O/c alternators

[Graham_Wright]

Now here's another spanner in the works.

What if (like I propose) you have two alternators? I propose to dedicate the smaller to the cranking battery and the larger (200A O/P) to the domestics. In emergency, it would be nice to cross connect. This imples switching (or maybe better) links. But there is then the possbility of leaving one alternator o/c especially with regard to the state of panic which may pertain.
So, how about a small (e.g. motorcycle or burgular alarm) battery wired through a fairly high resistance dropper permanently connected to each alternator. This would give a reference and provide a load and the dropper could be chosen so that the current available back into the alternator (in the very unlikely event of diode failure) would be insufficient to cause a fire.

 

[pvb]

No problem...

Having two dedicated alternators is, I think, the optimum solution - and it's what I've done on my boat. Biggest benefit is that it really simplifies the wiring; no need for splitter diodes, VSRs, etc. The smaller alternator can just be connected direct to the starter battery. The larger alternator can be connected direct to the domestic bank, and ideally needs a smart regulator. Then 2 simple on/off switches, one for the starter, one for the domestic bank.

If you wire the alternators direct to the batteries (with fuses) there's no possibility of leaving an alternator open circuit whilst in a panic. There's no need for other batteries.

In this set-up, the starting battery will always be kept well-charged and in good condition, so should have a very long life. It's difficult to see in what circumstances it would suddenly fail to work and therefore the need to be able to cross-connect might not be real. If you really want a cross-connect, a simple on/off switch would do the job, although it would be best to hide it away somewhere to avoid confusing the crew! I reckon the best place for an emergency cross-connect switch is close to the domestic bank, and I'd recommend wiring it to the load side of the starter battery on/off switch. If you need to cross-connect, the starter battery switch should be turned off (isolating the failed engine battery) prior to turning the cross-connect switch on. This is easy to achieve with on/off switches with removeable keys - the same key would be used for the starter battery switch and the cross-connect switch.

 

[Ships_Cat]

As pvb has said, there is no problem cross connecting two alternators.

In our own case we use a plastic keyed battery master switch in each of the negative and positive cross connections and the keys are hung elsewhere when not needed (I can only recall needing them twice in the last 8 years - once in the case of a cranking battery failure, and once when in very heavy weather the service batteries' alternator mounting frame was damaged) so that the connection can never be accidently left made. Take care that you assess all the loads, including windlass if one, that may be intentionally or non intentionally placed on the cross connection cables.

Regarding fusing the batteries I would strongly recommend anyone think carefully through their own situation on that as per my previous post. I work professionally in the industry and was involved in one country's regulatory body's discussion as to whether battery fusing should be mandatory on small commercial vessels. There had been an electrical fire on a coded yacht which could have been very serious but that it occured before the vessel had left the harbour.

Battery fusing would not have prevented the fire as the ignition was from arcing but during the investigation the merits of it were seriously discussed. On balance it was decided that there was a greater weight of safety matters on the side of not requiring mandatory fusing than there were for mandatory fusing and the proposal for it being mandatory was dropped.

As I pointed out this decision may depend on the quality of the boat's systems, whether it is twin engined or not, etc, etc, but my personal view is that if it was my own boat (a sail boat) and I thought fusing at the batteries would increase its overall safety then that could only be because the electrical systems were deficient in installation and I would replace them so that the safety balance would lead to there being no need to fuse.

John