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Old 04-21-2016, 07:31 PM   #1
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Is isolated start battery important?

Good day and thanks in advance for your thoughts. Just had a survey done on my next boat, I hope. One thing that came as a surprise to me was that the original isolated start battery and associated small alternator had been removed. Main engine starts off of large house battery bank which is charged by single large alternator on main, shore power charger or generator. Seems imprudent, to me, to not have a separate battery and alternator dedicated to starting the engine but perhaps I am just showing my age. Or my ignorance. If you were buying a boat set up this way would you think it necessary to change it? Thanks again, Dave.
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Old 04-21-2016, 07:39 PM   #2
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My sailboat was setup this way, one house bank charged by alternator (crappy alternator) and shore power. No generator. It always worked out great.

A separate start battery is a good idea, but not a necessity. One question, what starts your generator? If the house bank also starts it, then I think I would look at eventually getting a separate start battery that would start your genset and engine.

Depending on how much power you need to start your engine, I would use whatever flavor of battery you like, and simply use an Echo Charger to keep it charged up. With this setup, your shore power, engine alternator, and generator all will still be wired to charge the house bank. The Echo Charger simply charges from the house bank to the start battery once your house bank has reached a given charge level. Very easy to wire up and relatively cheap. It requires no management as well.
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Old 04-21-2016, 07:55 PM   #3
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... If you were buying a boat set up this way would you think it necessary to change it? Thanks again, Dave.
I would, but there are certainly plenty of folks who have a different setup. You could install an ACR (Automatic charging relay) between the house bank and separate engine start battery and keep all of the other existing setup. The ACR only closes when there is charge current available and opens at other times to keep the start battery isolated.
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Old 04-21-2016, 08:28 PM   #4
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for me the only way is each engine(s) and generator(s) need is own batterie and service need to be definitively separate with the possibility to be crossover
I love the "new" way to have automatic charge relay (from the old system on isolator diode) that also nice option of temporary crossover to make emergency start.

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Old 04-21-2016, 08:45 PM   #5
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I would take a look at common failure points and redundancies. If you only have one battery bank then it can get run down at anchor and then nothing will start. That is not good.


I would at least have a separate starting battery for the genset isolated from the house with an Echo Charger, ACR, combiner, etc. That gives you one point of redundancy. Better would also be a separate starting battery similarly isolated.


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Old 04-21-2016, 09:20 PM   #6
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I am not intelligent when it comes to anything electrical, however I prefer the Echo Charger over the ACR as I understand them. The ACR automatically combines the batteries when charging, and isolates them when starting or in the absence of a charge.

The Echo Charger does NOT combine the batteries, ever. It simply feeds a small amount of charging current from the house bank to the start battery when it senses that they are being charged. Once it senses about 13v at the house, it starts to feed a max of 15amps of charge to the starter battery.

If your house bank has a major fault, and you start to charge it, the ACR will combine those batteries, potentially giving your start battery problems. The Echo Charger can never do that since the two batteries are never combined. If your house bank has a major interior short, and you start to charge it, the Echo Charger will still sense the charge current, and feed no more than 15 amps of it to your start battery.

So, an ACR is great, but it doesn't completely isolate your start battery.
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Old 04-22-2016, 04:50 AM   #7
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An isolated battery makes sense, but it doesn't have to be the engine battery. Many prefer the generator start battery, which is smaller. Isolating the start battery for the main or one of the main engines requires a battery combiner so that the alternators are charging the house bank underway. Isolating the genset battery avoids this. Also with an isolated genset battery you get full benefit from the genset alternator. If the genset alternator charges the house bank then the 110v/220v chargers running off the generator will reduce the output of the genset alternator significantly as the alternator will sense the charge from the chargers.

I have taken this a step further and run everything off the house bank so that when the generator is on the chargers are charging everything. My exception is a small isolated battery under the helm for a VHF radio and the SSB. It is charged by a separate charger that turns on whenever there is shore or generator 110v power.
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Old 04-22-2016, 07:34 AM   #8
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For long term anchoring out a dedicated start batt is a fine idea.

With a huge house set there is usually enough to get a modest engine started , even when way run down.

Its all just a matter of sufficient plate surface area , as even at 10V most mechanical injected engines will start in warm weather.

I would hook the main alt with marine V regulator to the House batts for the best charge and use an $18 RV relay to charge the start

. Your rotary switch could select the start if required , otherwise it would be seamless , nothing to do, most of the time.
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Old 04-22-2016, 12:40 PM   #9
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...If your house bank has a major fault, and you start to charge it, the ACR will combine those batteries, potentially giving your start battery problems.
Actually that is not what would happen. ACR's such as those from Blue Seas have various safeguards against combining batteries that have problems. An ACR will only close when the battery voltage of the bank being charged goes above about 13.0V. So if the charging bank is low, it would not close until that bank came up higher than the normal fully charged resting voltage of the second bank - no current would ever flow from the second bank to the first. Also, they will not close if the second bank is very very low.

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Old 04-22-2016, 01:14 PM   #10
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"potentially giving your start battery problems."

A very low batt set when hooked to a battery in full charge does no damage while charging.,

Most crap car alts can raise the voltage of the near dead set to at least 12v.

Your good batt should be at 12.8 and the little difference in voltage will not discharge the good batt much if at all.

once the dead batts get up to 12.8 the good batt wont even notice.

Finally when you get fully charged 14.4 the good batt still wont notice .

In theory the good bat might use a bit of water at 14.4 , but this is the norm for most autos and nobody complains.
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Old 04-23-2016, 11:45 AM   #11
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The ACR works fine when the start battery and house bank are the same make or type. Often though the house bank is comprised of deep cycle batteries while the starter battery is set up for cranking. They accept charge in different ways. Since the start battery is seldom deeply discharged, there is some risk of applying too much current to the starter bank whilst charging the house bank (source- Calder). It's best to ensure you have some sort of a series voltage charging regulator for this purpose, rather than just an automatic charging relay.


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Old 04-25-2016, 06:39 AM   #12
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" Since the start battery is seldom deeply discharged, there is some risk of applying too much current to the starter bank whilst charging the house bank"

What charges any batt is a voltage , usually at least 1 volt above its current charge state.

A DEAD start batt with a huge alt charging it might have a V applied large enough to heat it , but that would be rate.

All a good start batt sees is a voltage difference, the only current flow is caused by the difference in voltage .

If both are the same , there will be almost no current , which is why some alts stop showing RPM readings.

For most boats the house will be down, the start almost 100% and simply joining them underway , solenoid or rotary switch is fine.

The parts sellers try to create a complexity and mistique when its really straight forward.
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Old 04-25-2016, 06:50 AM   #13
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My system has two banks of batteries that run the house and (1) engine. They can only be combined at the helm via. solenoid switch or in the engine room via. on/off combiner switch. The DC panel will only run off of one of the banks 1-2 option. Therefore you will never run both banks down unless you do it intentionally.
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Old 04-25-2016, 09:40 AM   #14
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You have the generator as a back up to charge the main batts and could even use the genny battery with jumper cables. 99% of the time what you have will work fine.
Had the same setup for a long time with many dead batteries without any need for change.
Some people however, do wear belts and suspenders.
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Old 04-25-2016, 11:26 AM   #15
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"Had the same setup for a long time with many dead batteries without any need for change."

With "many dead batteries" their service life is vastly reduced.

A State Of Charge (SOC) meter with an alarm is $150 to $200,

lots easier and probably cheaper than wheeling replacement batts down the dock!
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Old 04-25-2016, 11:29 AM   #16
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I think it depends a lot on how you use the boat. With a fishing boat I had, we were always starting and stopping as we moved from spot to spot. The boat had no generator, but continued to draw for bait tank, electronics, music, etc., so it was important that the house be isolated from the start battery.

With my current boat, although start batteries are isolated, when we are away from the dock the genset is always on (so batteries are always fully charged), and mains only shut down when we anchor. So, my only risk is a bad battery/bank that gets out of control before I can stop it from running down all the batteries in the bank, making isolation much less important.
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Old 04-25-2016, 11:45 AM   #17
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Actually that is not what would happen. ACR's such as those from Blue Seas have various safeguards against combining batteries that have problems.
Thanks Ken.

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The ACR works fine when the start battery and house bank are the same make or type. Often though the house bank is comprised of deep cycle batteries while the starter battery is set up for cranking. They accept charge in different ways.
This is another reason I like the Echo Charger. The charger profile for the house bank doesn't change how the Echo Charger charges the isolated battery. In effect, the ACR combines the batteries, the Echo Charger simply charges the isolated battery. I also felt that if I wanted to combine the batteries, I could use a switch for that.

Whatever system the OP picks, it can be a relatively inexpensive and simple solution. The only thing difficult in many installations is finding the space to locate the separate battery and then running the cables. (Another plus for the Echo Charger is that the wiring is very small).
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