One Starting Battery For Two Engines?

[nnyerges]

Hello all,

I am remodeling the entire electrical system of my boat and need advice to make my decision with respect to use two or one battery for starting the two Ditroit Diesel 310HP engines. I know that the most common is to use one battery for each engine and currently have two starting batteries (see DC-GENERAL-ORIGINAL.JPG)

Becouse space and weight problems, I need to remove one battery.

Obviously I can take out the HOUSE battery. Neither the GENSET battery, as it has its own alternator/battery charger and cannot be connected to the isolator. So I can only remove one of the engines starting battery (see DC-GENERAL-NEW.JPG)

The idea is to place the current HOUSE 1500A 8D battery, to start both engines, eliminating the 1200A. Place a new 8D DEEP CYCLE battery for the HOUSE. Switches A and B are used to draw a emergency parallel between any of the 3 batteries (including the GENSET battery). Switch C is for shutting the service when should start with the house battery. The two engine alternators are responsible for charging the starter and service battery through the isolator. Likewise, the AC / DC CHARGER is responsible for maintaining the 3 batteries charge, when the engines are stopped and AC is present.

I hear suggestions, criticisms and above all, I need the pros and cons that can give me, since if cannot remove the battery, I will have to do a big complicate job of remodeling the hull.

Thanks in advance,
Nicolas

EDITED: I mistake by placing AH on lead acid batteries, where A should say for cranking current of 1500A or 1200A. Deep Cycle 255AH data its ok

 

[bruceb58]

Re: One Starting Battery For Two Engines?

What kind of engines are these?

 

[nnyerges]

Re: One Starting Battery For Two Engines?

Two Detroit Diesel 671N engines
Two ONAN MDKAL Gensets

 

[bruceb58]

Re: One Starting Battery For Two Engines?

Those engines are going to pull a lot of current to start. What is the cranking spec on the battery?

Also, if you ever need your house battery to start your engine, you are now going through two switches which is going to cause a voltage drop. I sure like the first setup way better.

I think I would prefer having the genset starting off the house battery with an option to start off of one of the engine batteries if you have to get rid on a battery.

 

[Scott Danforth]

Re: One Starting Battery For Two Engines?

with only one battery, you will have dueling alternators and other issues. I wouldnt recommend it myself, however I would recommend a pair of group 31's vs an 8D for each engine to save room.

 

[CaptainKickback]

Re: One Starting Battery For Two Engines?

My first thoughts are how do you use your boat and how far from shore and other boat traffic will you be. Being stranded is no fun, but worse if help is a long way away.

I have always thought more batteries is better. As for hooking the generator to the house battery - never made sense to me. The engines cam recharge the starting batteries. But it is the house battery that you often run way down. How are you going to start the genny from the battery you just drained? I moved mine to a starting battery. Then, whatever motor (engine or genny) will start will charge tje batteries.

I would also look into a charging system.

 

[nnyerges]

Re: One Starting Battery For Two Engines?

Thank you guys, very useful comments.

First I need to clarify an error in the diagrams: I mistake by placing on lead acid batteries AH, WHERE A Should say for cranking current of 1500A or 1200A. 255AH Deep Cycle dates it?s ok

bruceb58

What is the cranking spec on the battery?

1500 max. 1650 amps

going through two switches which is going to cause a voltage drop

The nominal voltage drop of a battery disconnect switch its about 0.02 V to 0,025V for a 100A load. Assuming the worst, 1650A in 2 or 5 seconds, there would be a voltage drop of 0.4125V per switch, for a total of 0.825V for the two switches, which equates to less than 7% voltage drop, below the recommended 10% for starting, so having two switches is negligible.

I think I would prefer having the genset starting off the house battery?

I agree with CaptainKickback. The genset have its own alternator and regulator directly connected to the input of the battery post, so I do not think it very convenient to have the genset alternator constantly connected to the ship's service load.

Scott Danforth

with only one battery, you will have dueling alternators and other issues.

The two alternators have being connected to the isolator for 20 years without problems, so I think that having the isolator, you get rid of those type of issues.

I would recommend a pair of group 31's

I have understood that group 31 inrush current it?s too low for my two 671 engines. Besides that here are no facilities for group 31 batteries, so it will be a maintenance headache.

CaptainKickback
Again, I agree with you about connecting the gensets to the service battery its not a good idea. By the other hand. In the diagram you can see that there is a charging system running for 20 years and I do not mind running out of battery, since in more than 20 years, I've never had to use more that 2 batteries to make a emergency power procedure, so 3 batteries appear to be enough.

Thanks for all your responses. I still think that the new configuration is viable, ie not yet see nothing against it.

 

[nnyerges]

Re: One Starting Battery For Two Engines?

By the way i have place this post in another forum and here a link for a very useful reply. Please take a look to it and make your comments: http://www.cruisersforum.com/forums/f14/one-starting-battery-for-two-engines-71472.html#post816304

P.S. I help that I?m not getting in trouble, posting links to another forum. My apologies to the forum administrator if I do. I think as a man of the sea, we help all at sea and on land, so I guess that also on the Internet

 

[skargo]

Re: One Starting Battery For Two Engines?

A high compression engine like a diesel is going to draw a lot on start up. Heck my Cummins in my truck has 2 batteries to itself.

 

[nnyerges]

Re: One Starting Battery For Two Engines?

One issue I see with your plan is that you will have two alternators trying to charge one battery and feed the boats 12 volt systems.

Yes, but through the schottky diode isolator and thats the way it works for more than 20 years, without issues.

Starting both from one battery should not be an issue as long as you aren't cranking them over at the same time. Typically, starting an engine will discharge the battery by about 5% to 7% with normal cranking times. (A few seconds to start them.) Start one, give the battery 60 seconds "rest' and start the second.

In that, we are very clear. It would be unwise to attempt to start both machines simultaneously. Usually, whenever I start a engine, wait a reasonable time and then start the other engine, even, I monitor the battery charging condition.

By the way, a question remains: I place the switch C, ostensibly to prevent any reverse current to the equipment of the ship, in the case i need to start the engines with the service battery. Is "C" necessary? I already have a master ship service breaker, do i have to turn it off when starting engines from house battery?

 

[nnyerges]

Re: One Starting Battery For Two Engines?

In an emergency start, I prefer to start the engines with the GENSET and not the house battery. Suppose that you are navigating with all your equipments ON, and for any reason your engines stop and your engine battery is death. You can't just set switch A from 1 to 2 and then set B to 1 (or your knife switch on).

It?s insane to start the engines directly from house battery, with all loads connected. It is preferable to stop the GENSET (if ON) and switch B to 2 to start the engines from the GENSET battery.

Using the house battery will be the last thing to do in a emergency at sea, because you don?t want to have issues with your ship equipments, you must have to turn off house load before. By the other hand, there is no problem to use the house battery anchored or at safe shore.

I was thinking in another option in the design: Having two engines starting batteries, the House battery only to service (just emergency switching) and start the GENSETS from one of the engines or house batteries. But how insulate the two gensets alternators? (GENSETS: Onan 9KVA 9MDKAL3367295)

 

[nnyerges]

Re: One Starting Battery For Two Engines?

Based on all observations, I will rethink the design, to create version 2.0. I have only two last questions:

1. In case of an emergency, if I need to start the engines with the GENSET battery, can I leave the GENSET ON or Should I turn it off?

2. The ISOLATOR of my drawing is a 2ALT IN and 3BATT OUT type. The outputs are connected in parallel with the INVERTER/CHARGER. That?s OK. The question is: Can I connect the third remaining ISOLATOR output in parallel with the GENSET alternator (genset battery positive bus) all the time? I think its the same case as connecting a parallel switch between to starter batteries (paralleling a starboard & port engines alternators), but with the advantage of having a diode through them

Added:
Case1
If my design where with the ideal of one starboard engine battery, one port stating engine battery and the house battery, it was a perfect match for using a dual VCR or ACR. My design is only with only one starting battery for both engines and one house battery, with two alternators charging the starting battery and the house battery. The third battery is only for the GENSET (depending of question #2). I want/need only 3 batteries, if I can use two independent batteries for each engine and the house battery and have the GENSET's starting from one of those batteries, I will be the happiest marine in the world and I?m sure I will use an ACR. I think the original post could take a 180 degree turn, analyzing how to connect the GENSET's to any of those three batteries.

Case2
How about a battery for each engine (charged by the respective alternator) and use the house battery to start the genset. The house battery can be charged with two VSR and the two engines, or charged by the genset?

 

[nnyerges]

Re: One Starting Battery For Two Engines?

How about my new design, based on the three basic boat charging condition cases:

1. Cruising: I'll keep using my ISOLATOR (2INx3OUT) connected to the alternator of each engine. The outputs go to the starter battery, house battery and the third output, optionally the generator battery through a switch, to keep that battery charged, just in case that for some reason, do not count with both generators, which is quite unlikely, but can occur. Inverter / Charger OFF.

2. Anchored: Genset ON, as need it, charging his battery and connected to the house and starting battery using two ACR. Inverter/Charger OFF. Both ACR are going to be overridden by the engines starting ON switch, so both ACR's are disabled when engines are running (case 1), to prevent any issues of paralleling alternators.

3. ShortLine. Inverter/Charger ON for all batteries as need it.

NOTES:
1. Negative bus and other protections not show for simplicity
2. Optional two fuses or two ON/OFF switch or one OFF/1/2/1+2 selector switch, to avoir starter-solenoid shorts or failures
3. Optional In case of both gensets unavailable
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DIAGRAMS
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Original topic: Twin Engine + Genset + 3 Batteries Configuration
at http://www.boatinghowto.com/showthread.php?517-Twin-Engine-Genset-3-Batteries-Configuration

 

[nnyerges]

Re: One Starting Battery For Two Engines? (Final Design)

Re: One Starting Battery For Two Engines? (Final Design)

Happy New Year to all,

This is the definitive design for my 3 battery/2Engines/2Genset main DC switchboard. I start to connect the panel and planning to finish in the middle of February. Then I will upload some pictures of the installation. The following pictures are in los resolution, but I have uploaded a PDF hi resolution file for each of them.

Circuit diagram:

Simple working diagram:

Board Layout: