Split charge relay

[RLWP]

I'm having trouble getting the charge warning light to function properly on an installation, so I thought I'd go back to basics to see if anyone can help.

 

 

In the diagram we have

 

• 12Volts from the battery
  
• The ignition switch
  
• The charge warning light
  
• The alternator with it's field coil
  
• Engine earth (connected to battery negative)
  

 

I have labelled two points on the diagram A and B. As I understand it, it goes something like this

 

State 1: Alternator stationary, ignition switch off: Voltage at A = 0 Volts, Voltage at B = 0 volts, light is out

 

State 2: Alternator stationary, ignition switch on: Voltage at A = 12 Volts, Voltage at B = 0 volts, light is on (current flows through alternator to excite field coils)

 

State 3: Alternator rotating, ignition switch on: Voltage at A = 12 Volts, Voltage at B = 12 volts, light is out (alternator becomes self exciting)

 

So, is that right?

 

The current through the bulb in state 2 is I = V / R, where V is 12V and R is the resistance of the bulb plus the resistance of the field windings

 

Richard

[RobinJ]

I'm having trouble getting the charge warning light to function properly on an installation, so I thought I'd go back to basics to see if anyone can help.

The diagram you have drawn is not technically correct.

The warning light is connected to a set of diodes in the alternator that come from the rotor windings in parallel with those that provide the output to B+.

If the alternator is fitted with a negative switching regulator, then the warning light connection may also provide initial startup current for the field windings, if it is fitted with a positive switching regulator, then this probably comes from the B+ connection (i.e. Battery).

However in essence, what you say is true, provide the alternator fires up!

[RLWP]

The diagram you have drawn is not technically correct.

The warning light is connected to a set of diodes in the alternator that come from the rotor windings in parallel with those that provide the output to B+.

If the alternator is fitted with a negative switching regulator, then the warning light connection may also provide initial startup current for the field windings, if it is fitted with a positive switching regulator, then this probably comes from the B+ connection (i.e. Battery).

However in essence, what you say is true, provide the alternator fires up!

 

Let's see if it is near enough for what I'm trying to get straight in my head. As I was writing it I did think I was remembering dynamos...

 

 

We now have the same as before with the addition of the split charge relay connected to the same terminal. so now

 

 

State 1: Alternator stationary, ignition switch off: Voltage at A = 0 Volts, Voltage at B = 0 volts, light is out, relay is unenergised, batteries disconnected

 

State 2: Alternator stationary, ignition switch on: Voltage at A = 12 Volts, Voltage at B = ??? volts, light is on, relay is energised, batteries connected

 

State 3: Alternator rotating, ignition switch on: Voltage at A = 12 Volts, Voltage at B = 12 volts, light is out, relay is energised, batteries connected

 

In state 2, the voltage at B is now dependant on the resistance of the bulb, and the resistance of the alternator and the relay in parallel (I think that might be wrong

 

Also, the relay is energised, so it is possible that current to start the engine is now coming from the domestic batteries

 

Have I got that right?

 

Richard

[Keeping Up]

Not quite, Richard.

 

In your earlier diagram you showed the voltage at B in state 2 to be zero. This is almost correct because the resistance of the field windings is very very low.

 

In your later diagram you are placing the relay across that same point to ground, so the potential at B is still virtually zero (in fact slightly less than before because the relay is in parallel with the coils which is why sometimes the relay causes the alternator not to fire up so easily)

 

Because the potential at B in state 2 is almost zero, the relay doesn't operate yet. It only operates after the alternator has got itself all excited.

[RLWP]

Thank you Allan. so it is:

 

 

We now have the same as before with the addition of the split charge relay connected to the same terminal. so now

 

 

State 1: Alternator stationary, ignition switch off: Voltage at A = 0 Volts, Voltage at B = 0 volts, light is out, relay is unenergised, batteries disconnected

 

State 2: Alternator stationary, ignition switch on: Voltage at A = 12 Volts, Voltage at B = ??? volts, light is on, relay is unenergised, batteries disconnected

 

State 3: Alternator rotating, ignition switch on: Voltage at A = 12 Volts, Voltage at B = 12 volts, light is out, relay is energised, batteries connected

 

In state 2, the voltage at B is now dependant on the resistance of the bulb, and the resistance of the alternator and the relay in parallel. This voltage will be lower than without the split charge relay

 

Have I got that right?

 

Richard

 

MORE: I should revisit the circuit in post 1 and change the voltage at B in state 2

Edited September 12, 2012 by RLWP

[Gibbo]

Sort of

 

There appears to be some confusion about how the reg is wired up. Point B runs to the field and to the reg to provide a power feed for the reg. The other side of the field is connected to the transistor (in the reg) which connects it to ground in order to regulate the field current.

 

The reg needs a certain amount of voltage to work. It will switch on the controlling transistor which shorts one side of the field to ground. The field is very low resistance so the other side (connected to point B ) would also try to go down to ground.

 

However, if it did that, it would remove the power feed to the reg which would switch the transistor off again so point B would rise again. Obviously this can't happen. Instead it settles with point B at around 2 to 4 volts (depends on the reg). It can't go any higher because the reg would switch the transistor on harder. It can't go any lower because that would remove the power feed to the reg so the transistor would switch off.

 

In state 2, point A is at 12 volts, point B at somewhere between 2 to 4 volts. This is sufficient to energise some small relays (yes even 12 volt relays). A bigger relay will not do so. However, a bigger relay will rob current and sometimes stop the alternator from firing up. The commonly seen solution to this is to increase the wattage of the charge warning light, but that reintroduces the problem of the relay possibly connecting just with the ignition on but the engine not running.

Edited September 12, 2012 by Gibbo

[RLWP]

<snip>

 

The commonly seen solution to this is to increase the wattage of the charge warning light, but that reintroduces the problem of the relay possibly connecting just with the ignition on but the engine not running.

 

Right - that's where I am with my current installation. I'm not sure I can up the wattage of the bulb, so what is the alternative?

 

On the drive back I was considering a relay across the bulb (A to that switches the big relay, I guess the voltage will be pretty small though

 

Richard

[Gibbo]

Resistor in parallel with the charge warning light.

[Keeping Up]

You could even try two charge warning lights in parallel

[RLWP]

Resistor in parallel with the charge warning light.

 

 

You could even try two charge warning lights in parallel

 

A resistor in parallel with the light of the same value as the resistance of the bulb?

 

Richard

 

MORE: Is this a fool-proof, cast iron, copper-bottomed, guaranteed to work every time solution?

Edited September 12, 2012 by RLWP

[Gibbo]

A resistor in parallel with the light of the same value as the resistance of the bulb?

 

Well that will double the current available. It might need more, it might need less. Depends how much current the relay coil needs.

 

MORE: Is this a fool-proof, cast iron, copper-bottomed, guaranteed to work every time solution?

 

No

[Keeping Up]

Well that will double the current available. It might need more, it might need less. Depends how much current the relay coil needs.

 

 

 

No

 

Wot he sez too

[RLWP]

So, If I find the resistance of the coil in the relay, we could do some sums?

 

Richard

[Gibbo]

So, If I find the resistance of the coil in the relay, we could do some sums?

 

Richard

 

You need the pull in voltage of the relay too.

[dor]

One boat I had for many years simply had the relay wired to the 'ignition' switch. It is possible that the switch terminal was isolated in the start position.

Never had any problems with it, even when starting with very low cabin batteries as long as the start battery was well charged.

Edited September 12, 2012 by dor

[RLWP]

One boat I had for many years simply had the relay wired to the 'ignition' switch. It is possible that the switch terminal was isolated in the start position.

Never had any problems with it, even when starting with very low cabin batteries as long as the start battery was well charged.

 

The danger comes the other way around. If the starter battery is low and the cabin batteries high, the starter current is drawn through the relay - BANG!

 

Richard

 

You need the pull in voltage of the relay too.

 

Research needed

 

Richard

[Gibbo]

The danger comes the other way around. If the starter battery is low and the cabin batteries high, the starter current is drawn through the relay - BANG!

 

Only if you use a toy relay.

[dor]

The danger comes the other way around. If the starter battery is low and the cabin batteries high, the starter current is drawn through the relay - BANG!

 

The starter drew 130 amps. I would like to think that my relay could handle that - after all, many alternators now are rated higher than that.

 

Anyway, it didn't blow, even when the start battery was so low it could barely turn the engine over and I had to resort to holding a spanner across the terminals between the start and service batteries. But I agree, you do have a point if the relay is not the right sort (which the boat had originally - one of those little 1" cubes which also dropped the voltage by about a volt due to poor contacts).

[RobinJ]

Only if you use a toy relay.

Or toy wire!

 

One morning when the domestics were low, I found smoke coming out of the engine hole. It was the wires on the relay taking the current from the starter to domestic starting to melt!

 

There now 4 times thicker (starter cable)!

[smileypete]

The danger comes the other way around. If the starter battery is low and the cabin batteries high, the starter current is drawn through the relay - BANG!

Could connect the other side of the relay coil to the starter solenoid. Best to include a small fuse in the relay coil supply.

 

cheers, Pete.

~smpt~

[Gibbo]

You can easily get rid of all these possible problems by using huge cable and massive relays. It's far simpler than fannying around trying to stop the relay doing this and that under various conditions. You can then also use the relay as an "emergency start from domestics" in the event of a flat starter battery. The split charge system should be designed to handle starter current, not designed to try to avoid it.

[RLWP]

You can easily get rid of all these possible problems by using huge cable and massive relays. It's far simpler than fannying around trying to stop the relay doing this and that under various conditions. You can then also use the relay as an "emergency start from domestics" in the event of a flat starter battery. The split charge system should be designed to handle starter current, not designed to try to avoid it.

 

Yes, I can see that now. It isn't particularly obvious until you have to think it through

 

Richard

[Timleech]

You can easily get rid of all these possible problems by using huge cable and massive relays. It's far simpler than fannying around trying to stop the relay doing this and that under various conditions. You can then also use the relay as an "emergency start from domestics" in the event of a flat starter battery. The split charge system should be designed to handle starter current, not designed to try to avoid it.

 

BEP used to do a Voltage sensitive relay with an integrated override, seems to me a good idea but they appear to have dropped it from their range. Anyone know why, and does anyone else make a similar product?

 

Tim

[smileypete]

Oh, just spend another £150 on a Smartbank!

 

cheers, Pete.

[bizzard]

My split charge relay is a £4 car caravan one and has worked magnificently for 13 years. It came with 4 wires sprouting from it. Here's the wiring instructions.

 

1. Connect red wire to car battery +ve via 15amp fuse.

2. Connect black to earth -ve.

3. Connect brown to car ignition warning lamp circuit WL or IND on alternator or D on dynamo.

4. Connect green to caravan battery +ve via the 7 pin supplementary socket pin 2.

Easily reinterpriated for boat use.

The auxiliary battery is capable of holding at least 10v charge.

 

Ok i only have one cabin battery, so i suppose a more beefier one would be needed for more batteries.