Dumb question, from Ed....

[ex925]

Hi, guys, can I get away with being intellectually lazy?
Could someone give me a "rule of thumb" guesstimate of this:
What is the likely bulk of a 72v. dc Lithium battery-set totalling 7.5kwh?

I need a starting-point for what and how many batteries I should buy early next year for my TWEV
My ambition with this bike is to compete in the new "Formula 75" class in TTXGP in 2011
This class has a limit of 7.5kwh.......

Any advice will be gratefully received
All the best
Ed

[ex925]

In case it helps, here is a piccie, (I hope)

[ex925]

it's presently set-up for 6 in number, 65a/h Optima YTs... (yes six! "Lead-Sled" or what!)
There is one empty YT in one of the battery-positions for scale
The battery-holder is detachable by 4 bolts (my first attempt at a "Cordless-bike")
I hope to fit 7.5kwh of Thundersky or similar in a better configuration, say lower overall
The lithium battery-set would be built up into a detachable frame designed to fit the same fixing as the YT-set frame
I hope to use lead for short or drag races, perhaps, and maybe lithium for longer events, or road-range?
Looking forward to replies
All the best
Ed

[mattcarr]

I think your best bet would be go get the biggest size battery that you could. Potentially 23 of these
http://www.thunder-sky.com/pdf/20081122102551.pdf
to give you your 72 volts and around 7.5kwh or storage.

They are 3.2kg each, so a total weight of around 74kg for just the battery pack. I would have thought size would not be too much of a problem - I think about the same volume of the optimas your currently designing for.

[ex925]

Very grateful, gives me a set of figures to aim for
(including the fact that I need to find some more dosh than I had set aside so far.....)
Many thanks
Ed

[Jeremy]

Pouch cells will give you the best energy per unit volume (and probably the best per unit weight, too). A typical 20C rated LiPo pouch cell will give you about 150Wh per kg assembled into a pack, so around 50kg for the 7.5kWh pack you're after. Volume wise, you need to allow space for terminations, but I reckon your 7.5kWh pack will take up around 21 litres of space, scaling up from my experience with these pouch cells.

LiPo pouch cells come in a variety of different sizes, so can be packed into an awkward space more readily than the larger format cells. The downside is the need to build housings for them.

On a small scale, this is a 370Wh, 2.4kg pack with a 20C max discharge rate that is driving my folding bike at the moment:



The bike has a range of over 20 miles with this pack and will pull wheelies at full throttle.................

Jeremy

[ex925]

Very grateful, and a little out of my depth, please forgive simpleton questions that follow
1 I have a Li-Ping 20a/h ("LiPo"?) EAPC pack, very satisfied with it, is that an example of a "Pouch-pack"?
2 Li-Ping does not yet produce "big" (higher amp-rate capable?) packs, such as 7.5kwh in one pack
3 Could you recommend a source of pouch-packs who could:
3.1 Tolerate my lack of knowledge
3.2 Sell me a 7.5kwh, 72v.dc set of (LiPO?)pouch-packs with:
3.2.1 A complete BMS, including:
3.2.1.1 Charger
3.2.1.2 Battery-protection auto cut-out with rapid, safe, re-set
3.2.1.3 A reliable long-term out-of-service maintenance charge
3.2.2 Internal-strength and terminal-sizes capable of up to (500-amp?) draw
3.3 Meet any reasonable warranty-claims
3.4 Provide up to 3 years / 500 cycles pack-life tolerant of intermittent use
3.5 Good Mid-Spring 2011 price
Hope I am not pushing my luck in some of this, but, as a pensioner, money is scarce and precious
Also, my natural instinct is to seek maximum value for money, sometimes losing sight of the main objective.............
"Get it done NOW, (remaining) life is SHORT"
Many thanks for your advice
Really looking forward to your response
All the best
Ed

[Jeremy]

The Ping packs are pouch cells, but they are low discharge rate LiFePO4, rather than LiPo, so have a lower energy density. They are fine for low current use (I've had a Ping pack for a couple of years now) but not much use for a higher peak current vehicle.

Most of the items on your wish list remain impossible or very expensive to achieve. The only way to get an affordable LiPo pack suitable for something like your project is to build it your self and face the inevitable lack of warranty etc that this entails.

The cheapest way to get a lot of power from a light weight, low volume, battery system is undoubtedly to use RC type LiPo packs connected together. These things are outstandingly good at delivering lots of power with very little voltage sag from very small packages. They can only be purchased at a decent price from places like Hobby King in China though, which makes any warranty near on impossible to take advantage of.

When it comes to battery management, there are off-the-shelf balancing chargers that do the lions share of the job, again the best prices are from Hobby King (who are very reputable, BTW). The low voltage moitoring can be done in a variety of ways, but there is a chap on the Endless Sphere forum who has had some cheap 6 channel LVC modules made up. These can be used in multiples for packs that are bigger than 6S.

Your peak current requirement is trivial - my humble 370Wh pack will deliver 200 amps peak. You won't even come close to stressing a bigger capacity pack with a mere 500 amps.

The market just isn't big enough to support any good suppliers of ready-built, high capacity battery packs, so pretty much everyone has to do it themselves, I'm afraid. It's not hard to put a pack together, as all the parts are fairly readily available. The learning curve can be a bit steep though, plus the cost is still quite high.

Jeremy

[ex925]

It's always a sign for caution if I think I have got a grip on most aspects of a new concept, but......
I think I have only one more question, presently, (and that because of my lack of knowledge in depth)
If I assemble a pouch-set battery-pack:
Are the pouch connection-posts and connections between plates within each big pouch enough for 600amps?
(600amps would give me a sufficient margin for development, I think.....)
(if a 7.5kwh pack of any kind will give that much?)
Many thanks for your time
Ed

[Jeremy]

To give you an idea of the interconnect current handling, the 5Ah cells I'm using have 100A capable connectors and cable that's probably good for maybe 50 to 70A continuous. When paralleled up, the connectors all current share, so for your 100Ah nominal pack you would be looking at 20 off, 100A capable connectors working in parallel, so not really a problem.

You would need to terminate all these paralleled 10g wires into something more suitable for connection to the controller, but that should be easy.

The connectors for both the pack power leads and the smaller balance leads, are cheap and readily available, too.

Bear in mind that your battery pack is going to be made up of a lot of small plug-in packs - my 370Wh pack is made from 4 off 5Ah, 5S, packs. In other words, I have a pair of parallel connected 5Ah sub-packs connected to another similar pair, giving me a total of 20 cells, connected as 10 paralleled pairs.

Jeremy