Battery Emergency Tie switch

[goboatingnow]

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.

as an absolute recommendation,, this advice is nonsense and would preclude you from jump starting anything

[GreenWave]

Originally Posted by wingssail

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.

This post and Stu's right before it are pure GOLD!
Thanks!

[Bycrick]

Perhaps you feel comfortable "clip, clip, clipping" under bad conditions, but after being in the electronics business since 1958 and sailing some 40k miles since 1976, I wouldn’t do it.

[ayates]

For our house bank (12V, 740 AH flooded lead acid), I have it split in half. Normally both halves are in parallel, as a single bank. But either half can be isolated in the event that a cell is lost in one of the batteries.


Allan.

[ayates]

For a planned upgrade to the distribution system, I went with the tie bus concept. So any/all of the engine start, generator start, and house batteries can be tied together. The objective being to easily start the engine and/or generator in the event one of their batteries fail



Allan.

[wrwakefield]

We have a seperate 1; both; 2; off battery switch dedicated to only supplying power to engine starter motors. [Main and generator in our case.]

It has nothing to do with charging sources, so it can be turned off when the alternator is running without repurcussions.

The default setting is Starter battery. [#1; starter battery is shared by both engines.]

The back-up is House Bank. [#2]

Emergency is Both.

Note this switch can be turned off to prevent either engine from being started [SOP when the through hulls are closed, etc.]

This set up works well for us, and provides full flexibility in an emergency engine start situation.

In case this permutation is of interest.

Cheers! Bill

[noelex 77]

If you are using jumper cables, be careful . There will often be a spark when the batteries are joined and this can ignite hydrogen. Defective batteries can sometimes vent a lot of gas. A battery explosion can ruin your whole day.

It is better to attach the jumper cable well away from the battery itself (as you would when jumping a car) but this can be difficult to do on some boats.

A dedicated connection with battery switches is safer. With some of the options listed in this thread it is possible to leave the start battery connected or not depending on the position of the switches.

[rondelais]

This is an interesting discussion. Originally, when I rewired my boat, I put the paralleling switch on the battery side. Then, after some thought about the chances of a battery shorting out, I switched it to the load side, allowing isolation of the batteries. My other reasoning was that there is almost no situation I could think of for having both batteries driving the house load. I guess maybe the best answer might be to have two 1-2-B switches - one for the engine, and one for the house loads. Then, every combination is possible. Of course, if one of the switches is in “B” the batteries are paralleled. The only downside is that you have to think. With my current setup, I just turn on the house switch and the engine switch, and I’m done unless something goes wrong.

[jmschmidt]

Don't guess, Blue Sea makes a rotary battery switch that does it all.

[sailingharry]

OP here. Lot of good thoughts, and I appreciate them. I'm not 100% sure I'm motivated to do anything on it immediately -- it's been there for two decades. But I am convinced the correct answer is to move my tie switch to the load side of the battery switches. It may happen this winter, it will certainly happen before a lithium upgrade.

Many of the suggested alternate wiring methods are reasonable methods to wiring a boat. And on a previous boat, I have used and highly recommend the classic AB switch as main and reserve, as Stu suggested. But I certainly don't see the benefit in completely redoing the existing system. In particular, while I don't see the existence of a dedicated start battery (as I have) as being better than using the house bank as the everything bank, I equally do not see it as something that is bad.

With regard to the risk of alternate feed to the engine or any other accidental cross connect. A post-up thread talked about the key hanging on a lanyard. On my boat, the key is actually screwed to the bulkhead beside the switch. If I was sufficiently motivated, I could yank it right off the bulkhead without tools. But there is zero risk of an accidental cross connect!

[CaptTom]

Good discussion. Following.

The only thing I can add is that I've only seen the parallel as a momentary switch.

The point is, it's just a convenience, in case you kill one bank at anchor or drift fishing, and don't feel like climbing down into the engine room to adjust the main battery selector switches. This is especially useful if you start one engine off the house bank. You know what the problem is, you just want to get going quickly.

Anything beyond that, and all the other concerns (shorted cell, etc.) come into play, and you should be manipulating switches and/or jumper cables, not just pushing a button.

[Bowdrie]

I think that generally speaking moat of us could agree that our various boats each pose some kind of constraints on space for batteries, or space around the engine for alternator(s), or difficulty of cable runs.
So, we each figure out something that will work for our individual needs and also supports our individual philosophy.
I suppose that given enough switches it becomes possible to do any job with any battery or isolate any battery of choice.
I use two alternators, the "stock" internally regulated auto-type unit only charges the two start batteries, and the big unit only charges the house bank.
The two systems share nothing in common, neither hots nor grounds.
With two starting batts and a 1-2-B switch the chances of ever having to cross-connect the house to start the engine is extremely small.
It's simple, switches are few, runs are short.
No, it's not perfect, few things on a boat are.
Sometimes we have to choose between the rock and the hard place.