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EMP now offers inherently safe lithium ion battery pack
July 24, 2010 |
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Ft. Myers Beach, FL. Electric Marine Propulsion now offers inherently safe lithium ion battery packs in a range of sizes for the E Motion plug-in hybrid system.
The batteries, from International Battery, Allentown, PA, are significantly smaller than lead-acid AGM batteries of comparable capacity, weigh less than half as much, can be discharged nearly twice as far and last three to four times as long.
"Lithium batteries are what electric vehicles - on land and sea - have been waiting for," says Dave Tether, EMP CEO. "They're going to transform the industry."
The International Battery (IB) cells we've selected are based on lithium-iron-phosphate chemistry. They are chemically unable to undergo the kind of catastrophic breakdown responsible for explosions in laptop and cell phone batteries.
"We've all heard those stories," Tether says, "and we don't want them at sea."
Conventional lithium oxide batteries are vulnerable to what's called thermal runaway. When overcharged, the oxide dissociates and releases oxygen, which can recombine explosively with the highly reactive lithium. But in lithium iron phosphate (LiFePO4), oxygen remains tightly bound within the phosphate ion (PO4), even when the battery is overcharged.
Cells into batteries
Lithium batteries are manufactured as individual 3.2 volt cells in various amp-hour capacities. EMP uses 160 amp-hr cells, although 60 amp-hr cells are also available. The cells are series-wired and boxed together to make a module, i.e., a battery, of the desired voltage, size or weight.
A battery management system (BMS) keeps cells balanced during charge and discharge and protects the complete battery pack from a cell failure. When balancing, the BMS compares average pack voltage with each individual cell and diverts current to or from the cell to bring it back to the average. |
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| Lithium cells can be packaged into modules to make batteries of a desired voltage, size or weight. Battery capacity is the same as cell capacity - 160 amp-hr in this case. Battery voltage is cell voltage (3.2 v) times the number of cells - eight in standard IB configurations (above and below) or nine in the configuration designed for EMP . |
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| Lithium ion cells are all 3.2 volts, a value inherent in the electrochemistry. EMP offers IB cells with 160 or 60 amp-hr capacity. |
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EMP lithium batteries are available in two voltage sizes: a nine-cell, 28.8 v module and an eight-cell, 25.6 v module. Five of the 28.8 v modules are connected in series to make a 144 v battery pack (or "string"). The 25.6 v modules are used to power the boat's 24 v system.
With 160 a-hr cells, the 144 v battery pack's total energy capacity is 23 kilowatt hours (144 v x 160 a-h). A 23 kwh string has sufficient energy capacity for an approximately 45' monohull with one E Motion motor.
Two strings in parallel make a 46 kwh battery pack - more than enough for a large, dual-motor catamaran packed with all the electrical amenities anyone could ask for.
With 60 amp-hour cells, the energy capacity is 8.6 kwh for a single string and 17.3 kwh for a dual string battery pack.
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| A large high-voltage pack designed for a hybrid bus. The BMS is visible as a balancing board at the end of each module, with wires going to individual cells. A BMS supervisor (not pictured) controls the entire pack. Thick metal straps (visible below) connect cells in series, while the slender wires conduct BMS signals. |
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For comparison, the nominal energy capacity of a 144 v pack of standard 4D lead-acid AGM batteries is 30 kwh. But a 23 kwh lithium pack actually has more usable capacity, because it can be discharged more deeply. And the 4D AGM pack weighs 1,620 lb compared to only 536 lb for the 23 kwh lithium pack. Even a dual-string, 46 kwh lithium pack at 1,072 lb weighs only two-thirds as much as a 4D pack with far less energy capacity.
On the other hand ...
So with all these benefits, what's not to like? In a word, cost. Large lithium batteries are extremely expensive. But any calculation of their true value in a boat must also include their advantages over the AGM alternative: |
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- Three to four times the life. This longevity factor must be divided into the purchase price to calculate the real long-term cost.
- More electricity with less weight. A high energy-capacity-to-weight ratio is critical in order for a boat to carry multiple electrical amenities and still sail fast.
- Faster charge acceptance. The onboard generator has to run less often and for shorter periods of time, which translates into longer life and less maintenance.
- Deeper discharge. The same nominal energy capacity provides two-thirds more usable kwh.
- Lower relative cost with a higher boat price. For large luxury yachts, the lithium battery price can be a comparatively small fraction of the total cost.
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"We're looking at a classic early-adopter scenario," Tether says. "People who can afford a high-priced technology establish a niche market. Production ramps up to fill it, the price comes down, more people buy it and prices keep dropping."
That's what happened with small lithium batteries for consumer electronics. As the market for I Pods, cell phones and laptops expanded, the price of lithium batteries plummeted - by 90% in 15 years, according to one study. Although the market for vehicle-size lithium batteries won't grow as large or as rapidly, it should continue to expand enough to bring the price down to more affordable levels.
"They won't make sense for a 30 foot Hunter anytime soon," Tether says. "But if you're considering a big boat with plenty of electricity-hungry gadgets aboard, you might want to take a look." |
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