Electrics and charging on twin engine installations

[gravygraham]

Is there a rule of thumb regarding which engine charges which batteries, eg port for starter and stb for service batteries, or is this too simplified a notion? I've got two 200hp Nanni 4.390's and currently just one does all the charging and I can't work out if the service batteries are up the duff or if they're not getting charged properly? The alternators are both poking out between 13v and 14v.

Any ideas or comments would be appreciated.

Graham

 

[Wiggo]

Most builders have one bank for cranking one engine and the other bank doubles to crank the other engine and as domestics. The rationale is that keeping the two completely separate means you will always be able to start at least one engine.

I rewired ours slightly - one 85Ah cranks the port lump only, the other side is 4 x 85Ah and 2 x 110Ah and that does starboard cranking and domestics. The port alternator in consequence had next to nothing to do, while the starboard one burned out twice as it was overloaded. I now have both alternators with smart charge controllers feeding a big electronic charge splitter. This allows both alternators to feed either bank in parallel (so I have up to 120A charging capacity for the domestic bank if needed).

Not exactly common, but it works very well for us. PM me if you want more details.

 

[hlb]

Dunno, both my engines seem to charge everything up, as does the battery charger. Think it has a little brain, that says hey up a bit, to much power. No idea where the extra electric is sent to.

 

[halcyon]

There are a number of options.

Three battery banks.

One bank for each engine, engine alternator is connected straight to it's engine battery. There is then a relay between each engine battery and the main service battery bank.

Each engine charges it's battery, before relay comes in and charge goes to service bank.
Advantages,

Both alternators are perminantly connected to engine battery, any fault in in additional charge system will not effect engine battery charge.
Both alternators are charging service battery, giving max available amps, and also max available standard charge volts.
If one alternator fails, the system will roll over to charge the engine battery with failed alternator.
Service loads are not effected by spikes or low voltage due to engine starting, which normally occures with battery at low volts.
No service drain on engine batteries.

Two battery banks

One for engine starting, and one for engine starting and and service loads.

One engine charges it's battery, and thats it. The other engine charges it's battery, and supplies service loads i.e. navigation equipment, radar, heating, fridges, lights etc, any loads when the boat is in use.

Advantages

Cheap system for boat builder, saves a proper charging system

Disadvantage

One alternator is running all ship loads. and trying to charge a battery that is normally low due to service loads impossed when the engine was not running. The service loads take a large % of alternator output ( unless big alternator fitted ) which means the alternator voltage is low, and recharging of batteries are to a low level.
Alternator failure will loose both the engine start and service load charge.
On engine start you will get low voltage and spikes from starter motor, can effect electronics and memory. If you have sat at anchor for a time you have a low service / start battery, so the system is pulled to a even lower voltage.

Alternative

You can start both engines of one battery, charged by one engine, and the service loads from a second battery charged from second engine alternator.

Gets round volt drop and dirty power supply, but service loads can limit recharge levels, and there is no alternator redundancy built in.

Avoid diode splitters, you will have to fit alternator controllers to cover volt drop, and all charging goes through splitter. so a diode failure can loose engine battery charge, and you are adding more electronics, so more to fail, and more cost.

Keep it simple.

Brian

 

[Nigelhg]

Another way that boat builders have done it in the past is to have the domestic bank ONLY charged from shorepower/generator via the battery charger. I have seen this on my present boat (sunseeker) and on my dad's boat (princess). While this may suit a lot of users it doesn't suit those of us who keep their boat on a swinging mooring. I am currently in process of rewiring my boat from 3 battery banks down to two with a dual sense BEP VSR (both alternators charging together) and adding sterling regulators. Your owners manual may have a circuit diagram of the charging arrangement (mine did).

 

[leomagill]

Have a look at some of the 'x-split' controllers here http://www.dg2k.co.uk/
I have one and it works well, no connection to the company but they are only 1/2 mile from me.
Both alternators charge both batteries and they are isolated the minute the engines go off.

 

[PCUK]

Contact Adverc for chapter and verse!
Their equipment is used on ambulances and fire engines as well as boats so it has to be very good!