Interstate and Everstart are both made by Johnson Control. I'm guessing they are the same battery. Going with Group 31 really takes it to the next level in capacity. I like the AGM feature, but I have to ask if the price is worth it.

One question that keeps popping in my mind is how the two batteries will like each other.

What I mean is this. Both batteries will be identical. One will be the starting battery so it will see a small drain. The other will be used for the stereo so it might be discharged further.

Let's assume that my starter battery is at 97% capacity while my house battery is at 65% capacity because I played some music on the hook. Now I start the boat for a cruise and the combiner kicks in and both batteries are being charged by the alternator. Will the laws of electronics force more charging current to the battery that is more discharded or will each receive the same current resulting in an overcharge situation.

I am fairly ocmfortable that if these two were in a series, it would be possible to overcharge one battery if their capacities were different. But in a parallel situation, what would happen?

I don't think I will ever really regret buying the more expensive batteries. I MIGHT regret buying the cheaper battery. In reality, since the stereo battery and starting battery are isolated, I will likely never be stranded. Just without tunes which isn't the end of the world.

As you can see, I tend to overbuild things which often comes at a high cost of funds. Just trying to be smart about how I spend my money.

Lead acid batteries charge in accordance to the voltage difference between the battery and charge source. Anytime a battery sees a higher voltage, it will charge to that voltage.

Lead acid battery chemistry is such that the voltage changes with the amount of charge, so the voltage supplied by the battery depends on its state of charge.

So if you connected the two batteries together in parallel without a charger or engine running, the battery having the higher charge will discharge into the lower charged battery, thereby charging it - until both batteries reach equilibrium. That means, having the same voltage, or state of charge.

This is one good reason why the battery switch should never be left in the Both position, as if one battery is completely dead, you will at best end up with two half-charged batteries.

But with the alternator running, it will also be charging both batteries, so there is some dynamic goings-on here as one battery will be charging the other battery whilst the charger is charging both batteries.

That kind of X-Y-Z thing is somewhat complicated, and at what rate one battery discharges then charges; no one knows the answer, but one battery will not become overcharged before the other battery is fully charged. The two batteries will first reach equilibrium, then be equally recharged.

Of course this only happens when the battery chemistry is the same, which is the reasoning you should not normally mix battery types.

So mixing battery chemistries should be avoided but putting a lead acid starting battery and a lead acids deep cycle battery in my system would be ok since how they charge will be similar. Not saying I will do that, but just wanting to understand.

Make sure both are either agm, gel, or lead acids.

A deep cycle battery is a lead acid battery. The only difference from a starting battery is the size of the plates are optimized for one task or another; the chemistry is the same, hence the charge characteristics will also be the same.

So, mixing a lead acid starting and lead acid deep cycle battery is not violating their chemistry.

And speaking of combo batteries (deep cycle and starting), some battery manufacturers, such as Interstate have gone to just a combo battery for their marine batteries. Other brands such as WalMart/Everstart still have deep cycle, starting, and combo types.

Even though Everstart and Interstate are made by Johnson Controls does not necessarily mean they are made the same. Since the Interstate is inherently a combo battery while the Walmart are separate (and less expensive) batteries, I am going to believe they are different models.

What you generally want to do is to avoid mixing battery chemistry for batteries that might be commonly charged.

That means not mixing them for the typical setup you have with engine charging both batteries via combiner.

And most marine chargers - even those that can handle multiple batteries actually charge from a common bus. One charger I am aware of can charge different battery chemistries simultaneously, and has a battery type switch for each battery bank.

But even then, charging those mixed batteries should be avoided with a combiner/engine alternator.

what should be done in that situation, say you have a twin engine boat with 2 lead acid batteries for starting and a AGM battery for the house. You should not use a combiner from either lead acid battery to charge the AGM battery, but only rely on a triple bank charger with separate charge profiles for each bank to charge the AGM house battery.

This means of course, the AGM battery can only be charged via shorepower, but hopefully it will never be needed for engine starting as you have a prime and backup battery due to the twin engines anyway.

As mentioned before, I have two wet cell combo batteries of different sizes, and makes. I have been running them a few seasons now, discharging them at different rates and except in off season, have relied solely on the engine/ACR to recharge them. Despite fully discharging my starting battery twice, both batteries are working great, and I have no plans to replace either in the near future. In my opinion, you'll be fine with the less expensive wet cell combo type batteries. I suppose by have two identical group 29s, like rotating auto tires you would have the option to swap house for starting batteries every season, spreading the greater wear & tear from the house system onto both.

On many, if not most, boats, the batteries are installed in battery boxes, not secured to trays. The boxes are secured to the boat with straps. This arrangement protects the terminals from shorting and contains any acid spillage or leakage.

Battery boxes are inexpensive and available at marine stores or Walmart, etc. They come in several sizes.

I have a pair of group 27 dual purpose batteries in my old 88 FW and I used boxes for that same reason, one thing though is depending on the fit if the battery inside the box, it may be free to slide around a bit in there. I made some shim panels out of wood to keep both batteries from moving around inside the box. If you found a good battery hold down (tray) that screws into the deck of the boat (not strapped) that would be more secure but you'd have to get the terminal covers to prevent short circuits.

I like battery boxes. You must use some cover of some sort over the positive terminals anyway, and the battery box does this, as well as contain any spills that may occur.

The USCG regulation for battery boxes requires:



183.420 - Batteries

(a) Each installed battery must not move more than one inch in any direction when a pulling force of 90 pounds or twice the battery weight, whichever is less, is applied through the center of gravity of the battery as follows:

(1) Vertically for a duration of one minute.

(2) Horizontally and parallel to the boat’s center line for a duration of one minute fore and one minute aft.

(3) Horizontally and perpendicular to the boat’s center line for a duration of one minute to starboard and one minute to port.




Narrative:


If a battery was allowed to reposition itself indiscriminately at the will of the forces that occur when a vessel (especially smaller boats with quicker motion) is operated in waves, or while being transported over land with its battery installed, the battery could become damaged. If the battery casing ruptures, the electrolyte may run out. If a battery terminal loosens, poor contact or sparking could occur. The electrolyte is usually sulfuric acid which can severely attack many metals and other materials. Such attack on fuel system components has the potential of causing fuel lines and fittings to fail and leak fuel. Movement of a battery could cause the battery terminal to come in contact with grounded items, resulting in sparking.

Therefore, the regulation calls for restraint of the battery in the horizontal and vertical directions. A battery as installed by a manufacturer may not move more than one inch when subjected to a test force for a one minute duration in each of three directions: vertical, horizontal - fore and aft, and horizontal - port and starboard. The test force selected is 90 pounds, which is approximately twice the weight of the popular size of marine battery used in small boats, which may be subject to high accelerations. For this reason, 90 pounds was selected as the maximum that needed to be applied. For batteries weighing less than 45 pounds, a force of twice the battery weight is specified. This test force is to be applied through the center of gravity of the battery. (See Figure 12).

The commercially available plastic battery boxes may be used to install a battery; however, it may be necessary to provide means within the battery box to prevent excess movement of the battery. The materials used for restraining battery movement within a battery box should be selected with regard to potential deterioration by the electrolyte. The battery box must be fastened in such a manner that the battery installation will comply with this section of the regulation.

It is recommended that the use of materials to wedge the battery in a battery box be described in the boat’s owners manual to affirm its intended use. (See Figure 13).



Figure 12

Figure 13

Some "experts" have criticized the USCG regulation as being outdated. They state for batteries like D4 size (Trojans, etc), weigh more than 90lbs, so the regulation does not apply, as it is not up to date.

But I think there is a lot of good information in the regulation and narrative, and is worth following (it is the law afterall).

OK, so the Trojans weigh more than 90lbs... so use some common sense and test with twice the weight of the battery... duh!

If you use battery boxes (I use them in my boat), then it might be a good idea to use some 3/4" square strips of wood or Starboard at the base to help secure the battery, then use straps to hold them down (which is also the way my batteries are held down). In a sense, this is both a battery box and a tray; perhaps the best of both worlds.

(Both of my Four Winns boats just had trays, no battery boxes)

My biggest issue with battery boxes is the sloppy fit and the 4 small screws that hold the straps down. The ones I have seen, have a single strap that is held to the boat with 2 small screws and a plastic plate that is about 1/4" wide and 2" long. That is secured to the boat, the box sits on top of the strap and battery goes in it, the lid goes on and then the straps holds it all down.

I just am not that comfortable with that. It probably works fine, but I don't like small screws holding 90 pound batteries down.

Basically I agree even though the gp 27s only weigh about 50 lbs...
West Marine sells a nice battery tray that is screwed down with 4 screws and hold the battery from moving with a bracket that goes all the way around the top of the battery but they only go up to gp 27 size...

Still though the box is good in some ways as I thought to myself as I did my mid season oil change balancing the metal Topsider vaccum pump on top of the battery box right next to the engine dipstick, I would not do this if I had a battery tray even with the rubber insulators....

On my battery boxes, the straps run under the tray rather than having those metal strap holders.

Is your box screwed down to the boat or is the strap secured to the boat and the strap holds the battery box.

I have terminal covers already so I am not to worried about that piece.

Boxes would be nice and would probably be a good idea.

Maybe I can make one since I have made everything else it seems? Not going there.

I was able to pick up some group 30 battery trays from CP Performance which is a high performance boat place. They are copies of the Gil Marine battery boxes as Gil is no longer around. I would have to do a little mod work to make them work which is why I am probably going to return them.

You can use larger screws as long as you don't go through the hull of the boat. You don't have to worry about holding the battery down, gravity will take care of that. Your worry is side to side and front to back movement.

The boxes and their mounting are approved and used on many production boats.

The box isn't screwed to the boat.

The battery boxes sit on a shelf at the rear of the aft cabin, in a separate compartment behind the headboard for the "mother-in-law" bed.

We call it the MIL bed as the boat has a full sized bed on the portside, and a single bed on the starboard side. The single bed is called the MIL bed, and is typically used for storage. We store our two bicycles on the MIL bed.

The shelf has a tray made of 3/4" wood glued to the shelf that makes a rectangle, so that the battery box fits into the tray.

The strap goes under the 3/4" wood pieces. It is pretty strong, and since the batteries are at the transom, there is not much movement anyway.

When you start your engine and the combiner "kicks in" the weaker battery will be charged. In reality, the charged battery will also attempt to charge the discharged battery but that's not something to worry about here. The combiner is no more than an automatic 1 2 all switch. Designers took this all into account when they designed these systems. There's no need for users to re-engineer them.

In a series connection, both batteries would have been discharged to the same point so they will charge equally.

I have one battery in a box, and the house battery in a tray.

I like the box a lot better, and will convert the tray at some point.

The strap that holds it down with the 4 small screws, 2 at each side, secures it very nicely. It never moves, and I don't think it will unless maybe you are in a head-on collision at speed.

In which case you have bigger problems to worry about.

In terms of more expensive vs. cheap, I am of the opinion that most boat owners pay too much for just about everything, I tend to go cheap. ]

After years on this and other forums, I think it's a split verdict. Lots of complaints from folks who bought expensive gel cells or Rolls batteries that said they gave out way too early, many times el cheapo batteries lasted 5-7 years.

I don't think you can predict battery life. So I buy Wally Worlds best battery and call it even.

I ended up buying some group 31 boxes from Walmart. I like them as they have adjustable dividers for different sized batteries.

I have not made a final determination on which way I am going, cheap or expensive for batteries.

I vote cheap.

I haven't had any problems with the walmart marine batteries. Even sitting outside in 0 degree winters not plugged in, we still get 5-6 years out of them.

Since you've been wrestling and haven't pulled the trigger after all this time , I also vote cheap, and see how they do for a season or two...