Identify this component?

colind3782

colind3782

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After years of procrastination I'm getting the urge to sort out the rats nest in the battery compartment. I think the attached might be an old relay of some description but can anyone positively identify it? The wire going in behind it is from a + battery terminal and the two wires from the front terminals go to two separate isolating switches.
IMG_20210618_104929[138].jpg
 
Apart from the number of wires and colour - it looks similar to the split diode unit - that is in my engine bay - to split charge to two battery system.
Mines disconnected because it was not fitted with voltage compensator - the diode system drops the voltage about 0.7V.
 
Hi Colin and just to fill in the details (or teach granny to suck eggs). They look like silicon power diodes. They are like one way water valves in that current can flow form the one wire behind to the 2 wires connected to the cathode of the diodes. These connect to 2 different batteries. But current can not flow from one battery pos to the other through the diodes. Sounds like a perfect arrangement to charge automatically 2 batteries while maintaining complete isolation of the 2 batteries. (engine start and domestic load)
However the problem is that silicon diodes don't let the current through easily. In fact there is around .7volt drop when current flows through the diode. A bit like you might imagine a spring in a one way valve has to be overcome. This voltage drop is peculiar in that it is near constant from very small currents to large currents.
So the problem is that if the voltage from the alternator is regulated at the alternator to 14volts you can only ever get the batteries up to 13.3 volts. This is far short of fully charged. The answer is to get the alternator to regulate (deliver) more volts so if you can adjust the regulator up another .7 volt no problems. Unfortunately most regulators are not easily adjustable. A regulator does however does have a point where it senses the output voltage of the alternator. So if you can find this connection and move it to one of the battery positive terminals the alternator will sense the loss in the diode and make up for it. So called smart add on regulators usually have this wire come out for connection for battery sensing.
I tell the OP this because he needs to check actual voltage of the batteries on charge. Should be around 14volts. Any less might indicate that someone has connected the diodes without regard for volt drop.
You can however now days buy a similar device using field effect transistors that do not give a volt drop.
Another perhaps more popular option is to wire the alternator straight to the engine battery and fit a voltage sensing relay which will automatically connect house battery to engine battery for charging when the voltage of the engine battery rises because it is being charged. (and disconnects when engine stops charging) ol'will
 
William_H said:
Hi Colin and just to fill in the details (or teach granny to suck eggs). They look like silicon power diodes. They are like one way water valves in that current can flow form the one wire behind to the 2 wires connected to the cathode of the diodes. These connect to 2 different batteries. But current can not flow from one battery pos to the other through the diodes. Sounds like a perfect arrangement to charge automatically 2 batteries while maintaining complete isolation of the 2 batteries. (engine start and domestic load)
However the problem is that silicon diodes don't let the current through easily. In fact there is around .7volt drop when current flows through the diode. A bit like you might imagine a spring in a one way valve has to be overcome. This voltage drop is peculiar in that it is near constant from very small currents to large currents.
So the problem is that if the voltage from the alternator is regulated at the alternator to 14volts you can only ever get the batteries up to 13.3 volts. This is far short of fully charged. The answer is to get the alternator to regulate (deliver) more volts so if you can adjust the regulator up another .7 volt no problems. Unfortunately most regulators are not easily adjustable. A regulator does however does have a point where it senses the output voltage of the alternator. So if you can find this connection and move it to one of the battery positive terminals the alternator will sense the loss in the diode and make up for it. So called smart add on regulators usually have this wire come out for connection for battery sensing.
I tell the OP this because he needs to check actual voltage of the batteries on charge. Should be around 14volts. Any less might indicate that someone has connected the diodes without regard for volt drop.
You can however now days buy a similar device using field effect transistors that do not give a volt drop.
Another perhaps more popular option is to wire the alternator straight to the engine battery and fit a voltage sensing relay which will automatically connect house battery to engine battery for charging when the voltage of the engine battery rises because it is being charged. (and disconnects when engine stops charging) ol'will
Click to expand...

You can cheat a regulator by adding a diode to it so the regulator sees the 'in-house' drop ... far cheaper and easier than addon bits of kit.

Alastair Garrods "Electrics Afloat" may be an old book - but its still worth the pennies to buy. ISBN 0-7136-6149-6. Produced in association with PBO ...
Page 64 ... "Fooling the senses"

Basically the regulator if machine sensed (not battery sensed) will be cheated by this inexpensive diode inserted - thinking its only seeing 13.3V .. so it bumps up rate to what it thinks is 14V ... but in fact is 14.7, so the split diode system when it drops the 0.7V - you have your 14V at battery as needed. Cheap and effective.
Not only that - but it does not rely on one battery to sense because in fact it doesn't need to.
 
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