Hi Carter,
It's been a while since we covered the topic of Peukert's Effect in detail, with the number of new members, it might be time to delve into it again.
The battery choice can be difficult. Here's some general information that is generally accepted on this board. The specs are not always what they seem.
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 but he quantified the math and gave it a name. Here's a link to a page that explains the Peukert Effect is 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 battery bank size of 10kWh 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 charge 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 = 633lbs, cost about $1500
AGM – 10.5kWh to 70% DoD at 2500W = 2.5 hours = 520lbs, cost about $2500
LiFePO4 – 8.25kWh to 80% DoD at 2500W = 2.5 hours = 200lbs, cost about $3500
So now the lithiums are 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.
That's it for now. Some of you may look at this info and come to completely different conclusions and there are certainly other factors to consider. I know that this is a personal choice for everyone, but I wanted to give you food for thought before committing to any single technology.
Fair winds,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion Marine drive, 8Kwh TS LiFePO4 batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Carter Quillen <twowheelinguy@...> wrote:
>
> Myles,
> Â
> 6V Trojan 105REs or a comparative golf cart battery is the lowest cost per amp hour storage with the best overall performance I can find in the southeast continental US. 8 of them in series would give you an outstanding 48V battery pack. As long as you don't draw them down more than 50% they'll keep on going and going and going too. Rumor also has it that VLA batteries really like running with photovoltaics and that sulpherlidificaton(sp) is significantly decreases when you combine them with PV(Solar). Any chemistry majors out there that want to correct my spelling, please do. I just read that lead acid batteries perform way better if you have some PV in your system. I'm not even sure this is actually true, I just read it on the internet but it does seem to make sense from an electro-chemical analysis standpoint.
> Â
> Granted, 8 golf cart batteries is a lot of lead to put somewhere but if you figure the average 30' sailboat needs a couple thousand pounds of ballast, find a place below your boats center of gravity and strap them in. If you want to, or need to, spend a fortune to lighten and condense your batteries then Lithium Ion is for you. The manufacturer's performance specs on some of these batteries are awesome and being able to draw them down to nothing and back as often as you like is nice too.  It's the future and the more you buy, the faster prices will come down because lithium is almost as naturally abundant as lead.  They just need to get the supply chain developed and a lead acid battery will become like a video cassette, it will still play a movie but it's sooo inconvienient. Figure another 10-20 years or so for that though, depending on oil supplies and how long it takes the mainstream auto industry to get on the EV bandwagon.
> Â
> Sadly though, for now anyway, vented lead acid still appears to be the best value if you have a place to put them. If anyone can prove me wrong, please tell me where I can buy and don't forget to figure shipping and handling to SW Florida in your economic calcs.
> Â
> Viva the Electric Propulsion revolution!!!
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