Hi Michael,
Well, it's not as dramatic of a difference as you have stated, but you're on the right track.
First of all, lithium batteries should never be drained to 0%, using my Thundersky LiFePO4 prismatic cells as an example, a single event of dropping the cell voltage to 2V can ruin that 3.2V cell forever. This intolerance to complete discharge is one of the drawbacks to lithium cells in EVs and electric boats. That is also one of the reasons that many people run Battery Managemnet Systems (BMS) that will prevent over and under-voltage situations by disconnecting the batteries before they are damaged.
So for our purposes, the following dicharge levels are what most of the members here agree provide an acceptable balance of usable capacity and battery life.
Flooded batteries (FLA) can be regularly discharged to 60% depth of dicharge (DoD).
AGM batteries are good to 70% DoD.
Lithium cells are good to 80% DoD.
Like James said, you also need to consider how fast you drain your battery. We all know that a battery that will deliver 200Ah at 10A (20 hour rating) will deliver less at higher discharge rates. You can see that in the manufacturer's spec sheets. Using the Trojan SCS225 12V FLA battery's specs as an example, it's 20 hour rating is is 216Ah, at 25A it's down to 225 minutes or 94Ah and at 75A the battery capacity to 100% discharge is 57 minutes or less than 72Ah.
http://www.trojanbattery.com/Products/SCS22512V.aspx
My first post here about different battey types and Peukert's Effect was back in Nov 2009 (post # 12716), but I've refined the explanation each time it comes up. Here's the latest version "Choosing a battery type" from Jan 26 , 2012 (post #20750)
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Choosing a battery type for your electric boat conversion can be difficult. Here's some information that is generally accepted on this board. The product specs from battery vendors don't tell the whole story.
FLA (flooded lead acid) batteries are typically run to about a 60% DoD (depth of discharge). Some people go deeper, but many consider this depth to be a good compromise between energy delivered and maintaining a long life, measured in charge cycles.
AGM (absorbed glass mat) batteries can be discharged to the 70-80% range. I generally consider a 70% discharge for AGM systems. Both FLA and AGM weigh about the same for the same rated capacity.
Lithium Batteries (LiFePO4) can be discharged even deeper, for range estimates I use 80% DoD to keep a small safety reserve. LiFePO4 batteries are about 50% the weight of lead/acid batteries for the same rated capacity.
Then there is a trait called the Peukert Effect, this describes a battery's decreased ability to deliver energy at higher amp loads. I won't go into the math here, but that loss is predictable through Peukert's equations. Brace yourselves; it's going to get technical for a while.
In fact, the concept was known for many years before Peukert was able to
quantify the relationship between the load and capacity lost. Here's a link to a page that explains the Peukert Effect in greater detail:
http://www.smartgauge.co.uk/peukert2.html
The content can be a little difficult to work through, but it is correct. There are additional pages that provide validation of their version of the formula, where the results can be verified against the published specs of almost any battery. If you don't want to work through all of the math in the middle of the page, the second to the last paragraph refers to how different battery types have different Peukert's Exponenets and what that can mean.
If you can accept that different batteries can have different Peukert Exponents depending on how they are made, then the following link shows graphically how a different Peukert Exponent affects the available capacity of different batteries. Look about halfway down the page, in the section labeled "The Peukert Effect":
http://www.vonwentzel.net/Battery/00.Glossary/
Finally, while few manufacturers publish their specific Peukert Exponents, some do and some other numbers have been published for common batteries. Peukert Exponents are typically calculated through empirically measured capacity at different loads. Here are a few examples (values closer to 1.0 are better):
Trojan T-105 = 1.25
US Battery 2200 = 1.20
Optima 750S = 1.109
Thundersky LiFePO4 = 1.03
So what does this mean in the real world? Let's pick a battery bank size of 10kWh (208Ah @ 48V) and a constant load of 2500W (52A @ 48V). That will drive most of our boats to somewhere between 4 and 5kts, your results will vary.
The simple math would indicate that there are 4 hours of capacity at that discharge rate in the bank to 100% DoD, so FLA batteries like T-105s would have 60% of 4 hours or 2.4 hours of usable range. Likewise AGM would be 2.8 hours and Lithium would have 3.2 hours of usable range. But this is where Peukert's effect raises its ugly head.
FLA with PE of 1.25 – 10kWh to 60% DoD at 2500W = 1.60 hours (that's a loss of 33% to Peukert's Effect)
AGM with PE of 1.1 – 10kWh to 70% DoD at 2500W = 2.38 hours (15% lost to
Peukert's Effect)
LiFePO4 with PE 0f 1.03 – 10kWh to 80% DoD at 2500W = 3.05 hours (less than 5% loss)
You can see that AGM have almost 50% more usable range than T-105s for this size battery bank at this load. Reducing the load (slowing down) reduces the effect and speeding up makes it worse. Even though quality AGM are about twice the cost of T-105s, the extra usable range makes them a very good alternative. Add in the much lower self discharge rate so that constant maintenance charging is not required and they look even better. People also recognize that AGMs seem to last longer in the real world and now the AGMs are cheaper in the long run. If weight is a concern, then LiFePO4 is the way to go. The high price per Ah can be intimidating, but you can reduce the Ah and achieve the same range, saving money and even more weight. Let's do the math for a battery pack that has a 2.5 hour range at 2500W.
FLA – 14.25kWh to 60% DoD at 2500W = 2.5 hours = 633 lbs, cost about $1500
AGM – 10.5kWh to 70% DoD at 2500W = 2.5 hours = 520 lbs, cost about $2500
LiFePO4 – 8.25kWh to 80% DoD at 2500W = 2.5 hours = 200 lbs, cost about $3500
So now the lithiums are only 2.3 times the cost of T-105s but they are less than 1/3 the weight for the same range at this load.
So for most boaters, I recommend AGMs as a good balance of price to range.
So the bottom line is: it's your money, your boat and you know how you use it. You get to figure out your priorities and pick the storage system that best fits your particular needs and constraints.
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So Michael, you can see in the last table that 8.25kWh of LiFePO4 batteries delivers the same usable capacity as 14.25 kWh of regular flooded batteries at a 2500W load.
Notice that I'm not talking in Amps, Ah or Voltages any more. This math works for any voltage, 12V, 24V, 36V, 48V, 144V, it doesn't matter. Even I was surprised to find that out. Since the boats in this group are a bunch of different voltages, it is easier to compare them in Watts because Watts are a more consistant measure.
Fair winds,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion Marine drive, 8 kWh Lithium batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> it's a question.... if one has a 400ah battery bank of lead acid batteries and one can only pull about 40% from them then one is only getting 160ah of actual work right.... wouldn't the equivalent batteries of a lithium type only need to be 160ah to get that same amount of useful work because lithium batteries can be completely discharged?
>
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