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Old 04-02-2019, 05:42 PM   #1
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Supercapacitors for starting & thrusters

I've been reading about super capacitors used for starting (cranking) heavy equipment, like diesel truck and tractor engines and commercial generators. They do not "store" power like conventional batteries but are most effective at delivering high energy for short bursts. They are light weight, relatively petite compared to 4-D or group 31 type marine batteries, and can be fully recharged in minutes when connected to a standard battery. So the thought occurred to me that a super capacitor might find good uses in marine applications.

For example, in my Selene, I have two 12V G31 batteries combined to run the Side-Power stern thruster, plus separate starting batteries for the main engine and the generator. Those batteries, boxes and heavy power cables take up a lot of valuable floor space in the engine room. I'm thinking that a single 24V super capacitor measuring 16"x3" and weighing less than 12 lbs would deliver more than enough amps (900 CCA) to power the thruster; and that a single 12V super capacitor could crank over the 5.9 Cummins main and the 8kW generator. Super capacitors are being produced and sold by Maxwell (in the form of a Group 31 battery) and by Polar Power. Is anyone on the forum using a super capacitor for engine starting? What do folks think about this concept?
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Old 04-02-2019, 06:19 PM   #2
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You have to think about failure modes. A battery which might be considered medium energy density has some interesting failure modes. There is another post about that here recently. A super capacitor that can be charged to a high energy state in a short period probably also has some interesting effects if that energy is inadvertently released quickly.

Unless you can gain some benefit from using a super capacitor (maybe starting a big engine with a small battery), then what is the benefit? I'm guessing that most boats use more batteries for house power than for starting engines. Is a super capacitor useful for house loads?

In the case of a thruster I'd rather have a deep power reserve than the "sprint" mode that a super capacitor is likely to provide (never know when a docking is going to go wrong and don't need the thruster to run out of juice in the middle of things).
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Old 04-02-2019, 06:41 PM   #3
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Neither company, Maxwell and particularly Polar Power, gives much information on their products which is always troubling.

In a perfect world it only takes 2-3 seconds to start your diesel engine. Yes, lots of amps- 300-500, but a a super capacitor can supply those for a short period. How long, we don't know- 3 seconds, 10 seconds maybe.

After that the capacitor is dead and must be recharged with- guess what, a battery. How many 10 second cranks do you want? That 10 AH Li ion battery that comes as part of Maxwell's Group 31 form factor starting battery won't do that very often and it takes 15 minutes to recharge.

And don't forget, the amps for the first start must come from the battery as well. Super capacitors can't hold a charge for hours much less days. So you need a super capacitor and a battery. Are you really saving anything?

The situation is even worse for bow thrusters. They draw about half of the amps as a diesel starting battery, but they are also used for much longer- routinely 10-20 seconds to swing your bow against a stiff wind. You need a battery to charge the capacitor up for the first use and if that runs down the capacitor you sit for what, 15 minutes while the battery recharges the capacitor for another shot.

These are this issues facing the use of super capacitors for starting and bow thruster use on boats. These are never discussed in the manufacturer's literature.

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Old 04-02-2019, 10:08 PM   #4
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Reminds me of a diesel engine that had an air starter. Had about a 50gal tank. Guy called me up as it would not start. Did my troubleshooting and figured pump was air loaded. That air tank could only crank for like five seconds, not enough time for me to chase the air out in one or two goes. And took forever for tank to recharge. Took me all afternoon to get the thing running.

I'd figure the capacitors would be similar. All ok for a normal start, but clearing air from fuel, PITA.

If they do not publish actual amp-hour or kWh energy content, then whatever the numbers are, they are not proud of them. And those numbers matter when spec'ing out equipment.
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Old 04-03-2019, 06:51 AM   #5
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Quote:
Originally Posted by Ski in NC View Post
If they do not publish actual amp-hour or kWh energy content, then whatever the numbers are, they are not proud of them. And those numbers matter when spec'ing out equipment.


Maxwell does give cranking amp data- 1,000+ as I recall, but only for 5 seconds, unlike conventional Group 31 batteries which can deliver 1,000 amps for 30 seconds. After 5 seconds if it hasn't started the included battery, Li presumably has to recharge the capacitor just like your air tank and it takes 15 minutes to do so.

The whole Group 31 package of capacitor and batteries weighs just 20 lbs which tells me that the battery can't be very big, maybe 20AHs.


So if all goes well, 5 seconds is enough. But in cold weather, worn rings, etc it will take longer, then you have to wait 15 minutes and go through the whole cycle again.



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Old 04-03-2019, 08:42 AM   #6
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Quote:
Originally Posted by Ski in NC View Post
Reminds me of a diesel engine that had an air starter. Had about a 50gal tank. Guy called me up as it would not start. Did my troubleshooting and figured pump was air loaded. That air tank could only crank for like five seconds, not enough time for me to chase the air out in one or two goes. And took forever for tank to recharge. Took me all afternoon to get the thing running.

I'd figure the capacitors would be similar. All ok for a normal start, but clearing air from fuel, PITA.
Other than air starts on diesel subs, the only other one was an old one lung diesel, on his boat, and he used a SCUBA for his air supply.
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