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Need to know how to incorporate an 8 pin relay into a battery charging circuit

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nofky

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I need someones help how to use an 8 pin relay in a circuit. I need it to allow a standby cranking battery to be charged by a SCR charger when the standby generator is not running and then switch to an engine driven alternator when the standby generator is running.
 
The fact that the relay has 8 pins really says nothing. could be a 2pdt +coil , could be 3pst +coil could be 2pdt + balanced coil. Need the specs of the relay.
 
here are the relay specs

the relay is a DPDT relay. the pin configurations are 4-5 contacted all the time, 3-6 are the other 2 contacts to be switched to, 2-7 are to the 24 VDC coil, and 1-8 are the inputs. The coil has a 10A rating and is a type 60.12
 
Is the relay breaking the connection between the battery and the starter motor?
 
Is the relay breaking the connection between the battery and the starter motor?

I am not sure if is has to or not to be honest. What it is , is a schoool project and I have to design a shematic to use this relay. the relay is a KRPA-11D-240 I have to use it in a schematic to change over from the SCR charger to the alternator after the engine has started. Any help you can give me would be greatly appreciated.
 
Since the starter probably draws in excess of 50A through a #4AWG wire, then the relay should not break that path. Since it is a REALLY bad idea to start an engine with the battery disconnected, the relay should not break the path between the alternator and the battery, either. Of the roughly 20 battery chargers I own, all could be left connected to a vehicle battery as the engine is started, so why do you think you need the relay at all?
 
Since the starter probably draws in excess of 50A through a #4AWG wire, then the relay should not break that path. Since it is a REALLY bad idea to start an engine with the battery disconnected, the relay should not break the path between the alternator and the battery, either. Of the roughly 20 battery chargers I own, all could be left connected to a vehicle battery as the engine is started, so why do you think you need the relay at all?
This relay is to be incorporated into a standby generator system. The instructor has asked us to make a schematic that involves this relay switching from the SCR to the alternator after the engine is started. Could this be a trick question he has asked us?? LOL
If not....can you think of a way this can be done?
 
Is the relay switching supposed to be automatic?
 
I would consider it a trick question being a battery charger can be used on a battery while the alternator is still hooked up. Also a battery charger can be hooked up to battery while its power is off as well.

In practical real life applications both can stay connected to the same batery all the time.
 
so in short....is there a way this can be accomplished?? and if so can you please tell me how to do it.
 
You have a few minor problems. The first is trivial but should be addressed. The relay you mention is a very common relay and yours has a P&B (Potter & Brumfield) part number:

KRPA tells us an enclosed case model.
11 tells us a DPDT (Double Pole Double Throw
D tells us a DC coil
240 tells us a 240 volt coil and you mention 24 VDC coil.

Not a big deal.

Here is what I think your instructor is getting at. Emergency generators with automatic transfer generally just sit around and don't do much unless they are needed. These generators have a battery start circuit as trying to pull the cord would not be fun. Forget the starter motor as we don't care about it right now. Focus on the battery because when the generator is needed that battery is the ball game. To maintain a charged battery there is a "trickle charger" which is just a battery charger to maintain the battery. The trickle charger as well as all other standby electronics in the generator system is fed by mains while the generator is idle doing nothing.

However, when power fails at the mains things start happening. The generator will wait a few seconds (a preset time) and then the starter will engage starting the engine. More unimportant stuff happens and the generator comes what they call online replacing mains power and a big contactor switches from mains power to generator power. The battery charger is not fed by the generator. We also just drew current from our battery to start the generator. Now a set of windings on the generator take over the job of charging the battery. This is the battery charge winding.

A few caveats here. When you see the term SCR in this case it is not a Silicon Controlled Rectifier but what is called the Starter Contactor Relay and you have a BCR which is the Battery Charge Relay. What you have in your problem is the BCR. You want to switch the battery to charge from the generator DC output and not from the battery trickle charger when the generator is running.

Now before I forget looking at your relay. That is an octal base relay with 8 pins and you have the pins right. Pins 2 & 7 are your coil. Pins 1 and 8 (the common pins) would go to your battery, pins 3 & 6 are your NC (Normally Closed) and would go to the charger and last but not least pins 4 & 5 are your NO (Normally Open) and would go to the generator charge output. Your pins 2 & 7 (relay coil) would go to the generator control board.

I don't think your instructor expects a full blown drawing of a generator control board or the brains of the system in detail. He wants you to outline how the BCR would be configured. Additionally label your relay contact pins with + and - polarity making sure you keep things together.

Just My Take
Ron
 
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No problem and I hope you understood a post that had multiple interruptions. :)

Again when you run to your battery just make sure you keep the polarity straight from battery. That is likely why the instructor had you use a DPST relay.

Ron
 
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