Tesla’s Powerwall on a boat?

Clark September 17th, 2015

This summer Tesla unveiled its Powerwall, a battery large enough to power an average home with a solar system, and give this home independence from the grid. Elon Musk’s announcement was met with giddy excitement, and the batteries are already sold out for the foreseeable future.

I wonder how long before a Powerwall finds its way onto a boat? Tick, tick, tick.

Crunching the numbers, it may not make economic sense yet, but the price may come down in a few years. The Powerwall, the 7 kWh version, sells for $3000. The slightly larger 10 kWh Powerpack sells for $3500. If we compare these to a size 8D battery (generally the largest size, and common on boats), here’s how it stacks up.

8D battery:

An 8D holds about 3 kWh (kilowatt hours). You can buy an 8D battery for as little as $230, but for our purposes we’ll compare a top quality AGM 8D battery, say from Lifeline or Trojan, which sells for $650-$700. So in pure kilowatt hour terms, the Tesla battery costs about 40% more. You’d need three 8D batteries (a common arrangement, and what I’ve got on my boat) at about $2100 to supply to same number of kilowatt hours as the Tesla Powerpack at $3500.

But wait! With the lead-acid batteries we normally follow the 50% rule, meaning we only use 50% of the battery’s capacity. Tesla doesn’t expressly say this, but since the Powerwall/Powerpack is a lithium-ion battery I’m guessing it can cycle through it’s entire capacity without damage. This alone might make up for the difference in price. Also, lithium-ion batteries can take a charge must faster than lead-acid batteries. The Tesla also comes with a 10-year warranty, and I don’t know of too many marine batteries that see ten years.

The Tesla is WAY cheaper than other quality lithium-ion batteries of similar capacities.

The voltage on the Tesla batteries is stated as 350-450 Volts DC (huh?) so there would have to be some kind of DC to DC step-down converter. I can’t find much information on these, and they don’t seem to be common, but we can assume this will get expensive…and be one more device aboard that can fail.

Another advantage of the Tesla is that it’s lighter at 220 pounds. A single 8D weighs about 160 pounds, so the Tesla would be about half the weight for the same capacity. The Tesla is a big, flat battery at 51″H x 34″W x 7″ deep, so it might lend itself well to fitting under a bunk or mounting in the back of a locker. It’s meant to mount on a wall (duh) and it’s all sexy-looking, so maybe it could just mount in plain sight on a bulkhead.

It’s a deep cycle battery, so I have no idea how it would do for starting loads. Might have to have a starting battery too, which would provide some redundancy.

At the moment I’m going to say it’s a bit premature, especially with regard to stepping down the voltage, but the Tesla batteries are a VERY interesting prospect for powering a boat.

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Comment by Ron Porter
2015-09-18 11:07:25

In electric car use, lithium ion discharge/charge/nominal capacity ratios rarely get close to 1. Battery management for maximum life typically charges to something less than true 100% of nominal capacity and it’s recommended that you never fully discharge.

I’m not sure what typical values are, but I think of lead-acid as using the ‘top’ of the battery and lithium ion as using the ‘middle’.

Comment by Clark
2015-09-23 14:20:42

Hi Ron, Someone else told me that because of the huge step-down converter needed, the Tesla battery wouldn’t really be practical for boats yet. Also, apparently it’s water cooled, and probably wouldn’t be happy on its back or bouncing around.

Comment by Jason
2016-12-14 14:29:39

I’ve done it.

The “sheets” inside a model s pack are ~24v, which is the nominal voltage if my boat’s equipment. I have 4, stacked in parallel for a total of 21.2 kilowatts, or ~800 usable Ah at 24v.

These batteries, even when charged / discharged at over 150A don’t get hot enough to warrant the water cooling.

Comment by John
2017-08-24 12:36:50

JASON, I read your ” I’ve done it” comment with interest. Do you have any compatible stats? Are you using the system in a launch or yacht? You’ve had it a while now. Any reliability comments. Have you considered a powerwall? If so what advantages disadvantages did you find? I’ve a yacht and I’m thinking about comparing weight as a key factor too.

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