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Old 01-26-2018, 01:42 PM   #1
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Tesla Powerwall instead of lead-acid battery bank

Could anyone explain why someone with a large battery bank and occasional large loads (a/c, washer/dryer, water maker, fridge) couldn't replace it with a 24kwh Powerwall? I am assuming that it could be charged by the existing genset, shore power, or other source such as wind, solar, thermonuclear, etc. I realize that at $5500, it would be an expensive battery, but is there a technical reason it would not be better than lead-acid?

It would be nice to have a quiet air conditioned boat while we were sleeping at anchor, if we could recharge while underway. I also understand that these batteries do not need the lengthy float charge of lead-acid thus avoiding a long period of lightly loaded generator run. I am thinking of a scenario where one anchors out 4 or 5 nights a week at most and hooks up to 50a shore power the rest of the time.
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Old 01-26-2018, 01:48 PM   #2
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Could anyone explain why someone with a large battery bank and occasional large loads (a/c, washer/dryer, water maker, fridge) couldn't replace it with a 24kwh Powerwall? I am assuming that it could be charged by the existing genset, shore power, or other source such as wind, solar, thermonuclear, etc. I realize that at $5500, it would be an expensive battery, but is there a technical reason it would not be better than lead-acid?

It would be nice to have a quiet air conditioned boat while we were sleeping at anchor, if we could recharge while underway. I also understand that these batteries do not need the lengthy float charge of lead-acid thus avoiding a long period of lightly loaded generator run. I am thinking of a scenario where one anchors out 4 or 5 nights a week at most and hooks up to 50a shore power the rest of the time.
No reason at all, except cost. It's just a lithium battery bank, albeit very large. Lithium batteries are MUCH smaller volume per kWh (called power density) than traditional cells. They are also MUCH more expensive by that same parametric. There are plenty (now) of vendors for marine lithiums. You could build your own.

https://www.gonewiththewynns.com/tag/lithium-batteries

I'd be willing to bet a large number of the $10MM+ new-build yachts are now being specified with lithium house banks.
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Old 01-26-2018, 01:52 PM   #3
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How many amps would the Tesla 24 kwh Powerwall provide? I am used to thinking about house battery banks in terms of amp hour capacity and comparing that capacity to loads.
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Old 01-26-2018, 01:53 PM   #4
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How many amps would the Tesla 24 kwh Powerwall provide?
Depends on the voltage.

P (watts) = I * V

For example, 24kWh/12V = 2000Ah (ignoring losses, efficiency, etc.)
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Old 01-26-2018, 02:05 PM   #5
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How many amps would the Tesla 24 kwh Powerwall provide? I am used to thinking about house battery banks in terms of amp hour capacity and comparing that capacity to loads.
The Powerwall specs say it will put out 7kw @ 120v for 10s and 5.8kw/120v continuous, it will also charge at the same rate.........


https://www.tesla.com/sites/default/...rthamerica.pdf
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Old 01-26-2018, 02:44 PM   #6
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Other questions would be:
- Is the Tesla Powerwall ruggedized enough to work in a marine environment?
- Do you have the right sized space to install one (batteries clusters can be arranged for the available space)?
- What does it take to charge the pack (i.e. do you need 240v to charge it in a reasonable time)?
- Do you want to have all your eggs in one basket (multi batt clusters can be reconnected to omit a failed component)?
- Don't know what the internal working voltage of the Powerwall is, but it outputs 120VAC. Can you get 12VDC out of it?
- Can your boat support 300# in a single unit about 40x20 inches? Yes batteries are heavy, but multi-batt installations can distribute the batteries to avoid concentrated weights both for the structure loading and hull balance.
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Old 01-26-2018, 02:54 PM   #7
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Some models of the Tesla Powerwall have built in chargers and inverters. So instead of a three part system (battery charger, Inverter and battery bank) you would have one compact unit. It also allows the user to monitor charge rate, use rate and battery conditions over the network. The Gen 2 power wall doubled the power capacity in the same footprint as the Gen 1, and kept the same price.

There was one battery system that was a better deal (less money, higher power density) than the tesla, but not if you factored in the cost of the inverter and charger.
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Old 01-26-2018, 03:21 PM   #8
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Other questions would be:
- Is the Tesla Powerwall ruggedized enough to work in a marine environment?
- Do you have the right sized space to install one (batteries clusters can be arranged for the available space)?
- What does it take to charge the pack (i.e. do you need 240v to charge it in a reasonable time)?
- Do you want to have all your eggs in one basket (multi batt clusters can be reconnected to omit a failed component)?
- Don't know what the internal working voltage of the Powerwall is, but it outputs 120VAC. Can you get 12VDC out of it?
- Can your boat support 300# in a single unit about 40x20 inches? Yes batteries are heavy, but multi-batt installations can distribute the batteries to avoid concentrated weights both for the structure loading and hull balance.
I assume that it is sufficiently rugged, given that a garage installation in a beach house is as much a marine environment as a closet or battery room on a motor yacht. We are not talking about bolting it to the transom after all.

Charging rate is a function of generator capacity: bigger genset=faster charge. Converting 120vac to 12vdc is proven technology that uses existing components and 120vac output eliminates the need for an inverter. As far as the "eggs in one basket" idea, some people think having fewer failure points is also a good thing.

At 40x20 it could be mounted in many different places. Regarding supporting a 300# 40x20 load, one could have the mother-in-law stand on various parts of the boat and see if the structure fails or the boat lists prior to installation. Or you could just mount it where you removed the batteries.
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Old 01-26-2018, 03:23 PM   #9
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My opinion is "Not Yet." Let it get refined for various uses and any issues get worked out and then it might well become the way to go. We've been through the entire process of getting design and quote on our house. Just can't make sense of it yet.
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Old 01-26-2018, 03:30 PM   #10
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For sleeping at anchor with no generator running, you can do it with a bank of 6 golf cart batteries as long as the air conditioner isn't too big.

You want a small 5-7,000 btu unit that just cools your sleeping area to minimize power requirements. Dometic has a 6,000 btu marine unit that draws 4.6 amps plus 0.3 A for the raw water pump. Assuming a duty cycle of 50% that means 0.5*4.9*120*8 = 2,350 watt hours. If you were to take that from a FLA battery/inverter system it would require about 260 Amp hours from your battery bank. A bank of six golf cart batteries would cost about $700 and would stay below 50% discharged overnight. A low cost Xantrex 1,000 watt inverter would power it fine at about $400.

Why would you want to spend the money for a Powerwall system when you can do what you want for about $1,200 in major parts plus another $100 for wire, fuses, etc. If you want a bigger charger which would minimize your recharging time, then figure $1,000 for a 2,000 watt inverter/100A charger which brings it up to less than $2,000.

You do have to recharge that 260 Amp hours the next day. It will take 3-4 hours of genset running time and you probably want a 200 watt solar panel to top off your batteries for another $400 in major parts cost.

Those 6 GC batteries will be heavy and bulky- about 400 lbs total. But most 40' trawlers can easily fit that down in the engine room.

David
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Old 01-26-2018, 03:34 PM   #11
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I assume that it is sufficiently rugged, given that a garage installation in a beach house is as much a marine environment as a closet or battery room on a motor yacht. We are not talking about bolting it to the transom after all.
Most beach houses are not subjected to movement from wave action over long periods of time.

For beach houses that are subject to wave action, the duration is fairly short and the Powerwall will probably be damaged.
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Old 01-26-2018, 04:26 PM   #12
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............... It would be nice to have a quiet air conditioned boat while we were sleeping at anchor, if we could recharge while underway. ............... .
before you spend thousands of dollars on this "miracle battery", remember that whatever method or device you use to store energy, you're going to have to replenish it the next day.

Figure out the energy used to run your AC overnight, convert it to amps at 12 volts DC and calculate how many hours of engine running time it's going to tale to replace it.

You may find that to replace that energy in a realistic time you'll need a very high powered alternator. Possibly one that has an affect on your boat's speed.
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Old 01-26-2018, 04:40 PM   #13
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There are better, proven systems than Tesla. https://www.sonnenbatterie.de/en-us/sonnenbatterie

Not for a boat, unless itís a big boat.
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Old 01-26-2018, 10:36 PM   #14
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The Power Wall is awesome but you can ONLY recharge it with 220V shore power or 220 Gen Set.
I decided against it when I found out my hi amp 12V alternator would not recharge it or keep it topped off
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Old 01-27-2018, 12:48 AM   #15
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[QUOTE=
You want a small 5-7,000 btu unit that just cools your sleeping area to minimize power requirements. Dometic has a 6,000 btu marine unit that draws 4.6 amps plus 0.3 A for the raw water pump. Assuming a duty cycle of 50% that means 0.5*4.9*120*8 = 2,350 watt hours. If you were to take that from a FLA battery/inverter system it would require about 260 Amp hours from your battery bank. A bank of six golf cart batteries would cost about $700 and would stay below 50% discharged overnight. A low cost Xantrex 1,000 watt inverter would power it fine at about $400.
David[/QUOTE]

Good plan David, except I think using 6000 BTU to cool people and an entire stateroom, including overhead, ports, ceilings, drawers, drawers in the drawers, doors and floors, will result in a lot more than a 50% duty cycle.

The trick is to change from area cooling to personal cooling by installing a mosquito net (or 2), which you probably should already have if in the tropics. The smallest available AC directed only into the netted area should save a lot of watt hours.

And there is the cool blanket, a damp towel over your body with a fan blowing over it will FYA.
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Old 01-27-2018, 04:07 AM   #16
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For sleeping at anchor with no generator running, you can do it with a bank of 6 golf cart batteries as long as the air conditioner isn't too big.

You want a small 5-7,000 btu unit that just cools your sleeping area to minimize power requirements. Dometic has a 6,000 btu marine unit that draws 4.6 amps plus 0.3 A for the raw water pump. Assuming a duty cycle of 50% that means 0.5*4.9*120*8 = 2,350 watt hours. If you were to take that from a FLA battery/inverter system it would require about 260 Amp hours from your battery bank. A bank of six golf cart batteries would cost about $700 and would stay below 50% discharged overnight. A low cost Xantrex 1,000 watt inverter would power it fine at about $400.

Why would you want to spend the money for a Powerwall system when you can do what you want for about $1,200 in major parts plus another $100 for wire, fuses, etc. If you want a bigger charger which would minimize your recharging time, then figure $1,000 for a 2,000 watt inverter/100A charger which brings it up to less than $2,000.

You do have to recharge that 260 Amp hours the next day. It will take 3-4 hours of genset running time and you probably want a 200 watt solar panel to top off your batteries for another $400 in major parts cost.

Those 6 GC batteries will be heavy and bulky- about 400 lbs total. But most 40' trawlers can easily fit that down in the engine room.

David
David,

Your system would be pretty marginal for a 40ft boat and the cost for something that will work is MUCH higher. You don't get a quality battery for $125 each, and the wiring and install will probably approach $1000. I'm doing an install on a similar system right now (granted, a bigger inverter) but my costs are triple what you post. But I have a quality Magnum inverter and installing with a ABYC marine electrician. But that's what it costs.

Besides most folks have existing equipment and want to power that, not put in a whole new installation.

Lithium is not for my, but I can see the application when one needs more power from less space. FWIW, my electrician is installing one right now. Well get more specs and numbers from him, but it ain't cheap.
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Old 01-27-2018, 07:25 AM   #17
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I accept your point that the system I described above is marginal. Part of the reason I described it was to show how if you design the whole system for night time air conditioning, then it can be done using less than 300 Ah overnight and recharging in the morning with about 3 hours of genset time.

If you want to power a normal 16,000 btu A/C as well as some 120V appliances at night then you need a whole lot more battery storage, maybe a lithium battery system, and much longer genset running to put all of those KWs back.

If you use a Powerwall as the storage system then you will need at minimum a 7.5 KW generator which will be heavily loaded for an hour if the PW's batteries are fully discharged.

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Old 01-27-2018, 08:43 AM   #18
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In a recent discussion of battery storage a friend wondered if a battery pack from a electric car could be used on a boat. He thought even after its use in a car it would still have adaquate capacity.

I have no idea although a battery pack out of a wreck might be a better choice. Charging might still be an issue. Just don't know.

Rob

p.s. Bought 8 Trojan T-105's for $125.00 ea. this past spring.
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Old 01-27-2018, 10:04 AM   #19
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In a recent discussion of battery storage a friend wondered if a battery pack from a electric car could be used on a boat. He thought even after its use in a car it would still have adaquate capacity.
I don't doubt that some enterprising soul will do just that if it hasn't already been done. The problem will be the inverter. Auto battery packs produce DC voltage not AC as the Powerwall. Not sure what the voltage is; the internal DC voltage of the PW is 50V. You will need an inverter that accepts whatever voltage the batteries produce.

Magnum, Outback and probably others make 48V inverters and those should work with a 50V battery pack.

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Old 01-27-2018, 10:54 AM   #20
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For sleeping at anchor with no generator running, you can do it with a bank of 6 golf cart batteries as long as the air conditioner isn't too big.

You want a small 5-7,000 btu unit that just cools your sleeping area to minimize power requirements. Dometic has a 6,000 btu marine unit that draws 4.6 amps plus 0.3 A for the raw water pump. Assuming a duty cycle of 50% that means 0.5*4.9*120*8 = 2,350 watt hours. If you were to take that from a FLA battery/inverter system it would require about 260 Amp hours from your battery bank. A bank of six golf cart batteries would cost about $700 and would stay below 50% discharged overnight. A low cost Xantrex 1,000 watt inverter would power it fine at about $400.

Why would you want to spend the money for a Powerwall system when you can do what you want for about $1,200 in major parts plus another $100 for wire, fuses, etc. If you want a bigger charger which would minimize your recharging time, then figure $1,000 for a 2,000 watt inverter/100A charger which brings it up to less than $2,000.

You do have to recharge that 260 Amp hours the next day. It will take 3-4 hours of genset running time and you probably want a 200 watt solar panel to top off your batteries for another $400 in major parts cost.

Those 6 GC batteries will be heavy and bulky- about 400 lbs total. But most 40' trawlers can easily fit that down in the engine room.

David
What about the float charge, doesn't that last for several hours of light genset usage? In my experience that period is much shorter with lithium batteries.
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