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Old 04-28-2018, 08:07 PM   #1
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24 volt bow thruster battery and charger upgrades

The boat has a 24 v bow thruster powered by 2 8D batteries that in turn are charged by a 24 v 30a charger. The batteries are shot ( probably 9 years old from what I can tell). Looking at options 4x 6v -3x 8 v - 2 x 12 v. What are the pros and cons? Is a deep cycle battery appropriate as the work load is more akin to starting an engine. Cheap and cheerful G31 or something else?
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Old 04-29-2018, 05:57 AM   #2
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If high cranking amps are the right medicine for electric thrusters, you might look at a pair of Odyssey PC-2150s (Group 31) AGMs. Major cranking amps compared to many other G31s. The AGM-hood-ness means little off-gassing if your thruster batteries are located nearer to living quarters.

Not inexpensive. OTOH, our oldest dual-purpose bank lasted for about 12 seasons.

-Chris
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Old 04-29-2018, 06:54 AM   #3
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Maerin has an 11 HP Sidepower bow thruster. It's powered by a series pair of Optima D34 Blue top batteries. They were installed when we got the boat, I replaced them in 2014. The install also included a tray/cover setup for the pair. The Optima's have held up very well, and I've never had any issue with capacity.

The batteries are charged with a Charles 30A smart charger, it's powered by 120V. There's no other means to charge them, so it requires a generator run or shore power, and I ran it for years that way. Our trip last summer included lots of locking and thruster use, so I powered the charger from the inverter, wanting to keep the thruster bank charged while under way. Acknowledged it's a bit of a kludge, but it's worked perfectly. I've not seen any difference in the house bank consumption. I probably could have come up with a 12V-24V charger, but didn't really want to spend the money for the gear and the wiring it would have entailed. I didn't see any advantage over powering the charger off the inverter.

The Optimas aren't really a deep cycle battery, but a great match for high amp short draw, just the ticket for thrusters. No where near the weight of a pair of 8D's. If I had it to do over, I would keep the system just as it is.
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Old 04-29-2018, 09:15 AM   #4
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Two Group. 31 either AGMs or FLAs, starting batteries will work fine. They can easily supply 500A. As you note, bow thrusters are more like starting- high load for a short period.

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Old 04-29-2018, 10:17 AM   #5
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Even not knowing your thurster's HP rating, 2 8D batteries sounds like overkill. Check your thruster's manufacturer's battery requirements. ranger42c's recommendation of Odyssey 2150 (group 31) batteries sounds about right to me. 24 volt motors are more efficient than 12 volt motors.
AGM batteries and AGM rated chargers are the way to go.

I have a Vetus 9512D, 8 HP Bow thruster powered by a 8D, 12 volt, 245 AH, AGM battery, charged by a 30 amp Charles charger. This works well for me. (battery life - 13 years and counting)
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Old 04-29-2018, 12:04 PM   #6
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The best battery value by far is Duracell (actually Deka/East Penn) FLA deep cycle golf cart batteries, 2x6V, around $400 for 4x 200+AH @24V from BatteriesPlus or Sam's Club.

As FLA, will be less fussy about their charge profile and last longer than even the pricey AGM.

Just need to periodically check the water level.
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Old 04-29-2018, 01:42 PM   #7
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I am also an Optima (spiral wound) fan...but the D34 is suboptimal for this application. The D stands for dual-purpose. The better model is the 34M. It is designed specifically for cranking applications. I have a bank of x2 and this starts my twin 370hp Cummins; powers my thruster; and supplies my davit. I should have put the windlass to it too. 9 years old and still strong.
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Old 04-29-2018, 07:16 PM   #8
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Thanks everyone for your input. Your suggestions have focused my thinking.
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Old 04-29-2018, 07:35 PM   #9
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I would not use deep cycle 6 volt batteries for a thruster. I would use a starting battery since a thruster is similar to a starter motor. I have a 12 volt AGM for my stern thruster, works great. I would series a pair of them for 24 volts.
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Old 04-29-2018, 08:24 PM   #10
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It all depends on the power required by your thruster. I have 2 8Ds as well for my 24V thruster. The only thing you can be sure of is that the builder would not put in 2 8D batteries for the thruster if he could get away with 2 group 31s.
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Old 04-29-2018, 09:06 PM   #11
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Quote:
Originally Posted by gsholz View Post
It all depends on the power required by your thruster. I have 2 8Ds as well for my 24V thruster. The only thing you can be sure of is that the builder would not put in 2 8D batteries for the thruster if he could get away with 2 group 31s.
Not sure how old the boat is, but the batteries probably didn’t come from the builder. If is is someone anal like me then a PO may have upsized the batteries. I usually end up going way overboard on everything I do on the boat. Just ask my wife...
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Old 04-29-2018, 09:19 PM   #12
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It is true that thrusters require high amps discharge rate, and if used very seldom and briefly, only discharge a very tiny AH amount like starter motors.

However batteries designed for deep cycling - if of sufficient AH capacity - have **zero** challenge with that load pattern, the thin plate design of Starter batts is only an advantage with undersized banks.

And sometimes, thrusters do get used for more than a second or two. If such usage is regular, then a deep cycle thicker plate design is required for bank longevity.
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Old 04-30-2018, 06:36 AM   #13
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Quote:
Originally Posted by john61ct View Post
It is true that thrusters require high amps discharge rate, and if used very seldom and briefly, only discharge a very tiny AH amount like starter motors.

However batteries designed for deep cycling - if of sufficient AH capacity - have **zero** challenge with that load pattern, the thin plate design of Starter batts is only an advantage with undersized banks.

And sometimes, thrusters do get used for more than a second or two. If such usage is regular, then a deep cycle thicker plate design is required for bank longevity.

Some examples, just using a couple (AGM) batteries I'm familiar with:
Odyssey PC-2150 (G31): HCA = 1545, MCA = 1370, CCA = 1150
Lifeline GPL-4CT (GC2): HCA = 1085, MCA = 925, CA = 750

I think I remember cranking amps for multiple batteries in a bank are calculated similarly to the way capacity (amp-hours) works. If so, cranking amps remain the same when 6Vs are series-paired into 12V or 24V or when two 12Vs are series-paired to 24V, and added when two 12Vs are parallel-paired, etc.

So a pair of PC-2150s at 24V would give HCA = 1545, MCA = 1370, CCA = 1150. (Or at 12V, a pair of these would give HCA = 3090, MCA = 2740, CCA = 2300.)

And four 4CTs at 24V would give HCA = 1085, MCA = 925, CA = 750

Is that math/concept correct?

If so, maybe it comes down to a "highest cranking amps versus fewest batteries (cost and weight)" comparison.

I remember discussing my current set-up with Lifeline ref whether the 4CTs would be sufficient for starting one of our diesels... minimum MCA 1560 and minimum CCA 1250 at 12V... and the answer was yes, as long as we had at least 4 of them in the bank...

I dunno how much use thrusters actually get (given we don't have 'em) and it probably varies widely by user anyway... but the difference in overall lifespan for a starting versus deep cycle bank may be 9 years versus 10 years (or whatever, for example, and using OP's 9 year estimate), and I wonder if one could really pick which of those might be longer.

-Chris
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Old 04-30-2018, 08:28 PM   #14
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For batts that do actually go below say 20-30%, the cycle lifetime would easily be 3-4x.

In any case I'm talking about $2 / AH at 24V, not exactly paying extra for that longer life, may even be cheaper.

Plus lots easier to carry when needed.

But NBD your boat your choice.
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Old 05-03-2018, 05:57 PM   #15
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Thanks Everyone. I have gone with a combination of your suggestions; 12 v starter batteries with a 12v to 24 v converter. I ended up with this change as the charger was damaged once I got into the job. The area looks a lot better for the overhaul and this summer should provide sufficient use to prove if my selection of your input was correct.
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