Two domestic batteries wired in parallel or not?

[noswellplease]

For the last couple of years I had two batteries wired in parallel (domestic supply) and all seemed fine however on a recent visit to my boat which is in the boatyard for the winter I discovered that both batteries were flat! I separated the batteries and charged individually and found the faulty battery which basically dragged down the other one. Thankfully one battery now appears to be OK and holding charge at 12.6 Volts.

I am wondering is it better to simply keep a fully charged battery separate on board for any emergencies that may arise and can then be substituted if required. My good domestic battery is a very large capacity and well capable of supplying my minimal requirements. Any thoughts?

 

[VicS]

For the last couple of years I had two batteries wired in parallel (domestic supply) and all seemed fine however on a recent visit to my boat which is in the boatyard for the winter I discovered that both batteries were flat! I separated the batteries and charged individually and found the faulty battery which basically dragged down the other one. Thankfully one battery now appears to be OK and holding charge at 12.6 Volts.

I am wondering is it better to simply keep a fully charged battery separate on board for any emergencies that may arise and can then be substituted if required. My good domestic battery is a very large capacity and well capable of supplying my minimal requirements. Any thoughts?

You could consider a large capacity battery for normal use with a second smaller battery charged via a VSR for emergency use.

Maybe that is what you were thinking anyway

 

[Reptile Smile]

Vic, what does VSR mean?

Cheers

 

[Spi D]

A device to separate batteries when voltage drops:

The VSR, or Voltage Sensitive Relay, is a very handy little box that solves a load of traditional charging problems on marine electrical systems. It essentially serves as a smart battery switch deciding automatically when either one or two batteries are charged – or discharged. It works great on almost any boat with multiple batteries – and eliminates all of the guesswork that used to come with manual battery switches.

What a VSR does

The VSR is installed between two batteries. Many people are surprised to learn that it is NOT connected to either the alternator or charger output wires! Its setup is much more clever.

If either battery goes above 13.7 volts (due to either alternator or charger output), the VSR connects both batteries together. Both batteries are now charging – without the boater ever having to throw a switch.

Alternately, when the system voltage drops back below 12.6 volts, i.e., no more charging, the relay opens and the batteries are separate. This means that both batteries now discharge independently.

 

[reeac]

You could keep a jump starter aboard. That way you could start the engine and so have some charging capability. They cost around £40 upwards. I've had one aboard for several years but never needed it yet. I take it home and give it a charge once a year but it never needs much. Essentially it's a spare battery but in a convenient package with its own leads, voltmeter, switch and charger.

 

[jac]

The only problem with two separate batteries like that is that the domestic load you put on the one battery means that it discharges much further than if the two were paralled. E.g 50ah discharge from 100ah sole battery takes it to 50% charge (or 30%) if only charged to 80%. Which will shorten it's lifecycle significantly more than having two 100ah in parallel. The same discharge will take you to 75% (or 55%)

Adding in an emergency battery on top of the existing bank is an option of course but that will be adding cost, weight and complexity for minimal benefit IMHO. You already have an emergency battery - the engine start battery. Just remember to use it for it's proper purpose, starting the engine.

If you are thinking of spending the money on upgrading, invest it in either improving your charging or in reducing your consumption.

 

[PhillM]

You could keep a jump starter aboard. That way you could start the engine and so have some charging capability. They cost around £40 upwards. I've had one aboard for several years but never needed it yet. I take it home and give it a charge once a year but it never needs much. Essentially it's a spare battery but in a convenient package with its own leads, voltmeter, switch and charger.

I bought a small one from Halfords (100 model) which delivers 17AH. When I had a flat battery on my range rover I tried to use it. It didn't work. You should have heard the recovery man laugh when I showed him. Apparently it takes about 10x that to get the car started.

The point is to make sure you have one large enough to start your engine. Anyone any idea if 17ah is enough to start a beta 14?

 

[LP1]

For a starter battery, it's CCA that is more important than Ah.

 

[vyv_cox]

For a starter battery, it's CCA that is more important than Ah.

Provided it starts immediately. My experiment with a Red Flash 900 has not been a success. Although the CCA delivered is sufficient for the starter motor needs, its low capacity of only 13 Ah means that if it is slightly low on charge, or if the engine does not fire immediately, then it won't happen.

 

[Alex_Blackwood]

I bought a small one from Halfords (100 model) which delivers 17AH. When I had a flat battery on my range rover I tried to use it. It didn't work. You should have heard the recovery man laugh when I showed him. Apparently it takes about 10x that to get the car started.

The point is to make sure you have one large enough to start your engine. Anyone any idea if 17ah is enough to start a beta 14?

I think that Beta recommend 60ah for a 10/14Hp.

 

[ghostlymoron]

I bought a small one from Halfords (100 model) which delivers 17AH. When I had a flat battery on my range rover I tried to use it. It didn't work. You should have heard the recovery man laugh when I showed him. Apparently it takes about 10x that to get the car started.

The point is to make sure you have one large enough to start your engine. Anyone any idea if 17ah is enough to start a beta 14?

For a start 17ah is not the figure you want, you need the cold cranking amps capacity. A beta 14 is only 600cc ie only a third of a car engine so it doesn't take many amps to turn it over. I once started my yanmar twin cylinder with a jump start device so they do work. But only if its charged - its no use leaving it in a locker for years

 

[PeterWright]

You say two domestic batteries, from which I imply that you have a separate start battery for the engine. If this is the case, I would say that the domestic batteries are best wired in parallel. However, for this to work well, the paralleled batteries need to be as near identical as you can achieve, i.e same make and purchased from the same supplier at the same time. This means that if one fails you should replace both (worse in our case as we have 3 domestic batteries on parallel). You just have to console yourself with the thought that, if they have seen the same duty from new, when one fails, the other(s) are on their way out.

For domestic duty I use leisure batteries, and even these should not be allowed to discharge below 50% capacity - doing so shortens their life. Standard car batteries are even less tolerant of being discharged. Having a separate start battery (car battery fine for this duty) means that if you do discharge the domestic bank too far, you can still start the engine and charge it up again.

You can get batteries more tolerant of being discharged (e.g traction batteries), but the substantial extra cost rules these out for me.

 

[sailinglegend420]

Provided it starts immediately. My experiment with a Red Flash 900 has not been a success.....

I have a Red Flash 1100 lying on its side in the bilges - it has worked perfectly for 10 years. It's 37Ah, 775 CCA, starting a Yanmar 56 HP. Size is only 250mm x 97mm x 205mm. They are very small, but also very expensive but you can hide them away anywhere to provide more space for extra service batteries.

 

[vyv_cox]

I have a Red Flash 1100 lying on its side in the bilges - it has worked perfectly for 10 years. It's 37Ah, 775 CCA, starting a Yanmar 56 HP. Size is only 250mm x 97mm x 205mm. They are very small, but also very expensive but you can hide them away anywhere to provide more space for extra service batteries.

Yes, that's the way I am going. I can just fit one vertically alongside the gearbox in the tunnel to replace the 900. Not ordered yet as I will leave it until last minute before heading for Greece. I enquired during the 2013 season but there are no Red Flash agents in Greece.

 

[halcyon]

I think that Beta recommend 60ah for a 10/14Hp.

The reason for start battery capacity is to allow at least 10 starting cycles in cold weather, while maintaining a minimum capacity of around 40%. This is to minimize low voltage during starting, as the battery voltage falls the current drawn rises, the lower the battery capacity the greater the voltage fall.

It used to be common with production boat builders to fit common batteries to both service bank and engine start, these were never any problems. If you look at the current draw by say an inverter, it's not that different to normal starter motor loads.

If in doubt check the ratings for your starter motor, engine size is not a guide to amperage, it's the type and design of the starter motor. 1GM10 can pull upto 400 amp stall and 200 amp cranking.

Brian

 

[neil1967]

I fitted an Optima RTS4.2 red top battery this season as a dedicated starter battery. It's not as small as the red flash, but is about half the size of the wet acid battery it replaced (an of course can be installed anywhere). It's 50AH and 815 CCA. It is used to start a 50HP Perkins 4108, which it does effortlessly. Even when I struggle to bleed the engine and have to turn it over repeatedly on the starter, I have had no problems at all. One of the reasons I went for the Optima was that the charge regime is not too dissimilar to a wet battery (I have 3 x Trojan 130AH wet batteries in the domestic bank), so the Optima should not be damaged by the mains charger or the alternator booster.

Neil