Batteries and alternators

[Col_T]

The domestic batteries in our boat (5 off 110Ah) each have two pairs of terminals (one pair +ve, the other pair -ve), and they are interlinked (paralleled) to be a 12v, 550Ah bank. If I connected them in series I would get a 60v, 110Ah bank. However, if I connected a circuit to the unused +ve and -ve on a single battery, I think that I would get 12v from that pair of terminals. Correct?

 

I don't understand alternators in the slightest, other than that spinning a coil of copper wire in a magnetic field will generate electricity. So, what is it in the build that determines whether the output is 12v, or 24v or something else - length of copper windings, diameter of the copper wire, strength of magnets, speed of rotation, what??

[Tony Brooks]

2 minutes ago, Col_T said:

The domestic batteries in our boat (5 off 110Ah) each have two pairs of terminals (one pair +ve, the other pair -ve), and they are interlinked (paralleled) to be a 12v, 550Ah bank. If I connected them in series I would get a 60v, 110Ah bank. However, if I connected a circuit to the unused +ve and -ve on a single battery, I think that I would get 12v from that pair of terminals. Correct?

 

I don't understand alternators in the slightest, other than that spinning a coil of copper wire in a magnetic field will generate electricity. So, what is it in the build that determines whether the output is 12v, or 24v or something else - length of copper windings, diameter of the copper wire, strength of magnets, speed of rotation, what??

 

1. Yes, but its a very bad idea because in effect you will unbalance the bank so one battery works harder than the rest so is likely to suffer premature failure.

 

2. Simple answer is the voltage regulator. I have lit mains light bulbs from an unregulated 12V alternator as "party trick". I suspect the number of windings on either or both the stator or rotor would also vary. It is unlikely to be diameter of wire on its own. More turns (longer wire) on the rotor will give a higher magnetic field providing the current flow was the same so that implies lower resistance so thicker  wire. Likewise, the more turns on the stator the greater the voltage induced  from the same magnetic field strength but it may not need thicker wire here because the current at 24 volts would be half that at 12V.

 

@Sir Nibble can give you a  definitive answer I expect.

[Col_T]

Thanks, Tony.

 

The battery thing - presumably the rest of the bank would keep the battery with the 12v circuit 'topped up', so would that reduce the wear on that battery?

[Tracy D'arth]

4 minutes ago, Col_T said:

Thanks, Tony.

 

The battery thing - presumably the rest of the bank would keep the battery with the 12v circuit 'topped up', so would that reduce the wear on that battery?

No, it does not work like that. In fact the reverse is true, the overworked battery restricts the rest of a series battery bank to equalise the charge.

It is the same inside one 12v battery where six 2v cells are actually connected in series. One flat or defective cell destroys the whole battery.

[Tony Brooks]

5 minutes ago, Col_T said:

Thanks, Tony.

 

The battery thing - presumably the rest of the bank would keep the battery with the 12v circuit 'topped up', so would that reduce the wear on that battery?

 

Are you talking when they are in series or in parallel? In parallel then yes they would tend to try to charge the lower charged battery but it is still a bad because that one battery would be using up more cyclic life than the rest. When in series then no, they can't until a circuit is complete. That is charging or supplying power, but it would still be using up cyclic life on that one battery.

[Col_T]

Many thanks, both.

[MoominPapa]

2 hours ago, Tony Brooks said:

2. Simple answer is the voltage regulator. I have lit mains light bulbs from an unregulated 12V alternator as "party trick". I suspect the number of windings on either or both the stator or rotor would also vary. It is unlikely to be diameter of wire on its own. More turns (longer wire) on the rotor will give a higher magnetic field providing the current flow was the same so that implies lower resistance so thicker  wire. Likewise, the more turns on the stator the greater the voltage induced  from the same magnetic field strength but it may not need thicker wire here because the current at 24 volts would be half that at 12V.

 

@Sir Nibble can give you a  definitive answer I expect.

The simple answer is that the voltage is proportional to the number of turns in the stator and the rate of change of the magnetic field through them. The rate of change of the magnetic field is proportional to the rotor RPM and the magnetic field intensity, which depends on the current through the rotor. HOWEVER, the current through the stator creates a magnetic field which opposes the field from the rotor and depends on the stator (or alternator output) current.

 

So voltage is proportional to speed and rotor current and inversely proportional to output current. Which explains why alternators need a regulator which controls the rotor current to keep anything like a steady output voltage.  I guess you party trick lit one or two mains bulbs, and therefore was at very low current. At full rated output current, a well designed alternator will output about it's rated voltage with the field current achieveable at the rated voltage.

 

MP.

[Tony Brooks]

13 minutes ago, MoominPapa said:

The simple answer is that the voltage is proportional to the number of turns in the stator and the rate of change of the magnetic field through them. The rate of change of the magnetic field is proportional to the rotor RPM and the magnetic field intensity, which depends on the current through the rotor. HOWEVER, the current through the stator creates a magnetic field which opposes the field from the rotor and depends on the stator (or alternator output) current.

 

So voltage is proportional to speed and rotor current and inversely proportional to output current. Which explains why alternators need a regulator which controls the rotor current to keep anything like a steady output voltage.  I guess you party trick lit one or two mains bulbs, and therefore was at very low current. At full rated output current, a well designed alternator will output about it's rated voltage with the field current achieveable at the rated voltage.

 

MP.

 

All true, and yes just one 40W Heavey duty bulb. However, none of that addresses exactly how 12v and 24V alternators differ, which is what Col-T basically asked.