Battery solution for electric sailboats

Battery solution for electric sailboats

There are many battery solutions for electric sailboats available on the market and therefore it is hard to make a choice. In this blog post we will describe technical details of batteries that will help the reader in making a decision.

In an earlier post we explained the battery solution for electric sailboats we prefer and why.  We listed  the technical criteria, summarized below, but most of all we wanted lithium iron batteries for a good price! However criteria are important for a battery solution for electric sailboats and we going to look into them one by one.

• Charge and discharge rate, or C-rate,  (C2 = the batteries rated capacity is provided in 30min.)
• Price
• Weight
• Stable, not explosive
• Long life time
• Memory effect (how many cycles before the battery looses power capacity)
• Self discharge  (what is the power after 3 weeks no sailing)

Charge and discharge rate, or C-rate

In describing batteries, discharge current is expressed as a C-rate, which is a measure of the rate at which a battery is discharged relative to its maximum capacity.  A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. A C/2 rate for this battery would be 50 Amps. First I didn’t understand the difference between C2 and C/2, however now I understand it: C*2 and C/2 (divided by 2).

LiFePO4 batteries have a continues maximum discharge rate of 2, (C2), while lead-acid batteries have a much lower C-rate as we will explain below. 

Lead-acid batteries

Rated capacity of lead-acid batteries is typically indicated as C/20, (discharge in 20 hours).  At a C1 discharge rate the rated capacity drops to 65%, see the table below from Victron.

Lead-acid batteries are sensitive to the depth of discharge. The battery life cycle reduces significant when the batteries are discharged to 80% see table below from Victron.

What does this mean? We can conclude that the rated capacity of lead-acid battery must be reduced significantly. An AGM lead-acid battery of 220amp and 12Volt has only 65% of its capacity at C1. Rated capacity is 220*12 = 2,6kWh , 65% of that rated capacity is 1.7kWh. The maximum depth discharge is 80% resulting in 1.3kWh.  Please note that at this discharge rate the lifetime of the battery is reduced to less than 500 cycles.

Battery price for electric sailboats

Prices of Lithium ion batteries have fallen considerable in the last three years. The price per kilowatt hour (kWh) in the automotive industry has fallen to 150 euro per kWh. That means that your average 10kWh battery solution for electric sailboats would be around 1500 euro. Consumer prices are a factor 4 to 5 higher. The good news is that we found a European supplier offering lithium ion batteries for 350 euro per kWh.  We would argue that lithium ion batteries reach the same price level as the much heavier and bulky lead acid batteries. Batteries have a significant effect on the total cost of the electric propulsion system, therefore it is good to take your time and compare prices.

Weight of batteries

Although weight may not be an issue at many sail boats, I know very few sailors that prefer heavier battery solutions above a lighter solution. Lead-acid batteries weigh roughly 4 times as much as lithium-ion batteries and take 6 times as much space on your sail boat. Barcelona Sail is very clear about this and selected a lightweight and small battery package. Our 5.2kWh package weighs 55 kilo, less than one 220amp 12volt lead-acid battery (65kg).

Long life time and slow memory effect

LiFePO4 batteries have a longer cycle life than most other lithium-ion batteries. The expected lifetime of the LiFePO4 batteries for sailboats is 7 to 10 years.  The battery life depends on the depth of discharge, in other words, how much juice you take from the battery. LiFePO4 batteries can be completely discharged (100%) but are rated for  2000 cycles at 80% discharge. More than most sailors are sailing in their entire life!

Environment

LiFePO4 batteries are 100% recyclable and non-toxic.