[mattW]

I was a little worried about the current rating Of the thundersky cells I was planning to get, I have heard they over play their specs, but here are some encouraging real world numbers from our Aussie EV forums, I asked about pulling 5C:

as far as doing it healthy, i would say so, rod dilkes pulls 450A out of his 90AH (also 5C) pretty regularly in his conversion. his pack is now a year old and going strong.

Most i have pulled out of my pack is 250A (just over 6C) and it sagged to 2vpc.
The thundersky batteries seem to be pretty consistent as far as volage sag goes, 0.2v per 1C of current.
so at 250A my pack sagged from 3.2v to 2v, at 70A it sags to 2.7v.
rob masons old 200AH packs in his mustang fall to 2.6vpc at 600A (3C).
the new 200AH pack in his triton ute fall to 1.7vpc at 1600A (8C)

Those are the packs i have seen in person.

Id doubt your bike would pull 300A continuous, at 60kmh, mine pulls [email protected] (i also have a yesa booster pack in series with the thunderskys in case you were wondering).

 

[Manntis]

I was a little worried about the current rating Of the thundersky cells I was planning to get, I have heard they over play their specs, but here are some encouraging real world numbers from our Aussie EV forums, I asked about pulling 5C:

Rod is the TS guru. I know he sells TS cells, but he's one of those mates who sells them because he knows they're good - not pushing crap on people for a quick buck. I've had the pleasure of talking with him at length on the phone (haven't been fortunate enough to visit Australia in person yet) and he comes across as a real stand-up bloke.

 

[mattW]

So that means that the current levels are confirmed, the quality control is confirmed and all that is left to see if the cycle/shelf life lives up to the various 1000, 1500 and 2000 cycle claims I have seen.

By the way I just got a quote for my 72V 60Ah pack delivered for $1.80 per Ah or AU$2484 for the whole pack. The supplier just waits for a big enough group buy and then buys them in bulk, seems like every 3 months or so.

 

[david85]

This sounds promising.......

 

[DVR]

By the way I just got a quote for my 72V 60Ah pack delivered for $1.80 per Ah or AU$2484 for the whole pack. The supplier just waits for a big enough group buy and then buys them in bulk, seems like every 3 months or so.

Matt who quoted you that? Was it Trev at F&F?
Have you had any discussion about BMS?

 

[mattW]

Yeah it was trev, he said he has an electrical engineer working on a prototype and wondered whether EV power would sell me a BMS without buying the batteries from them. I am hoping I could just get the components from EV power and solder up the 24 units myself to save the labour time/cost but I haven't followed that up yet.
The batteries will still cost close to 5 grand once the BMS and charger are paid for, but I think it will be worth it. Performance estimates look good too.

 

[JeffPritchard]

Is a BMS for lithium battery charging only critical when charging the cells in series?

If one were to come up with a clever switching "mechanism" to charge the cells in parallel, and use them in series, would that negate the need for a multi-segment BMS?

thanks,
jp

 

[david85]

I sent an Email to this guy a little while ago: http://www.evalbum.com/1497

He had nothing bad to say about the TS batteries, but he also mentioned that he only had about 50 cycles on the pack at the time.

 

[alex]

Is a BMS for lithium battery charging only critical when charging the cells in series?

If one were to come up with a clever switching "mechanism" to charge the cells in parallel, and use them in series, would that negate the need for a multi-segment BMS?

thanks,
jp

So what? a 3.2v charger system?

Don't you get the battery balancing problems whether you do it in series or parallel?

 

[Manntis]

parallel is, in theory, self-balancing but adding all those switchers would be complex.

 

[alex]

Would it really be?

I've already got a simple switching mechanism I made up with relays that can switch 3x 12v batteries between parallel and series for different speeds.

I'm thinking this would be a hell of a lot cheaper than a BMS. Is there anyone who can qualify this idea?

There is no mention of parallel in the BMS wiki http://www.diyelectriccar.com/forums/showthread.php?t=14778

 

[Manntis]

Would it really be?

I've already got a simple switching mechanism I made up with relays that can switch 3x 12v batteries between parallel and series for different speeds.

Yes, it would. With TS cells (or many LIon cells, for that matter) nominal voltage is <3.5V, so you need several dozen switches and a way to control them all. May as well use a BMS.

 

[alex]

As per the image with my MS Paint skills...

Without power connected to the relays, the NC circuit is connected (series). When power is connected, the top relays are turned on, so the NO circuit is closed.

THEN the bottom relays are turned on, and you go from having a series circuit to a parallel one. The catch is that each TOP relay would have to be rated for max Voltage/Amp use plus you have to wire up all of those relays.

But, depending on the price of a high amp, 3.2v charger - this might be a poor man's BMS (eg my kind of BMS )

 

[Manntis]

The catch is that each TOP relay would have to be rated for max Voltage/Amp use plus you have to wire up all of those relays.

bingo Therein lies the problem

 

[alex]

Heh, lets not jump to conclusions!

Ok, so for 144v we're looking at 45 cells. Ouch

So that is 44 high current relays ($???) + 44 lower current relays. Suddenly a ~$1000 BMS is looking cheap.

I guess for someone using 12v lead acid, it might be more practical? :/

Back to the drawing board


Maybe I'll go for the water powered car instead

 

[JeffPritchard]

So what? a 3.2v charger system?

Don't you get the battery balancing problems whether you do it in series or parallel?

Somebody will no doubt correct me if'n I'm wrong, but I think the problem is that with big juice across a series combo of batteries, the real problem is that the individual characteristics of the different cells determines how much voltage appears across each one. Basically, ohms law. With a certain amount of current flowing through the whole lot of 'em, the voltage across any individual cell is going to be proportional to it's resistance.

The worst battery in the group winds up getting too many volts across it, and thus the potential for explosion and fire.

If they are all in parallel, the charger controls the voltage across the whole lot of 'em. If one of the batteries had an internal short, it could still cause a big problem, but I think that is not the typical failure mode for these batteries. Also, the switching mechanism of which I speak so vaguely, could just as easily have an individual current limiting resistor or even a fuse in series with each cell during charging.

jp