Choosing a BMS for 8S (24V) prismatic LiFePO4 cells

[aybabtme]

Seems like the REC can drive more amps for the contactors on the charge/discharge outputs. That'd save some hassle, since the Orion Jr can only drive 250mA (or about, from memory), which isn't enough for large contactors and requires an additional external circuit.

[ronstory]

I haven't gotten the battery fully installed in the boat, but I'm playing around with using the Victron Battery Protect modules, specifically the BP-220. I'm just setting the voltage limits out of range of the Li voltages and then just using them as fancy smurf-blue relays.

With them being MOSFET, the drive current is negligible. Even the Orion could do it.

[Audeamus]

For our refit we are going to use 123 smart, calb cells. and Victron Inverter & Solar chargers. This is based on the advice of the supplier we are working with. They have around 60 installations both marine & onshore. I'm sure if there were any issues with 123 then they would not use or recommend it. Can't comment yet on performance as we are a couple of months from installation - I'm about to start opening the boxes

[nebster]

Originally Posted by ronstory

I haven't gotten the battery fully installed in the boat, but I'm playing around with using the Victron Battery Protect modules, specifically the BP-220. I'm just setting the voltage limits out of range of the Li voltages and then just using them as fancy smurf-blue relays.

With them being MOSFET, the drive current is negligible. Even the Orion could do it.

Be careful with those BPs.

First, they are asymmetric like most FET gates: they only stop flow in one direction. So you can't really use one of them with an inverter-charger, unless you only care about stopping one scenario.

Second, they are very easy to melt with high inrush current. A large inverter on the other side will probably have big buffer caps, and those will overdraw the FET and destroy it in a few milliseconds. (Even worse, the inrush may simply destroy it into a wire -- so it will "work" until you need it to do its job, at which point it will quietly NOT work. )

Whether you elect to stick with the FETs or use a contactor, you may also want to have a precharge mechanism if you want to maximize the reliability/life of your switch.

[ronstory]

I was looking to do a separate charge / discharge busses but have been pondering how to interface that with an inverter/charger without adding too much complexity. It looks like the answer is would be going with a separate inverter and charger. More space, but better flexibility for failures and workarounds.

Versus what we had 20-25 year ago, modern MOSFETs are pretty darn durable today and frankly amazing. I remember building a motor controller capable of 6-8 amps drive capability years ago... but kept seeing these modern and tiny Arduino drive boards with the same rating. So I bought one, pulled out my own thru-hole designed drive board and put the $10 SMT Amazon wonder in its place. It's still working after 18 months with a miniscule heatsink. I expected to last only a day or two before the magic smoke was liberated.

Completely get your the point of one-way current flow, and if I end up using a combo inverter/charger I'll definitely use a contactor instead.

[Ken Fry]

Originally Posted by aybabtme

Hi there,

- Batrium
- 123 Smart BMS
- Orion Jr BMS
- REC Q BMS

They are a bit different from each other and I find it confusing to compare them. Has anyone had any experience with them?

I don't have direct experience with those. However, I do have experience with BMS systems from Elite Power Solutions. The first system, from about 10 years ago, worked quite well, via top passive balance (and would still be working if it were applicable to the new cell arrangement). In general, the GBS cells tended to retain their balance quite well, so the top balancing did not need to work very hard.

(BTW, my opinion, for what it is worth, it that any passive balance strategy is like to work OK: top, mid, bottom. I had planned to design an active system, with just feels better to me -- for esoteric reasons -- but the last thing I need is an additional project in this already complicated boat. "WHAT??? I am going to convert hard-earned charge into heat???!!! Instead of using the high cells to charge the low cells?!!!" What a profligate wastrel!) The second system takes care of two paralleled strings, totaling 24 cells. The single BMS systems tracks all the cells, although having it do so required a free (I think) tweak from the vendor... it is usually set up for series strings.) It is in my current project boat and I have not yet really used it other to test it through several charge-discharge cycles. So far so good.

However, getting here with the second system was difficult, because I was shipped several boards that did not function: DOA. In addition, (amazingly) I received two strings (of four boards) that could not fit the cells, because of a large error in cutting and soldering the ribbon cables -- about 1/4 inch too short. A person with average eyesight could see from two or three feet away that the string could not fit -- and therefore one could easily conclude that quality control was stunningly awful.

So... The vendor eventually supplied boards that worked, and I think that they are pretty good well-intentioned people, but I wish I could provide a better review. My first system was just fine, (albeit with a certain cheap feel that is echoed in the second system) so had I not bought the second system, I would have a better review.

Once you have a system that functions for a couple weeks, you are likely "good to go", so I am not too worried about the system. (In other words the smoke more often comes out right away, rather than months later.) However.... that's a lot of gut feel, and I concede that capacitors, in particular, can age and fail.

I was all set to just give up and design a system from scratch when I finally got the EPS system working. Do you like puzzles and science fair projects? I frankly kinda do, and I am more patient than many re this stuff, but even so, it was a royal pain.

[Ken Fry]

Have you considered Elithion? They say it is the worlds most installed BMS, or some such. Davide Andrea, the founder, has been generous with advice for more than a decade to many people building systems of various sorts, from tiny to quite large. Have you read his book?

[Ken Fry]

Another thought. I had a good deal of trepidation about using parallel banks, because a shorted cell in bank 1, (for example) can cause bank 2 to deliver very high amperage into bank 1. ("Feeling a little low over, there, bank 1,? Here, let me charge you right up at 2456 amps... should only take a few seconds.) This might not be seen by the amperage sensor, (of which there may be only one -- on the combined output.) It is worth considering, if you have not, the various ways that things can go bang, or burst into flames.

I ended up with several circuit breakers (six in my case) to limit that transfer amperage. BMS systems often do not deal with such things directly (other than via LV shutdown, but that might not be a solution in some cases... or may require that the system is functioning well. (Typically disconnecting the pack from the load automatically -- via LV signal -- has no effect on this sort of internal fault.)

Not too long ago, many folks would strongly suggest that lithiumX banks should not be paralleled.

[Ken Fry]

Gadzooks!!! More verbal diarrhea:

My boat bank's prior life was in the prototype for my Zing, a micro plug-in hybrid (series hybrid) car. Voltage control and amperage control during charging (in the engine-running mode) was only via engine speed control (the engine drove a PMDC generator). Pretty much just blasting in electrons at whatever rate the engine could produce and the batteries felt like accepting. Sometimes, while the engine was running, I would also be breaking (via regen-- which was, I found to my alarm, powerful enough to cause skidding... so I cranked it down a little in the controller.) So charge rates were all over the place, and sometimes very high (maybe 5C?) The pack, ten years later is still working fine, even alongside some brand new cells. The cells are not fragile, I'd say... but definitely do your own testing, and talk with your cell manufacturer, etc.

[nebster]

Originally Posted by Ken Fry

I ended up with several circuit breakers (six in my case) to limit that transfer amperage.

It's pretty easy just to fuse each string at a little over the maximum design draw. DC breakers at higher voltages and currents are pretty hard to come by, expensive, and big.

And, as you suggest, you definitely need to do it. Not just so that string A can't dump into string B, but so that if string B drops out, string A isn't overloaded.

Not too long ago, many folks would strongly suggest that lithiumX banks should not be paralleled.

There's a decent amount of old literature that suggests, but does not really articulate precisely how or through what experiment they showed, that it shouldn't be done. Most of it written by people who make and sell BMSes, which inconveniently become intractably expensive with large numbers of parallel strings.

[mikedefieslife]

Originally Posted by ronstory

Just checked my invoice. The REC-Active (12V only) was 250 Euros with harness, the PC link was 55 euros and canbus cable 13 euros. Shipping was $40 for 1 week delivery.

From where? Rec doesn't have an online store, and so far just the PC link cable is €150 in Germany, which a joke for a pretty simple RS422 cable to USB and software that should be free with the purchase.

[Ken Fry]

Originally Posted by nebster

There's a decent amount of old literature that suggests, but does not really articulate precisely how or through what experiment they showed, that it shouldn't be done. Most of it written by people who make and sell BMSes, which inconveniently become intractably expensive with large numbers of parallel strings.

Yes. http://www.cruisersforum.com/images/smilies/thumb.gif

When considering the mediocre quality of the second bms system I used (and am using, yikes) I asked myself: "Really, am I more likely to cause a fire with or without all these little boards?" A couple hundred amps through a tiny board can produced impressive temperatures.

[nebster]

Originally Posted by Ken Fry

When considering the mediocre quality of the second bms system I used (and am using, yikes) I asked myself: "Really, am I more likely to cause a fire with or without all these little boards?" A couple hundred amps through a tiny board can produced impressive temperatures.

My concerns exactly.

Truly, if the cells wouldn't be so darn stable and perform so well by themselves, it would be a lot easier to justify the added complexity. Maybe it will become necessary in older packs that were run conservatively for their whole lives; I guess time will tell.