Lithium battery BMS questions

[Ryland]

Originally Posted by TurnNBurn

Ok. That explains it. I didn't know the charger only does the full pack. When I see some chargers they ask how many cells, what chemistry, things like that and I assumed this was because they auto detect each individual cell voltage.

Nope, not that I've ever seen, the charger wants to know the number of cells and the chemistry so it knows the charge curve to use and the total pack voltage to stop at, because each of those is going to be different depending on the battery, some chargers will try to float charge the batteries no matter what, this is ok for every type of battery other then lithium, so having a way to turn off the charger is a good idea and you want the charger to turn off before you over charge any single cell.

Always install fail safes and if you can, an LED or something else that tells you when that fail safe has been triggered, the solid state relays that I use have an LED on them so you know when they are turned on and my mechanical contactor makes a click when it closes.

[TurnNBurn]

I was looking at this youtube video:

https://www.youtube.com/watch?v=yF3vf4YXSDI

The user states, "The alarm(s) can be set to go off to any value within the cell-log specs.". So can the celllog be programmed to trigger an alarm when charging and when it reaches a certain voltage send a cutoff signal which will then disconnect the charger?

On a side note....why is a BMS such a MacGyver thing? Why must I rig one like this? Is there no market for a programmable BMS?

[P-hack]

production EV's have BMS's from the factories, in fact they have a well thought out and tested and integrated charger/bms/controller and they know the battery and motor parameters ahead of time. Some even use the large induction of the motor and the inverter circuit as a basis for a high power onboard charger (if it is plugged in, you aren't driving it).

But the DIY market either has really expensive and somewhat flexible (and more complicated) BMS systems or kinda cheezy ones, plus the wide variety of controllers and chargers (from manufacturers with no real agreed upon bms interface standards) that may or may not interface with those BMS (or any bms), so it isn't a huge market but has a number of players, including a lot of diy'ers who make bms's for fun.

[TurnNBurn]

So what options do you guys see? Am I stuck with the DIY route? I wouldn't mind the arduino route if I only had instructions. But being on my own is scary. I just re-caulked the bathtub and I did okay. Can't say I'm satisfied 100%, but I can't imagine tackling a small soldering electronics project. I always enter something with confidence, but when the parts are on the table and it's time to put everything together I lose it.

-edit-
What about the iCharger 1010b? It's a balance charger that can handle up to 10 cells.

http://www.amazon.com/ProgressiveRC-iCharger-1010B/dp/B005X0470G/ref=sr_1_1?ie=UTF8&qid=1398920521&sr=8-1&keywords=icharger+1010

Two of them? Can I find a single power supply to run two of these?

[P-hack]

learn how to bms by hand for one. then you will know what the pack is doing. The other thing to keep in mind is that you are looking at a $1800 investment in batteries whereas some cars have like 10's of thousands invested. The fancy bms makers are targeting the latter so the overhead is much larger in your case. As well you are using a rarer battery chemistry that has different voltage requirements and is more heat sensitive, which makes a stronger case for a bms and one with lots of thermal probes (more $$$) which could eat up what you saved on them in the first place (and the different chemistries limit your options).

The non bms route is well travelled with lifep04, it is considered safer for a number of reasons, and as long as you keep an eye on the pack voltage and current you *should* be fine. The leaf cells have a much shorter life cycle too.

LiMnNi Batteries recommends bms for LiMn*, you are asking for a lot of extra work even if you do know exactly what you are doing.

The charger options are much more plentiful for lifep04 as well.

I know we kinda hijacked your thread here but dealing with a special needs battery is either rather diy or somewhat costly (and still bms's introduce their own complications).

The main thing with lifep04 is not to over discharge as that causes plating, and they are more resistant to overcharge, and better temperature range, and 4x the life cycle.

So probably the best option is the well traveled one. Some 60ah calb cells to get you to 72v and shop around for a charger that has two phase charging and a "safe" cutoff limit, keep an eye on your pack voltage when driving so you don't overdischarge it, and check the balance after charging/riding by hand (18 voltage readings) frequently at first (while it is charging initially) and less as your confidence in it grows.

The equipment for manually balancing is a voltmeter and a resistor and patience. And that simple led circuit will tell you if something is horribly out of whack.

Here is the battbridge circuit again, that's it, couple leds and resistors:


more info: http://www.evdl.org/pages/battbridge.html

[mechman600]

Wow, I have heard of the battbridge before but never actually looked into it. Nice and simple. However, isn't a 2V alarm threshold a bit dangerous, especially when the entire pack is discharged to a point where any cell will quickly drop below the knee? That's when catastrophic cell damage occurs - when one cell is dead and the 17 other cells still have enough jam to force a bunch of current through it.

I believe bottom balancing and an amp-hour counter (EASILY done with simple Arduino code) it the ticket. When you've used enough Ah to be at 80% DOD, STOP! Bottom balancing allows for all cells to kit the knee at precisely the same time, and if your fuel gauge shows "E" long before then, you are good.

After everything is nicely bottom balanced, your first [carefully monitored] charge will show you the weakest cell. It will be the first one to hit cutoff voltage. This cell will always be the weakest, so technically, this cell is really the ONLY cell you need to monitor for charge cutoff, potentially making an Arduino based charger cutoff extremely simple.

Summary. Bottom balance. Use an Ah counter. Maybe use a battbridge to make yourself feel better.

Parts list:
1 - Arduino ($15)
1 - Charger relay ($5)
1 - 20X4 LCD display ($10)
1 - 2N7000 transistor (to power relay, $0.20)
1 - HTFS400 current transducer ($30)
1 - MUX Shield (only if you feel like reading all voltages instead of only the weakest cell, $25)

[P-hack]

I'm concerned about getting the basic bike wiring going (As well as dealing with less safe limn batteries without understanding everything possible, i.e. they are far more prone to thermal runaway).

Per the link though, the batt bridge can be adjusted, probably a good idea for a smaller pack, you just choose different resistors to get the desired sensitivity:
" 40ma gives you high sensitivity (less than 1v difference to light an LED)."

http://www.evdl.org/pages/battbridge.html

Plenty of on the road experience:
https://www.google.com/#q=%22batt+br...r.com&safe=off

less than $1 in parts.

[TurnNBurn]

I think in the future I'm going to go the DIY arduino route. That way I'm not rushed, I have time. if I mess up, oh well.

So there's really two aspects to this that need attention:
1) Monitoring while charging to make sure the packs aren't over charged and killed
2) Monitor cell voltages while being driven to make sure a cell didn't die suddenly.

As for balancing and charging, would the iCharger1010b be recommended? That way I can setup balancing and charging and monitoring from those two ichargers. This way cut off from over charging will be handled.

As far as discharging while riding, I'm going to look into the Batt-bridge or a similar monitoring device.

-edit-
Hmm. I'm still considering the DIY route, but only if I can confirm a DIY BMS will be able to integrate with those chargers I listed.

I still like the idea of separate chargers that balance charge and then have a monitor separate for when I drive.

[P-hack]

I dunno about chargers and the leaf cells.

But lifep04 (which is safer, and has 4x times as many charge/discharge cycles! and is WIDELY supported) isn't terribly expensive either.
60 ah are like $66. Assuming you are paying ~ $105 per leaf cell * 9 = $945 for 65ah at 67.5v.

GBS cells are a known quantity. You would need 21 for the same voltage and about the same ah, or $1386. $441 more, but they last 4x as long, so really you would save $2394 over the long haul with lifep04 gbs 60ah cells. Plus you aren't experimenting with leaf cells and can lunch off others experience with lifep04.

GBS Battery : GBS 60Ah LiFePO4 lithium battery

It makes a lot of sense to me given where you are at, but I think I will just grab my bucket of popcorn now

[TurnNBurn]

The battery vendor recommended dual ichargers. With the ichargers and celllog, shouldn't that be good? ichargers to monitor charging, celllog to monitor charge while riding.