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Old 17-02-2022, 08:11   #1
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Battery Emergency Tie switch

So, I'm doing a bunch of high power work. New inverter, solar upgrade, revising some fuse layout, etc. And I got to pondering. It's a disease....


I have an "emergency tie" that allows "Start" and "House" to be interconnected. I know it's an exceptionally rare situation, but it's a good idea. But I think it's done wrong.

The tie connects the Battery hot bus to the House hot bus. Direct. This is essentially connecting the battery + posts together. I think it should connect the load side of the respective switches. In fact, if I were to re-do it, I'd replace the 3' long cables with 8" cables, and actually connect on the load side of the switch.

Here's my reasoning. The primary reason you would use this switch is if one battery was toast. Internal short, or just DEAD. If you close the Emergency Tie, you connect the batteries together. Except for a direct internal short in the start battery, the house bank probably won't notice (but still, you'll be dumping serious amps into the dead battery that you may not want to) -- and if you did have a direct internal short, then it would really be bad to connect them. And if for some reason, you had a dead house bank and wanted the engine battery to run the instruments, that wouldn't be good! As wired, this is not optional -- it is what the switch does! If it were wired AFTER the battery switch, you could disconnect the defective battery, close the tie, and be off to the races. (Of course, since my alternators are connected directly to the house battery, that might cause some other issues...LOL).

The only other reason for the tie is if somehow your means to charge the start battery were OOC, you could tie them together to charge them. In my proposed re-wire, you could still do that -- get the charger online, and then close the tie and keep both battery switches closed -- and the charger would charge both at the same time.

It's not a real big deal -- the chances of ever needing that switch are miniscule. But it seems the way it is wired would likely defeat the intent!

Oh, and as an aside, if a LiFePO4 upgrade is in the future, connecting both banks together is even less desirable.

Anyone have thoughts?


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Old 17-02-2022, 09:39   #2
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Re: Battery Emergency Tie switch

Following. And hoping to learn something.
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Old 17-02-2022, 10:06   #3
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Re: Battery Emergency Tie switch

I carry jumper cables, had a start battery die once, learned from that.
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Old 17-02-2022, 10:13   #4
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Re: Battery Emergency Tie switch

Two possibles: The starter battery is partially discharged and will not start the engine. In that case joining the positives makes sense.

Second case: The starter battery is seriously toast, and shorted. Then you need to replace the function, rather than paralleling the two.

Remember that the negatives from everything are always connected together through your bonding.

So, really, a single jumper cable is a better tool than battery cables and switch. In the first scenario you connect the two positives with the cable. In the second, you disconnect the positive cable from the starter battery, and connect the jumper cable to it but not the battery.

You are talking about unusual but foreseeable events. A jumper cable gives you versatility without complicated wiring.
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Old 17-02-2022, 10:15   #5
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Re: Battery Emergency Tie switch

I came to the same conclusion years ago. I have two ON-OFF switches, one for each battery. The bridging switch connects the LOAD side of the two switches.
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Old 17-02-2022, 10:17   #6
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Re: Battery Emergency Tie switch

Old big trucks, and my boat until I removed it, had a relay that let you put two batteries in parallel or series. Normal was parallel = 12v. Series for starting on cold mornings was series = 24v, which sure as hell will spin a 12v starter.
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Old 17-02-2022, 10:27   #7
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Re: Battery Emergency Tie switch

The ONLY time battery banks should be tied together is when charging source(s) are present. NOT if/when one bank is either dead, broken or seriously discharged.


Here are different ways to wire different types of switches together properly:


OEM 1-2-B Switch Wiring History http://c34.org/bbs/index.php/topic,4....html#msg30101

1-2-B Considerations (New 2020 - Rod finally got around to diagramming what I had done in the above link in 2009 )
https://marinehowto.com/1-2-both-bat...onsiderations/

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: http://c34.org/bbs/index.php/topic,6604.0.html

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowner...d.php?t=137615

This is a newer primer for boat system wiring design with a thorough diagram: Building a Good Foundation (October 2016)
http://forums.sailboatowners.com/ind.../#post-1332240

The Short Version of the 1-2-B Switch Stuff: http://c34.org/bbs/index.php/topic,5....html#msg38552 This is a link to the Electrical Systems 101 Topic, reply #2

What are ACRs, Combiners & Echo Chargers? (by Maine Sail) [scroll to the top]
http://forums.catalina.sailboatowner...d.php?p=742417 and http://c34.org/bbs/index.php/topic,9....html#msg70131

Making Sense of Automatic Charging Relays (2019)
https://marinehowto.com/automatic-charging-relays/


************************
************************


These links come from my series:
Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html
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Old 17-02-2022, 15:26   #8
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Re: Battery Emergency Tie switch

Well, Stu wants you to read a few books. Good idea.

But here is a simple alternative:

Install Off-1-Both-2 Battery switch.

Connect House Battery to 1

Connect Start Battery to 2

Connect ALL LOADS to the output side of that switch (include engine starter, bilge pump, etc. Everything. Use breakers or switches to allow disconnection.

Connect ALL CHARGING sources to the same output side as the loads. Everything. Use breakers or switches to allow disconnection.

You can run your house off of either battery or both.

You can start your engine off of either battery or both.

You can charge 1, 2, or Both under your control from all available charging sources.

Keep you brain intact and avoid switching to OFF with charging sources engaged, (particularly the Alternator).

Remember, you are in charge of this system, not some little black boxes.

Throw away about 10 useless bits such as combiners, isolators, redundant fuses, tie cables, etc.

When you go to LiFePO4 get a BMS which can switch off the charging sources rather than drop the batteries out of the system.
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Old 17-02-2022, 15:53   #9
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Re: Battery Emergency Tie switch

Wings,


My last boat, I did a major rework on the battery system. There never was a "start" on that 1979 Sabre 34! I had a pair of golf carts for the "main" bank, and a Group 24 for "emergency," spots 1 and 2 on the 1/2/both switch. Worked a treat! I have drunk the Cool Aide of Maine and Stu, and believe the 1-2-both switch has an undeserved reputation. In the 8 years I sailed with that combination, I think I need the "emergency" battery maybe twice (remember, that's only 220Ah of battery, with refrigeration included!).



I'm not looking to reinvent the wheel on this boat. The installation is pretty rock solid. But I'm really tempted to remove the 6 feet of wire that is cluttering up my wiring -- and doing a DISservice to the system -- and replace it with 2 8" wires to the load side of the switches. For the cost of two 2/0 lugs, I get what I see as a big improvement (but see my other thread about how to get lugs attached....LOL).


Oh, on both boats, I do disagree slightly with your connection. My alternator, bilge pumps, battery monitors, alternator sense, and a few other key loads are on not the switch. They are on a small blade fuse bank connected through a breaker directly to the main battery fuse (well, the alternator has a breaker and then directly to the battery fuse). I don't want any of those items to be switched off EVER. I also think an ACR is an incredible little device, although my current boat doesn't have one.


And finally, with LiFePO4, you are 100% spot on. I am NOT going to disconnect the alternators! And I'm NOT going to have them feed the start AGM and then buy a bank of DC-DC chargers. My BMS will simply disconnect the ignition wire from the regulators. It will also shunt the "disable" pins on the inverter charger. Those two actions will deal with almost all of the high power loads and chargers connected to the battery, and the rest will be much easier to deal with.




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Old 17-02-2022, 19:52   #10
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Re: Battery Emergency Tie switch

connect the batteries together.

connecting the load side of the switch is dangerous. because then when the engine switch is off, it's still possible to get power to the motor if the parallel is accidentally left on. someone working on the engine could be badly injuried or killed if this happened.

if the engine switch is off there should be 100% no power getting to the engine ever.

the chances of having a physically bad battery is tiny. the most common use of it is for a drained battery. and jumping a drained battery works just fine.


if you really want to be able to start from either battery. then the correct way is to use a 1-2-all swtich. from engine battery, house battery to engine. then you can start off either or from both. but when the swtich is off, it's off. so no safty issue.

you would still need a 2nd swtich for the house loads. it could just be an on / off from the house battery to panel. or it could be a 2nd 1-2-all to allow the panel to be fed from either battery as well.

you would need an acr or simular to allow both batteries to charge.

using a single 1-2-all to feed the engine and house together as posted above is horrible. this is not the 1980's any more.


same goes e for acr / vsrs. they must be on the battery side for same reason of allowing power to bypass an off switch.
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Old 17-02-2022, 20:58   #11
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Re: Battery Emergency Tie switch

It doesn't matter, there is no best practice - there is only good practice. Making sure the conductors and terminals are suitable and well made is way more important than the final design used. Get to know the design used and the ramifications of your choice and all will be good.

The two posters below are both well experienced, know their electrons and mostly I would listen to their advice. Note they are on opposite sides of the fence on this question. This tells me either is good and neither is best.

Quote:
Originally Posted by Bycrick View Post
I came to the same conclusion years ago. I have two ON-OFF switches, one for each battery. The bridging switch connects the LOAD side of the two switches.
Quote:
Originally Posted by smac999 View Post
connect the batteries together.

connecting the load side of the switch is dangerous. because then when the engine switch is off, it's still possible to get power to the motor if the parallel is accidentally left on. someone working on the engine could be badly injuried or killed if this happened.

if the engine switch is off there should be 100% no power getting to the engine ever.

the chances of having a physically bad battery is tiny. the most common use of it is for a drained battery. and jumping a drained battery works just fine.


if you really want to be able to start from either battery. then the correct way is to use a 1-2-all swtich. from engine battery, house battery to engine. then you can start off either or from both. but when the swtich is off, it's off. so no safty issue.

you would still need a 2nd swtich for the house loads. it could just be an on / off from the house battery to panel. or it could be a 2nd 1-2-all to allow the panel to be fed from either battery as well.

you would need an acr or simular to allow both batteries to charge.

using a single 1-2-all to feed the engine and house together as posted above is horrible. this is not the 1980's any more.


same goes e for acr / vsrs. they must be on the battery side for same reason of allowing power to bypass an off switch.
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Old 18-02-2022, 08:33   #12
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Re: Battery Emergency Tie switch

"Dangerous?" Not really. With a separate house/starter battery system, the only reason to connect the batteries together is to charge them. In an emergency where one side "fails," one would normally turn off the switch to the failed battery BEFORE bridging the two loads.

In my case, the battery switches are simple ON-OFF. The bridging switch has a key (attached on a lanyard next the the switch), so you’re unlikely to bridge them accidentally. Because my memory isn’t always perfect, there’s even a warning placard on the switch that I can move over to the voltmeter area, which I look at frequently.

I don’t like charging multiple batteries from a single charger. So I’ve rules out ACRs/VSR.

YMMV.
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Old 18-02-2022, 08:36   #13
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Re: Battery Emergency Tie switch

a jump lead is the best solution , simple and can be deployed in the most advantageous way depending on the situation
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Old 18-02-2022, 08:43   #14
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Re: Battery Emergency Tie switch

If all connections are easily accessible and lighted, everything is properly marked, you know what you’re doing electrically, the weather is calm, and you’ve had plenty of sleep, maybe you’re in a position to play with a jumper wire with clips, when the dirt hits the fan. But I don’t trust that solution in a real emergency. But, feel free …..
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Old 18-02-2022, 08:45   #15
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Re: Battery Emergency Tie switch

Quote:
Originally Posted by Bycrick View Post
If all connections are easily accessible and lighted, everything is properly marked, you know what you’re doing electrically, the weather is calm, and you’ve had plenty of sleep, maybe you’re in a position to play with a jumper wire with clips, when the dirt hits the fan. But I don’t trust that solution in a real emergency. But, feel free …..
clip , clip ,clip and clip , its not rocket science
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