LiFePO4... Why Use A BMS?

[bobmcree]

Bob,

Those eBay 3.3V DC-DC converters look like a bargain, but checking the spec sheet I've found that they have a minimum current output of 10% of rated output, which means that they have to deliver at least 2A all the time. I guess a shunt could be used to take the excess, but that seems a bit wasteful.

Had you got any bright ideas for getting around this snag?

Thanks in anticipation.

Jeremy

that spec is just the range over which they guarantee all the other specs. as far as i can see they work fine at a milliamp or even a lot more than 20A. they just might not regulate as tightly at a lower current but it is certainly not an issue with a battery charger. they are totally cool! i have as many as i want or i would not be telling you guys. if you could hook them up to ac through a fet voltage divider you might be able to run them without a big 48v supply but they will take 36-72v in and it does not have to be well regulated. he will only charge $1 shipping each after the first one and will give you free shipping on 50.

 

[bobmcree]

To BMS or Not to BMS - Part Two

Here's another thought that might be junk, but here it goes...

I think you should just preface all of your posts this way. When most of us have an off the wall idea we spend a little time thinking it through before we waste a lot of other peoples time helping us figure out how little we know

 

[Link]

What about Zeners? Instead of pots, stick two Zeners oppositely biased on each cell. Once the cell reaches a certain voltage, the Zeners start conducting and dump off all the excess power.

Apparently you haven't been following all the subtle problems that have been cropping up with automated BMS systems.

Not really. Still waiting on them supercaps. No need BMS or even proper chargers, yay !

One of the big problems with LiFePO4 is that when people try to do things like set a cap voltage this still allows that cap voltage to grind away at a full cell for a longer period of time that it ideally should endure.

Perhaps use the Zeners to flip on a solid state relay or two and take the cell out of the string? Then the charger voltage would need to drop, though. Or maybe throw in a dummy load? Hmm...

You are in a sense "beating up the runt kid" when you do this and in the cruel, cruel world of LiFePO4 that runt dies pretty fast.

Maybe he should get more protein.

 

[Jeremy Harris]

Bob,

Thanks for confirming that, real world experience wins over the spec sheet any day!

I'll risk buying some and see how I get on, at that price, even with international shipping and taxes they're still a bargain.

Thanks for spotting them and taking the time to pass on the good news.

Jeremy

 

[fechter]

Oh, yeah. This reminds me to ask something: Does anybody know where I can get some electronics SIMULATION software? The only things I can find are like $3000 (eep!).

Try this:
http://www.catena.uk.com/site/index.htm

I haven't had a chance to try it, but it was recommended by a co-worker.

 

[fechter]

So imagine a trim pot for each cell.

You keep constant track of how your cells are behaving and when one needs some help you turn the trim pot so that it gets the help it needs. The really strong cells are left so that they get no bypass, the weak ones are given a lot of bypass.

8) Cool idea... or has insanity taken hold?

Er, uh, sitting around for several hours tweaking a bunch of trim pots and measuring the voltage of every cell pretty much qualifies as insanity in my book. :wink:

What happens when the batteries are done charging and just sitting around?
The resistors will be draining the cells. Bad.

The other problem is due to variations in cell manufacture and evironmental conditions, the correct settings for the trim pots would be constantly changing, requiring constant adjustment. If you fell asleep or weren't around, you could easily fry your battery. The other problem is the pots need to be able to handle the bypass current, which could be as much as the full charging current. They would need to be pretty big or you would be

smoking pots.

 

[Link]

Alright! Thanks, Fechter!

The trial version is "limited on circuit size", but that limit is quite high. I placed a couple hundred parts on the board and it didn't stop me. It's more than adequate for making anything short of a Pentium, apparently.

What happens when the batteries are done charging and just sitting around?
The resistors will be draining the cells. Bad.

That's what I was worried about. Any resistance value low enough to allow sufficient current to pass up the battery would also be low enuogh to discharge the batts really fast.

 

[safe]

I had moved this idea to the "Manual BMS" thread. The problem is that there simply is no way to limit cells short of something like a PWM controller on each one. It's an interesting concept and I'll keep it in the back of my mind and if there's anyway to make it a reality it's pretty good.

But as a practical matter there is a simple rule:

"No resistors in the battery"

...that would be going backwards in time. (pre-PWM)

So if there's any way to do it it's going to involve something other than resistors. (probably not possible)

 

[Eric G]

Hey Link
Here's a link another simulator...


http://www.5spice.com/?referrer=GoogleAd5&gclid=CKn5kNnZ85ACFUV0OAodT1XQ0Q

Eric

 

[safe]

Why Use a BMS... When You Have Long Range?

I was doing some spreadsheet calculations on the Thundersky cells that Patrick has now at $80. (3.2v 40Ah) If you make a 48v pack with those (15 cells $1200 1920Wh ) the range you could get on a bike with a "legal" motor is going to be in the range of from about 50 - 75 miles. That's "worst case scenario" so on a day where you aren't racing that might mean closer to 100 miles on a charge. (and if you pedal it would be longer) I just DON'T NEED to go 100 miles or even 50 miles most of the time and would be happy with a nice little 30 mile trip. In such a situation there is virtually no way to reach your low voltage cutoff because you would only be using 3/4 of your capacity... with lot's of room to spare.

On charging you just need to buy 15 chargers, one for each cell, but for discharge (with such a large pack) you just don't need to worry too much.

So one way to deal with the situation is to simply buy more battery... that way you aren't pushing the limits of a small battery all the time. 8)

 

[recumbent]

Why Use a BMS... When You Have Long Range?
So one way to deal with the situation is to simply buy more battery... that way you aren't pushing the limits of a small battery all the time. 8) [/color]

That's basicaly what I said on page one on this thread, but it still won't protect you from that one time your in a hurry, or it' starts raining, or your friend borrows your bike and BAM! you just cooked your $1,200 battery pack. Or at least that's how i dreamt it would happen.

 

[safe]

All self monitoring, self controlling battery management techniques rely on the user to not be an idiot. I can't prevent anyone else, but I'm personally self disciplined enough to do it on my own.

For the general public they will need "idiot proofing" there's no doubt about that...

 

[disadvantage]

Here is VisualSpice, a schematic editor and circuit simulator I've been considering:

http://www.islandlogix.com/html/products.html

disadvantage

 

[lawsonuw]

The various Open Source circuit design and PCB layout software has been getting quite well developed lately. I'd expect the open source spice simulators would be getting similarly polished. Still, I don't think any of them do a good GUI out of the box. Not that many of the commercial spice packages do a very good job either... Of the two I've had experience with, the most expensive one (PADS) did the worst job of hiding all of the text based spice code underlying the GUI. Worse yet, the documentation assumed that any user would already KNOW all this stuff so it just went over the minimum needed for operating the GUI. Moral? check the help file before you buy any software

Anywho, I'd want a good BMS for LiFePO4 to insure that I get every last cycle out of the pack.

Marty