Different lithium battery manufacturers use the term BMS to mean different things. It’s important to know what you’re getting because the battery management system is used to keep the system running smoothly and protect the battery from permanent damage.
Some BMS systems might just be an idiot switch that disconnects the battery if it sees an internal voltage outside some predefined range (say, 10v and 16v). Their range of operation is designed to prevent catastrophes, not to condition the pack or encourage longevity.
Others include cell-level monitoring and some degree of balancing in addition to having the overall protection. Our lithium pack BMS does that. It looks like the one that the electric car parts company sells is very similar if not the same one.
Obviously, if you buy a sealed “drop-in” lithium battery you normally can’t get to the individual cell terminals to measure voltage and see whether they’re in sync. You have to trust the BMS. You have to trust that when the manufacturer says BMS they mean a true BMS rather than just an under/over voltage cut-off.
Our BMS communicates using little flashing LEDs on each cell and on the main board. We can tell the status of each cell in the battery. We don’t need that on a daily basis, but it’s good to check on your battery’s health every now and again. Other BMS systems don’t allow that level of granularity. That might be OK for you, but the lack of information shown on the outside of the battery probably means it’s not being monitored on the inside either. Without cell-level monitoring, your battery can get out of balance. That can damage it.
Why is balancing important?
Batteries are made up of a stack of cells. In lithium batteries, four cells are wired in series to create the correct voltage range for a (nominal) 12v system. As the cells charge, small differences in how they are manufactured mean they might absorb current at slightly different rates.
This drift happens over a long period of time, but it can end up causing issues. If one cell gets seriously out of whack from the rest of the cells, it might get overcharged (and damaged) before the rest of the pack reaches full charge. A battery-level sensor won’t pick this up because the overall voltage isn’t too high, but the voltage of one cell could be much higher than it should be.
A good BMS will detect each cell’s individual level of charge and start shunting power past a cell that is ahead of the rest. In practice, we’ve found that the conditions for balancing to take place are within a pretty tight tolerance, and it’s been hard for us to “force” the BMS to balance. Basically, for our BMS (and probably others too) the voltage has to be in a set range, just over the recommended bulk charge voltage but not so much higher that it causes a fault. That’s between 14.2 and 14.4v for us (other lithium battery chemistries may differ). Even in that narrow range, the bypass circuitry on each cell can only handle a pretty minor amount of current, so the balancing must occur over a long period of time.
If you are trying to use raw alternator power as your charging source, you’ll probably never get the benefits of cell-level balancing. The voltage isn’t controlled enough to create the right conditions for balancing to occur. The engine isn’t on for long enough for balancing to finish. A good BMS will still protect a cell that’s seriously overcharged, but it’ll be harder for the system to keep things well maintained.
In that situation, you’d have to top balance on a bench with a highly controllable power source or by disconnecting the charged cells and reconfiguring them in parallel to equalize over several days. With a drop-in replacement battery, that’s not an option because the cells are all hidden away inside a sealed container.
Other fun BMS features
Our BMS also has relays that can trigger other events based on cell or pack level faults. It has a low voltage relay that can (for instance) be used to start a generator. It has a high voltage relay that can (for instance) shut off the charging source. As you can imagine, there are many possibilities with this type of level of control.
Again, the drop-in replacement lithium batteries don’t offer this type of feature. You give up control for “convenience.”
BMS does not mean scary or complex
You may come away from reading this feeling depressed about the amount of care and attention that a lithium pack seems to require even with the magic BMS.
I’d argue it’s no different from what people should be doing with their lead-acid batteries. Wet lead batteries need frequent electrolyte checks. Most lead batteries need desulphation cycles too. There’s a higher requirement to keep the batteries in a full state of charge. It’s still possible to over-charge the batteries and hurt them. In other words, they still need maintenance if you want them to work to their full potential and last a long time.
If you decide that you’d rather have a lithium battery than a lead-acid one, then I’d suggest you carefully research the battery management system that comes with it.
The drop-in replacement batteries you can buy seem appealing because they are sold as sealed, “maintenance free” units. However, without a BMS that allows cell-level monitoring, balancing, and status reporting the fact that they are maintenance free actually counts against them. If you can’t maintain them, they aren’t going to last as long as they could.
Even if you don’t use all the features that a BMS has, like high and low voltage relays, it’s good to check that the BMS on the lithium battery you are considering buying has at least cell-level monitoring and preferably cell balancing as well. If not, all you really have is a failsafe switch, not a true battery management system.