series or parallel

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themotorman
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Joined: Sun Jan 17, 2010 10:15 pm

series or parallel

Postby themotorman » Tue Jan 19, 2010 5:09 am

I need 400 volts 20 Ahr total and can get 8 packs of the same size into my own designed ecar. I can either get 50 volt 20 Ahr packs and connect them all in series or get 100 volt 10 Ahr packs and have two strings that are connected in parallel.
Cost is the same as the total cell count is the same BUT with the 50 volt 20 hr packs they will have cells in parallel in the pack as the battery company cannot make the size pack I need using 20 Ahr cells, with the 100 volt packs I can use only 10 Ahr cells and will have no parallel cells. I'll have two strings each of 4 x 10 Ahr 100 v packs with 400 volts total that could run the system so a complete failure of one string would not bring me to a dead stop!
The reason for all of this is that my first set of 8 packs failed when the BMS failed. .. this lead to one group of cells in each pack becoming a dead short!
With LiFePO4 batteries at around $USD1.00 per Ahr it is too expensive to keep making mistakes like this.
I would welcome some comments on this topic and advice is always welcome.
Thankyou

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Jeremy
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Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Tue Jan 19, 2010 7:59 am

Generally speaking, it's best to parallel lithium chemistry cells at the cell level. This means connecting each pair of 10Ah cells together in parallel then connecting the sub packs in series. This has two benefits. Firstly, it means you only need a single BMS, rather than two. Secondly, the paralleled cells will tend to average out the variation in capacity that you get with lithium, which is one of the factors that can lead to cells getting out of balance.

I've got a couple of LiFePO4 packs built this way and they've been pretty reliable. My electric boat packs have 8 10Ah Headway LiFePO4 cells connected in parallel in each sub pack, with a home made high shunt current BMS monitoring each cell.

Jeremy

themotorman
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Joined: Sun Jan 17, 2010 10:15 pm

series or parallel and types of LiFePO4

Postby themotorman » Tue Jan 19, 2010 5:49 pm

thanks for comment. What is opinion on cylindrical cells or flat prismatic cells? I can use either depending on my enclosure shape and size. The prismatics look good but that's probably because they stack nicely but they are heavier due to the case and also have no natural compression to keep the plates together. There is also the screw connect or welded tab question, the battery suppliers seem to have diverging opinions on this, I even found a manufacturer who uses soldered connections which seem to me to be at the bottom of the list.. Lot's of choices and not much data.. any input from actual users of LiFePO4 's please.

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Jeremy
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Postby Jeremy » Tue Jan 19, 2010 6:29 pm

I can only comment on the cells I have and the use they've been put to, neither of which may be directly comparable to your circumstances.

I have a small prismatic LiFePO4 cell pack on my electric bike. This is doing really well, around 500 cycles or so with no sign of any loss in capacity. The cells are arranged as paralleled pairs of 5Ah pouch-type cells, with 12 sub-packs connected in series. There's a BMS that controls charge and discharge. The pack doesn't compress the cells at all and they don't seem to need it.

The boat pack is much heavier and uses old-style cylindrical Headway cells. I've had a mixed experience with this pack - out of 64 cells I've had 8 fail for no apparent reason, other than poor Chinese quality control. I understand that Headway have markedly improved these cells, though, and that the newer, screw end cap cells are much more reliable. So far, these cells have not been cycled more than a dozen times, but they have been stored properly (and regularly part-charged) for about a year.

Based on the experiences of others, I would be inclined to look closely at either the BMI cylindrical cells or the newly released 20Ah A123 prismatic cells. I know of lots of people who have had excellent results from the BMI cells, they seem to offer a good combination of value and quality. The A123 prismatics initially look to be very good, as we should expect from A123, and now that they are available for a pretty good price they might well be worth going for.

There is some useful data on the Endless Sphere forum on batteries, although it tends to be focussed on high performance ebikes and motorcycles.

Jeremy

themotorman
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Joined: Sun Jan 17, 2010 10:15 pm

Lifepo4 batteries follow up

Postby themotorman » Tue Jan 19, 2010 7:00 pm

thanks Jeremy, It is really great to get info from actual users. It seems that the BMS is absolutely critical to the Li chemistry and the QC in China is suspect if it even exists! I tried to get the A123 20 Ahr cells but cannot find them anywhere for sale. I am trying to build about 100 cars a year and so need a reliable source for both packs, BMS and Chargers. It seems the prices and QC is all over the place and hopefully will mature into a reliable few eventually. After all you can buy reliable Pb, NiMH and NCd's batteries now!
I am hoping that the Vanadium cells end up available in smaller sizes for EV use as they have an intrinsically long life. The cost /mile for just the LiFePO4 is really far to high especially when you consider the recycling of all these in a few years!
Have you read my Battery Exchange article?
http://members.cox.net/rdoctors/evs.html

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Jeremy
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Location: Salisbury

Postby Jeremy » Tue Jan 19, 2010 7:30 pm

If you're looking for a stable source of supply, then I think you're stuck with trying to deal directly with reputable manufacturers, which can be an interesting challenge with some Chinese vendors.

I've heard mixed reports about Thundersky and SkyEnergy cells, although I know that some have found them to be OK. Although the big A123 cells look good, and are undoubtedly of good quality, I understand that their current availability is due to them being "dumped" on the market.

Overall, I suspect that the BMI cells (a.k.a. "LifeBatt") might be one of the better long term supply options. The only problem for you might be dealing with the US vendor, who is, to put it mildly, a bit of an "interesting" character. I would think that dealing direct with BMI in Taiwan might be a much better option, as you'd undoubtedly get a better price and competent technical support. They have a range of BMS systems that are well-matched to their cells and seem to have a better approach to quality control and customer service than some of the Chinese vendors.

Jeremy


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