Systems+Engines

The Price of Power Page 2

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The ‘Throughput’ Cost

Let’s put all these numbers together, assuming a 100 amp-hour, 12-volt battery.

The lead-acid battery can be cycled through 30 percent of its capacity 1,000 times (this is being generous). At each cycle it delivers 30 amp-hours x 12 volts = 360 watt-hours (Wh). Over 1,000 cycles this is 360,000 Wh, so the lifetime energy throughput is 360 kWh. Looking at a range of battery prices, we find the amortized cost per kWh throughput is between $0.50 and $0.90. This is a “storage” cost over and above the generating cost and the efficiency loss during each cycle.

The TPPL battery gets cycled through 60 percent of its capacity for the same number of cycles (this is probably an underestimation) to deliver 720 kWh. Up front, it will cost up to 50 percent more than a quality conventional battery, but over its lifetime, it will be substantially cheaper (The amortized throughput cost will be less than $0.50 per kWh, based on current pricing) and at all times it will perform better. In particular, its high charge-acceptance rate allows much larger charging devices to be used, which in turn enables any fossil-fueled engine to be more fully loaded which drops the amortized engine cost.

The lithium-ion battery gets cycled through 80 percent of its capacity for 2,000 cycles to deliver 1,920 kWh. It can cost over five times as much as the lead-acid battery and still break even in terms of its lifetime kWh throughput cost. It perfoms much better throughout its life in terms of its charge-acceptance rate, energy-delivery rate, charge/discharge efficiency, and voltage stability. This dramatically reduces the cost of operating the battery and also substantially improves the performance of the DC system in which it is installed.

Arbitrary Assumptions

Clearly, I have made many arbitrary assumptions here. For any given application a fair amount of research would be needed to derive accurate numbers. Nevertheless, in spite of their higher price as compared to conventional lead-acid batteries, the TPPL batteries come out looking good, and the amazing conclusion is that in some circumstances even a $5,000 lithium-ion battery may make sense!

For those who don’t want to spend this kind of money on batteries, but must repeatedly charge them at anchor, aninvestment in solar panels and wind generators can be cost effective. Finally, a small portable generator on deck powering a powerful battery charger is a lot cheaper per kWh of energy produced than running the main engine or a permanently installed generator at anchor. But I usually recommend against this unless particular care is taken to marinize the electrical connection and eliminate the risk of carbon-monoxide poisoning from the exhaust.


Nigel Calder has written many technical articles and books, among them Nigel Calder’s Cruising Handbook, published by International Marine.

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