Adding second alternator to Beta engine

[pvb]

Check math:-
1 metric hp = 0.7355kW
100amp alternator charging at full amps and 15 volts = 1500watts ie 1.5kW
1.5/0.7355 = 2.04 metric hp.

Your calculation makes no allowance for efficiency losses, heat produced, etc. I quoted 3.5hp because that's what Prestolite quote for a typical 12v alternator. See this curve if you don't believe me http://www.prestolite.com/productinfo/alternators/8MR2325U/8MR2325U_curve.jpg

A 3YM20 @ 2000rpm will be putting out (at crankshaft) 10.5kW and the alternator will (theoretically) be absorbing 14-15% of that output. A not inconsiderable deficit.

I was responding to OldBawley's comment about alternator loads, and he was talking about a "big Perkins".

 

[William_H]

Also, I believe we currently have a simple splitter diode to isolate the engine and domestic banks, so nothing clever. Would a smart charge controller be the first port of call if considering upgrading the system?

If you have simple ie silcon splitter diodes then there will be drop of min .7volt often more like 1 volt between the regulated 14v output of the alternator and the battery. This will reduce charge current hugely. It is possible that the alternator is "battery sensed" which means that a small wire from the battery (either battery) tells the alternator what the output voltage is. So the alternator senses the loss in the didoes and adjusts accordingly.
So a first thing to do will be to check actual charging voltage at the battery. If this is 13v and yet you have 14volts at the alternator output terminal then you will really be restricting your charge current. ie not much current into batteries with only 13v applied.
An amp meter int he charge circuit might also show this shortcoming up.
Cheapest simplest remedy if this is the case is to fit a VSR. A relay which makes a hard connection so no volt drop. You could also fit a switch to bypass the splitter diode for more charge. (2 switches one across each diode or one for each battery. Engine and domestic.)
The other option is to leave the splitter diode and modify your regulator to battery sense. (likely quite difficult as sensing is in the regulator and alternator body) or fit a new smart regulator with battery sensing.
So check the charge voltage with Digital Volt Meter a the battery.
Fit an amp meter to see what is happening
Then go for smart regulator or be satisfied with what you have . good luck olewill

 

[bloke]

Just to add another 'smart' alternator regulator - I use an Stirling Alternator to Battery Charger - which has the advantage that you simply hook up all your alternators in series and it does the job of making them 3-stage charge by 'fooling' them into thinking that the batteries are lower than they are (think it 'feeds' a lower voltage so the stock alternator regulator goes into 'full output' mode as required, and the box does voltage step-up for the other charging stages than bulk). I think it's DC-DC charger technically. Anyway this removes a lot of faff involving adding wires to field coils and needing to do it again if you replace alternators. I know if you have lucas etc it's easier with studs already there but if you have a hitachi, it's an internal job. I have used it on a liveaboard english canal boat for over a year and it's been flawless.

 

[Molteni]

As Mistroma pointed out, unless these are AGMs you'll struggle to put much more than 100A into your bank. I have a 660Ah domestic bank and find that my 90A Leece-Neville alternator happily recharges it. The engine battery can be disregarded, as these are almost always fully charged.

We've just upgraded our battery bank to 7 batteries totalling about 720Ah (600Ah domestic) . The engine is a Thorneycroft T80 35hp with an A127 alternator rated at 70Amps. Additionally we have a Stirling external regulator and a modern Victron splitter.
When the batteries are all at around 60% capacity, I've yet to see more than about 40 Amps going into the bank.
There's probably not enough space to fit a second alternator, but I'd really like to up the charging current so as to shorten the charge time.
Any ideas?
Thanks
Mike

 

[jwfrary]

We've just upgraded our battery bank to 7 batteries totalling about 720Ah (600Ah domestic) . The engine is a Thorneycroft T80 35hp with an A127 alternator rated at 70Amps. Additionally we have a Stirling external regulator and a modern Victron splitter.
When the batteries are all at around 60% capacity, I've yet to see more than about 40 Amps going into the bank.
There's probably not enough space to fit a second alternator, but I'd really like to up the charging current so as to shorten the charge time.
Any ideas?
Thanks
Mike

What size cables are you using?

 

[Molteni]

As came with the boat from new, a Moody 376.

 

[jwfrary]

As came with the boat from new, a Moody 376.

I would expect then that they are on the light side which is usual for factory fitted and may be the cause of your slow charging.

Equally how long in the tooth is your alt? It may be past it's best!?

 

[William_H]

We've just upgraded our battery bank to 7 batteries totalling about 720Ah (600Ah domestic) . The engine is a Thorneycroft T80 35hp with an A127 alternator rated at 70Amps. Additionally we have a Stirling external regulator and a modern Victron splitter.
When the batteries are all at around 60% capacity, I've yet to see more than about 40 Amps going into the bank.
There's probably not enough space to fit a second alternator, but I'd really like to up the charging current so as to shorten the charge time.
Any ideas?
Thanks
Mike

My reaction to 40 amps is that you are doing well. The rating of an alternator is a max usually temp limited. Much more likely limited by engine /alternator RPM and as said possibly wiring size. Also possibly any switches in the charging circuit. ol'will

 

[Molteni]

My reaction to 40 amps is that you are doing well. The rating of an alternator is a max usually temp limited. Much more likely limited by engine /alternator RPM and as said possibly wiring size. Also possibly any switches in the charging circuit. ol'will

Wire from the alternator to the splitter seems to be the original 6mm2 so have just ordered some 16mm2 . I'll crimp on some new connectors as well.
From the Victron splitter to the 3 banks I'll keep the original 6mm2 for the time being, as the wire from the alternator is clearly taking the biggest load.

 

[Molteni]

Previously having never seen more than 40 Amps going into the batteries, this morning I ran the engine to charge batteries around 30% discharged and saw ---- 54Amps !
This is a massive improvement and will result in less engine time to recharge. And it's all down to heavier duty alternator cables!!

 

[Graham_Wright]

Another use of a high output alternator is to run an inverter. Our inverter will just run the wet vac which is very useful. It will also run the electric kettle, microwave etc.
Straight off the battery would be cruel!