Batterie/Alternators & Charging


Active member
25 Sep 2003
I am currently reviewing the battery & charging arrangements on my boat since a lot of new electrical kit has been added over the past couple of years, radar, fridge etc.

The boat is a 34ft yacht marina based & is typically used for short cruises with a 3 or 4 longer trips (couple of weeks) each year. We are rarely away from mains power for more than 3/4 days.

The current arrangement is two sealed lead acid delphi marine freedom batteries 105 amp. One serves domestics the other engine starting. They are charged by an alternator (capacity unknown-see below) with a Sterling smart regulator & switched using a battery switch. They are also conected to a Navcom 15amp switch mode charger which can apparently deal with 3 battery banks. This tends to be permenantly on when shore power is available.

What I am thinking of doing is getting another Delphi marine freedom 105amp & putting it in parallel with the existing domestic battery to double the capactiy. However the following thoughts occur.

1. Would I be well advised to invest in a bigger battery charger to deal with the bigger domestic bank ? Ideally I would like to be able to charge the batteries from say 50% charge overnight. What capacity charger would I need to do this? Any recomendations as regards type of charger?

2. How do I find out the capacity of my aternator. The engine is a Yanmar 3 GM30F. There is a lable on the alternator but it is badly worn & illegible. Any ideas. Anyone know what was fitted to this engine as standard-I doubt it has been upgraded?

3. Is it advisable to leave sealed lead acid batterie on permenant charge-Delphi suggest not but I'm not clear what harm a trickle charge can do.

4. Anyone know anything about Navcom chargers. I came with the boat & I don't have any manuel or info about it & google doesn't bring up anything.

Any thoughts appreciated.


24 Jun 2002
co.Wicklow, Ireland
1. I've just bought a 15amp charger from Dolphin to charge 2 * 110amp domestic batteries (assuming my engine battery is topped up by alternator). I thought this would be adequate to charge 50% (ie. 110amp) over 8-10 hours (10hours*15amps=150) But, I'm considering asking if they'll change it for a 25amp, based on a friends recommendation. The rationale suggested was (a) the charger wont give full rated output as in my simple calculation above, and (b) if you have 12vfridge, lights etc. on while connected to shore power, this will take some of the output capacity.

2. havnt a clue!, but it sounds like a current engine, maybe a Yanmar dealer would help if no-one else knows.

3. I dont know of any reason not to have sealed batteries permanently connected to a charger as long as it is properly regulated smart charger. You could try run it past Merlin - they sell Delphi sealed batteries with smart chargers in packages and will certainly know the answer if you can get the right person. I think I've read that the RNLI had a special algorithm built into their chargers to charge Delphi batteries in particular because of the charging regimin expected. Dont know if it's true.

4. pass.


Well-known member
28 Jul 2003
West Australia
You don't say if you have an ampmeter on the charger or the engine system.
I would suggest firstly get an ampmeter either portable or built in.
If you find that your battery charger is (as is likely) not normally reaching it's rated output then a larger charger will not do any more.(A smart charger may get more in by increasing voltage) The rating of the charger is it's current capability before melting or the voltage drops. More often the current going in to the batteries is a function of the voltage of the charger (fixed) and the size of the batteries. The same comments go for the engine alternator. It is probably rated at 40 amps but this figure will seldom be reached if it is then a larger alternator (or charger is indicated) if it is not reached then a bigger alternator or charger probably won't do any more.
I would suggest a Voltage sensing Relay to connect all your batteries in paralell as that should get more total current into the pack than doing one battery at a time. (can do the same by using "both" position but then you may leave it in that position which can see all discharged with no backup) keep the battery switch but don't use the both position unless you really need it for engine start. yes another battery would allso get more amps into the total system at any one time if they are in paralell however you might consider a smaller specialised battery for the engine and keep the two in paralell for services. I reitterate though. get an ampmeter on to alternator and charger see what is really happening. olewill


Well-known member
20 Apr 2002
Re: Batterie/Alternators & Charging

If fitting an ammeter, fit it to the service battery lead, not the alternator output lead. Else you can have a mater showing a charge, but due to service battery loads, the battery is being discharged. Electronic shunts have the advatage of not being capable of being overloaded, so cover engine starting or high loads, and do not have a volt drop. Or if you want a simple answer, you can get cheap clip on ones.