One thing to keep in mind in doing energy density comparisons is to note the battery configuration and whether or not the comparisons are at the same capacity scale or not.
Example: Each of my Enerdel-made THINK battery modules configured as 2P(12S2P) has 31-48v @ 70ah for nom. 3kwh of energy at a weight of about 30kg, or 100wh/kg. While that may seem low, the lithium cells themselves are rated at 147wh/kg. Why the huge difference? Well, the battery module includes 2 BMS circuit cards, a host of nickel-plated interconnects and a large steel shell around the enclosed cell stacks---plus plastic lids. So if a manufacturer indicates "cell energy density" of 200wh/kg it doesn't mean much at the module level, especially if they're using tiny cylindrical cells that then get strapped to other cells in parallel, then all stacked in series and then packaged after. The more cells strapped together per KWH, the more interconnect mass there may be as compared to another module that uses larger cells and fewer interconnects. Each of the Enerdel Moxie cells in my pack is rated at about 50wh each, and is about 10"x7"x ¼".
Anyway, it seems to me that with boats, we are more interested in better volumetric energy density than we are mass energy density. Cylindrical cells as Tesla uses may not be the best way to go for volumetric energy density---they leave a lot of space unused ( Ncells * Lcell * (1-pi/4) Dcell^2 ). Still, compared to the energy density of lead acid (volumetric or mass), it's pretty dang good… J
-MT
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of 66b6dcd5b59507e7d751ea81382ea1f6
Sent: Monday, June 26, 2017 5:03 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Battery specific energy and power in relation to volume and density
Hi Hannu,
A little confused by your 2nd para but if I've understood you then the 'old' Tesla cell in a package form is around 200Wh/Kg. The new variant is impressive indeed. And as you say the Alta pack is good, if not the best as they claim. Mind you we haven't looked at volume and on a motorcycle, along with weight, that is very important.
Forgive me for forgetting about efficiency and heat. I guess, at the powers I use, on my boat it is negligible.
Like you I keep an eye on battery tech and density but of how much relevance it is to small sailing boats is debatable, unless you have a small race boat or like me very restricted space with weight needing to be in just the right place.
On the above, I checked my own Wh/Kg for my LiFePO4s. 61Wh/Kg and whilst somewhat better than lead the prime reason is safety and cycle life and the effective lack of peukert for a small pack. Recently though we have (Victron Energy) released a high energy Lithum NMC which is 175Wh/Kg. Of course this whole chemistry thing and safety thing vs C rates and cycle life is a balance for boats. These radar charts I find very useful:
http://batteryuniversity.com/learn/article/types_of_lithium_ion
Having said all this I have to say in principal I think for small sailboats up to around 30ft and 10,000 lbs unless space/weight is critical then the Northstar TPPC AGMs are excellent, just as James Lambden at Electroprop recommends. One thing I think folk neglect though when comparing such batteries with Lithium chemistries is just how much useable energy is available vs lifetime costs, when loads are taken into account.
For instance say such a sailboat typically requires around 1kW to go at 3.5 to 4.0 knots in calm conditions, if we take the cycle life of 2,000 cycles to 80% DOD for LFP and 50% for TPPC, also giving 2,000 cycles, then once we have considered peukert we find that an Ah of around 210 (20hr rate for the lead) for each battery type, requires twice the number of TPPC batteries compared to LFP. In other words for my 24V system that is 2 x 210Ah LFPs (actually 200s) whereas the TPPC boat would need 4 x 210 Ah batteries to go the same distance at the same power which I calculate is about 15 miles range at 3.5 to 4 knots in calm conditions using 1kW of power for both packs. That of course impacts overall system costs to the extent that LFPs will then likely only cost about 2 to 2.5 times that of the lead pack.
Having said all that I'm a realist and Lithiums have capital risk and higher initial cost and to be frank the cycle life of mine, at my age and health, will probably outlast me for a weekend summer sailing cruising boat. Indeed it begs the question whether most using LFPs will hit calendar life limits long before cycle life ones.
My message for all this is to minimise batteries for electric small sailboats and use a generator for higher energy demands and sail whenever possible!
John R
Posted by: "Myles Twete" <matwete@comcast.net>
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