One thing that I can’t see mentioned anywhere in anyone else’s post’s on this topic… Is the “Usable” Capacity difference between Pb/Acid and lithium cells…
If you discharge a lead acid battery totally it is very bad for the battery’s health, and means you cant fully utilize all of a lead acid battery’s current/power.
As quoted in this article below… Lithium batteries give a usable current/potential/power of closer to 80 to 90% of their potential capacity!!!!
Usually with lithium cells they need their own battery management circuit to manage charging and discharging rates of each cell. Also as previously mentioned… Discharge rates are very very important. If you exceed the designed discharge rate of a lithium cell you will probably end up with a fire, or at least overheating problems. Its all in the design… and for the purpose of PSUB’s…. how hard would it be to modify existing off the shelf cells with water cooling? Closed loop of course… with a heat exchanger to a heatsink
Here is a real life Electric vehicle Lead acid VS Lithium battery comparison! J It is for electric cars… but it is defiantly applicable to your application.
http://www.diyelectriccar.com/blogs/2008/05/lithium-vs-lead-the-great-cost-debate.html
The first step is to source a lithium pack with 10kWh of usable energy. The Lithium batteries I have chosen get 2000 cycles to 80% DoD and I estimate that at the 1 hr rate they will deliver 95% of their rated energy due to the Peukert’s effect. So to give our total energy we multiply our usable energy by 1.35 meaning we need 13.5kWh to get out 40 mile range with lithium batteries. If we assume a 120V conversion this means we need 112.5Ah. The cheapest Lithium batteries I have seen in Australia are Thundersky LiFePo4 LFP prismatic cells $2.50 per 3.2V per Ah. So our 13.5kW pack would take 38 3.2V cells at ~110Ah and would cost $10450 at normal prices (no group discount), without shipping or BMS.
For a Lead Acid pack we also need to keep the batteries at less than 80% DoD and at the 1hr rate we can only expect to get 55% of the rated energy of the pack back due to the Peukert’s effect. That means we need to multiply the usable energy by 2.25 to get our total energy, in this case its 22.5kWh. Trojan T605 batteries could make up a 22.5kWh pack with 18 batteries (108V, 210Ah) at $225 each or $4050, it didn’t say how many cycles it would take on the website but let’s guess around 650 to 80%. That means we’ll need to replace our lead Acid pack around three times for every lithium pack we buy, meaning our total cost for the lead packs goes up by a factor of 3 to $12150 over 2000 cycles.
Now I must admit that I didn’t look very hard for the cheapest batteries and I am only guessing the cycle life of the Trojans but even if it’s not precisely accurate it does show that the myth that Lead is clearly cheaper than lithium is not well founded. Lithium’s greater efficiency and cycle life makes up for its higher initial cost. I didn’t know what the results would be like before I started. The10kWh number was chosen just to make it easy to calculate, it has little influence on the result one way or the other. I thought the results would be close but not this close. Please note that the Lithium pack would require a BMS, which would cost $1270 but that still means you are going to pay around $12k whether you go with Lithium or Lead. It would also be worth mentioning that you would be paying for more electricity over that time with Lead; 36MWh costing $3600 compared to 21.6MWh $2160 @ $0.10 per kWh and recharging 80% of capacity. You would also need to water the batteries if you went with the Trojans, while the lithium’s would be maintenance free. It’s also worth mentioning that there are apparently disputed copyright issues with the thundersky cells and their previous record with customer support apparently not good, but this was just a cost comparison and the number look pretty convincing.
The total cost per km for the 2000 cycles is $15 750/ 130 000km = 12.1c per Km (19.7c/mile) for Lead Acid and $13888/ 130 000km = 10.7c per km (17.4c/mile) for the LiFePO4. These would obviously increase slightly when factoring in tire and brake wear. Just for comparison a the cost per km of a bunch of small ICE cars are listed here the cheapest being 41.44c/km but only 33.5% of that cost was for fuel and servicing (the rest being common cost for EVs as well) so that’s 13.9c/km for the cheapest ICE using fuel at $1.25/L. Clearly then EV have a price advantage over ICE’s especially now that normal unleaded is averaging $1.48/L.
In summary, while Lead Acid may be cheaper up front new lithium packs are more cost effective in the long run as well as being lighter, smaller and maintenance free. Obviously battery choices are highly dependant on individual conversions and budgets but it should not be assumed that Lead is the budget option, since it’s just not true anymore.
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Firebolt
Sent: Friday, 1 October 2010 3:34 PM
To: Psusbs
Subject: [PSUBS-MAILIST] Batteries Bank
Hello, this is a short question, 250 kw electric motor running for 8 hours what type/many batteries do I need? ex. ampere Hour, voltage