Okay, I'm circling back to this post of mine after spending a good amount of time researching, thinking about, and discussing BMS. I still feel like a newbie regarding BMS' in the realm of things, but a knowledgeable-newb
.
I'm going to use this post to put some of my thoughts into writing, and to answer some of the questions and misconceptions that come up all the time in the beginners forum. For starters:
DZL's FAQ:
not official, authoritative, or set in stone, but better than nothing
Which is the best BMS?
There is no best BMS, the BMS that is best for you will depend on the type and specifics of your system, your priorities/goals, your experience/comfort level, and the context of your situation. The best BMS for a small hobby system where the user wants something that 'just works' and requires no user intervention/management, will be different than the best BMS for a multithousand dollar battery bank on a sailboat setting out on a arctic expedition.
To get better recommendations, explain a bit about your system and goals/experience (at a minimum what is your cell arrangement, what is the max current, will the batteries be exposed to freezing temperatures, what is your use case)?
Do I need a BMS?
If you are building a battery using raw cells and you are asking this question, then the short answer is YES, you do. Like a seat belt, its not absolutely needed to operate a vehicle, your car runs fine without it, and if you are a careful driver and don't have bad luck hopefully you will never need it, but its cheap security in the event that you do need it, and a fundamental piece of a properly designed system.
I have a Drop-in lithium or lead acid battery do I need a BMS?
No. Lead acid doesn't need it, and drop-in lithium batteries already have the BMS built in.
What is a BMS?
A
Battery Management System is a
system for monitoring, managing, and protecting lithium batteries
. The defining features are that it is aware of
cell level voltages and it can take take actions based on cell level voltages. These actions most commonly include but are not limited to automated
low voltage disconnect (LVD), high voltage disconnect (HVD) and
cell balancing. Additionally many people look for features such as
low and high temperature protection, over current protection (OCP), and
State of Charge (SOC) monitoring.
If there is one thing you remember about the role of a BMS, its
cell level monitoring and protection. This is what fundamentally distinguishes it from other components of your system.
What isn't a BMS?
An active cell balancer is not a BMS, it can monitor and balance but not protect.
The protection features built into other components like chargers and inverters work hand in hand with a BMS but on their own do not comprise or replace a BMS.
But my Inverter/Charge Controller/Battery Protect/etc has LVD and/or HVD
These are useful features to have, but do not replace the need for a BMS. Recall the distinction of
cell level protection. Your inverter or charger only has knowledge of
pack level voltage, it is ignorant of cell level voltage, and won't be aware of cell level problems.
You said a BMS is a system, not a thing, what does that mean?
A BMS is a system, designed by what it does, not what it is. It is a system for managing and protecting your battery at the cell level. Often this is accomplished by a single component, in this case a BMS is a system and device. These devices are often referred to here as "FET based BMS'" (and occasionally inline BMS' or commodity BMS'). But even with these single device BMS some aspects of battery management are often accomplished by other devices.
On the other end of the spectrum, there are decentralized BMS' or modular BMS' like the Batrium Watchmon4
Then in the middle, there BMS' like the SBMS0 and the Chargery, that have centralized command/monitoring but rely on external components for control and additional features. Within this group there is a distinction between BMS' designed around relays like the Chargery, or BMS' designed to integrate with and control the other components in your system.
How do I know what type of BMS is right for me?
That is a complicated question that is beyond the scope of this FAQ, but we can make some generalizations.
Inline "FET based" BMS' are often the simplest most affordable option for simple systems. They are the closest to a 'plug and play' approach to battery management. They are limited by the current that they can safely handle. There are a few high amperage models (Daly for instance), but most are <150A, and most experienced forum members reccomend derating this type of BMS below the stated max current.
If you desire more flexibility, customization, or higher current you may want to look beyond FET based BMS' at options like the SBMS, Chargery, or TinyBMS which are not 'in the main current path' and thus are not current limited in the same way that inline FET based BMS are. These types of BMS usually offer an expanded featureset and greater flexibility and capability (SOC monitoring for instance).
I bought [this brand/type/capacity] cells what size BMS do I need?
Battery capacity is irrelevant to BMS selection. What you need to know is how many cells
in series and what the max current in/out of the battery will be.
What is the difference between common port and separate port?
As far as I know, Daly is the only company that makes this distinction. And its only a distinction relevant to FET based BMS'
Common port means that there will be one universal current limit for charge and discharge, and the BMS can only connect/disconnect the battery, it cannot control charging and discharging separately. This is not ideal for a number of reasons, but its cheaper to produce, and allows a higher charge current than their separate port BMS' which tend to have much more limited charge current limits.
Separate port allows controlling of charge and discharge current separately.
Most other BMS do not make the distinction between common and separate port (with the added bonus of not having limited charge current). Some refer to these as Hybrid Port BMS, as they operate like a separate port BMS but have some technical differences.
My advice, get a BMS that is capable of controlling charging and discharging separately unless you have an explicit reason not to.
What are some BMS that people on this forum like/use/recommend?
Daly BMS
Daly Smart BMS
Ant BMS
JBD-SP04S020 ( AKA the
DYKB /
Overkill Solar / Battery Hookup BMS)
Chargery BMS
SBMS0
What are some other notable BMS' worth exploring?
TinyBMS
DIYBMS
Batrium
Orion Jr
Most of these BMS' are available on different platforms from different sellers, and sometimes with different configurations, I'm not endorsing, promoting, or reccomending these configurations or sellers. The links above are just starting points for your research, due diligence required.
