Electric Motorcycle Primer “InsideEVs Style” – Part 3: Battery Care and Handling (the BMS)


We’ve covered a few things that a gas veteran will probably not find too foreign.  Let’s look at something that’s entirely new – the care and feeding of a lithium battery pack.

We’ve talked a little about batteries in the preceding post, as well in our Nerdgasm battery post, but we haven’t gone into any detail about how it works, the pack’s maintenance and some best-use practices.  Assuming that we all bring some preconceptions to the table based on our experience with lead-acid batteries, maybe some Ni-Cad appliances from Days of Yore, and even laptop and RC car batteries, let’s start by talking about how lithium batteries are different.

First off, unlike lead batteries, lithium has no real mechanism for shedding excess energy as heat.  This particularly comes into play in charging.  A lead battery can sit there, taking a charge way past its capacity, and turn that extra energy into heat.  It’s not great to do it to a lead battery, but it doesn’t cause significant damage, and can happen quite a few times before the battery starts showing some wear.  Lithium packs, however, can’t do this.  When you overcharge them, you damage them, often immediately and catastrophically.

Lithium chemistry has a different charging and discharging “profile” than lead.  No doubt you use a “trickle charger” on your lead-acid batteries, and this works by feeding the battery full current and voltage, and as the voltage of the battery rises, the charger slows the charge down gradually.  As you use the lead battery (let’s say it’s a 12V automotive type), the voltage starts at maybe as high as 14V, and drops down to below 12V.  Again, it’s a gradual drop, and the voltage can be a good indicator of the battery’s state of charge (SOC).  You guessed it, lithium doesn’t work that way.

The charge/discharge profiles are a lot more dramatic with lithium chemistry.  A typical lithium pack will stay “stiff”, or keep its voltage up to very nearly its maximum charged voltage for a considerable portion of its capacity.  When it drops, however, it plummets.  Here’s the problem.  Once it plummets like that, you’ve damaged the cells.  Where you can use voltage to indicate the state of a lead cell’s charge, it’s virtually impossible to do with lithium without damaging the pack permanently.

What this all adds up to is that you have to keep your cells from being overcharged, and you have to keep them from being over-discharged.  While that’s not a tall order if you’re using some sort of Watt meter to gauge your capacity, and a good charger with a lithium profile, it gets tricky when you start to understand that your individual cells are what matters, not your pack as a whole.  If you’re looking at your whole pack, you could have individual cells that are very low, or very high, and you wouldn’t know it.  What you need is cell-level monitoring.

Sound like it’s getting complicated?  It is.  Enter, the BMS.

The BMS (Battery Management System) tries to, first, keep all the individual cells topped up to the same level.  If there’s a cell that’s high, the BMS will shunt that cell out and let the others catch up.  If one’s low, it shunts over to that one.  It also can control the overall pack charge levels with high-voltage-cutoff (HVC) and low-voltage-cutoff (LVC).  As a byproduct, you get all sorts of other cell monitoring – capacity, temperatures, like that.

2013 Zero battery module with integrated BMS

2013 Zero battery module with integrated BMS

Any commercially available EV will have a BMS integrated into the pack.  Some of the more sophisticated will have the BMS integrated into the control systems on the vehicle as well.  Needless to say, if you’re building your own pack, it falls to you to build a BMS into it, if you want to protect it safely.  The good part of this is, because a commercial vendor doesn’t want you messing up their batteries by misuse, the BMS serves to take a lot of the bother out of your hands.  The pack maintenance, and the operation of the BMS should be pretty much invisible.

That said, there are some things you should take into account if you’re trying to get the best performance and life out of your pack.

There’s a great post over on the Electric Motorcycle Forum called Lithium Ion (Li-ion) Batteries used in Zero Motorcyles – all you need to know, but you can apply it to just about any lithium pack.  Here’s a rundown of the top basic tips:

1.   If you run your bike to empty, do your best to give it at least a partial charge (ideally to 40-50%) as soon as you can.
2.   Charge and discharge your bike freely and at your convenience.
3.   Don’t worry about keeping the bike topped up, like standard car batteries.
4.   Don’t feel you have to run the bike to empty before re-charging, it’s fine to charge if you’ve only used 10%, for example.
5.   If you can afford extra chargers to reduce charging time, Zero’s fast chargers won’t hurt the batteries.

Want to read the last word in battery information?  Wander on over to Battery University.  There’s none better for information on virtually any kind of battery.

Here’s a personal favorite tip.  Don’t top the charge up on the pack in the cold of winter.  Why?  When the pack is cold, the cell voltage drops.  When it heats, the voltage rises.  If you charge the pack to its maximum when it’s cold, and later the pack heats up, your cells are going to pop over their maximum safe voltage.  Some cell chemistries will actually explode.  Others will just be unceremoniously and irreparably damaged.

One last question – safety.  Unlike a car, bikes can do silly stuff like tip over.  There’s all sorts of fearmongering about batteries catching fire and exploding, and, to be honest, some chemistries, like what they use for RC cars and planes are prone to doing that.  You probably won’t run into those if you’re riding a motorcycle, though.

You’ve been patient and waded through all this, so far, we think you deserve a video of what happens when you puncture a pack.  Fun times – shooting a CALB LiFePO4 (pretty standard EV chemistry) cell with a nail gun…  lots of smoke but no fire, and no going boom.

Catch up on the whole Electric Motorcycle Primer “InsideEVs Style” series:

Electric Motorcycle Primer “InsideEVs Style” – Part 1: The Ride …where in we talk about why we’re doing this in the first place. Because riding electric motorcycles is awesome.

Electric Motorcycle Primer “InsideEVs Style” – Part 2: The Drivetrain The bits and pieces, and how they’re different, but also how they’re the same as your gas bike.

Electric Motorcycle Primer “InsideEVs Style” – Part 3: Battery Care and Handling (the BMS) How a lithium battery pack is different than any battery you’ve ever owned.

Electric Motorcycle Primer “InsideEVs Style” – Part 4: The Rest … the other funny stuff about an electric bike.

Electric Motorcycle Primer “InsideEVs Style” – Part 5: Reading the Specs Wrapping up, using what we’ve learned by reading the spec sheets (and actually knowing what they mean).

Category: Bikes

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One response to "Electric Motorcycle Primer “InsideEVs Style” – Part 3: Battery Care and Handling (the BMS)"
  1. Robert says:

    Puff, the Magic Dragon!

    Actually, I have seen – just one LiFePO4 cell made to burn: it was an A123 M1 cell, the 26650 size, and it was hit repeatedly with the sharp edge of the bucket on a 966 Front End Loader, against the middle of the cell, which was on a road! It still took 4 or more hits to trash it enough to go off!