To Parallel or not to Parallel ?

[bobgosling]

I know that it is common practice to design onboard electrical circuits to allow Engine and Domestic batteries to be "parallelled" in an emergency to allow the engine to be started off the domestic battery when the engine battery is flat but I was wondering ; how many people have ever been in a situation where they have had to do this AND WOULD NOT have had enough time to just swap the battery leads over ?

Besides which, if you parallel a flat and a full battery won't you just quickly end up with two half charged batteries ?

 

[snowleopard]

...Or have the house and engine batteries 15 ft apart.

I provide the extra volts by putting a short jump leads across the output terminals of the blocking diode. I'm thinking of wiring in a switch to save taking the engine cover off (which sometimes involves turfing the occupant of the aft cabin out of bed)

 

[Guest]

But why swap leads over when the bog std 1-Both-2-off switch caters for that already ??? Click Click - done.

And yes - I agree about flat and charged batterys ... IMHO foolish to connect them together ...

 

[srm]

My boat came with a changeover switch but I keep starter (100ah) and domestic (300ah) bateries in parallel nearly all the time for wind turbine charging. Using the full bank for engine starting causes little voltage drop if fully charged. However, if the bank voltage drops below 11.5v at sea I either start the engine to top up, or isolate the starter battery for later use. A small digital voltmeter display on main bulkhead lets me keep an eye on battery state all the time.
I know, its the opposite to what most folk do, but it has worked well for 3 years and the batteries still seem fine.

When on a boat with changeover switch and two similar sized batteries I would start engine on its dedicated battery which should be fully charged. If this failed I would then parallel them as domestic and engine batteries will probably then be at a similar state of discharge, rather than link a fully charged to a partly discharged battery. This is an intuitive response and I would like to know if any techies agree or otherwise.

If your batteries are close together, easilly accessible in a hurry, and you keep a dedicated jump lead for quick connection you can do without a switch. However, a disgruntled owner did tell me how a charterer lost his boat, when its anchor was dragging and the engine refused to start due to a flattened battery. A correctly wired change over switch to put all batteries in parallel may have saved the boat.

 

[Guest]

Re: To Parallel or not to Parallel ? Confused ???

Quote <<If this failed I would then parallel them as domestic and engine batteries will probably then be at a similar state of discharge, rather than link a fully charged to a partly discharged battery>>

Surely the only reason you would look at changing anything from start battery is if the start battery was flattened in failed starts ... then surely parrelleling them would be daft ?? And what is difference between parrelel and connecting the two batterys ?? (no-one is daft enough to series or short-connect the two batterys ?????). As I understand it parrelel the batterys and full one will discharge to flat one pretty damn quick ???????

 

[peteandthira]

Re: To Parallel or not to Parallel ? Confused ???

As I understand it, no matter what the state of charge of your two different batteries, when you connect them in parallel the good one will attempt to charge the bad one. However, when you attempt to start the engine on both, you will get the benefit of the energy available in both batteries. Even the bad one will provide some of the current providing it has sufficient charge.

Once yer engine's going again you can probably leave them in parallel and get them all charged up at the same time.

Prepared to be "enlightened" here!

Pops

 

[Talbot]

no need to parrrallel the batteries for wind or solar charging as decent regulators allow you to charge different battery banks and also to give priority to one bank if necessary.

 

[bazobeleza]

bobs right of course. I'm just lazy I suppose. its easier to have a batteries connected to a changeover switch, off, bat 1, bat2, or both. I carry a spare taxi battery in my engine room just in case the other batteries fail. i wish they did a three battery changeover switch, the battery charger has three lines out so the spare is always fully charged but I'd have to into the engine room to change it over if needed.

 

[srm]

True, but I am a strong beliver in keeping things as simple as possible afloat. Also, my turbine has a built in regulator.

I also read somewhere that two batteries used in parallel have a longer life than if each was used singly until it failed. Its to do with them being kept at a higher state of charge throughout their lives and avoiding deep discharging.

 

[Guest]

I know the argument for ...

But still feel it is better to use full charged or better on its own and not run risk of dragging it down ... Should it get to sluggish stage on starting - then put them together to get the greater potential ?

My main question though was the statement about connecting and the suggestion of two ways of putting two batterys together ... ??

 

[srm]

Re: I know the argument for ...

Its your choice, but I would fit a change over switch, its easy to use (even by charterers or crew) and if you ever need to swop batteries in a hurry you will be glad you did. With luck you will never need it in a real hurry but regard it as a tangible form of insurance.

Personal experience; the yard had left a boat (catamaran) on blocks to float off on the night tide. I had never handled it before. As the tide rose so did a strong cross wind. The boat had to be moved as it would not settle back on the blocks which could damage the hulls if it dried out. I was solo as the boat was only being moved a few hundred metres to a pontoon berth and the afternoon had been nice and calm. "No problem, I can do that for you".
As the boat started to float I tried to start the engine only to find a dead starter battery. Swopping batteries over involved physically moving them in a tight engine bay as they were either side of the engine and the cables would not reach across. Holding down bolts were mild steel, and rusty. 40 minutes of hard work in a small space left me with greasy bleeding hands and a running engine. I got the boat to its pontoon.

A correctly wired change over switch would have avoided all the hassle and adrenaline.

 

[Guest]

Why me ??? It was not me that started the post ???

I have a batterey change-over switch and am very happy with it ........... why am I the one getting the advice ??????

And I was the one who said fit the switch ......... not Bob !!!!

Please go to top of thread and check who has not got the switch and was talking about swapping leads etc.

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[srm]

Re: Why me ??? It was not me that started the post ???

Many apologies.

I had conflated the original question:

"how many people have ever been in a situation where they have had to do this AND WOULD NOT have had enough time to just swap the battery leads over ?"

with your question:

"My main question though was the statement about connecting and the suggestion of two ways of putting two batterys together ... ?? "

then wrongly assumed that you were restating the original post to bring us all back on track. Blame it on my age and trying to do something interesting during a coffee break. . . .

 

[shmoo]

Why not connect to batterys together?

This may get me flamed but here goes.

If you connect a fully charged battery across a more or less flat one very little current flows.

"Current flowing into a discharged battery" is the same saying "charging a discharged battery". The chemistry says you need 12.9V before ANY current will flow into(ie charge) the battery. Even 13.5 V would only be thought of as trickle charge. A battery at full charge (but not actually being charged) is 12.6 V. Not enough to "charge", ie push current into, the discharged battery - hence no current flows.

I think, IMHO etc

Peter

 

[shmoo]

Re: Why not connect to (oops, two) batteries together?

Sorry about spelling mstackes...

 

[Guest]

Re: Why not connect to batterys together?

I have a truck with a pair of batterys making up 24V .... ok they are in series ... but I think what happens is enough to illustrate that a battery does not care what voltage is available as long as it is more than it has ....

I often leave the truck standing between jobs and it needs a hell of a "battery" punch to start when really cold. So we charge batterys while standing for more than 3 days. (Tacho / other bits and bobs are still ticking away at systems ...) We used to charge each battery separate .... but found actually that we could charge one battery and that would be enough to get truck started.
Now I have actually checked this .... Charged one battery, started truck ... run for little bit and then stopped. Metered the batterys before and after ... one battery showing significant difference to other .... Meter next day and both batterys are very similar with lesser charged battery actually increased.
Now having spoken to local auto electrics guys here - they were the ones who said charge one battery then start .... they agreed that is what they would expect to see.

IMHO - I am no auto-electrician - but internal resistance and voltage needed to overcome is dependent on what level of charge is in the battery ... If its near flat - then any extra voltage in better charged battery will flow creating ampage charge to lesser battery .....

They also today reckoned that they would not put batterys together until both batterys separately have failed to do the job .... they reckon that putting good battery to bad and then cranking will flatten the good battery faster as it is cranking and also balancing out both batterys ....

I am not one to argue with them !!!

 

[SvenglishTommy]

Re: Why not connect to batterys together?

If you think of the emergency parallel switch as a diverter, then this isn't a problem. You would usually have the islotaors set up so you can have either 1, the other, or both banks powering either the starter bank, house bank or both.

It takes a charger long enough to get the AH back into flat battery at 14.8v, I doubt a few seconds at 12.5 volts is going to do much before the alternator kicks in.

 

[beneteau_305_553]

For years now I have had split charge diodes and Iv'e never had to do it. The engine starting battery is dedicated to that and the domestic batteries are similarly dedicated.
.

The battery switches only isolate each bank and cannot switch to "both" so the engine battery is always fully charged. If the domestic voltage gets low I just start the engine and dont have to think about switches.

 

[jfkal]

Re: Why not connect to batterys together?

Humble opinion but correct. Unless onebattery is totally wrecked there will be little flow across. A few amps at best which is nothing compared to the what a near empty battery can still provide once presented with a low resitance (i.e. Starter Motor).
However-- if the culprit is suspected to be near dead (i.e. internal cell short cut) it becomes a different story.

So the save procedure is to:

1. Switch to second battery and crank from there
2. If 1. does not work put them in parallel and try
3. If that does not work, Hand crank the engine (if you can)
4. If that does not work sail home or else quit boating

Enjoy.

 

[halcyon]

If you have a single engine, turn the isolator switch off, and start from service battery using link system.
If you have a twin engines, start the one that will start, isolate flat battery and start from the engine battery that is running.
If you have a large service bank, then the drain to the flat engine battery would not be bad, if the service and engine are about the same the voltage will fall, but not rapidly, you will need around 13 volt to charge the flat engine battery, and the service is around say 12 volt. So you will see more of a slow equalisation, how long will be dependant on your system, cabling, tempreature, etc etc.


Brian