Thread: cranking issue solved?

alternatively, you dont have to use a relay and splice into your starter solenoid cable - you could put an isolater switch somewhere on your dash. A DPST Switch that you flick before cranking, then flick back when the engine is started. The above diagram is just to take the manual bit out of it - but a switch could be put in its place should you want to.

basically, the parallel circuit between the 2 batteries is complete when no power is going to the relay

Only problem is that when your car is on B2 will be powering the computer but will not receive a charge. It will begin to charge when the car is off. This could be a problem if you run b2 very low. Then when you shut the car down b2 will drain b1.

Another issue is the current you are sending across the coil of the relay. Your alternator is going to pump out ~100A across an ~80ohm coil. It is going to get really hot really fast and pop.

I think you are on the right track though. You can just use switched power across the coil and use the relay(solenoid) to seperate b1 from b2 when the car is off=).

Grab one of these relays and you will be set.

thankyou thankyou and thankyou again. this is the godsend i have been looking for. well, the relay to be exact. Ive gone through several hundred relays trying to find a suitable one. I know what youre saying about blowing the relay,that was one of the biggest concerns of mine when designing it - "can i get a relay thats gonna be man enough?!"

I dont see why B2 wont get a charge whilst powering the PC. AHA - i see where youre coming from. The batteries will ONLY be disconnected during the couple of seconds of engine cranking. When the engine is started, the relay will go back to closed and the batteries will be connected again. B2 is only going to be separated from b1, so B1 alone can start the engine, without getting a huge v drop on b2 thur rebooting to PC. as soon as the engine is running and cranking is over, B2 will again be connected to B1 (and the alternater) and recieve a charge whilst powering the PC.

Hope that explains it a little better. (nb the relay will ONLY recieve power and sever the circuit when the starter sends out a current (to start the engine only) when starter stops sending current (after engine is started) the relay will close again and link the 2 batteries)

there are many sources of power that you can tap into that remain on during cranking. fuel pump,ignition,ect all you need is to use a relay and then power from battery , no need for coils and complex wrinig.

Hi guys this is my first post, hope to help u. I've reading about the cranking shutdown problem. The solution suggested by Draco is right, but the component between the two batteries is just a diode with amp rating above your PSU amp requirement (suppose 20A will be good enough for most systems @ less 5US$).

This is how its go: connect your two batteries in parallel. Then between the the two positive poles put your diode forward to the PSU battery, this is anode to car battery, cathode to your PSU battery (the cathode is the line marked terminal). Done. When you crank, if the voltage on the car battery goes low enough, the diode will be reversed so no current will draw from the PSU battery, wich will be supplying your PC system, while you crank. When you finish cranking the voltage rises and your generator will charge your PSU battery without need of manual intervention.

The diode will not waste power when you stop your engine. It will just get a little hot when recharging the PSU Battery (maybe some heat sink needed). It wont lower the full charge voltage of the PSU battery.

I suppose that any small UPS battery like 12V@5Ah at ~30US$ will work for your PSU Battery.

This is the correct setup to correct this problem, cause is very healthy to the PSU, and should be the solution used when the PSU dont hold while cranking.

Remember, no manual switches, no relays, just a diode.

Thankx. Please send your comments and questions.

Originally Posted by Mr Crown

I suppose that any small UPS battery like 12V@5Ah at ~30US$ will work for your PSU Battery.

Is that true? I was going to pick up a $10 7.2 Ah sealed lead acid battery from a local store to see if this works. However, at 200w potential power usage for my PC, that's something like 16A! Can a cheap battery really supply that?

Also, what happens if there's a voltage spike at the end of the crank? Does this help protect against that?

Here's the diagram.



Really i haven't tried, BUT THIS WORK, maybe if your system is heavy loaded its better 7ah, but dont think bigger than that is needed. For example my 450VA APC UPS uses one 12V@6Ah, and holds about 5 minutes at full charge, REALLY I DONT THINK YOU'LL CRANK FOR 5 MINS, really i dont think your system takes 450VA (thats a full pc and >15" monitor), and your using it directly not loosing energy on an inverter.

5Ah should work for most systems, 7Ah WILL WORK for "any" system.