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Old 14 January 2007, 16:59   #1
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Type and size of batteries for dual system

I am going to install a new main battery and a second one and wanted to know what would be suitable.....

The main battery would be for engine cranking only and would need to start a 90HP 2 stroke outboard.

The second battery would be primarily for running the electronics (GPS/Sonar, NAV lights, VHF, Bilgepump, accessory socket) and should the main battery bit a little low this battery could be tied in to give a boost using one of these...

http://www.power-store.com/view-item...d=1153&id=198&

I am short on space and do not want to add loads of weight so my thinking is as one battery is for electronics only (unless it is being tied in to boost the main battery) can it not be smaller?

Likewise if the main battery is for cranking only and no electronics it presumably does not need to be so big either?

Chris
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Old 14 January 2007, 17:32   #2
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2nd battery

Hi Chris,

Just wondering why you need a 2nd battery. i had thought about this two and over the years of sodden batteries and shagged engines, nearly everything let me down from time to time, However that was half the fun (until out at sea). Having thought about this and given power and stuff is my backround, I figured that a High Torque Out Battery from Good supplier such as Lucas or Bosch for Marine use is best.
All you should need is one of these but that depends on the use ( demand and recharge rate and regularity of charging).

In a nut shell I believe that unlss one is expecting much overnight use a 2nd battery is of minor use on any engine with a low usage and recharge rate.
It is usual for Yachts to have these where there can be extended use on Battery without Engine power.
You will need a Battery Switch (relay/diodes etc) and correct fitting (including "Tinning or soldering on Terminals" not just Crimping.

Have you thought of one of these Mobile Hot Start Units. Lidl Ireland)have them on offer at the moment at €38. They are a must on a boat and may well do away with the need for a scond battery on anything but the most useless of Engines. You will always know that they are fully charged when arriving to your boat.
Itis also very useful as a separate power source and Light Source. Plus do not forget that you can use it to lift your engine up or down if the Power trim and Tilt does not work...
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Old 14 January 2007, 17:56   #3
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I'd also vote for a single battery. Unless you are running instruments for a long time without the engine running, you'd have to work pretty hard to flatten a battery to the point where it would not start the engine (typical cranking current for small-mid outboards is a lot less than a car for example - my 50HP has 40amp inline fuse for engine start). I guess one good case for 2nd battery would be where the bilge pump was running unattended.

When I was investigating this, a deep cycle 80Ah leisure battery seemed a good compromise.

Standard lead acid batteries don't like being discharged down to low level (even 50%) and this will damage them over a period of time. If you are going to a 2 battery installation, the service battery (electronics etc) should definately be deep cycle. Reason I mention that is deep cycle batteries are typically quite big - I think the smallest is 70-80ah. It may be you could have a small car battery for engine start and deep cycle for instruments.

If you were concerned about battery useage, you can get good electroic battery computers that can estimate remaining charge and tell you when you should re-start engine to charge the battery.
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Old 14 January 2007, 18:08   #4
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2 x 50Ahr will be fine. But I'd do it differently. Use just one battery for both cranking and all electronics. Connect the second battery through a battery isolating switch but wire a chunky schottky diode across the switch terminals in the direction of the charge current. This way you'll always have an almost fully charged battery as a backup.
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Old 14 January 2007, 18:48   #5
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Quote:
Originally Posted by jwalker View Post
2 x 50Ahr will be fine. But I'd do it differently. Use just one battery for both cranking and all electronics. Connect the second battery through a battery isolating switch but wire a chunky schottky diode across the switch terminals in the direction of the charge current. This way you'll always have an almost fully charged battery as a backup.
Hi J Walker, well done, I have not heard of the Schottky diode for 20 odd years, I had tocheck it up on http://en.wikipedia.org/wiki/Schottky_diode.

Yeah have to agree. What do you think of the idea of a Power pack. I expect to get one at Aldi Ireland special offer this thursday for 40.
They are most useful and can be used to ballast bow. I would be probably better of buying a Bosch one. DOnt know what it is bit Bosch always seems to work better.
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Old 14 January 2007, 19:07   #6
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Quote:
Originally Posted by jwalker View Post
2 x 50Ahr will be fine. But I'd do it differently. Use just one battery for both cranking and all electronics. Connect the second battery through a battery isolating switch but wire a chunky schottky diode across the switch terminals in the direction of the charge current. This way you'll always have an almost fully charged battery as a backup.
As you mention, the blocking diode will cause the 2nd battery to be "almost" charged. With lead acid batteries, does that not cause a problem with sulphanation? Generally the alternator regulator will top out at approx 14.4V to prevent boiling the battery. With the blocking diode in the way, with a 0.4 to 0.7V drop over the diode (depending on diode and current), the battery will potentially never enter the absorbtion phase of charging so plates may suplhanate. This will kill the battery pretty quickly! (Diode will also get hot)

A better alternative could be a split charging relay which connects the 2nd battery when engine is running. In an ideal world it would be a split charge cct so each battery was individially monitored but that's probably overkill and expensive.

Would still recommend a deep cycle batt if it's going to be discharged over a period of a days fishing for example without the engine running.
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Old 14 January 2007, 19:13   #7
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http://global.exide.com/exideGlobalW...n_maxxima_page

best batteries I have come accross.

It is a requrement when coding a boat to have a twin battery set up, I think it's a very good practice to have twin batteries. Having a back up at sea is always good! I don't reckon the power packs much and I certainly wouldn't take one to Sea instead of a proper backup!

If you don't want to splash the cash for the batts above then deep cycle leisure batteries work pretty well, 85 amp would do you and you can get a pair for 70 quid in towsure
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Old 14 January 2007, 19:38   #8
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Originally Posted by Rogue Wave View Post
I think it's a very good practice to have twin batteries. Having a back up at sea is always good! I don't reckon the power packs much and I certainly wouldn't take one to Sea instead of a proper backup!

If you don't want to splash the cash for the batts above then deep cycle leisure batteries work pretty well, 85 amp would do you and you can get a pair for 70 quid in towsure
Yes I absolutely agree twin batts is best scenario. I guess my earlier comment re single was one good sized marine battery is possibly better than 2 small car batteries. I was assuming that short of space meant not enough room for 2 leisure sized batts (Chris, how much space do you have?)

On coded RIB's do you have electronc charge distribution to handle both batt's or are they just linked with a switch?
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Old 14 January 2007, 19:55   #9
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Cool,
As far as I know there is no spec that says you have to have a split charging system, I don't see it as a bad thing though, but I kep it simple and use the selector switch to pick which batery to charge.

re no having enough space, you could always buy a big sturdy toolbax and fit the batts in that and then fit that to your boat !

like this one

http://news.thomasnet.com/fullstory/31410/1424
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Old 14 January 2007, 21:03   #10
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Quote:
Originally Posted by al40 View Post
... With lead acid batteries, does that not cause a problem with sulphanation?
No.
Quote:
Generally the alternator regulator will top out at approx 14.4V to prevent boiling the battery. With the blocking diode in the way, with a 0.4 to 0.7V drop over the diode (depending on diode and current), the battery will potentially never enter the absorbtion phase of charging so plates may suplhanate. This will kill the battery pretty quickly!
No. The normal charge voltage provided by a regulator is 13.7 - 14.2v. In practice, I've found them to usually give close to the maximum. The schottky diode will give a voltage drop of about 0.4v so this is within range.
I've run this system for many years and the main battery has been the one to fail first.

Quote:
A better alternative could be a split charging relay which connects the 2nd battery when engine is running. In an ideal world it would be a split charge cct so each battery was individially monitored but that's probably overkill and expensive.
KISS.
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