packs with a BMS, or need to repair/replace a BMS, or otherwise use a BMS, here is what you need to know.

The BMS is typically a printed circuit board (PCB) containing surface mounted electronic components, at least one connector socket for a removable plug with the cell balance leads (wires), and several heavy-duty solder pads to make the relatively permanent connections to the battery cells and to the pack's recharge port. Some BMS boards will also support optional features such as an ON/OFF power switch, LED voltage meters, and a USB port to allow cell phone charging while you ride your bike.

Probably the most serious shortcoming of the *typical* BMS, found in the majority of consumer devices, is that they aren't really a battery "management system" at all. A reasonable expectation for a BMS would be that it automatically and continuously kept all cells in the pack in perfect balance (to whithin several millivolts). This expectation is only met by an "active" BMS, which most aren't. The typical "passive" BMS found in ebike battery packs is designed to prevent massive overcharging, and to prevent using the battery when any of the cells are depleted to some minimal voltage level. The overcharging prevention is usually accomplished by "bleeding down" cells which happen to arrive at the maximum of 4.2 volts before others in the pack. The undervoltage protection is usually accomplished by the battery just being 'turned off' during your ride. The fact that an active BMS is much more expensive than a passive one likely explains why your battery pack doesn't contain one.

The following section shows some well-prepared information found on the web in the middle of 2020.
(text and diagram courtesy of


Understanding the wiring

Please look at the diagram your BMS comes with, either in the box or from the place you bought it from. Not all BMS's are the same. Most BMS's have three 'solder spots' which should be labeled CH- (or C-), B- and P-, and a balance cable connector. Some BMS's may differ (from) this.

The BMS won't connect to any positive wires.

Let's start with the solder spots.

Each of the three solder spots needs to be wired into different parts of the circuit, that's why they're labeled differently.

Typical Specs: FOC, up to DC 90 volts, 3,600 watts, 40 amps continuous/90A peak; up to 60,000 eRPM (actual motor RPM x pole pair count

Typical connections to BMS, batteries, and controllers.


When they fail

We have several batteries that have failed BMS boards, or have had the BMS boards removed because they were proprietary and other components in the system failed (like the BionX 13S/48V pack). The cells in these batteries are still good, and the packs tend to stay balanced even when simply bulk charged and used normally (with controllers capable of protecting the batteries by having an appropriate LVC capability). Our plan is to continue to use them this way and to occasionally balance them, as necessary. To make it possible to balance/charge them as a single process, the BMS can be a generic one not mounted inside the battery pack. In the diagram above, the controller ("VESC" or any other controller) is not involved, so the BMS main negative power output (large) wire going to the controller can be left disconnected. The "P-" (power negative) solder pad on the BMS can be ignored.

The charger power supply will connect its positive (red) wire directly to the positive output of the battery pack, and the negative to the "C-" (charger negative) solder pad on the BMS. since we like to charge at around 1 amp (50 watts for 50 volt batteries), the charger cables can be relatively small gage wires. We'll probably use 12AWG silicone wire anyway for robust handling. Since we have standardized on 15-45 amp Anderson PowerPole (APP) connectors for our higher capacity power supplies, we'll also use the APP connectors for our charger ports. We already have APP-XLR adapters as well, in case we want to use chargers with XLR plugs. All battery packs we want to charge this way will need to have balance leads attached to their internal cell series and brought out to an externally accessible balance plug. The Luna Mighty Mini 14S pack, whose BMS failed, is a good example of this, having the 14S JST-PH female plug hot-glued to the outside of the case. The following photo shows what a bit of exposure to rain can do to corrode the BMS and causing it to fail. This particular battery, even used without the BMS and bulk charged (no balancing) for several years, has all of its cells remain balanced to within a few millivolts. There is no substitute for packs made with high quality *matched* cells at the time of manufacture.

When charging a

















Balancing cells in battery packs

Balance leads (wires) are typically 20 to 22AWG and use JST-XH (2.54mm-0.1") or JST-PH (2.0mm) connectors. Be careful about encountering JST-EH connectors, whose 2.5mm pitch is very close to the JST-XH 2.54mm spacing. Balance connectors will have one more wire than the number of cells, so a 13S battery will use 14 conductor balance wire harness. Traditionally, 13S/48V batteries and lower have used JST-XH connectors, while the 14S/52V batteries and higher have used the JST-PH connectors, presumably to keep the connectors from getting too wide (the JST-PH family has a 'finer' pitch than JST-XH).

Battery Balancing tools and methods

PCB to solve the multiple pitches challenge

The following shows an attempt at making a tool to deal with the three commonly encountered connector pitches in the current LEV lipo battery pack world. Soldering connectors and loose wires to a printed circuit board (PCB)is much easier and more reliable than trying to solder wires directly to a 2.00mm pitch connector, especially if you're over 70 years old (which some bikers are). This "breakout" PCB simply connects the 15 pins on one connector to all of the corresponding pins on all of the other connectors. There is no requirement to populate any of the connectors, or to utilize any of the pins, other than those needed for a specific task. Breakout boards are becoming increasingly common tools as electronic devices are made ever smaller, and manual access to their connections becomes more challenging.

For the cost of about $5 for the actual manufacturing, plus $20 for shipping, the chinese company Seeed Studios will accept your design files and send you 10 high-quality finished PCBs. In this example, because these breakout boards are small (40mm by 50mm), we'll "panelize" them and fit four individual boards on each PCB (80mm by 100mm), so we'll end up with a total of 40 breakout boards. Given the total cost of materials and shipping at about $25, this results in a cost of about 63 cents per breakout board. The software being used here is by Eagle, version 9.5.1., which is not the most recent version at the time of this writing (Aug 2020).

The widths of the traces have been made as wide as practical and have been routed on both the top and bottom of this two-sided board to allow as much current as possible. Since we rarely charge packs at more than 150 watts or balance them quickly, none of the traces are ever likely to see as much as 2 amps, which these will easily handle.











Battery Resources

Micah Toll's great video on tearing down a Hailong SO-39 pack:







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