The Naming of Parts

[mick]

I know there's a wee thing you can buy that automatically switches alternator charge from one battery to another depending on need. I don't know what it's called; I don't know where you might buy it; I don't know if it's expensive; I don't know how easy, or otherwise, it is to fit; in this particular instance I obviously don't know much. Could someone please enlighten me. Thank you.

 

[LadyInBed]

Maplins - Split Charge Module

But only good for upto 20 Amps. Big ones get expensive.

 

[kengill]

Do you know if one can use two of these in a cascade for a three battery setup?

 

[William_H]

The device depicted looks suspiciously like a 2 diode set up. Possibly with Schotky low volt drop diodes. As you say only good for 20 amps.

The device Mick is enquiring about might take a little explanation.

A battery is charged by presenting it with a voltage across the terminals. Depending on the difference between this voltage and the inherent voltage of the battery current will flow in. The inherent voltage depends on the charge state of the battery varying between about 11 volts (quite flat) to 13.5 volts or so well charged.

So the simple time honoured charging system for a car or boat has the alternator producing a voltage which is closely regulated to about 14 volts. It senses the battery voltage which is the alternator output voltage and adjusts by field current the output to ensure it never rises above 14 volts.

The battery then takes the charge current depending on its inherent voltage (charge state) and as the battery recharges the inherent voltage rises so current into it falls until it settles at a small current.
The 14 volts is chosen so that it can be applied to a battery.

Now it is quite satisfactory to connect another batter in paralell with the normally charged battery. ie positives connected together and negatives also connected together.
The current into the batteries is apportioned according to charge state until both are charged.

The problem is that we normally want both batteries isolated from one another for discharging. So we want to be able to discharge the domestic services battery with lights while quarantining the start battery for reliable engine start.

You can simply have a switch which you use to connect the engine (normally charged battery) to the service battery when the engine is running and disconnect when you stop the engine. All you have to do is remember each time.

A VSR (voltage sensing relay) is a relay (electrically operated switch) which makes a connection only when the engine is running the alternator is charging and the engine battery has risen to about 13volts. The service battery then gets the charge.

Another method is to use diodes. A diode is like a one way valve for fluid. Current can flow in one direction but not the other.
2 diodes are needed, one from the alternator to the engine battery and one from the alternator to the service battery. The current can never flow from the engine battery to the service battery and never flow from the service battery to the engine battery. Only from the alternator to both batteries.
Now the difficulty is that a diode actually drops a little voltage as it allows current to pass. This voltage is around .7 volt for silicon diode the most common high current diode. Another type called Schotky diode has a loss of around .3 volt.
Now the volt drop is not a great problem if we sense the alternator charge voltage at one of the batteries instead of at the output of the alternator. It just means the alternator produces 14.7 volts before the regulator suits the voltage back (regulates it) Unfortunately most alternators have a built in regulator and the wire which senses the output voltage to the regulator is not easily accessible.
But it can be done. Purpose built smart regulators overcome this problem by bringing out a connection which goes to the battery terminal. It doesn't matter so much which battery is sensed as it is assumed they will both take as much current as they want at the same voltage on the terminals.

So I suspect the suggested product is a pair of diodes possibly schotky type. They will charge the 2 batteries but with a drop of .7 volt (or even .2 volt less so) the batteries will never be fully charged before the charge current diminishes.

I suspect that you want a VSR for a simple installation.

You need to have the negatives of all batteries connected together. Use heavy cable. Then the engine battery is connected to all the engine circuits including alternator usually via the switch for the engine. (ignition switch).
All the services loads need to be connected to the service battery via switch and fuses.
The VSR is then connected from the engine battery positive to the service battery positive with its sensing wire going to engine battery positive via the ignition switch.

Some VSRs boast an engine emergency start function. This is good and allows you to operate a separate emergency switch which allows the engine to start on both service and engine battery. This is in the case of the engine battery being dead. and hopefully your service battery is OK or at least has enough power to boost the engine battery.
This function can be duplicated by a jumper lead. (you only need one because you have fitted permanently heaving wiring for the negative) or by a purpose fitted high current switch.

If you need distribution to 3 batteries and you don't want to paralell the 2 service batteries. Then you can buy another VSR or fit a third diode with it attendant problems or use a switch to paralell the service batteries for charge.

I hope this explains the problems and solutions olewill

 

[Tranona]

The BEP Marine VSR is the most obvious one to look at. Better still for a 2 battery system is to install one of their switch clusters which give you the choice of switching each bank or paralleling them for emergency start. Merlin www.power-store.com have them in their catalogue and can make up an installation kit for you with instructions. The switches and VSRs are also available through chandlers.

Hope this helps

 

[VicS]

A voltage sensitive relay (VSR) is the device that connects a second battery for charging once the first (usually the starter battery) is charged. It does not suffer the voltage drop of a diode split charging system does so there is not need to convert the alternator to battery sensing (or "cheat" the sensing circuit by the addition of a diode, which was sbc's favourite trick).
The "X-split" is an electronic device which operates in a similar way to a VSR but has no mechanical parts.

 

[bryeomans]

We completed installation of a BEP set up last year with a VSR to regulate charging to Engine Start (26Ah) and Service Battery(2 x 108Ah), with a further Anchor Winch Battery (also 26Ah) charging off a Xantrex Echocharge. The Echocharge gives a max 15A charge to the Winch Battery only when Service Battery is charging(and Winch battery needs it). Would never have worked scheme out for myself, am pleased with how its working and supplier (Merlin) excellent - very helpful and supplied guidance and instructions extremely clear.

 

[rogerthebodger]

[ QUOTE ]

The device depicted looks suspiciously like a 2 diode set up.


[/ QUOTE ]

Thats what I thought but it has shows a "ground" (battery negative ? ).

The Vetus uses MOSFETS in diode mode which has a very low volt drop, could this be the same type as Vetus