adding a second battery....is as simple as?

[bakes]

i would like to add a second battery to my jag 27. my plans are to have two battery banks alternated in house use with the unused bank kept in reserve for cranking.

is it as simple as getting two deep cycle batteries of the same capacity(replacing the current cranking one), connecting them in parallel and introducing a isolation/changeover switch?

currently the anode, electrics bus and an unknown wire(!) connect to the neg terminal of the battery which in turn is connected to engine ground. Should these now all be grounded directly to the engine block?

and one other question. when adding the second battery can its neg terminal be connected to the neg terminal of the number 1 battery which in turn is directly connected to engine ground? or do they need to go to engine ground separately?

any help would be greatly appreciated

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[LORDNELSON]

I do not know the current demand of your starter motor but it is customery to use a deep cycle battery for domestic use and a starter type battery for the engine. Whilst you may well be able to start an engine from a deep cycle battery the theory is that they are not built to regularly meet the sudden large current demand made by the starter motor.

It is not unusual, in my experience to have a common ground for all batteries.

I expect you will receive much more detailed and more technical advice from the boffins who contribute to this forum, just wanted to say wait a little while before you buy the new batterie/s.

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[alex_rogers]

My Vancouver 28 has two identical 100AHr batteries - one for engine starting and one for domestic use. They both have seperate breaker switches and one other switch beyond these two switches that allows either one to start the engine in an emergency.

There is also a charging relay which connects them both together for charging when the engine is running.

The negative terminals are connected together at the batteries and a single earth runs to the engine block and anode.

I physically swap the batteries over each year so they get an easy year as the engine starting battery. This system seems to work well but the loads on the battery are quite light - little electronics and no fridge.

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[Talbot]

As has already been said, the engine start battery should not really be a deep cycle battery. The starting will put a load on that battery that a standard deep cycle cannot cope with and will considerably reduce its life. Likewise using a car battery for domestics - the charge/discharge cycle would reduce the life of a normal car type battery.
Solution:
One car type for engine start, and one deep cycle for domestics
or
Slightly more expensive option but possibly more in line with your thoughts:
Carbon Fibre type batteries that are deep cycle, but also built for cranking loads see <A target="_blank" HREF=http://www.elecsol.com/html/products.html>http://www.elecsol.com/html/products.html</A>

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[bakes]

thanks for all your help

does that mean i could keep my 90amphour cranking battery (1) and buy just a 110amphour deepcycle(2)? will charging be an issue with different size batteries? - alt output 35amps

start the engine on (1) and charge both and switch over to (2) for house use.

thanks



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[roger]

You should be able to get away with a physically smaller battery for engine starting only. What matters is the cold cranking amp capacity and matching that to your engines requirement.
I intendely dislike the 1/2/both/neither battery switches. You have to get it right every time or you damage battery or alternator. There are a lot of alternatives including the split charge relay system already mentioned and low loss diode splitters. Theres a lot of stuff written about battery charging and there are a number of pitfalls - good luck

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[Dave99]

Not as simple as you describe - how are you going to charge them?? In order to charge both batteries you coulr just connect them in parrallel and then use your battery charger/ alternator circuit.

However there is one main problem... If one battery is half full and the other is empty they will 'drain' into each other and equalised. This can be avoided by using split charge diodes to allows charging wothout the equalisation affect.

Then the battery switch simply controls where the power for insruments etc. is drawn from - 1, 2, or both.

The problem with the diodes however is that there is a voltage drop, meaning you would never be able to fully charge each bank. (I think there may be some diode type units which dont do this) The ultimate answer to this is a charge contoller such as the one made by sterling. Another method is to attach a sensing wire to the alternator from the domestic battery but i dont know how this works

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[VicMallows]

Over 20 years, I've tried both the two-identical-batteries; and the 'one cranking (car) battery and one heavy duty'. To be honest, I can't see much difference between the two approaches, especially since the cost of 110Ah so called heavy-duty ones has fallen so much. Am currently using a car battery for cranking after removing a nearly new one from (an otherwise scrap) car before 'selling' it.

Charging should not be a problem if both batteries are OK. The voltage will settle at about 14.2v and each battery will take what it wishes. If one battery is seriously on its way out it may take so much current that it drags the charge voltage down enough to prevent the other charging adequately. Easy to check with a voltmeter.

Vic

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[andyball]

Despite their disadvantages, 1-2-both switches serve on many boats without incident.

sure, it's possible to get it wrong, but if you get into a routine of switching to 1 before you start the engine, both as soon as it starts & 2 when you stop the engine it's possible to be quite happy with them.

They're available in alternator protect versions, if you feel there's any possibility of switching to off while the motor's running.

You won't get perfect charging with such a cheap,simple system, but how much do you want to spend?.

With relatively small batteries/alternator, a battery combiner is feasible & would automate charging both batt's.




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[dickh]

I also use two identical batteries - Exide 110M - which are classed as liesure batteries and suitable for both engine starting and domestics, and a changeover switch. Never had a problem yet.

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I'd rather be sailing... /forums/images/icons/smile.gif

 

[kds]

Have you considered the Vetus batteries. They claim to be designed for both starting and deep cycle purposes and they certainly are lighter and smaller than competitors of same capacity. Pricier, but Gael Marine are doing good offers and easy transport.
I will let you know about life in another 5 years - I hope !
Ken

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[SimonD]

One more thing to bear in mind. It seems that you have the MD7 with the standard 35 amp alternator supplied? I understand that its regulated output isn't up to much (vertainly never got my singel battery fully charged). Even if it was, it's a bit small for the capacity you're planning. You may therefore want to keep an eye on charging and be prepared to ditch it for a bigger one.

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[William_H]

I can't resist making a comment that reflects my own short arms deep pockets philosophy.. First keep your existing battery for engine start . Buy for domestic services whatever you have the urge to buy. Sure if you anticipate punishing it with high current loads for fridge or heater then buy a good deep cycle. if not go for anything that comes along including an old car battery. The thing is with two batteries you get twice the chance to recover from a failed batteryu situation. Your cranking battery is protected from domestic discharge while hopefully if it does fail then you can jump start with the domestic battery as well. As for the domestic battery you can discharge it without concern because you can always recharge with the engine.
No the alternator will be adequate and will probably never get near it's rated current whnen charging. I would suggest the negative of the new domestic battery be connected to the negative of the existing battery.. Get a heavy cable jump lead one wire with two clips to reach from one battery to the other for emergency starts. You may do better to put terminals on each end of a heavy cable and bolt the terminals onto the battery terminals when you need it in emergency. rather than use crockadile clips which tend to not carry hhigh current so well.
Get two switches of medium current carrying capacity ie 20 amps. One is an isolation switch to the services which should be disconnected from the existing engine battery circuit ( don't forget to ensure there is a decent fuse in circuit) The other switch is turned on when the engine is running to chaerge the services battery. This must be switched off when the engine is stopped. This wiring must be short and well safe from any short circuit to ground or have fuses at say 20 amps.near each battery. You could use an oil pressure switch or a voltage sensing relay to do the same job automatically . The auto matic way is recommended. An ampmeter or voltmeter on each battery would be a nice luxury. This will enable you to confirm both batteries are being charged. Don't worry about uneven charging. Regards will

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