May 21, 2015

Update 2018: This article was written more than five years ago. During that time, we have revised our Pulse lithium motorcycle battery product line twice, with the latest version, the Pulse IPT, being a clean sheet design which includes an advanced BMS. The concepts in this article are still accurate- what has changed is the availability of technology which has allowed us to produce a battery with a full BMS- at a lower cost than our old product which did not have a BMS.

Please keep the above in mind as you read this entry. 

This week I am going to write about another topic relating to lightweight lithium batteries like our Pulse batteries. Last week, I talked about caring for a lightweight lithium battery during the winter, and explained why you should not use a Battery Tender® on a lithium battery. Judging from the response we got via email and our Facebook page, I think it was helpful information.

What was really interesting to me, was seeing some of the discussion which popped up on forums about this topic. One of the things that I noticed was that there were terms being used improperly, and I want to use this post to explain one of them: Battery Management System, or BMS.

First, lets look at the official definition of a BMS:
“A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and / or balancing it.[1][2]”

Now, definitions aside, how does this information apply to lightweight lithium start batteries like the Pulse? In the case of our Pulse batteries, it doesnt apply, as we do not use a BMS in our Pulse lightweight batteries.
Our Pulse batteries do have an IPC (Integrated Protection Circuit), but this is not a BMS. The IPC is designed to protect our batteries from short circuit damage and to protect you and your vehicle from damage under a forced discharge situation. This is quite different from a BMS. You can read about out IPC on our Technology Page.

In my opinion- and the opinion of most every battery industry person, ever- a BMS must be an integral part of the battery, to be appropriately called a BMS. There are also functions which a real BMS performs while the battery is in use in a vehicle- I will talk about those functions in a bit.

Here is the reasoning for the above statement. If the job of a BMS is to “manage…by monitoring...calculating…reporting…protecting…” and so forth it would of course follow that in order to do those things, the BMS would have to be either a part of the battery itself, or at the very least connected to the battery while the battery is in use, right?

Apparently not- at least according to several lithium motorcycle battery manufacturers. These companies have seen fit to call their balance changers by other names, like “external BMS” or “BMS Charger”. This is pure marketing B**lS**t designed to fool customers.

As an example, imagine that your ECU (Engine Control Unit), whose job is to ”manage the engine…by monitoring…calculating…reporting…protecting…” and so forth, wasnt actually connected to your vehicle while it was running. See the problem? The vehicle wouldnt run without the ECU, just as a battery with a BMS cant function without the BMS.

Again, not all batteries have- or need- a BMS as a part of their design. Our Pulse batteries dont have one, and they have won more races and championships than all other lightweight battery companies combined.

So, why are people calling this a BMS, and what does a BMS actually do?

Well, I believe that companies are claiming to have a BMS because it sounds flashy, and technically complicated…and cool. No other reason.

In other words, they are lying.

So what is a BMS, and how would it work in a lithium start battery?

In a lightweight lithium battery, a BMS would have several functions that would be a bare minimum, when looked at in context of BMS’s used in larger batteries. In short, the BMS would provide:

1. LVP: Low Voltage Protection- the BMS would stop the battery from working below a certain voltage to prevent damage to the battery. Every BMS I have ever worked with, or heard of, has this feature. This would be helpful in a start battery left in storage with the battery connected. After a certain voltage the battery would stop supplying current, and would not get killed by over discharge.

2. OVP: Over Voltage Protection- the BMS would stop the battery from being charged with voltage in excess of design parameters. This feature might be helpful in vehicles which have damaged charging systems which charge in excess of 14.7 volts.

3. Short Circuit Protection: the BMS would protect the battery, the vehicle, and possibly the user from excessive current being discharged during a short circuit. Imagine dropping a wrench across your battery terminals, and you can see how this would be helpful.

4. Continuous Current Output: the BMS would limit the amount of current the battery could put out over a given time frame. By example, the BMS would limit the battery to 40 amps continuous discharge. The BMS would shut the battery down to avoid excess discharge.

5. Pulse Discharge Output: the BMS would limit the max amount of current that the battery could discharge over a 10 second Pulse or burst. This is not a feature you would want in a start battery, obviously.

6. Active Cell Balancing: the BMS would continuously monitor and adjust the voltage of every cell in the battery. This could be useful in a start battery, if all of the other things were also available, but it is of little use as a standalone feature.

These are the absolute minimum controls that a real BMS would have on a lightweight lithium battery. It is very common for BMS to include all of these, as well as Fuel Gauging or real time State of Charge monitoring, temperature monitoring and interruption, and so on. It gets much more complicated as the size of the batteries increase, but these are the basics.

Oh yeah, and it needs to be inside the battery itself. But then that should be obvious, right?


It should be said that adding a BMS to a battery is usually a very expensive proposition. For example, on our small systems like our Pulse batteries including a simple BMS would add more than $150 to the cost of each battery.

[Note: There are lithium start batteries on the market that do have real BMS's. They all cost two, three, and four times what our batteries cost and are mostly marketed to...well...I don't really know who they are marketing these to. $1000-$2500 seems expensive for a start battery, so while there may be some people willing to buy them, this just isn't a market that we are looking at right now.]

A few weeks ago, I talked about the proper way to care for a lightweight lithium battery- and why you should not use a Battery Tender® as part of that program. I understood when I was writing the last post, that people would ask other questions about charging and battery care, and I am going to address those questions. I added another post about understanding battery capacity a week later, which provides even more background on these batteries.

With this post, there should be little confusion about BMS’s and their role in a lightweight battery. What I have written here is well understood by people in the battery industry. Unfortunately, the people producing lightweight batteries being marketed to the powersports industry are choosing to lie to their customers, so that they can sell more products.

Next week I am going to post another lightweight battery topic, which is going to be the first in a series about the technology we use, with comparisons to other lightweight batteries.

As always, if you have any questions, feel free to send us an email or give us a call.