I am sure the answer is somewhere among the million battery posts but I thought I would ask any how. What is the proper way to use a two battery system? I have a 2007 Sea Ray 260 Sundeck, 496, DTS, etc. I recently had a battery issue with the one battery not being charged enough. I now have both batteries charged and wondered if there is something I should be doing to keep them that way. When I bought the boat the tech said I should use battery 1 during one outing then switch to battery 2 on my next outing. He said to use both if I was having problems getting the boat to start due to low voltage. I have yet to hear the same advice twice. My boat has the normal stuff no add ons. It does have DTS which is wired to battery 2. Any help???

what i've heard is that when set to "both", both batteries are either being charged or discharged but not necessarily at the same time. for instance, when on the hook and in the "both" position, one battery will be used for house stuff like lights, radio, tv, etc. at some point, the voltage will drop to a point where it can't supply any more and the other battery will supply the power. if this is true, than keeping the switch on "both" is a dangerous thing to do while the engines aren't turning because if you drain one batery and then the other, you'll have a tough time getting your engines started. however, having the switch set to "both" will charge both batteries when either your engine is turning or you're connected to shore power. for this reason, i've been told to keep the switch on "both" when on shore power and with the engines running and then to set it to "1" or "2" (but not "both") when on the hook or drifting with engines off. i've heard this frequently but it may not be true. Al or Seabuddy or someone else should be along shortly to confirm or deny the story.

I think you received 'standard' advice on how to use a two battery setup using an A/B switch. Sounds like the selected battery provides starting, and powers any load when the engine is off, and receives charge from the alternator when the engine is running. The unselected battery is held in reserve. By alternating batteries with each outing you are ensuring that each has an opportunity for a charge from the alternator. I'm not certain how much load your DTS (Digital Throttle & Shift, right?) system places on battery 2, but if you only run on battery 1, in time battery 2 will run down.

You might explore alternate wiring arrangements, such as dedicated House and starting batteries, using something like a Yadina or BLue Sea Systems ACR to ensure both batteries are charged when a charging source is present. With this type of arrangement, typically the switch is used only in situations where you are having difficulty starting due to low current.

When the battery switch is in the "both" position, both batteries will discharge more or less simultaneously.

Batteries are voltage devices, so they will try to equalize each other - that is, try to maintain the same voltage. Batteries change their internal voltage depending on their state of charge, so when maintaining the same voltage, both batteries will generally have the same state of charge.

For instance, should both batteries be connected together - but without a load, if one battery is charged more than the other, then the battery with the higher charge will charge the battery with the lower charge until they both have an equal voltage (and are discharged by an equal amount).

If a load is apparent, the batteries will attempt keep each other in balance voltage-wise as they are discharged by the load. This means both batteries will be discharged at the same time.

If a charger is connected to the batteries, either a single charger or the engine alternator, both batteries will also charge at the same time - again trying to maintain a voltage-balance between the batteries.

But if the batteries are not matched, some interesting things occur. The charge rate, in Amps for each individual battery will be different. Say one battery is a 20Amp Hour battery and the other is a 40Amp Hour battery, the higher capacity battery will receive more charge current, so the net result is the voltage on each battery will try to be the same, and means both batteries will become fully charged at about the same time.

This all happens because the internal voltage on each battery is a function of its percent of charge. As a battery goes from discharged to charged, the voltage increases.

One charcteristic worth noting, regardless if the batteries are identical or different, is if the two batteries have a different state of charge, one battery will have a lower internal voltage. The battery with the higher charge (and higher internal voltage), will supply current to the lower battery to bring it into balance voltage-wise with itself.

In fact, if you have a battery with a 50% charge on switch position 1, and a battery with a 100% charge on switch position 2, if you put the switch into the "both" position (assuming a charger is not present), battery 2 will discharge into battery 1, thereby charging it, until both batteries are equalized - say 75% charged.

Due to this equalization characteristic, it is generally never a good idea to permanently parallel both batteries together, or to leave the battery switch in the "both" position with the charger off.

If one battery ends up being damaged, it could completely discharge the good battery, and in a worst-case scenario, damage the good battery as well.

For this reason, I have always been against hard-wiring batteries together in parallel to increase the capacity. I would rather find two higher capacity 6V batteries and wire them in series. That way, you would not have the parallel discharge problem.

If you can visualize, you have a single engine charger and two batteries. The only way to simultaneously charge both batteries by the engine is to put the battery switch into the "both" position. However, you should realize by now that leaving the switch into the both position when the engine charger is not active, results in essentially both batteries being paralleled together, with the risk of at least discharging both batteries, and the potential for one battery to damage the other.

Therefore, the conventional wisdom of using battery 1 on day 1 and battery 2 on day 2 is that each battery remains treated as a separate entity, and they are never paralleled. And as the switch is put into the opposite position each day, the theory goes that if the battery not connected is not used, its residual charge is enough to keep it charged in reserve for that day.

If you accidentally discharge the "active" battery, then you can switch to the reserve battery to start the engine. Only in an emergency, say if both batteries have been discharged, maybe only by using the "both" position may you have enough power to start the engine to get the charger going.

So in the conventional 1-both-2 scenario, hardwiring anything to one or the other battery, such as a DTS system, kinda defeats the purpose of keeping one battery in reserve. It would be far better that it be switched along with the starting circuit, unless there is some characteristic whereby it doesn't like the switching process.

By hardwiring a device such as the DTS, it will put a drain on the battery, and if that battery is the reserve battery, its obvous that the battery will discharge - without a means to recharge it.

The first attempt to fix this was to use something called a battery isolator. It had one input and two outputs. The isolator consisted of silicon diodes. For those of you that do not know how a diode works, it allows current to flow in one direction but not the other.

The practice was to place the isolator such that when using a 1-both-2 switch in the "1" position, or in the "2" position, each battery would be accessable via the isolator. The diodes were positioned so that the current from a charger would flow into each battery, but one battery could not "see" the other as the current flow would be blocked in that direction.

This allowed a single charger to charge both batteries, but not allow one battery to discharge into another. But this only worked if the 1-both-2 switch was never put into the both position, as that essentially bypassed the isolator. However, luckily, since both batteries are always charged regardless of the switch position, there is now never a need to put the switch into the both position, and you could then disregard the practice to swap from one battery to the other on a daily basis (although it is still a good idea to "excerise both batteries).

And if a device was hardwired to one battery, such as the DTS system, the battery would be charged via the isolator.

This works well enough, except that the diodes in the isolator introduce a voltage drop. The voltage drop - remember that the batteries are sensitive to voltages, and like to equalize each other; the voltage drop introduced by the diodes is enough to either require a longer charging time, or at worse, never allow the batteries to fully charge.

To alleviate this condition, some alternators can be setup to compensate for a battery isolator by outputting a higher voltage, or to sense the battery voltage after the diode insertion point. This setup however, is beyond the ability for most boat owners to add an isolator to their charging circuits at a later date.

Then, a device called a combiner came to market, and pretty much replaced the isolator - at least for the do-it-yourselfer project.

A combiner can be thought of as a voltage-sensing switch. It is typically connected so that both batteries are connected in parallel together in the presence of a charge voltage. A charger, whether it be the alternator, or shorepower type, typically places a higher voltage on the batteries than they are capable of themselves. Again, since batteries are voltage devices, and since the charger voltage is higher, the batteries will be charged.

The combiner can sense this higher-than-normal voltage when the charger is active, and will close the switch that parallels the batteries so that they can be charged simultaneously. When the charger is turned off, the combiner also senses the drop in voltage, and opens the switch so the batteries are no longer in parallel and effectively isolated from each other.

So in summary, the combiner "combines" or parallels the batteries in the presence of a charge voltage on either battery, and disconnects the batteries when the charge voltage on both batteries is removed. This in effect acts like an auto 1-both-2 switch.

Just one more piece of info then I am done.

Some combiners are only intended to pass charger current - 10amps, or so, while other combiners can pass the entire starting current that the battery is capable of delivering. Therefore, depending on the model combiner you buy (and your wallet), you can use a combiner in several scenarios.

The two most common is to use a combiner in conjunction with a 1-both-2 switch, or using a combiner by itself (not needing a 1-both-2 switch).

I hope this explaination is not too long-winded.

Al, do you're fingers get tired?

just kidding... great info.

I use batt 1 on even days and batt 2 on odd days. This is to keep me thinking, therefore not to let my brain drain away.


Actually, I use BOTH and switch to 1 or 2 when we're sitting without engine running.

No switch on either of my last 2 boats. I use a Yandina combiner and have an Optima starting battery and an Optima house battery.

I never have to think about this set up or "do anything" to it.

Downside is that if the starting battery goes dead for some reason, I will have to remove the battery cables and reverse the batteries to get the boat started.

Which is why I usually carry a PowerDome portable jumper, Haven't had to use it yet, but you never know.

Troop; why not just install a switch?

Thanks Al. It sounds like I need to buy a combiner. I am guessing since I do not use the boat a lot I probably did not run it enough on battery 2 to keep it charged.

Or carry a set of very short jumper cables?

The logistics and cost just don't make sense for a switch in my boat. Never had any need for it yet, and like I said, the jumper box would work if needed.

I would still probably carry the jumper box even if I had a switch...seems like there is always some other boater at the ramp or nearby on the water who needs it.

I start my boat with battery one, then turn the switch to both so they can both charge up. Switch it to one while we are on the hook, then start it with two. Turn it to both to run and charge them up, and then two while we are on the hook. I store it in the off position, and they were fully charged when I had it at the house yesterday.

Wow, not sure I could handle that. Way more complicated than "turn on blower switch, start engine" No wonder I like the combiner.

How much is a combiner? It's a pain...especially because we have to take out the cooler to get at the switch...

About $69 from yandina.com