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Old 07-07-2009, 12:12   #1
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Adding a Second House Battery - Pitfalls?

I have a Chaparral with Mercruiser I/O. It has 2 identical batteries (engine & house). Some of the accessories are attached to the engine battery. I would like to add house battery capacity and move some of the load off of the engine battery -to reduce the risk of being stranded with a dead engine battery. My 120v charger has capacity for 3 batteries. I want to avoid any issues with alternator charging while underway. It seems that my best bet would be to replace the engine battery and use the two identical in size, age, etc in parallel for the house load? Does that create problems for the alternator or 120v charger?

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Old 07-07-2009, 13:23   #2
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Yes. And yes.

It certainly could create problems. What's the capacity of the shorepower charger, and of the alternator? If the alternator is only 35A, it makes no sense to, for example, add a 2nd 100Ah battery to the existing 100Ah house battery since the real max output of the alternator (considering ambient high temperature) would probably be about 20A or so.

Do you spend a lot of time at anchor? Are there other recharging options other than the alternator? A small gas-powered genset, perhaps? How large is the anticipated DC house load? How much discharge are you willing to tolerate (more discharge - fewer but longer recharge periods, versus less total life of the battery bank)?

In other words, more information would help the forum to help you.

Cap'n Jon (KB1HTW)
S/V Beausoleil -1979 Formosa 51 Ketch
"If it's gonna happen, it's gonna happen out there." - Captain Ron
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Old 07-07-2009, 14:59   #3
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Hey Capkern

Back in the day before moving to the dark side (sail), I had (still have actually) a ChrisCraft 283. We had three batteries; one for the house, one for the fridge and a dedicated starter. When you are at the dock or running, set the battery switches on All and they will all charge. When on the hook, set the switches to 1 and they will all be isolated from each other. This worked very well for us. We were always able to stay out longer without charging than our friends. BTW, always carry jumper cables in case there is a problem with your starter battery, you can boost it from one of the others which, hopefully are still alive.

Good luck!
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Old 07-07-2009, 16:56   #4
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The shorepower charger is a Professional Mariner, Protech 1220. I looked through the manual and cannot find the capacity and their website is down. I previously contacted Professional Mariner and they say no problem adding a second house battery wired in parallel with the other house battery as long as the group size is the same. I could not identify battery manufacturer but the tag had part # DP27, CCA @ 0f 625, CA @ 32f 770, 23 amp ave 175 min. Since I'm using 2 identical batteries the group size will be the same.
The boat is 27' with a cabin so it has a substantial DC house load (engine hatch, windlass, trim tabs, refrigerator, lights, stereo w/amp, etc). The boat has a generator but we like to anchor overnight, use the generator for a couple of hours but not all night. On one occasion with anchor light, refrigerator and misc other battery draw the CO detector started chirping during the night from low battery. I have a switch to parallel the batteries and that made the chirpping stop. Obviously, that made me concerned about draining both batteries.
The Mercruiser is a 496 Mag (2006) and Charging System spec is 65 Amp/917 Watt Alternator. I assume the alternator charges the battteries (house and engine) when the engine is running? It is not obvious to me how the alternator is connected to the batteries but I'm assuming that if I simply parallel a second house battery it will be connected to both alternator and charger in the same way the first house battery is connected.
I hope that more clearly identifies my situation and will help respond to my inguiry.
Thanks for the reply posts!
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Old 07-07-2009, 17:34   #5
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"DP27" indicates that it is a Group 27 battery.

Instead of adding a second house battery I would add a larger house battery. I would consider a 4D or even an 8D. This will keep things simpler and less expensive than adding a second house battery. Hopefully you have the additional space. If not a second house battery in parallel would work. I would then add a switch in order to isolate either one in case one or the other house battery fails.

You need to calculate how many amp-hours your boat is drawing at night. Then install a battery that has at least 3 times that capacity. Remember, the amp-hour rating for a battery is for a full charge down well below 12 volts and you never want to do that to a battery. You generally don't want to run a lead-acid battery below 12 volts for any great length of time if you want it to have a full life.

Your alternator is on the small side capacity wise. You could always buy a larger one but make sure your belt is capable of handling a larger capacity alternator. MerCruiser will be able to tell you this. MerCruiser of course will try to sell you their expensive alternator. Take your existing 65 amp alternator to a good alternator shop and they will be able to match you up with an equivalent with greater charging capacity.

You can be pretty certain your house and start battery is being charged when your engine is running. To test this, disconnect your shore power to take the battery charger out of the equation, put a multimeter across the house and then the start battery. Then start the engine and run it above an idle some. Did the voltage come up on both batteries? If so, your alternator is charging both batteries.

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Old 08-07-2009, 04:51   #6
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The ProTech-4 #1220 is rated at 20 Amps TOTAL, deliverable to up to 3 Banks. (See wiring diagrams on Page 5 of manual)
Products :: Battery Chargers :: ProTech-4 1220 12 Volt

Their website is "up"
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Old 08-07-2009, 08:22   #7
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The Protech is the charger that I own. I do mostly weekend trips with the boat in the slip most of the time hooked up to shore power. This charger keeps 4 6v golf cart batts for the house and an Optima start batt happy. I also have an 85 amp alt on the diesle. It has only let me down when the batts finally wore out.

I suggest you trace your battery cables and make a simple schematic so you know how the switch works. by Charly Wing's book on Electrical systems, (Can't remember the title) and be happy.
It's kind of like tearing up $100 bills while standing in a cold shower.
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Old 08-07-2009, 09:34   #8
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I see no problem with adding a second group 27 house battery in parallel to the first for your described usage. personally I would avoid an 8D battery like the plague...
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Old 21-07-2009, 14:04   #9
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Originally Posted by Cheechako View Post
personally I would avoid an 8D battery like the plague...
The 8D battery was surely invented by a chiropractor on a slow business day.

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