Voltage regulation - bulb or resistor?

[Ronald_M]

Hi all - could I ask for some advice please?

I'm trying to regulate the output voltage of my outboard motor's charge coil. The motor is a 9HP electric-start model fitted with a rectified, but unregulated, battery charging facility rated at 12v/70w. The manufacturers do not supply a voltage regulator for my particular model.

Measuring the charge voltage accross the battery terminals gives a reading in excess of 16 volts - too high for comfort. However, if I connect a 10w bulb accross the terminals, the reading falls to about 13.5 volts/1.5 amps - much better. As a temporary fix this arrangement is OK, but I would like to install something more permanent.

Would some kind of "power resistor" work? Something that will drop the voltage and be able to handle the current? Of course, the resistor would have to be switched out of circuit when starting/switched in when the engine is running, but this should be quite easy to arrange.

Any thoughts?

Cheers,

Ron.

 

[bobgarrett]

The reason the voltage drops with the bulb connected is because there is a load on the circuit. The voltage will also drop when you connect a battery for the same reason. How much it drops will depend on its charge state and the power of the dynamo. So you may not have a problem at all. Also if you do want to regulate it then since the battery being charged presents a variable load only a proper regulator will do.

 

[Rigger Mortice]

The reason the voltage drops with the bulb connected is because there is a load on the circuit.

OP said that he gets 16 volts when measured across the battery, so he probably does have a problem.
I had the same with a Mariner a few years ago. It churned out just over 16 volts and the battery boiled away quite merrily. I fitted a regulator and the problem was resolved.
The problem with using a bulb or resistor is that you end up losing some of the power you are generating. It's a short term solution, but really a proper regulator is needed.

 

[Ronald_M]

Thanks for the replies, chaps.

I agree that a proper regulator would be the best solution, but, as I say, the manufacturers don't supply one for my motor. I could try to fit a third-party regulator, but I fear that this might invalidate my guarantee.

The manufacturers recommend that the engine be connected to an "ordinary" 40AH battery.Trouble is, it's very difficult to define "ordinary" in this context. My understanding is that modern, "maintenance free" batteries are more efficient, ie can have a lower internal resistance, than the old-fashioned "top them up all the time" types. This could explain the high charging voltages.

But to be fair, although the engine is claimed to have a "charging" coil, it might be more accurate to describe it as a "lighting" coil - connect up some (incandescent) nav lights = no problem. Unfortunately, I don't want to leave my nav lights switched on all the time!

Interestingly, it would appear that some small DC-DC chargers (starter battery to domestic battery) draw a couple of amps when working. One of these might help the issue, although, of course, when the charger has finished its work it will cut out and we're back to square one.

Maybe one of these chargers linked to a voltage-sensitive relay might do; if the charger cuts out and the voltage rises to above, say 14 volts, the relay switches on a 10w bulb. inelegant, but it might work.

Which I suppose takes me back to my original enquiry - would a power resistor be the answer to my problem? After all, would this not be similar to how regulators work - above a certain voltage the power is dissipated via a shunt?

Dear oh dear - they don't call it "electrickery" for nothing!

Ron.

 

[noelex]

On most of these small outboards the charging can be disconected with the engine running with little risk of damage. If the bridge rectifier is accesible it is cheaply replaced anyway.
If you are prepared to take this risk. A simple solar PWM regulator can be used. These are cheap.
If you do not want to take the risk simply switch on a load (say a couple of lights) when the voltage rises. It will not happen very often with a reasonable sized battery as the output is low.

 

[Rigger Mortice]

If the engine is new then, personally, I would take this up with the manufacturer. That's what I did with the Mariner which I mentioned in my previous posting; they supplied me with a proper regulator F.O.C......the rectifier was charging at too high a voltage and was damaging my battery.
IMO the manufacturer of your engine has described it as a "charging coil" and that's exactly what it should be able to do.

I'm not sure what you are meaning by a "power resistor". AFAIK a resister will simply provide the same resistance to your power supply whatever the voltage. No problem with that when the battery is charged and you want to reduce the input to the battery, but if the battery is low, all you would be doing is reducing the input when you want to put as much as possible into the battery.

 

[Ronald_M]

I'm not sure what you are meaning by a "power resistor"

Well, to be perfectly honest, I'm not sure what I mean by a power resistor, either!

But, as I have discovered, connecting a 10w, 12v bulb across my battery terminals seems to keep the charge voltage down to a safe 13/13.5 volts. This voltage remains pretty constant whatever the state of charge of the battery and - idling excepted - at any engine speed.

So I am assuming (and here I might reveal my lamentable ignorance of electricity - sorry) that a load similar to my 10w bulb - a 10w resistor in other words, might do the trick. Of course, the bulb dissipates the power in the form of light and heat. A resistor would dissipate the energy as heat and would therefore have to be designed so to do?? So what I mean is a resistor that places a known load to the battery and can handle the high current without failing. Er.... I think......

Ron.

 

[Rigger Mortice]

That makes sense, though I would question what the voltage is when the engine is not running. The "static" voltage will give you a better idea of the state of charge.

I suspect that, by using the bulb you are slightly under charging the battery which is probably better than slightly overcharging it. So, if it works for you, fine! FWIW my battery charges at a minimum of 13.7 volts; usually a little more.

A resistor will have exactly the same effect as the bulb so, as you are happy with the bulb, you may just as well use that!

 

[Ronald_M]

The "static" voltage will give you a better idea of the state of charge.
FWIW my battery charges at a minimum of 13.7 volts; usually a little more.

I agree that the voltage of the battery "at rest" will give a good indication of its state of charge, and that it is normal for "trickle" or "float" charging to register about 13.5/13.7 volts.

It should be a simple matter to experiment with bulbs of various wattages in order to find the value that restrains the engine charging-voltage at a suitable level. Then all I'll need to do will be to locate my "power resistor".

Now, where did I put that Maplin's catalogue........?

Thanks for all the help,

Ron.

 

[elton]

Using a resistor or a bulb to reduce the charging voltage is a ludicrous idea. If the motor is fitted with a battery charging facility, then it should be able to do that without risk of damaging the battery, or causing it to burst open. If you do this, then what happens when the motor isn't running at high revs? Does the battery then discharge through the bulb?

Get on to the manufacturers of the motor. Or else get someone who knows what they're doing to fit an old motorcycle regulator to it.

 

[GrahamM376]

Looking at the voltage and current, would not a small and cheap solar panel regulator do the job?

 

[Ronald_M]

Using a resistor or a bulb to reduce the charging voltage is a ludicrous idea. If the motor is fitted with a battery charging facility, then it should be able to do that without risk of damaging the battery, or causing it to burst open. If you do this, then what happens when the motor isn't running at high revs? Does the battery then discharge through the bulb?

Get on to the manufacturers of the motor. Or else get someone who knows what they're doing to fit an old motorcycle regulator to it.

Hi Elton.

Well, I agree that it's disappointing that such roundabout methods need to be considered.The most likely cause of the problem is that the internal resistance of my battery is too low to provide a suitable match to the engine's charging coil. Although I have connected the engine to a battery of the specified capacity, the owners' manual fails to point out that not all battery types will provide the appropriate load. I will be taking up the matter with the manufacturers in due course.

In the mean time, I'm simply trying to rig up something that will allow me to continue sailing without ruining an otherwise pefectly good battery.

You will see from my earlier posts that I would incorporate a switching system that would isolate the load when starting, when the engine charge voltage fell below a certain figure or when the engine was at at rest, thus avoiding battery drain.

Ron.

 

[elton]

Hi Elton.

Well, I agree that it's disappointing that such roundabout methods need to be considered.The most likely cause of the problem is that the internal resistance of my battery is too low to provide a suitable match to the engine's charging coil. Although I have connected the engine to a battery of the specified capacity, the owners' manual fails to point out that not all battery types will provide the appropriate load. I will be taking up the matter with the manufacturers in due course.

In the mean time, I'm simply trying to rig up something that will allow me to continue sailing without ruining an otherwise pefectly good battery.

You will see from my earlier posts that I would incorporate a switching system that would isolate the load when starting, when the engine charge voltage fell below a certain figure or when the engine was at at rest, thus avoiding battery drain.

Ron.

I suggest an electro-mechanical voltage regulator off a motorcycle, from a breaker.

 

[Ronald_M]

Looking at the voltage and current, would not a small and cheap solar panel regulator do the job?

Hi Graham.

You're right that a solar panel regulator would protect the battery. However, if I understand correctly, such regulators work by simply disconnecting the source when the voltage rises above a preset value. Solar panels are not, I believe, damaged if they are "open circuit" but I'm sure that many alternator coils would be - the output voltage would go sky high. Indeed, my engine wiring combines the battery input and output into a single, shared pair of wires; the manufacturers specifically caution against disconnecting these wires whilst the engine is running.

Might get one for my solar panel, however!

Ron.

 

[bedouin]

Hi Graham.

You're right that a solar panel regulator would protect the battery. However, if I understand correctly, such regulators work by simply disconnecting the source when the voltage rises above a preset value. Solar panels are not, I believe, damaged if they are "open circuit" but I'm sure that many alternator coils would be - the output voltage would go sky high. Indeed, my engine wiring combines the battery input and output into a single, shared pair of wires; the manufacturers specifically caution against disconnecting these wires whilst the engine is running.

Might get one for my solar panel, however!

Ron.

There are different types of regulator available. Some just disconnect as you suggest but others are "proper" regulators and that would certainly be the best solution. (cost about £25)

 

[VicMallows]

Unless I have missed it (haven't read every word) you have not measured the CURRENT flowing into your battery. This is much more interesting than the voltage you are seeing across the terminals (IMHO) . I have a suspicion that this will be very little. Batteries age in strange ways and I have often seen this characteristic with old ones.

Vic

 

[Ronald_M]

Unless I have missed it (haven't read every word) you have not measured the CURRENT flowing into your battery.
Vic

Well, Vic, I haven't actually stated it in this thread, but I did in fact measure the current flowing into the battery with a clamp meter the other day - about 1.0 to 1.5 amps.

The plot thickens!

Ron.

 

[halcyon]

Unless I have missed it (haven't read every word) you have not measured the CURRENT flowing into your battery. This is much more interesting than the voltage you are seeing across the terminals (IMHO)

A voltage of 16 volt would indicate a fully charged battery, or a completly duff battery, you would see the same current in both cases.
Fisrt job is to let the battery rest and check the voltage, then connect the bulb and let it drain the battery. If the voltage falls slowly battery is probably okay, drain battery for 10 hours. Then start engine, if voltage goes to 16 volt check the wiring, it should take 1 or 2 hours to get there. The battery sets the voltage you see, not the charging source.

Also the battery is more a capacitor, so if you are charging with just a rectified output you may be seeing the peak voltage, with the bulb connected you are clipping these and seeing more the actual value.

Have you tried a analogue meter?

Brian

 

[VicMallows]

Have you tried a analogue meter?

That's a favourite suggestion of mine ....especially because we are usually talking about VERY cheap digital meters. But I usually get shouted down

Ronald M: 1-1.5A is certainly significant if correct. However I would put little faith in a basic clamp-on meter at such relatively low current.

Vic

 

[halcyon]

Ronald M: 1-1.5A is certainly significant if correct. However I would put little faith in a basic clamp-on meter at such relatively low current.

Vic

Lot depends on the meter, I recon on 0.1 amp no problem with Hall effect, but as all things battery, it's only an approximation, to many variables.

Brian