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Author Topic: Do lithium batteries make sense for off-grid?  (Read 11498 times)
Eleanor
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« Reply #15 on: February 27, 2010, 12:05:01 AM »

I don't think there's any doubt that battery technology is going to move on in leaps and bounds over the next few years. When we bought our new battery bank recently we did consider other technologies but there wasn't really anything we felt we could depend upon more than the standard FLA. As our experience of battery living (only 20 months so far) grows I think we'll probably trial other battery types on a small scale before we invest again. Hopefully there will be a competitive range of chargers and accessories to complement the new developments  stir
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"Very few batteries die a natural death ... most are murdered" stir
KenB
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« Reply #16 on: February 27, 2010, 08:33:31 AM »

Wookey, List

On a similar theme, ex-aviation NiCad cells were available for a number of years.

These were about 10" tall, 4" wide and 2" deep - rated at 40Ah.

The could be reconditioned by refilling with potassium hydroxide solution.  We used them in the early 1990s on an EV project.

NiCad cells of high capacity were also used for emergency lighting units.


Outtasight - we live in a world where everything ends up scrapped regardless of its condition. I have found some real gems at my local scrap metal dealers.  I bought brand new 22mm copper tubing - still in original packaging for 4 a kilo. That's about 1/3rd of the plumbers merchant's price.  I also got a 400 litre thermal store for 150.

Following the 2000 "dot com boom" there was a huge increase in the amount of UPS units installed in server rooms.  These are all at "end of life" and can be picked up from breakers for their scrap steel price.  Some good true sine wave inverters available for next to nothing.

Following my California Odyssey I have learned to Re-use, Repair, Recycle. 

Learn how to make use of these otherwise scrapped systems.  Learn how to mig-weld  and create things from metal.  Useful skills needed to re-purpose and repair of everyday scrap. There are those who will continue their hapless way in the disposable society - and those of us who will adapt our ways and face up to the limitations imposed by finite resources on a finite planet.

The only thing preventing this common sense movement is the nanny state and the HSE, however with inevitible government spending cuts - we can only prey for a decimation of head count in some of these counter-productive agencies.


Ken

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splyn
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« Reply #17 on: March 09, 2010, 03:11:52 AM »

I recently came across this site which bulk buys components for electric vehicles:

http://www.evcomponents.com

They collect orders for batteries and have them shipped directly from China to the end customer. They sell Thundersky and Esky lithium batteries up to 3.3v/800Ah for $1.1 per Ah which seems to be an excellent price. I don't suppose shipping will be cheap though.

I wouldn't put too much faith in the claimed cycle life figures of 5000 cycles at 70% DOD though - google 'thundersky failures' or similar to find several cases of premature failures with less than satisfactory back-up from the manufacturers. The battery design is supposed to have changed though so current ones may well be up to spec, but I doubt that many customers have had these batteries long enough to have got anywhere near 5000 cycles yet

If they do last as long as claimed though, then they are potentially cheaper than lead acid - $1.1/AH, 3.3V, 5K cycles at 70% DOD works out at approx 6.3p/KWh. (Ouch! And that doesn't include the cost of the electricity to charge them!) A Rolls 2v 1766Ah 5000 series lead acid at 580 with a claimed life of 3200 cycles at 50% DOD works out to 10.3p/KWh. Ok you can get much cheaper batteries but the claimed cycle lifes are much lower. The much simpler maintenance regime for lithium batteries has to be worth something too.

Also don't forget the rather better charge efficiency of the lithium batteries - I can't recall the exact figures but I believe that you get should get back > 90% of the power used to charge the battery whereas lead acid are rather worse (70 to 80%?), especially if you fully charge them. I believe the efficiency of the bulk charge phase isn't too bad but the last part is very poor as the battery warms up/gases etc.

...Ok. A quick search found this paper: http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf

This says that charging a lead acid battery from zero to 84% SOC (state of charge) is 91% efficient *BUT* the incremental charging efficiency from 79% to 84% SOC is only 55%! This has serious implications as it means that almost half of the power to be stored is lost whenever you are discharging  the batteries less than 20% before recharging. You could operate much more efficiently by using a regime which charges the batteries to no more than 80% full charge but then you run the risk of sulphation and/or over discharging the batteries when you have a long run of dark or windless days. Tricky business battery management - not that I've actually had to do it myself though...
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wookey
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« Reply #18 on: March 09, 2010, 04:31:11 PM »

I'm pretty sure evcomponents get a pile delivered to them from china, and then they ship them to customers (i.e in the normal way). Their shipping outside the US is pretty expensive. It was much cheaper for me to buy direct from the chinese supplier, even for quite a small order than to buy cells via them. Thundersky and Sky Energy are good cells, but there are cheaper ones with lower C-rates. For static applications low C-rates is fine. Ping battery (via ebay) is probably the best of the cheap suppliers. And there is also hipower (taiwanese  think) and Foxx Energy (US-based, but selling chinese cells - I just haven't worked out which ones yet). Ping batteries may not go quite large enough for static applications, whereas all the other supply large prismatic cells which make more sense for static applciations.

The 'thundersky failures' were nearly all with early product about 3 years ago. Check dates on postings on that subject. Recent cells have been good by all accounts, but yes I don;t think anyone has had them long enough to confirm total lifetimes. But again static use is much more gentle than EV use. Lifetimes are likely to be longer. It seems that the way to keep LiFePO4 happiest is to not go near either full charge or full discharge any more than necessary. (slow) degradation of the cell occurs at both ends of the charge range. So ignore the thundersky datasheet max voltage of 4.20 for example, and use 3.60 or therabouts as max charge for longer life. The difference in capacity is less than 5%.

splyn is right about charging efficiency. I've only just started taking measurements but for the same commute charging my lithium cells needs 790Wh at the wall, 714Wh into the batts for 630Wh delivered to the motor. That's 88% efficient charging (and a 90% efficient charger). With lead-acids I was using about 1kWh from the wall for ~630Wh to the motor (charger efficiency not measured). That's 20% better, although some of that may be due to charger differences.

So far as I can tell all chemistries have reduced efficiency as the last part of the charge goes in. It's is certainly true of NiMh and SLA. Lithiums have the advantage that keeping them below 80% charged is good for them in terms of longevity so that could keep efficiency up throught the system (at the expense of reduced capcity per ). I guess people are going to have to work through these considerations for the new tech and work out where the optimum price/longevity/regimen points are.
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Wookey
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