Cutoff Voltage

[mustangman]

 Ooops, I did not think of that scenario. Maximumizing profit ratio on new technology.

[myelectricbike]

There is a site somewhere I've run into in the past couple of days that starts with meta something or another that has brief commentary on stuff like subway musicians go to Korea or overlimit credit card charges that were followed by 3 to 10 followup offers for more credit cards.

Recently in fact my power company began sending out billing notices that instead of taking you to the "View your bill" customer login page took you to the home page where a "Pay your bill" was the only link. It took you to a page where you either could pay your bill online for an additional charge of $4.99 or to sign up for ebill which if you had already signed up for took you back to the home page. Why, I wondered?

Turns out that customers where unwilling to pay their bill without viewing it first so that by the time they finally got through to customer service or got a paper bill sent to them in the mail they had exceeded the deadline for payment and were given the option of having their power turned off until they paid the $35 turn on fee or to just pay the turn on fee and not suffer through having their power turned off.

Moral: The world is full of schemers. The ones you see in the tabloids are the ones who have gotten caught doing what the big utility companies and other corporations get away with doing all of the time. If someone is not scheming to find a way to maximize profit on battery technology then there minds are at work scheming to find a way to maximize profit on something else. Wal-Mart's scheme of low price and return anything including dead batteries and light bulbs without a receipt with grandmother types to greet you when you enter and to check your receipt when you leave plus handing your kid a smiley plus whatever else they do seems to be the best scheme yet.

[mn_engr]

ahend:
  I am using a compression spring from menards in each tube.  It is about 1" long, and I compress it about 1/2", so it has a relatively strong force. 

  My strategy was to keep everything as repairable as possible, that way if the monitoring circuitry tells you that a particular cell (or pair of cells in my case) is always going dead first, you can disassemble that tube, replace the cell, and even test the cell in question in the lab to determine why it discharges so early.

  I thought about other sizes, but I also presumed that AA cells, since they are so common, would give the lowest cost/W*Hr.  Plus, if I had to abandon project due to infeasibility, I could always use AA's,  I have few devices that use anything else.

  However, after 3 hours on the bench today, I've been having second thoughts about buying 10AHr D cells like these instead, which would decrease circuit complexity by a factor of four.

http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1878

  Four sets of these cost $184, plus you get 2 extras for backup.  That's only $40 more than my 120 AA's, and basically the same capacity.   As far as I know, this deal is still good if anybody wants to play with 30 T-energy D cells.

MyElecBike:
 As for the PCB between each cell, that was my original idea.  However, in the end I used steel wire through a small yet oversized drilled hole in the PVC.  When I drill the holes, I use a spacing jig.  When you compress the cells, you have to make sure you don't have any misallignment issues.


Everbody:
  BTW, as I develop this 'active monitoring' circuitry futher, if any are interested, I will share technology.  (a phrase from my playing Civilization days     If it works out, it would be a shame for me to spend all this time just to make a smart battery pack for myself...  You need to be handy, and it does take some perserverence...

[mustangman]

   Sounds Great, You could even build a few to sell to some of us who are not able to make it.(minus the batteries) 

[mn_engr]

don't mean to double post but here is some more info on batteries:

http://hardware.slashdot.org/article.pl?sid=07/09/06/0431237

One post here mentions the A123 battery and there is a IEEE Spectrum article on the company:

http://spectrum.ieee.org/sep07/5490

On page 3 they talk about calendar life for Lithium cells.  The article states some Li-Cobalt chemistries lose 20%/year in capacity, just sitting on the shelf.  (Not LiFePO4 chemistry, but the article leads you to think the effect is still there but perhaps reduced?)

I suspect this will be a big factor in acceptance of these cells, although as an engineer it will be difficult to access this risk when designing your product.

Besides not having a shelf life capacity reduction issue, NiMH and NiCd cells can be stored at partial/full discharge and not risk losing cycles.  This was one of the biggest reasons I want to make NiMH (or NiCd) work.

Since I live in MN, I expect I'll only be able to get 100 rides to work per year.  If I get 500 cycles out of my cells, my optimistic side tells me I can make these cells last 4 years, maybe more.  However, if there is a cobalt lithium effect going on, that would put a significant dent in that number.

I don't know what the best chemistry is out there for transit, but whoever gets it right will be famous.  Somedays I wish I was a ChemE rather than EE.  At work, I do a lot of testing of Energizer L91 primary AA's.  (LiFeS2 chemistry)  This battery is spectacular, to say the least but it's a primary.  I remember reading somewhere (can't find a reference) that this chemistry is also rechargable, but only at high temperatures (250C or something like that.)

[ahend]

30 T-Energy F Size 14000mAh NiMH cells from All-Battery are now $403.50

Just an FYI . . .
All-Battery has an eBay store where they sell quite a few items. In that store they are still listing 30 F sized 14Ah cells for $335. This is where I picked up mine. It is in contrast to their website so it might not be bad to check out if your in the market.

Andy

[mn_engr]

Andy,
  How many cycles do you have on your T-Energy pack?

[ahend]

How many cycles do you have on your T-Energy pack?

About 10, too early to give me any warm fuzzies about the decision. I was a bit worried about heat damage as I did some heavy duty soldering, both end to end, and tabbing with beefy battery bars.

Andy

[Dalecv]

Check out missbattery on ebay, they have some good prices on LiFePO4 batteries. They don't mention shelf life but do mention cycles and being good for 3 to 5 years.

[johnbear]

So has anyone been able to actually accomplish a real life working lower voltage cut off controller? Did I miss something?

Thanks! 

[OneEye]

The theory and resistor locations are all worked out, but I haven't heard of anyone actually implementing it.  Look through this thread for pdonahue's posts.  He cracked the code for lowering or raising the low voltage cutoff, and also found out what needs to be done to raise the operating voltage to 48V.

If anyone implemented this with good results please post your results.

-Mike

[pdonahue]

The theory and resistor locations are all worked out, but I haven't heard of anyone actually implementing it.  Look through this thread for pdonahue's posts.  He cracked the code for lowering or raising the low voltage cutoff, and also found out what needs to be done to raise the operating voltage to 48V.

If anyone implemented this with good results please post your results.

-Mike

I did implement it when I first went to 48V, but I put it back to the regular cutoff value so I could use my 36V battery as well when my 48 is drained.

[johnbear]

With my nickel batteries 42 volts seems to solve the problem. I had to add a 4 cell nimh pack in series with my 36 volts. Seems to work great.

[myelectricbike]

What is your range?

[johnbear]

I did the mod, now r73 is 10000 ohm and the cutout is 29.22. It works awesome, see attached pics!