twin battery charging and management - new to us sailing cruiser

[niccapotamus]

we have a typical two battery arrangement on our new to us old 30foot sailing boat.

we have trickle charging from an old solar panel which charges via crocodile clips - so you choose which battery to keep topped up
we have 240 v charging via a similar arrangement - connect croc clips, plug in and charge

engine charging when motoring is presumably straight from the old alternatore

I think I would like to try and get a better system particularly as we will be on the boat all year round and therefore will put more demands on lighting and we intend installing an eberspacher.

does anyone have any advice regarding a system to put in place to better regulate and manage the two batteries so that they get charged more efficiently by the engine, shorepower and from the solar panel

many thanks

nick

 

[Champagne Murphy]

http://www.power-store.com/view-item.asp?itemid=1158&id=195&
I've found this very effective. Charges the start battery first and once it reaches 13.7v the charge is directed to the next bank. Can run the solar through it too so the VSR does the switching automatically.

 

[Tranona]

Almost a piece of string question as there are several different ways of designing a system depending on your anticipated usage, space availability and sources of charging. Probably the essentials are to split your battery capacity into 2. A small high power battery solely for engine start and bigger capacity for the house circuits. A split charge device such as the VSR already suggested which charges the start battery first then the house, plus individual master switches for each bank and a means of using the house battery to start the engine in an emergency is the basic requirement.

To determine the storage capacity (size of battery) you need to do an audit of your expected consumption and this in turn determines what charging regime is required to keep your house batteries charged. There are many different solutions to achieve this, but first you need to know what the requirements are before splashing out lots of beer tokens on gizmos.

 

[William_H]

The best system involves using 2 batteries with one isolated for engine start so that even after flattening the house battery there is still a good battery to start the engfine for charging. So essentially a separate system for all your domestic services and a separate system for engine start.
The thing then is that you need to parallel the domestic battery with the engine battery when the engine is running to charge the domestic battery.
So the negatives of all batteries are commoned but the positives are connected only when the engine is running. This can be done by a switch. However you have to remember to close the switch when engine is running and open it again when engine stops. If you forget you may get no charge into domestic battery or dischharge both batteries and so engine can't be started.
The linked device at its heart has a relay which makes this connection. Now it is possible to operate this relay from an oil pressure switch alternator W output or the engine ignition switch. However with the latter you will have the batteries paralleled for start. This is not good as while it gives an enthusiastic cranking you may have one battery old and unable to start tthe engine and not know it becuase the other battery is helping.
The linked device is called a voltage sensing relay and senses the voltage of the engine battery. When this rises to over 13v as happens when alternator is charging it connects the 2 batteries so charging the domestic battery. When the negine stops this voltage falls to nearer 12.5 volts so relay senses this and opens the connection so providing isolation.
It is desirable to be able to connect both batteries together for the situation where engine battery has died and can't do the job alone. As a get you going emergency help.

The other option is to use diodes to feed the 2 batteries but will not allow current to flow in reverse so keeping thje 2 batteries isolated. Only specialised diodes will not lose too much voltage so under charging batteries or the other option is to have the alternator sense the battery voltage rather thna alternator output voltage so alternator voltage is inceased to allow for volt drop in diodes.

I would suggest that in practice you will mostly need the solar charging and shore power charging for the domestic battery as if the engine battery is isolated it will not lose much charge and if it does it can be jump boosted from the domestic battery.

Get an amp meter for charge current and volt meter for each battery so you know what is happening. You will see that after a while running the engine to charge batteries does not put much in so give up on engine charge. A smart alternator controller can get more into batteries with shorter engine run for charging. From there a meter which measures cumulative discharge and charge current in amp hours can be useful.
I would suggest you just get the basic isolation set up and proceed with the expensive stuff as you see the need and get experience with your boat and life style. good luck olewill

 

[john_morris_uk]

Before the thread descends into extolling the virtues of one system over another (and there are lots of ways of skinning this particular cat!)

Might I suggest that the Rolls Royce of simple systems is to have two alternators. One smaller one to charge the engine start battery in the traditional way and one larger one to charge the domestic battery bank via a 'smart' charging system. Top it up with decent solar panels with their own controllers.

Some monitoring and metering is a good idea, but assessing the need and doing an audit of power requirement etc is the first step. Then look at never discharging the domestic bank more than 50%. You might be surprised at how large a capacity of battery bank you need…

If you stick with one alternator then you need to consider a Voltage Sensing Relay or a Diode splitter and sense the batteries at their terminals to make sure the charge isn't being compromised by loss in the diodes.

This whole subject is large and can become quite complex fairly quickly.

 

[sailinglegend420]

You won't find a "simple" answer here, only lots of opinions which in the end will confuse you. The answer you need is in a book, or several books. You don't need ones that tells you how to mend an alternator or exactly how a battery works. You don't even need a whole chapter on Ohms Law! I don't think the book you need has been written yet - but try the RYA Electrics Handbook, and then search this forum archive for more specific answers.

 

[ghostlymoron]

I think the first step is to determine how much power you need to store by doing an audit of everything using electric. Fridges, Eberspachers and overnight sailing use a lot and so do laptops. IMHO what you want then is a system with an engine starting battery, a domestic battery bank, a dual sensing VSR, separate isolators for engine and domestics, a means for 'jump starting' if you engine battery fails and possibly a 'smart' regulator to boost your alternator output. If you're going to hook up to shore power a lot, a 'smart' charger can refill your batteries overnight.