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In an emergency, can yourun both motors off one battery?

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In an emergency, can yourun both motors off one battery?

Old 05-28-2018, 05:54 PM
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Default In an emergency, can yourun both motors off one battery?

I have twin opti's and two batteries with two Perko Switches (one, two, both, off).

I normally run one motor on one battery and the other on the other battery. I only put Perko switch on "both" if I need the extra juice to start one, then after motor is started put it back on switch one or two.

My concern would be, if a battery died or was drained from electronics or whatever, can I disconnect a battery on put the switch on both to start and run both motors. My thinking is if bad battery was disconnected it couldn't drain my good battery.
Old 05-28-2018, 05:59 PM
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If the battery isn't bad you could just start it. It's not good to switch switches while the engines are running as far as I know. I think it can ruin alternators.
Old 05-28-2018, 06:36 PM
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I have twin Optis and yes, you can definitely run them both off one battery. No problem.
Old 05-28-2018, 06:45 PM
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I've been told that if the switch is make before break than you can switch from batt one to batt two with the engine running and you won't do any harm to the alternator/regulator. Now running both engines off of one battery is not advisable.
Old 05-28-2018, 07:04 PM
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An emergency is just that... Why not install a crossover/parallel switch so you can jump-start one from the other?
Old 05-28-2018, 07:17 PM
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you can start both motors off of one battery. I do it on my boat all the time. I also switch between 1 - all - 2 while motors are running without issue. I have been doing it for 15 years without issues. I am not sure switching is as big an issue as some may make it out to be.
Old 05-28-2018, 08:21 PM
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Mmmmm I could be way off the mark here, but its my best shot. If any of THT is an electrical major etc... then pipe up, I'd hate to be giving misleading info, that I haven't even tried myself. If you're not an electrical aficionado--then please refrain from dissecting, as it will only add to the confusion if we are both wrong. So wait till some others pitch in who state their electrical knowledge/experience. Lastly this only applies to bad batteries--not one drained down at the sandbar listening to the radio etc.., which can be charged by switching to the "both" position when running without consequence---okay? So here goes:

If both engines alternators are good then running both off one battery is no problem. But if there is a short in the battery in question or a dead cell in it, then there is a problem. Its like putting one dead battery in a flashlight and a brand new one in a 2 cell flashlight. Yes the one battery will give light---but only for a very short time , it will drain quick. Same principal if you put the switch in "both" and have a short in one battery, regardless if the alternators are charging. It will bring the system down as a whole.The drain is quicker than the charge can possibly keep up over time. An alternator can only keep up with a specific drain amount. A short or dead cell is a 100% drain, there isn't an alternator made going to keep up with that.

Which going back to boats, means taking the bad battery out of that loop, Either by switching that engine battery out of the loop (cant use that engine now) or physically removing the bad battery by disconnecting cables, and closing the loop by joining its cables together to allow the "dead" engines leads to be looped into system again---without worrying about draining the good battery. Connect reds to reds and electrically tape, then blacks to black like the reds--but separate of each other--it completes the circuit sans the battery. Connect snugly using either small bolts or vise grips. Then protect with electrical tape (temporary till you get back in etc...) Then you can be sure, that no draining of the good battery occurs inadvertently. And both alts will be charging the one battery, which their respective voltage regulators will keep that in balance. Any thought guys? I'm just rationalizing this. Technically what I said is sound-- but I've never had to do it and I wouldn't want to be giving out bad info. Remember any electrical wizards in the THT trust? Please pitch in, I'm curious if what I said makes sense.

Last edited by antlerz22; 05-28-2018 at 08:29 PM.
Old 05-29-2018, 05:25 AM
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The only time an engine is "running off" a battery is while its cranking.
Once running at idle or above, it "makes" the electricity it needs - and more.
Now, a battery still needs to be connected,
or the charging system electronics can be damaged.
But there is no reason two engines can not "share" a battery.

Originally Posted by antlerz22 View Post
A short or dead cell is a 100% drain, there isn't an alternator made going to keep up with that.
A "shorted" cell, yes - essentially becomes a 10 volt battery.
A "dead" cell - since they are all in series - means a "dead" battery.


Originally Posted by antlerz22 View Post
Which going back to boats, means taking the bad battery out of that loop, Either by switching that engine battery out of the loop (cant use that engine now) or physically removing the bad battery by disconnecting cables, ......., I'm curious if what I said makes sense.
Overall, your point is valid - but you are overthinking the "emergency wiring" required.

In that scenario, all that would be needed to isolate the "bad" battery
is to simply remove it's ground cable(s).
Done. Nothing needs to be "insulated".

Last edited by Fwpratt; 05-29-2018 at 07:25 AM. Reason: fixed bad arithmetic!
Old 05-29-2018, 05:45 AM
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Originally Posted by antlerz22 View Post
Mmmmm I could be way off the mark here, but its my best shot. If any of THT is an electrical major etc... then pipe up, I'd hate to be giving misleading info, that I haven't even tried myself. If you're not an electrical aficionado--then please refrain from dissecting, as it will only add to the confusion if we are both wrong. So wait till some others pitch in who state their electrical knowledge/experience. Lastly this only applies to bad batteries--not one drained down at the sandbar listening to the radio etc.., which can be charged by switching to the "both" position when running without consequence---okay? So here goes:

If both engines alternators are good then running both off one battery is no problem. But if there is a short in the battery in question or a dead cell in it, then there is a problem. Its like putting one dead battery in a flashlight and a brand new one in a 2 cell flashlight. Yes the one battery will give light---but only for a very short time , it will drain quick. Same principal if you put the switch in "both" and have a short in one battery, regardless if the alternators are charging. It will bring the system down as a whole.The drain is quicker than the charge can possibly keep up over time. An alternator can only keep up with a specific drain amount. A short or dead cell is a 100% drain, there isn't an alternator made going to keep up with that.

Which going back to boats, means taking the bad battery out of that loop, Either by switching that engine battery out of the loop (cant use that engine now) or physically removing the bad battery by disconnecting cables, and closing the loop by joining its cables together to allow the "dead" engines leads to be looped into system again---without worrying about draining the good battery. Connect reds to reds and electrically tape, then blacks to black like the reds--but separate of each other--it completes the circuit sans the battery. Connect snugly using either small bolts or vise grips. Then protect with electrical tape (temporary till you get back in etc...) Then you can be sure, that no draining of the good battery occurs inadvertently. And both alts will be charging the one battery, which their respective voltage regulators will keep that in balance. Any thought guys? I'm just rationalizing this. Technically what I said is sound-- but I've never had to do it and I wouldn't want to be giving out bad info. Remember any electrical wizards in the THT trust? Please pitch in, I'm curious if what I said makes sense.
Boy, those are a lot of wrong words. First, the analogy to a flashlight is totally wrong. Those batteries are in series and the boat batteries are connected ("Both" position) in parallel. True, paralleling a bad battery with a good one will bring down the bad one if there's a shorted cell. If an open cell makes no difference. As previously said, running both motors off one battery is fine.
Old 05-29-2018, 06:04 AM
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If your switches are wired correctly you should not need to disconnect the dead battery, that is what the switches are for. If battery 1 is dead simply put both switches on 2 and battery 1 will be disconnected from the system.
Old 05-29-2018, 07:12 AM
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Originally Posted by Fwpratt View Post
A "shorted" cell, yes - essentially becomes an 8 volt battery.
A "dead" cell - since they are all in series - means a "dead" battery.
Well, probably more like a 10.5 volt battery (2.1v per cell, on a normal lead-acid battery).

It's pretty rare to have an actual shorted cell. More likely it's a high-resistance cell which doesn't contribute any power under load.
Old 05-29-2018, 07:18 AM
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Originally Posted by afteru View Post
I have twin opti's and two batteries with two Perko Switches (one, two, both, off).

I normally run one motor on one battery and the other on the other battery. I only put Perko switch on "both" if I need the extra juice to start one, then after motor is started put it back on switch one or two.

My concern would be, if a battery died or was drained from electronics or whatever, can I disconnect a battery on put the switch on both to start and run both motors. My thinking is if bad battery was disconnected it couldn't drain my good battery.
When I get offshore I switch both engines to one battery and isolate the other so if electronics drain the in use battery too much I know I have a hot one to restart engines. You can switch while engines running but don’t pass ‘Off’ when switching.
Old 05-29-2018, 07:24 AM
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Originally Posted by km1125 View Post
Well, probably more like a 10.5 volt battery (2.1v per cell, on a normal lead-acid battery).

It's pretty rare to have an actual shorted cell. More likely it's a high-resistance cell which doesn't contribute any power under load.
yep - I stopped counting one finger too soon!

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