CONTINUOUSWAVE

Right now [my boat has two] starting batteries [connected to] an OFF-1-BOTH-2 battery switch. For offshore fishing I'd like to add a third battery to the mix, a deep-cycle battery to power the stereo, electronics, Hydro-glow light, Chum Chucker, and spreader lights. Ideally, I'd like to isolate this battery and feed the above mentioned circuits from the deep cycle battery with a Blue Sea Systems 7649 Mini Add-A-Battery kit. I could isolate the circuits and feed the individual circuits with a 6-fuse panel, running a 8-AWG or 6-AWG conductor to feed the panel and a battery switch to parallel this battery if necessary.

If I use an automatic charging relay and am running on both starting batteries, will the third (deep-cycle) battery charge if paralleled with the starting batteries?

It is hard to know precisely how you intend to interconnect all the batteries from your narrative. Is there a schematic diagram available? Perhaps BLUE SEA SYSTEMS has one in its reference material that is applicable to your design.

One concern I would have from what I think you propose: it sounds like you might have all three batteries configured in parallel to be charged by a single charging output source. In general, I tend to dislike charging batteries in parallel if it can be avoided. Batteries charged in parallel do not necessarily behave as equals, and one battery may absorb more charging current than another wired in parallel with it.

Batteries accept charge when their terminal voltage is forced higher than it would be from their internal stored charge. Current flows into them from the charging source. However, when batteries are wired in parallel, you cannot really be sure which one is going to absorb the charging current.

If you must charge batteries in parallel, it is probably best if they are the same type of construction, that is, all of them of the same general design type (flooded cell vented, sealed valve regulated, absorbed glass mat, or gel cell), and all of the same brand and age. Mixing together different types is to be avoided because each type needs a different charging voltage to reach proper charge. And mixing old and new batteries in parallel can also be a problem due to differences in their internal resistance.

Does your outboard engine--I think you have a large HONDA V6 engine--offer the option of an auxiliary charging output?

I have thought about the circuitry I would use for an arrangement of:

--two engine cranking batteries
--one deep cycle battery for house loads
--one engine charging source without an auxiliary charging output

My first concept is shown in this sketch, an electrical schematic diagram:



NOTES ON THREE-BATTERY ONE-ENGINE CIRCUIT

I show only the positive circuit for clarity. The negative circuit is in common for all batteries, all loads, and all charging current sources.

Batteries B1 and B2 are of similar type and are engine cranking batteries.

Switch S1 is a conventional marine battery switch with OFF-1-BOTH-2 positions and three terminals. Either battery B1 or B2 can be selected to start the engine and receive charging current from the engine, or the two batteries can be combined in parallel (BOTH position) to allow starting when one battery alone would be insufficient.

Switch S2 is a newer style of marine battery switch with OFF-ON-EMERGENCY positions and four terminals. It has two independent circuits, one for engine starting and one for house. They remain isolated unless the switch is moved to EMERGENCY, which combines the two circuits.

Battery B3 is a deep-cycle battery and feeds certain isolated loads on the boat so their power is not affected by engine cranking and the accompanying battery voltage sag.

The ACR device permits Battery B3 to be charged from the engine at any time that the other battery--which is either B1, B2, or the combination in parallel of both B1 and B2--has reached sufficient voltage to be considered re-charged.

In normal operation S1 would not be in the BOTH position. This avoids any problems with having three batteries connected in parallel to the charging current source. The charging of B1 or B2 would be affected by the position of S1. As a standard practice, the operator would switch between B1 and B2 using S1 in order to maintain them both in a charged state. For example, if the boat is used daily, alternate B1 and B2 every other day. Or, use B1 for the voyage out on a particular day, and B2 for the voyage back.

If desired, a second ACR could be added between only B1 and B2 so that the battery not in use was also automatically charged. However, that could result in the automated action of all three batteries being in parallel on the single charging current source, which I do not think is a good plan.

Thinking about this more, could 2 new deep cycle batteries be paralleled on the "2" side of a regular selector switch, leaving the starting battery on the "1" side of the switch? If the 2 new batteries were identical, would they act as a single large-capacity battery with respect to charging them?

I believe you are just proposing changing the type of batteries B1 and B2 to become deep cycle, and changing B3 to be a cranking battery. You would also have to change the loads on S2 so that the HOUSE loads were fed from the output of S1 and the engine from the B3 source. Other than that, this change would not affect the circuit I have proposed for connecting THREE BATTERIES to ONE ENGINE.

You would also change the nomenclature I show in my diagram by moving "starting battery" to be associated with B3. The change does not affect the circuity.

I hope I have understood your proposed changes. The best and really the only way to show electrical circuity is in a diagram. Using narrative to describe electrical circuitry is tedious and leads to misunderstanding. I encourage you to create a schematic diagram of how you propose to construct your battery primary power distribution circuit. When readers can see your proposed circuit in a diagram instead of imagining it from your narrative description, you can be sure they will be clear about your intentions. Please give us a diagram.

You now seem more concerned about having high-capacity for electrical storage in the HOUSE circuit rather than the CRANKING circuit. Why have you made this change?