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Li-Ion/LiPo Batteries
Resources Online content to get you up to speed with lithium chemistry cell-based packs.
Battery Management System ("BMS") Connection diagrams, etc.
Balancing Cells Notes on cell balancing.







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 22AWG and use JST-XH (2.5mm-0.1") or JST-PH (2mm) connectors. 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 Resources

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







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