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Old 06-26-2021, 01:27 PM   #1
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Using Deep Cycle as Start Batteries

A current thread is discussing mixing start and house batteries, but I don't think it is dealing with an issue that I've been wondering about. I understand that LA start batteries have more plate surface and can put out a lot of juice. House batteries (deep cycle) have thicker plates and are designed to put out juice over time. But it would seem that once the house battery bank reaches a certain size the deep cycles could put out the cold cranking amps without a problem.

My 80 hp Leyman still had the old abandoned battery box next to it. Based on the size, it appeared to hold two Group 24 batteries (maybe Group 27). Probably one start and one deep cycle. My prior owner put in 4 Dyno GC batteries (in two banks) for a total of 520ah. He said, and I think he was right, that there was no longer a need for a separate starter battery.

At some point a large deep cycle battery bank would have the same plate area as a lone starting battery. Thus, no need for a start battery, right?
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Old 06-26-2021, 01:35 PM   #2
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Yes, as long as the deep cycles are big enough they'll start an engine just fine.

However, I don't like starting an engine from the house bank for a couple of reasons. First, I consider starting power to be safety critical, so it should be its own source (and all engines should have a choice of 2 sources of starting power). Second, unless the bank is really big relative to the starter load and also sufficiently charged, the voltage can dip low enough during start to reboot electronics. In an emergency engine start such as a dragging anchor, waiting for the electronics to come back can be a problem.

I'd rather have a start battery or 2 depending on number of engines and generators. If there's only 1 start bank, then being able to start from house as a backup is good, but I wouldn't make starting from the house bank a normal SOP.
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Old 06-26-2021, 01:39 PM   #3
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Right!!!

True deep cycle batteries, like golf carts probably only have 50% more lead than starting batteries (they don't weigh that much more). So if a single Group 31 starting battery works to start your Lehman (and it should) or two Group 27s then a pair of golf carts should start it fine. A single golf cart battery is about the same weight as a G31 starting battery which probably has 50% more plate area. So two GCs should give you plenty of plate area and therefore starting current.

Maybe when trying to start a bigger engine like a Cummins 6C I would go with four golf cart batteries to start it.

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Old 06-27-2021, 12:26 AM   #4
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Quote:
Originally Posted by Marco Flamingo View Post
At some point a large deep cycle battery bank would have the same plate area as a lone starting battery. Thus, no need for a start battery, right?
Right. But most people are afraid of having a dead start battery. A dedicated start battery is cheap insurance against that and provides a layer of redundancy.

I had a combined house/start bank on my last boat. If you're treating the GC batteries right and not discharging to low SOC it's a sensible simple approach.
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Old 06-27-2021, 01:32 AM   #5
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Redundancy is a good thing.
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Old 06-27-2021, 06:11 AM   #6
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Its all about plate area, start batts have lots of surface area , which helps keep the voltage up while cranking the engine. Sadly that surface area gets bubbles rapidly so in a short time the batt seems dead. Usually a 10 min wait will let the bubbles depart , and you get to try another start.

Deep cycles work fine if the plate area from a bank matches the area of the start batt.

Many engine starters are fine with 10V while cranking , so even well discharged house batts will spin the engine fast enough to start a diesel, in warm weather.
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Old 06-27-2021, 08:07 AM   #7
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Many engine starters are fine with 10V while cranking , so even well discharged house batts will spin the engine fast enough to start a diesel, in warm weather.

Exactly. The concern, however, is that many electronics won't be happy below 10.5 volts or so and may reboot when you crank the engine.
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Old 06-27-2021, 08:58 AM   #8
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My port bank is port start and house. 32V.

I just pulled out 4 8D sized 8V batteries. They were 90 pounds each. I replaced them with 8 (yes) Rolls deep cycle 8V batteries. 4 in series, twice paralleled. The Rolls weigh 60 pounds each. Total lead went from 360 to 480 pounds.

The Dekas that came out were rated at 1000 or so cranking amps and 180 Ah reserve capacity. (Lets not get into the hours/amperage for the test etc etc) The Rolls actually had a CCA listed, marine 540A. And, the reserve capacity is also around 180Ah (20h). BUT..... I now have two banks in parallel so I more than double these numbers and end up with 360Ah reserve capacity and 1100A CCA.

All i exactly the same boxes..... Win Win.

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Old 06-27-2021, 10:07 AM   #9
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The big question is why would you use deep cycle batteries to regularly start your engine.

There is zero logical reason.

We have already determined that you need redundancy in your engine starting. That means having a dedicated engine starting battery and a house battery bank.

Why not use a starting battery for engine starting?

Your question iv valid, but if you consider the bigger picture the answer looses relevancy.
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Old 06-27-2021, 01:02 PM   #10
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Why not use a starting battery for engine starting?
Because it can be a redundant and unnecessary complication in some cases (like mine, apparently). My deep cycles are in two banks, so I switch to one bank when anchoring and have a fresh bank for starting my little Leyman (redundancy #1) or use both banks (redundancy #2). I also have a group 27 starting battery for my generator that can be jumped to start the Leyman if both banks were ever drained (redundancy #3). Or start the generator to charge the drained DC batteries (redundancy #4). It seems that a dedicated starter battery for the engine is conventional, but not required in all cases.
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Old 06-27-2021, 02:02 PM   #11
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In your case, I'd combine the house banks into a single large one and up-size the generator start battery a bit. Make that start battery the primary start source for the main and gen (it's no problem for them to share), and add switches to allow either to start from the house bank if needed.
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Old 06-27-2021, 02:50 PM   #12
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Quote:
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The big question is why would you use deep cycle batteries to regularly start your engine.
Well, in my case there are a lot more 8V golf cart batteries than 8V starting batteries. Secondly, I believe you do less damage starting with a properly sized deep cycle bank than slow discharging a start bank. Plus, you get less out of a start bank on the hook.....
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Old 06-27-2021, 03:17 PM   #13
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The big question is why would you use deep cycle batteries to regularly start your engine.

There is zero logical .
If you look at AGM they do both well and the FLA guidelines don't necessarilly apply. You will find both CCA and AH speced for many AGMs... not so common with FLA.
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Old 06-27-2021, 04:38 PM   #14
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I don't think anyone would suggest it's optimal. But in the OP's case where he's starting a 80 hp diesel with a 520 ah GC pack there aren't many cautions apart from avoiding deep discharge and maintaining the batteries. Starting use will do no harm in this case. I'd leave that pack as one rather than trying to partition it.

If you take that as the sufficient but not optimal solution what's the next upgrade? I think I'd replace the 4 batteries with AGM and carry on rather than add a dedicated start bank. That's what I ended up with on my last boat. My case was like the OPs in that I had a genset with a dedicated start battery.
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Old 06-27-2021, 05:15 PM   #15
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Using Deep Cycle as Start Batteries

Quote:
Originally Posted by Marco Flamingo View Post
Because it can be a redundant and unnecessary complication in some cases (like mine, apparently). My deep cycles are in two banks, so I switch to one bank when anchoring and have a fresh bank for starting my little Leyman (redundancy #1) or use both banks (redundancy #2). I also have a group 27 starting battery for my generator that can be jumped to start the Leyman if both banks were ever drained (redundancy #3). Or start the generator to charge the drained DC batteries (redundancy #4). It seems that a dedicated starter battery for the engine is conventional, but not required in all cases.

On my sailboat I didnít have a dedicated start battery, nor a genset. The boat was setup by the factory with 2 x 4D batteries with an old school 1-2-Both switch. The switch was always on Both unless at anchor then I switch to one or the other so that I could start the small 54hp diesel when pulling up anchor. I ended up replacing the 4D batteries with 4 GC batteries combined into a single bank. My intention was to add a separate start batter but I never got around to it before I sold the boat. Yes, the GCs started the diesel just fine, even after a couple of days on the hook. So starting with a GC isnít an issue with a smaller engine, but it isnít necessarily ideal

The primary problem that with the two bank setup is that half of your house bank is being taken offline to ensure starting capability. This reduces your usable Ahs for your house bank more than by half. Iím not smart enough to explain all the reasons but look up the Peukert effect.

2 banks also take longer to get back to fully charged than a single, larger bank. Again, donít ask for the details because Iím too ignorant to explain it correctly.

Finally, if you are using just one bank at a time, even if you are alternating which bank is being used as the house bank from day to day, you will shorten the life of the batteries because you are drawing the batteries down to a lower SOC than if you were using a single larger bank.

So redundancy aside, two banks that you alternate give you less usable Ah, take longer to fully charge, and shorten the life of your batteries. How significant these are will depend on how you use your boat. If you rarely anchor out, then maybe it isnít important at all.

Adding a small engine start battery for your Lehman would give you a much better house bank, beyond the redundancy of the start battery.
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Old 07-02-2021, 09:36 PM   #16
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Why not use a "start" battery, designed for the short but big burst of power required to turn the starter, to start your engine(s). And use a deep cycle batt for what it's designed for? Then both will last longer. 20 years ago we converted our 2 8D batt configuration to 6 Trojan T105s (configured as 3 12v "batteries", 675 amphr)) for our house use, and a single Type 27 conventional battery to start either our engine or our genset. The engine alternator feeds the house bank, along with our solar and windgen (and 3- legged batt charger). A Balmar Duo-Charge off the house bank recharges the start batt, along with the genset alternator. And a 1-2-Both sw allows me to connect the house bank to the starter IF I should ever need it. I also installed a On/Off sw on each of the 3 "12v batteries", so any single (or double) T-105 can be removed from the system if one should fault or short. so far, 20-some years later, the system continues to work without a hitch...And we've never run out of batt power!
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Old 07-02-2021, 09:52 PM   #17
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Quote:
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Why not use a "start" battery, designed for the short but big burst of power required to turn the starter, to start your engine(s). And use a deep cycle batt for what it's designed for? Then both will last longer. 20 years ago we converted our 2 8D batt configuration to 6 Trojan T105s (configured as 3 12v "batteries", 675 amphr)) for our house use, and a single Type 27 conventional battery to start either our engine or our genset. The engine alternator feeds the house bank, along with our solar and windgen (and 3- legged batt charger). A Balmar Duo-Charge off the house bank recharges the start batt, along with the genset alternator. And a 1-2-Both sw allows me to connect the house bank to the starter IF I should ever need it. I also installed a On/Off sw on each of the 3 "12v batteries", so any single (or double) T-105 can be removed from the system if one should fault or short. so far, 20-some years later, the system continues to work without a hitch...And we've never run out of batt power!

Sounds like a great setup.
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Old 07-02-2021, 10:21 PM   #18
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I have a single-bank 12V boat (6x6v GC batteries). The USBattery 2200's are putting out 100A for a couple of seconds to get me the 300A it takes to fire up my Lehman 120's - a trivial load.

I wouldn't run it this way if I didn't have 700W of solar. Because of it, I have never (to date) run my pack down below 80% or so. The bank could start an engine even from a pretty severely discharged state, and a minute or two with a wrench would bypass a single dead cell or module. Finally, if I somehow badly ran down the pack, I could always wait for a few hours of daylight to get me back up. I think for us that's enough redundancy.

I think for some use cases solar can obviate the need for a lot of the complexity our boats were built with: generators, multi-bank battery switches & complex charging systems, etc. etc.
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Old 07-03-2021, 01:09 PM   #19
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We use our house bank for starting too. We also have an 8.8 kw genset with it’s own start battery and 540 watts of solar. The house and starting batteries are next to each other so if we ever do mess up and kill the entire house bank, we can jump the engines off from the genset start battery. Or we can just wait until the solar or genset has recharged them.
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Old 07-04-2021, 05:49 AM   #20
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One good reason to have a start battery might be if it served the bow thruster.

Thrusters like engine starting batts as the higher voltage is a help in producing max thrust.
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