Dual Batteries...can 1 dead one drain the other one?

[SoldTheBayliner]

I've never had/messed with a dual battery set-up, so I'm relatively clueless... I bought this boat (2000 SAN) in August, and it had been in storage for a long time. The seller had put a brand new battery in because it had been sitting. We used it for the end of summer with no issues, and I never messed with the other battery or the battery switch (it has a Perko dual battery switch). I think the switch was in the 'all' position the whole time. You can choose '1', '2', 'all', or 'off' on the switch. I'm assuming the new battery is #1, it has gobs of stuff attached to it (stereo related, big fat power, lots of grounds), and the other one just has two wires. I followed the positive wire from the second battery thinking it would go straight to the switch, but it heads up towards the dash (? Fuse?) Anyway this 2 month old battery was dead when I went out to winterize the boat after no use for maybe 4 weeks. I'm sure the ignition was off, but the battery switch was not off.

1. Can a dead 'second' battery drain a new 'main' battery?

2. Should I be switching the battery switch 'off' all the time between uses? Will this stop any drain from accessories?

3. What else could drain the battery during non-use? Stereo/amp stuff seems to be all off...no lights, etc.

4. Any basic explanation of how a typical dual battery set-up works would be most helpful.

The new battery seems to be taking a charge fine, so it's getting charged and going into room-temp storage for the winter, and I'm planning on a new second battery in the spring.

[DanielC]

I hate to say it, but if your boat is not in stock from the factory condition, it is any body's guess on how the batteries are wired.
Your boat sounds like it is not stock.

[hyparks]

I had a battery problem earlier this year and was going to wire in a second battery.
I chose not to in the end as I was told by several people that 1 dead battery can drain another.
I just replaced the one that was on the boat.
So not really sure how true it is but that is what I was told.



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[shag]

I switch mine to off when Im done. There could definitely be something draining them such as stereo memory, Etc. If there is a wiring problem, they can drain even faster. I believe the recommended practice is to have the switch on 'both' while running, then if you sit somewhere for awhile listening to the stereo, switch to one battery. This way you don't run both batteries down. Then switch back to both when running again so you charge the other one back up.

[SoldTheBayliner]

I'm sure it's not a factory set-up. But assuming that at least the switch is wired correctly, do you effectively cut off all of the potential battery drains just by putting the switch to "off" when you're not using the boat? I need to look, but should the hot wires from the stereo, amp, etc be going straight to the battery, or to the switch?
In my mind the only way the switch could 'kill' everything would be if there was one hot wire coming to the battery from the switch, and everything else was on the 'other side' of the switch.

[TxJole]

Stereo memory and bilge pump should be the only "always hot" items. If I remember correctly.

[SoldTheBayliner]

Does that mean the hot wires from those 'always hot' items should bypass the switch?

[TxJole]

You can or the hot side of the switch.

[SoldTheBayliner]

OK. I have some investigating to do on the wiring it looks like. But it sounds like I'm safe in assuming that it's a good chance that the one dead battery is what was sucking the life out of the other battery... so with two good batteries, if all of the wiring was done right, I won't see the drain like I did this time. I'll check out the wiring in the mean time, and see what happens with two new batteries in the spring before I start re-wiring stuff. Based on what I know about the previous owner, I'm sure he had a dealer install the dual battery set-up, so hopefully it's done right.

[kaikane]

Make sure you have some type of multi battery isolater on the system seperating the two batteries from draining each other. You may also want to make sure you have a battery selector switch that allows 3 seperate functions: Battery A / Battery B / Combined A+B. Blue sea systems has a combo pack that includes the isolator and switch for a multi battery set up. To answer your question, yes, a 'dead' battery will drain the power from a 'good' battery in any combined bank, hence the need for some type of isolation.

[SoldTheBayliner]

What does an isolator look like? Maybe I have one and don't know it? My battery switch has "1", "2", "all", and "off". Isn't that what you mean? And does the switch act like an isolator when it's in "1" or "2" mode?

[hyparks]

Although mine only has 1 battery my switch has on, off and bilge. Before this switch I have a large red isolator. Basically 2 big red wires. Obviously switching 12v directly from battery

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[mrbobolina]

YES your dead battery can drain your good one. Your battery is likely running in a loop system. Where the positives run through the perko and then the grounds are common. When your switch is on battery 1 or battery 2 only that battery is getting charged when you are running. If your battery 1 is dead and that is your stereo batter no big deal. If battery two is good and you leave your switch on battery two you are again okay.

If you switch your perko to both what WILL HAPPEN! You will use the good battery to feed the dead battery. You will take 14volts from the good batter and feed the bad battery which if were at 5 volts, and the two will now be 10 volts. Your boat will now not start. The reason this happens is because the perko does not segment good battery from bad battery when you flip the switch it is strictly for the alternator to know where to send the power.

In order to remove this from happening you need something like a Yandina between the batteries, which keeps you from having a dead battery kill a good one. People will say you don't need it which is true, however if you flip the switch wrong, oops you are fkd.

When you have the boat sitting on dry land and your bilge does not need to run, you should have the switch to off. If you need the bilge running you can leave it to the batter that you want to leverage.

When you are charging your batteries take them off the grounds and positive feeds. You are not going to charge them both at once and can overfeed your amps, as well as the starter and the alternator and the master solenoid. The alternator, and solenoid and starter are not real threats, however baking your amp with power it cannot release is.

With your stereo batter, be sure it is deep cycle. Between the winter and normal boat abuse, you will kill that batter over and over. A normal battery or a yellow cap will have a shorter life than it should.

Never move from battery1,2 or both to off with boat running. Nor do not ever go from off to 1,2,both when running. This will kill your alternator.

Finally, despite your best intentions it is good practice to keep a charged jumper on the boat. You just never know when you will have the batteries fail you>

Hope this helps.

[AllianceBJJ]

What is the popular opinion on what brand of "Deep-Cylce" battery is the best for the $$ these days?

[wake_fun]

YES your dead battery can drain your good one. Your battery is likely running in a loop system. Where the positives run through the perko and then the grounds are common. When your switch is on battery 1 or battery 2 only that battery is getting charged when you are running. If your battery 1 is dead and that is your stereo batter no big deal. If battery two is good and you leave your switch on battery two you are again okay.

If you switch your perko to both what WILL HAPPEN! You will use the good battery to feed the dead battery. You will take 14volts from the good batter and feed the bad battery which if were at 5 volts, and the two will now be 10 volts. Your boat will now not start. The reason this happens is because the perko does not O good battery from bad battery when you flip the switch it is strictly for the alternator to know where to send the power.

In order to remove this from happening you need something like a Yandina between the batteries, which keeps you from having a dead battery kill a good one. People will say you don't need it which is true, however if you flip the switch wrong, oops you are fkd.

When you have the boat sitting on dry land and your bilge does not need to run, you should have the switch to off. If you need the bilge running you can leave it to the batter that you want to leverage.

When you are charging your batteries take them off the grounds and positive feeds. You are not going to charge them both at once and can overfeed your amps, as well as the starter and the alternator and the master solenoid. The alternator, and solenoid and starter are not real threats, however baking your amp with power it cannot release is.

With your stereo batter, be sure it is deep cycle. Between the winter and normal boat abuse, you will kill that batter over and over. A normal battery or a yellow cap will have a shorter life than it should.

Never move from battery1,2 or both to off with boat running. Nor do not ever go from off to 1,2,both when running. This will kill your alternator.

Finally, despite your best intentions it is good practice to keep a charged jumper on the boat. You just never know when you will have the batteries fail you>

Hope this helps.

You can switch to one, two, or both as long as you don't pass through the "OFF" while the engine is running.