T-Pain Is On A Boat: Re: [Electric Boats] LiFePO4 battery experience?

Monday, November 2, 2009

Re: [Electric Boats] LiFePO4 battery experience?

Hi Eric,

I've got a pack of 16 x 200 ah ThunderSky LFP cells - the initial impetus for their purchase was the weight factor. I have a light trimaran and its very sensitive to weight so I wanted to keep that down as much as possible. Using the Thunderskys and a Torqeedo 4kw outboard I was able to limit the weight to the same as the 25hp yamaha outboard and fuel tanks that were removed.

But since getting them I've learnt more about their other properties which offer advantages over AGMs.

My recharge will come entirely from renewable energy - mainly solar - and so I need to make the best use of every watt-hour I extract from the sun. Because the LFP have a almost zero loss when charging - that is to put back 100 ah of usable energy into the pack the charging system needs to put in just 100 ah - I require less solar panels to get the same amount of energy into the pack as I would with AGMs.

With AGMs there is a loss when charging - known as Charge Efficiency Factor in the setting for battery monitors like Link-10 and TBS - and I recall this is around 10%. So just considering that factor alone, I have a 10% larger capture of available energy.

Another energy loss factor to consider with AGMs is the peukert effect. You know, when you draw power at high amps, the capacity of the batteries is effectively reduced, and so maybe there' another 10 to 20% loss of energy here.

And thats not all... the other thing I notice is the voltage doesn't sag so much under load, so to get the same power from the motor it takes less amps at this higher voltage - maybe only a few volts in 50, so probably about 5% I guess.

But putting all these together I get the picture that the amount of energy I can get back out of AGMs is way less than what I put in. But with the LFP batteries its nearly break even.

I got my batteries from Rod Dilkes of EV Power here in Australia and he runs an electric car and has done an interesting comparison between LFPs and lead acid batteries. see http://www.evpower.com.au/-TECHNICAL-ARTICLES-.html Over a long period he has kept records of the amount of energy needed to charge the pack, and the mileage that the car travels. See the article for all the details but the bottom line is that lead acids have a lot of losses which are not present with LFPs. Here are the results of the test:

LITHIUM ION - 301.98 Whr/mile

LEAD ACID - 421.18 Whr/mile

You can see that you need 30% more energy for the same mileage when using lead acids. This means either a bigger genset or more solar panels, both of which mean extra cost. So to make a fair comparison between the cost of LFPs and lead acids we should also consider these additional costs for energy. For those who recharge largely from a dockside power the difference probably has less impact because you probably get electricity "for free" when paying for a marina berth. But for those of us who plan to recover the energy on board it is significant.

The only downside with LFPs for me is that because it is new technology I have had learn as I go about the unexpected properties of LFPs. Such as the need to cycle them a few times when new to achieve full amphour capacity. And putting together charging/management system needed some cobbling together of various parts and building an understanding as I go.

And thinking about the importance of efficiency for the total package, the selection of motor and prop is also critical in creating good system. I changed from using an Epod to a Torqeedo and in doing so doubled the efficiency of the drive train. With the Epod I was a guinea pig experimenting with types of prop and trouble-shooting a poorly engineered motor. On the other hand the Torqeedo is well engineered and in my tests is twice as efficient as the Epod. That is, half the amps for the same boat speed.

Considering the overall package, and comparing say a system using lead acids and an Epod, with a system using LFPs and a Torqeedo, there is a 260% improvement in energy efficiency.

Right now Current Sunshine is on the hard and my actual use of these batteries has been limited and there is still some fine tuning to do related to the charging system. But I hope to be back in the water soon and I'll post again when I have more news about this.

Cheers

Chris

1986 Crowther 43 ft Trimaran

http://currentsunshine.com

On 02/11/2009, at 3:55 PM, Eric wrote:

Along these lines, has anyone in this group installed a large (greater than 6Kwh) LiFePO4 battery bank in their boat. I'm on the verge of ordering a set of batteries for my 30' Ketch conversion and believe that the weight and size of the Thunder Sky batteries compared to AGM alternatives is worth the extra cost. I'd really like to hear from someone with firsthand experience with this technology in our specific type of application.

Thanks for your help...

Eric
1964 Bermuda 30 Ketch - Serenity
SoCal