Rob Beckers
9th May 2015, 07:47
Unless you have been living on a deserted island (without Internet), you will have heard about Tesla's new PowerWall Lithium-ion battery pack for the masses. I am in absolute awe, not about the battery, but about the flawless marketing campaign! :amazed:
I'm a bit jealous too, if only I could do the same for my own business...
On the technical side there is just enough information released to be tantalizing, too little to actually figure out if this is of any practical use for off-grid life, or grid-tied battery backup, or as a living room wall ornament!
What we do know is that there are to be two versions; a 7 kWh pack, and a 10 kWh pack, with the latter going for $3,500 (US dollars). This price does not include an inverter, something that will be needed in both grid-tie and off-grid scenarios.
At $350/kWh it is very cheap in comparison to other Li-ion packs, but how does it stack up to old-fashioned deep-cycle flooded lead-acid batteries, the current main-stay of the industry? We are selling Rolls/Surrette S-550's for $285 (US dollars, to compare apples-to-apples), and each stores just about 2.5 kWh in energy (20-hour discharge rate). That makes a cost of $114/kWh, or to phrase it differently, Tesla's Li-ion battery clocks in at right around 3x the cost of lead-acid.
Those in the know will just about now point out that you can discharge Li-ion batteries much deeper than lead-acid, so a 10 kWh Li-ion battery will replace a much larger lead-acid battery bank. There seems to be a bit of confusion surrounding this, not in the least my own, and meanwhile I see colleague in my business put in these ridiculously undersized itty-bitty Li-ion battery banks for off-grid systems (at multiple times the cost of lead-acid, but, it's Lithium-Ion!).
Best I have been able to determine Li-ion batteries of the Lithium-Iron-Phosphate variety (the only Li-ion type that's suitable for deep-cycle longevity and stability) do not like to sit at above 80% state-of-charge (SOC), it cuts into their longevity. They cannot be discharged below 20% SOC either, that causes irreversible changes and a very short battery life. In effect, it looks like there is 60% of the battery capacity to play with long-term, occasionally 80%, but not too often if you want longevity.
Lead-acid deep-cycle batteries should not be discharged below 50% SOC in day-to-day use to get good longevity, with an occasional 80% being allowable. Not too different from Li-ion in my view.
News articles have been floating around the Internet that say Tesla's 10 kWh battery is not the right chemistry for day-to-day cycling (http://www.bloomberg.com/news/articles/2015-05-06/tesla-s-new-battery-doesn-t-work-that-well-with-solar), which would mean that it's only good for grid-tie battery backup. It would be nice if Tesla would release a few more technical details about these batteries, such as cycle life, calendar life, battery chemistry, the battery management system (BMS), and their view on longevity.
I wish too that I could get a little more clarity about Li-ion batteries in general. It is very difficult to piece together a coherent view from the information on the Internet. This is what I have so far (and feel free to correct me):
Sitting above 80% SOC cuts into longevity, battery should not float at this level
Discharge below 20% SOC is near-instant-death for Li-ion
3000 - 8000 cycles to 30% SOC (comparison: a Rolls/Surrette S-550 will do 1300 cycles to 50% SOC)
Absolutely must have battery-management-system (BMS) to get longevity
BMS should include equalize functionality for longevity (though many don't)
Calendar life unclear; I've seen anything from 6 years to 20+ years mentioned
Seeing the shortfalls of lead-acid batteries on a daily basis I desperately want "a better battery", and Tesla's new Li-ion battery pack is a step in the right direction. Prices of Li-ion are finally coming down! Many questions remain though, and I would like a little more clarity before going Li-ion whole-hog.
Tesla did get one thing absolutely right: Those battery packs really do look cool! Question is if it will be more than a wall ornament...
-RoB-
I'm a bit jealous too, if only I could do the same for my own business...
On the technical side there is just enough information released to be tantalizing, too little to actually figure out if this is of any practical use for off-grid life, or grid-tied battery backup, or as a living room wall ornament!
What we do know is that there are to be two versions; a 7 kWh pack, and a 10 kWh pack, with the latter going for $3,500 (US dollars). This price does not include an inverter, something that will be needed in both grid-tie and off-grid scenarios.
At $350/kWh it is very cheap in comparison to other Li-ion packs, but how does it stack up to old-fashioned deep-cycle flooded lead-acid batteries, the current main-stay of the industry? We are selling Rolls/Surrette S-550's for $285 (US dollars, to compare apples-to-apples), and each stores just about 2.5 kWh in energy (20-hour discharge rate). That makes a cost of $114/kWh, or to phrase it differently, Tesla's Li-ion battery clocks in at right around 3x the cost of lead-acid.
Those in the know will just about now point out that you can discharge Li-ion batteries much deeper than lead-acid, so a 10 kWh Li-ion battery will replace a much larger lead-acid battery bank. There seems to be a bit of confusion surrounding this, not in the least my own, and meanwhile I see colleague in my business put in these ridiculously undersized itty-bitty Li-ion battery banks for off-grid systems (at multiple times the cost of lead-acid, but, it's Lithium-Ion!).
Best I have been able to determine Li-ion batteries of the Lithium-Iron-Phosphate variety (the only Li-ion type that's suitable for deep-cycle longevity and stability) do not like to sit at above 80% state-of-charge (SOC), it cuts into their longevity. They cannot be discharged below 20% SOC either, that causes irreversible changes and a very short battery life. In effect, it looks like there is 60% of the battery capacity to play with long-term, occasionally 80%, but not too often if you want longevity.
Lead-acid deep-cycle batteries should not be discharged below 50% SOC in day-to-day use to get good longevity, with an occasional 80% being allowable. Not too different from Li-ion in my view.
News articles have been floating around the Internet that say Tesla's 10 kWh battery is not the right chemistry for day-to-day cycling (http://www.bloomberg.com/news/articles/2015-05-06/tesla-s-new-battery-doesn-t-work-that-well-with-solar), which would mean that it's only good for grid-tie battery backup. It would be nice if Tesla would release a few more technical details about these batteries, such as cycle life, calendar life, battery chemistry, the battery management system (BMS), and their view on longevity.
I wish too that I could get a little more clarity about Li-ion batteries in general. It is very difficult to piece together a coherent view from the information on the Internet. This is what I have so far (and feel free to correct me):
Sitting above 80% SOC cuts into longevity, battery should not float at this level
Discharge below 20% SOC is near-instant-death for Li-ion
3000 - 8000 cycles to 30% SOC (comparison: a Rolls/Surrette S-550 will do 1300 cycles to 50% SOC)
Absolutely must have battery-management-system (BMS) to get longevity
BMS should include equalize functionality for longevity (though many don't)
Calendar life unclear; I've seen anything from 6 years to 20+ years mentioned
Seeing the shortfalls of lead-acid batteries on a daily basis I desperately want "a better battery", and Tesla's new Li-ion battery pack is a step in the right direction. Prices of Li-ion are finally coming down! Many questions remain though, and I would like a little more clarity before going Li-ion whole-hog.
Tesla did get one thing absolutely right: Those battery packs really do look cool! Question is if it will be more than a wall ornament...
-RoB-