How to take the alternator out of charging if all power is needed to run engine???

[sailingharry]

Originally Posted by roverhi

The real question is why an 80 amp alternator on a 16hp motor. You'd need huge battery amperage, 400 amp plus or exotic batteries to be able to use 80 amp output and then only for a short period before the regulator cuts back alternator output. Think you'd be better off with smaller alternator with an external multi-stage regulator. The external regulator would give you the ability to cut the field current. Assume you have a Hitachi internal regulator alternator which makes cutting the field impossible once it takes over.

Actually, an 80A is quite useful. My last boat had two Trojan T105s, 225 Ah, and a 105A alternator. In the mornings, I could push 70+ amps for easily 30 minutes. Even with his little engine, I really wouldn't want much less than 80A. Especially since most alts never get rated amps (amongst other problems, it is VERY hard to get rated RPM required to produce rated amps).

[RaymondR]

Originally Posted by roverhi

The real question is why an 80 amp alternator on a 16hp motor. You'd need huge battery amperage, 400 amp plus or exotic batteries to be able to use 80 amp output and then only for a short period before the regulator cuts back alternator output. Think you'd be better off with smaller alternator with an external multi-stage regulator. The external regulator would give you the ability to cut the field current. Assume you have a Hitachi internal regulator alternator which makes cutting the field impossible once it takes over.

The problem is large banks of house batteries.

Auto alternators are rated for their maximum output for a short period of time with the assumption that the voltage of a single cranking battery under charge will rise fairly quickly and the regulator will consequently wind the current output back fairly quickly.

However large banks of house batteries are often heavily drawn down and do not respond as would a single cranking battery and oblige the alternator to high output for extended periods of time and consequently the alternator overheats. A large capacity alternator will suffer less from this treatment.

If your alternator has an external voltage sensing lead you could include a switched in circuit with say a 6 volt battery in series with the main battery so that the sensing lead sensed 18 volts rather than 12 which would cause it to wind back the alternator output current.

[Compass790]

Originally Posted by Cpt Mark

I'm with Parkstone Bay. Why would you ever need to do this?

I'll tell you why.
If you have a small engine with a large alternator you sometimes need to take the alternator load off the engine to be able to use maximum engine power. Also to take the load off the starter its useful especially if your batteries are a bit down.
We have a button on the instrument panel to interrupt the field wire circuit which we frankly cant do without using a 55 amp alternator on an over-propped 8hp engine. We would still need it even if we weren't overpropped. 1&1/2 hp is a lot out of an 8hp motor. Ask me how I know
Of course most would say get a bigger engine but he is trying to work with what he has.

[model 10]

Originally Posted by Compass790

I'll tell you why.
If you have a small engine with a large alternator you sometimes need to take the alternator load off the engine to be able to use maximum engine power. Also to take the load off the starter its useful especially if your batteries are a bit down.
We have a button on the instrument panel to interrupt the field wire circuit which we frankly cant do without using a 55 amp alternator on an over-propped 8hp engine. We would still need it even if we weren't overpropped. 1&1/2 hp is a lot out of an 8hp motor. Ask me how I know
Of course most would say get a bigger engine but he is trying to work with what he has.

I have a switch in the field to stop the alternator from overheating and to stop the belt from squealing sometimes. Usually I see about a 50 rpm increase when I turn the alternator off.

[NorthernMac]

I’m a big fan of having a alternator switch, I like to start with the alternator off, I also like to turn off the nav/com, alternator, then engine. Also very handy if you get a overcharge situation or other malfunction, one switch and it’s offline.

[CassidyNZ]

Need to be careful with separately switched fields. It’s easy to forget to switch them off after the engine stops running. Then they will draw up to 5A from your batteries u til you remember.

I have two alternators with switched fields and have on occasions forgotten to switch both off after stopping the engine - 11A draw for 5 hours runs a house bank down quite a long way.

[bouncycastle]

A lot of technical info to digest

For most marina to marina sail boats with occasional overnight 2 day sail or anchor and often over powered the need to control power systems is unlikely to occor

However live a boards often older boats often border line minimum powered anchoring for week or doing one week plus ocean passages can hit problems

Even large boats can hit power issues

Example ~65 foot Henkly 2 million dollar plus boat made for daily marina to marina in the burmuda region got crippilled going for 10 day ocean crossing sail

The onbord ~8 kW generator could not keep going for more than 24 hours supplying the ~7 kilowatt air con and ~1 kilowatt navigation and sail systems and ~2 kilowatt water maker washing machine and dryer hot water shower and other kilott power demanding systems.
After three days the engine alternator 200 amps with the 4000 amp battery system and 5 kW invertor all melted down and generator guit.
The rest of the seven day sail was back to hand winch sails and flashlights.
The boat for extended cruising needed 20 kilowatt generator but island to island marina to marina where electric shore power was there a 8 kilowatt generator was enough power.
Boat electrical system was dezined to close to the ragged edge of power demands

My boat the power demands from a alternator can vary from a few amps to top up battery that is fairly full to north of ~50 amps if the battery is empty.

~23 amps alternator recharge is about ~1 HP and ~50 amps is about ~2 HP and ~80 amps is touching ~4 hp demands from the drive engine.

On myn~ 13 HP engine I have then that maximum 80 amp load is some ~25% the power of the engine output at max RPM .

Many the boat on anchor in sheltered bay for long periods the battery fully charged and the weather changed
Then they had to punch upwind for some miles at Max RPM badly able to make 1 knot against the waves and wind conditions

Similar boats in similar situations with veryn depleted battery and larger alternator dragging power away from going against adverse conditions could not make any head way against the wind and waves .Those boats could choose to stop engine remove the alternator belt drive and start up again to solve the issue sometimes with anchor dragging and some boats didn't make it.

A simple switch can sometimes solve these issues.

I know one ~55 foot boat 120 Hp engine well sorted out good captain left the island of Majorca Spain and went one mile from marina .
A mistral 60 MPH wind sprung up.

Engine full power could not make head way
Although the boat was in Lee of large island with storm sails up and 120 HP engine max power it took 24 hours of 2 mile tacking gaining a few feet on e
ach tack to get back to the marina only one mile away

A depleted battery on that craft could result in putting large drag load on the engine and could have extended the time to get back to marina for a much longer period
Even a large boat can get maxxed out of power if they can't switch off excessive alternator demands

The main reason I went for 80 amps alternator is I have 100 amp deep cycle lead acid domestic battery
The logic is low RPM the too big alternator will top up battery faster than the OEM alternators of 35 amps and 55 amps meant to suit day sailor demands often with no domestic battery requirements

That ~ 80 amp slternator can overload engine in some situations

The charge cycles on empty lead acid is the first ~80% charge is the bulk charge and the alternator in theory will supply battery with north off ~50 amps for about ~ 2 hours .
That large 50 amp continious alternator recharge rate only applies to good quality marine alternators that do not heat up
My type of cheapo car type alternators after 15 minutes drop down to ~20 amps and therefore can take~4 hours to put in 80 amps .
The remaining 20 amps in my case will because of back pressure from battery reduce the recharge rate to battery down to some ~5 amps taking another ~4 hours to fully charge the 100 amp battery .
Total time to recharge battery from empty looks to be ~6 hours.

If I used the OEM Yanmar alternator another basically brain dead car type alternator with ~55 amps recharge rate that once it heats up after 15 minutes barely gives ~3 to ,5 amps continious recharge rate and it could take north of ~20 hours to recharge the same ,100 amp battery from empty

If I opt to only recharge 100 amp battery to 80% every time the cycle life of battery can drop from~ 1000 cycle life to south of ~400 cycle life. For live aboard mostly at anchor mostly engine running to recharge that means replacing deep cycle 100 amp battery every year if I abuse it with 80% recharging cycle life.

Many boats in my size 13 hp engine will have ~300 amp deep cycle lead battery and a ~55 amp alternator and a very small ~50 to 100 watt solar panel or worse even smaller watt wind generator and rarely recharge house battery to ~100% and then complain the batteries are broken after one year.

My regime will be to try to only use 10 amps per night on anchor and engine run for two hour at 1500 RPM to top up ,100 amp battery to 100% so ~ 5 amp demand from 80 amp alternator will be micro HP demands probably 1/4 HP .
In September the purchase of 100 watt panel should reduce the engine run times a lot on the sunny days

The Yanmar ~55 amp alternator on same battery at 1500 RPM will he lucky to give me ~2.5 amps and mean I could need a 4 hour recharge time

Needless to say larger better quality marine type alternator of ~120 amps plus coupled to suitable Lithium ion type battery can solve a lot of these issues and reduce engine run time to recharge batteries by factors of 4 times making it possible to recharge a ~100 amp lithium ion battery with less than 15 minute engine run time .Also lithium ion can do less than 100% recharging with no ill effects to 2000 plus cycle life.

However I tend to go the cheaper French sailing route keep it simple stupid KISS and invest in oil lamps and lanterns for candles and minimise engine size
Thrn over time as funds allow increase solar panel sizes and when the Solar power is a big enough system to always charge up domestic battery then remove the alternator drive belt

For Yanmar engine the water pump is separate belt so no cooking the engine issues but that isn't always the case for every marine engine

In the mean time it would be useful back up to me to find a simple switch way to decouple the alternator from engine if I needed for some reason maximum power

[NorthernMac]

Originally Posted by CassidyNZ

Need to be careful with separately switched fields. It’s easy to forget to switch them off after the engine stops running. Then they will draw up to 5A from your batteries u til you remember.

I have two alternators with switched fields and have on occasions forgotten to switch both off after stopping the engine - 11A draw for 5 hours runs a house bank down quite a long way.

This can be prevented with a relay or a dummy light

[Cheechako]

Originally Posted by sailingharry

Actually, an 80A is quite useful. My last boat had two Trojan T105s, 225 Ah, and a 105A alternator. In the mornings, I could push 70+ amps for easily 30 minutes. Even with his little engine, I really wouldn't want much less than 80A. Especially since most alts never get rated amps (amongst other problems, it is VERY hard to get rated RPM required to produce rated amps).

I agree. I've had huge alternators on small engines. Battery capacity is another question though. I've often installed a field disconnect switch. Install it in the wire from the normal key activated wire and it's pretty foolproof. Pretty handy if the belt starts squealing as you head out etc. Not used often but not hard to install. The smaller the engine the more useful it might be.

[witzgall]

Chris from Balmar here. With the MC-614 regulator, you have two options. 1/ Put a switch in the Ignition wire. When you Flip it, the regulator is turned off, stopping all charging. 2. Put a switch in place to enable what we call Small engine mode. Flip this switch, and the field is limited to 50%.

Chris

[RaymondR]

An alternative to switching the excitation off would be to arrange to switch in a large wattage resister in series into the alternator output and sense the voltage to the regulator from the alternator side of the resister. This way you can set the amount of battery charging to a very low value and still have some charging taking place.

I have done this to limit alternator output, and consequent alternator overheating, into the house batteries whilst cruising under engine power. My high wattage resister was a spare SS push bike spoke where I could tap the wire connection on one end back and forward to set the alternator output.

[Compass790]

Originally Posted by RaymondR

An alternative to switching the excitation off would be to arrange to switch in a large wattage resister in series into the alternator output and sense the voltage to the regulator from the alternator side of the resister. This way you can set the amount of battery charging to a very low value and still have some charging taking place.

I have done this to limit alternator output, and consequent alternator overheating, into the house batteries whilst cruising under engine power. My high wattage resister was a spare SS push bike spoke where I could tap the wire connection on one end back and forward to set the alternator output.

I like this idea though I'm no sparky so not qualified to judge.
Makes me ask another question. Can you achieve the same result by having an adjustable amount of field current? & if so an equally budget way of doing it would be nice if you can suggest one.
What i was thinking was a variable resistor in the field wire coming from the lamp circuit. I think our alternator gets all its current from that.

[Wotname]

Originally Posted by Compass790

.........
What i was thinking was a variable resistor in the field wire coming from the lamp circuit. I think our alternator gets all its current from that.

It might do but I doubt it. Mostly the field is initially excited by the lamp circuit until it gets up to a couple of hundred RPM (rotor rpm) at which time the alternator stator winding are producing enough volts for the field current to be supplied internally from the alternator.

Put another way, at start up, the field to externally excited but once it is running at speed, it becomes self exciting.

Easy way to tell is to tell is to get the alternator running and then remove the lamp. If it stops putting out current, then it was getting it's field current via the lamp.

[Maine Sail]

Originally Posted by Wotname

Put another way, at start up, the field to externally excited but once it is running at speed, it becomes self exciting.

Easy way to tell is to tell is to get the alternator running and then remove the lamp. If it stops putting out current, then it was getting it's field current via the lamp.

Spot on.. The "excite wire", on self excite alternators, and we've seen AFD switches with the excite circuit wired through it, usually does nothing once the alternator is excited. This is why folks still blow diodes when they use the "excite wire" as a field disconnect.

The easiest way to de-power the alt is to use external regulation. Then, all you do is flip off the ignition feed to the external regulator with a dash mounted switch. I have an alternator ON/OFF switch on my own engine panel. De-powering the external regulator kills the field.

With an internally regulated alternator, you'd need to interrupt one of the brush wires but, this can actually damage some internal regulators. This is why Balmar etc. don't want the field-output interrupted and prefer the regulator B+ or the regulator ignition feed be used to shut down the alternator..

[Compass790]

Thanks Wotname & Mainesail,
I should have said the current thru the lamp circuit must[ be the full field current as pushing the switch that opens that circuit cuts the alternator output off even at full revs.
Interesting you say cutting brush wire feed can damage VR as that is what we are doing. It interrupts it before it gets to VR. Lunched one VR in 7 years but it was a $6 Chinese special so no biggie.
Yes I know that external reg is the way it's usually done but I was asking if it can be done another way.