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I have just seen a diagram where it shows both a main battery and a standby battery however I am unsure what the differences are?
I thought the main battery was there as an emergency power source / APU starting etc.. so what is the standby battery there for? Or is it a secondary back up?
Depends on the aircraft.
In most piston driven aircraft, the main battery is there for starting. The main battery also sort of runs all electrical equipment, lights and avionics. The only thing it does not run is the ignition system of the engine. I say “sort of” because the alternator supplies the battery and aircraft with more than enough electricity to supply the aircraft’s needs and recharge the battery simultaneously and at a slightly higher voltage. If the alternator goes out, it should not interrupt the total supply of electricity to the aircraft systems.
Think of the battery as a bucket with a hole in it, placed under a faucet (the alternator/generator). As long as the faucet is open enough to supply more than the bucket needs to remain full, you are getting all of the faucets water. If you partially or fully close the faucet, you are only going to get the water that comes from the bucket. But, your water flow is never completely interrupted.
On the other hand, the standby battery is a separate, isolated supply of electricity. As long as the main battery/alternator combination is supplying electricity, the relay is self supporting to keep the circuit closed to supply electricity from the main battery to the aircraft’s Bus Bar (electrical connection to the aircraft’s systems). If the main battery/alternator combo is not supplying electricity because it is dead, the relay will open the circuit from the main battery. It will simultaneously close the circuit supplying electricity from the standby battery to the Bus Bar. This would happen if the alternator/generator was offline or inop long enough to drain the battery.
The relay is probably a spring and electromagnetic set up. The state of being ON makes the main battery active. The state of being OFF makes the standby battery active.
Think of it like the faucet and bucket in the previous example, if the bucket were attached to a spring loaded shut off valve for another faucet. That second faucet was being fed by a container of water. It also had a hose bypassing the bucket. Once the bucket became light enough, the second faucet would turn on to supply you with water. At least, temporarily in a no water emergency.
In the picture above, the piece of electrical equipment in the red dashed box is a relay. Relays work when electrical power is supplied to the relay and magnetism pulls the relay down. That is the reason why there are coils (circles) on the bottom of that box. In this picture, the relay is open connecting the standby battery to the bus bar.
The standby battery has a switch called NORM / EMERG. That switch is currently in the NORM position preventing emergency battery from connecting to the bus bar.
The main battery also has a switch called ON / OFF and it is in the OFF position preventing the main battery from connecting to the bus bar. Notice that the main battery powers the relay and when the switch is ON the relay closes and main battery power is routed to the bus bar.
All aircraft that I fly require main batteries to start the airplane before it's APU, GPU or engine driven generators will come online. The main batteries will also power selected electrical equipment in flight if all the generators fails. In this situation, the pilot will select EMERG on the NORM / EMERG switch to supply emergency battery power to the bus bar so that certain equipment can still be powered. In most aircraft, that equipment is the standby attitude indicator, possible tachometer gauges or other engine power gauges, possible landing gear and flaps. It is all dependent on how that airplane was designed.
