Battery Replacement

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Senior Member
Oct 8, 2007
Vessel Name
Vessel Make
Mainship 34
Both the battery banks on my boat are getting close to their expiration, I'm afraid, as they don't hold a charge for more than a day or so.* The house bank has one 8D30 and one 4D30 run together, and the starting battery is a 4D.* They're all connected together, and not separated thru the main electrical switch, which, to say the least,*is a poor setup.* These wet cell batteries were installed in Feb 02, and were rated as 30 month batteries, so they've served the boat well.

Here's the question: I'm contemplating buying new batteries, one for the starter and two combined for the house set.* I thought that having the alternator (and*battery charger when on the dock)*charge ONLY to the house set, and then use a combiner switch to link the house set to the starter battery would be a really simple method of assuring all the batteries stay charged.

I thought of first trying to equalize the batteries, as they may have never been done, but because of the years they've been operating I thought it may be too little too late.

Is this too simple, or am I missing something?

Not really - it's similar to how I want to set my system up when I get the time (& dinero).

Only change that I would make would be to connect the alternator to the engine battery and the charger to the house bank. This makes absolutely certain that there is voltage to excite the field of the alternator if somehow the house bank goes absolutely completely 100% out. And somehow there's just a warm fuzzy feeling about the engine battery getting its fill first.

Two options at this point: I know some of the CHB guys have put a second alternator mount on their Lehman. You could do that, and then not worry about having to parallel the batteries at all except when you leave the boat on the charger: one alternator for the engine battery and the other for the house set.

Other option would be to use an automatic charging relay - for example:

This closes automatically when either side gets and sustains a charge voltage - that is, if either the alternator or the charger are running and their associated batteries start to top off, it automatically bridges the batteries so they all charge at once. You can add a switch to bridge the batteries manually if you want.

The cool thing about the automatic relay is that you can't forget to open the switch when you get to your anchorage - sometimes the beer fairy calls me just as soon as the engine shuts down.* *

Thanks for the info.* I'd planned to use an ACR between the batteries, as I've used them in the past with other boats and they seem to work well.* But I hadn't thought about the dual alternators, and using one to power up each bank.* As luck would have it, the PO added a second alternator already (bless his little pointed head) and that would make sense to share the power duties with one alternator powering one bank*as you suggested.

You're right about batteries costing a LOT, but after fueling up yesterday I'm still in shock.* Maybe I'd better pony up for the new batteries before the shock wears off and I think clearly again!

I know it's all costly, but there's nothing as relaxing or theraputic as being out on the water.* Okay, maybe not the 'Perfect Strom' kind of water, but you know what I mean.

Now.....where's my plastic?

Mike Wiley
Brookings, Oregon
The house batts are the ones that get driven down the furthest so should be given the best chance at a good charge.

Any non mechanical connection (transistor combiner) will lower the voltage to the non monitored bat set.

My reccomendation of an RV $17.00 solenoid and engine key start with ACC position is still the easiest , seamless method.

Before purchasing the bat set , if the boat is a Cruiser,(Cruisers live on the batt set) I would install an E - Meter or similar.

Its the best $150 to $200 you will ever spend , if you USE the bat set.

As Bill Lungren from the movie "Office Space" says, "I'm afraid I'm going to have to disagree with you here"

True, the house batteries are going to need more of a charge. But if you had to pick a set to run down because they didn't get enough charge, would it be better to have the house or the start batteries depleted? In reality, it probably doesn't make any difference. And since Mike has dual alternators, not a big problem. If one of the two alternators is higher capacity, that's the one that goes to the house bank since it will be able to absorb much more current.

The solenoid is a much less expensive method of paralleling the batteries, but doesn't address the problem of keeping the start batteries on the charger back at the dock. You could add a switch to energize that solenoid when on shore power.
You could add a switch to energize that solenoid when on shore power.

The key switch to the ACC position every two or 3 weeks takes care of that.

OR leaving the rotary parallel switch in ALL , while on shore power.

No multiple output shore charger required.


-- Edited by FF at 05:34, 2008-06-20

Also - I wanted to emphasize the point you made about using the ACC position of the switch. My engine switches don't have one - they're strictly off-on-start. But you absolutely need to have a switch that doesn't energize the relay while the starter is engaged - if the house bank has been run flat, it'll try to suck down the start battery at the same time you're trying to turn over the engine.
Chris--- I don't think this is always the case. Older Grand Banks like our '73 GB36 had pretty simple electrical systems. We have not altered the original electrical system, so, we have two 8Ds that can be assigned to "house" duty by the "Off-1-All-2" battery switch. If 1 or 2 is selected, by default the other battery becomes the "start" battery.

But it really isn't a "start" battery because on these boats pushing a starter button fires a big solenoid in the engine room that connects both 8Ds together regardless of the postion of the battery switch as long as the starter button is held down. So the 8D that's not powering the house circuits becomes a "start" battery only by virtue of its not being drawn down by anything.

Not long after we got the boat I inadvertently left the engine room lights on all night during a cruise. By morning the "house" battery (port 8D) was pretty much dead. When we went to start the engines for that day's run, each one turned over and started as fast as it always had. Granted an FL120 starter isn't a huge load, but there was no hesitation or lack of starting power even though the start combiner relay was connecting a charged 8D and a dead 8D together. (We now have an indicator light on the electrical panel to tell us when the engine room lights are on.)

Some years later on a cruise a cell in one of the 8Ds went bad. I don't know much about electricicals so all I can say is that when tested with a hydrometer (?) the little ball didn't float. But it was the same deal--- when we started the engines the starters spun as fast as ever and the engines started right up. So connecting a good battery and a bad one in these two cases, at least, did not cause any starting problems.
In a cold engine room the results might be quite different.

Below 32F many diesels need to spin rapidly to create combustion heat, and the batts loose a bunch of ability.

Normally a good bat is 12.8 and the "dead" one at 12.0 ,
since the voltage at the starter will only be 9 or 10V while cranking , the 'dead" batt does no harm to the start sequence.

Should someore get stuck with dead near batts in cold weather crank for 30 seconds (to heat the cylinder walls from compression) and do it again in 45 seconds.

Usally works.better than the ether can.

Should it be COLD!!!

0'F the ether is usually the only way , a 2 or 3 second squirt , then crank does it.

Quite true - 99% of the time there will be no benefit from having the two banks separated at start time. In fact, most of the time the house bank will contribute to starting current. Worst case scenario, however, with a cold day and an absolutely dead house bank, and you're better off with them separated.

That's why I like the idea of the ACR with the control switch. Everything is kept separated until either side has a good solid charge on it -- then they're bridged. No messing with it unless there's an unusual situation where for some reason you want to keep them separated regardless (major electrical fault on the house system) or you want to bridge them regardless (you know the house bank is solidly charged, it's cold, and you want to spin as fast as possible).

But the odds pretty good are that you'll never actually run into a scenario where this'll be of benefit - so if the extra money for the ACR is an issue, or you don't trust it, or you're happy with your existing system, don't do it.
A tape measure might show that you could fit some Trojan L 16 in place of the 4D- 8D mix.

The same footprint , but taller might get more amps for the house .

I have found deep cycle 4D and 8D to be much harder to find than that size in Start or "Marine" batts. Both will do far less and have less life cycles than a true deep cycle house setup.

But install a E meter before hitting the batt store , and you may be able to get 6 or 7 years out of a set of deep cycles.

I always thought that when batteries are in a bank (linked) they should always be identical to ensure common charging across them. Hence, if battery types are different in a bank, premature failure is going to happen, including possible boiling!
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