Stuff I’ve Learned: Battery Management Systems

…or, “What the HELL is a BMS anyway?”

Here’s the issue.  In any electric vehicle you’re building up battery packs from cells with low voltage- lets keep it simple and say you’re using six 12V batteries- and wiring them together in series to add up the voltage.  Six 12V batteries in series will add up to 72V.  Nice.  Keep in the back of your mind- wiring the batteries in series adds the voltage but not the capacity.  If you want to add up the capacity- say you’re working with 20aH batteries, then you have to wire them in parallel.  Two 12V 20 aH batteries wired in parallel gives you 12V, with 40 aH.

So, OK, I have six batteries giving me 72V.  Let’s charge them up.  I can do it two ways- with a 12V charger, doing each battery separately, and the charger will monitor the battery, top it up, and shut off at a prescribed, and constant, voltage.  If I use the same charger on every battery, every battery will be exactly at the same state of charge.

The other approach is to charge the whole pack with one 72V charger.  This is easier, but you may or may not hit the same state of charge for each battery.  This is because every battery has a different set of characteristics- internal resistance, capacity, like that.  When you hook up a bunch of batteries in parallel to a charger, the charger gets access to both ends of each battery.  It can charge and monitor each battery directly.  In series, it’s more like a bucket brigade- the charge goes through each battery and on to the next.  Because each battery is different, each battery will find it’s own level, and not necessarily the level of the pack.

This doesn’t only apply to charging, it works for discharging too.  If you start with a bunch of cells at identical states of charge, they discharge at different rates and they charge at different rates, sooner or later you’re going to have a pack with wildly varying voltage between batteries.  For six lead batteries, it’s not so much of a big deal.  You can check and charge them individually pretty easily, and if you accidentally cook a battery, they’re cheap enough to replace.

Not so with lithium and other battery types.  These things are both delicate and expensive. Dangerous, too.  Discharging or charging at too high a rate can be damaging, or even catastrophic.  Enter the BMS.

The Battery Management System is set up to monitor the state of charge and discharge of each cell. It will see if one cell is low, and ask for more charge.  When a cell reaches a certain point, it will shunt the charge to the rest of the system.  It does the reverse on the discharge leg of the cycle.

I found this “mini-BMS” video to be the single most helpful explanation/demonstration of how a BMS works.  Take a look.

On a mini-BMS you get small modules on every cell.  The diagram at the top of the page is a conventional BMS with individual leads going to each cell, then connecting to the charger/controller circuits.

This is all swell, but the one issue is that there’s a lot of development going on in the BMS segment, and that’s because, well, they blow up a lot.  They’re expensive, too.  Within a year or two, the BMS concept may well be the biggest area of development and change you’ll see in the EV market.

One approach, by the way, is to avoid the BMS issue altogether- as per Ed “Juiced” Fargo’s “Non-BMS” solution.  Ed drag races electric motorcycles, and has melted more than his share of BMSs.  He’s stuffing enormous amounts of current through the system, and the normal BMS is just not designed for it.  His answer?  Forget about the discharge rate, and just make sure you’re starting with a constant state of charge- by using individual chargers.  He’ll have several Headway cells wired together in parallel- a total voltage of 3.2V, but a capacity of maybe 40aH, and have one charger wired to that sub-pack.  Then all the sub-packs are wired in series to get his voltage up to 72V.  All told, he has 24 individual, inexpensive chargers feeding his 24 parallel clusters.

Probably the single biggest reason, besides the cost, I decided to go with cheap AGM batteries rather than spring for the Headways I really want was the BMS issue.  I feel like this is something that just isn’t quite there yet.  Several manufacturers incorporate the BMS into the battery, (Valence, notably, the company that supplies Brammo) thus giving you a system that has some warranty protection for both the batteries and the BMS- as it is now, if you pull one battery and another BMS and one, or both, fail- both the battery manufacturer and the BMS guys can point at each other and say it was the other product’s fault.

They don’t call it the bleeding edge of technology for nothin’!

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