I have a vintage car, and corroded wireing is normal. if you turn on a load and check the voltage drop on the wire from the battery to the load, you can find many bad connections. on my car, I turn on the headlights and test the voltage on each fuse one at a time till I find one with low voltage. or turn on the wipers and check to the wiper motor to find the voltage drop. most radios these days do blank when starting, because they have two paths to the battery, the radio and display are turned off when the starter is run to prevent undervoltage problems, while the clock wire is hot all the time to keep the clock runninng, ogtherwise you would have to reset the clock every time you start the engine.

 

Okay different approach. Battery voltage is 12.6VDC. You are measuring 8.5 VDC. This is excessive. What is the current draw from the starter? What is the voltage drop on the negative side of the circuit? What is the current draw on the positive side of the circuit? Do you have a schematic?

 

Another possibility is the starter bearings (bushings) are bad and causing the started to draw too much current.

Or, the battery is aging and internal resistance is high under load. Bring it to the automotive store (pepboys) and they can do a load test. Load test measures voltage drop under a known load. If too much drop, time to replace the battery. If not too much drop, see suggestion in the paragraph above.

 

Okay different approach. Battery voltage is 12.6VDC. You are measuring 8.5 VDC. This is excessive. ...
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It's common enough for a battery to drop as low as 8-9v at the wires of battery terminals when starting the engine.

 

It's common enough for a battery to drop as low as 8-9v at the wires of battery terminals when starting the engine.

If that is the case, you are you then suggesting that the resetting clock is an issue with the clock/radio or the power cable from the battery to the clock?

 

If it's like the gas carts I know they use the generator as a starter, so it's a little like trying to start you car when it's still in gear (for those of you that remember manual transmissions) so I can see a big dip. Is your problem with the radio or the clock? To keep the radio working probably takes another battery and #12's diode. If it's just the clock the cap might do it. Might be a good application for a super cap, but they aren't cheap.

 

It's common enough for a battery to drop as low as 8-9v at the wires of battery terminals when starting the engine.

My background in DC motors is fairly extensive. I had been a rebuilder of starters, generators and alternators for several years and have worked on gas and diesel engines for most of my life. A battery shouldn't draw down past 9.5 VDC when cranking. If it does, there are other issues at work. It could be a bad starter/generator, voltage drops caused by excessive resistance or a bad battery. The OP should have his battery load tested first of all, even if it is a new battery. Some sit on the shelf way too long and sulfate before they are used. As I said earlier, he should also check the current draw while cranking which could indicate worn bushings, armature problems etc. He should also check voltage drops on positive and negative sides to ensure cables and connections are all good. You are looking for a voltage drop of near 0.0VDC to a max of 0.3VDC. After that and only after that would I start looking at the radio and caps/supercaps etc. Just for a quick reference, a 600 HP diesel engine usually starts with 10VDC still showing at the starter solenoid "S" terminal while cranking.

 

A battery shouldn't draw down past 9.5 VDC when cranking.

Anything less at the starter is considered a fault condition. The drop across the cable in an auto should only be ~0.1V.

 

It may indeed be a fault condition, but I have seen 8-9v range when cranking on older vehicles. Maybe it's a high resistance battery, maybe bad wiring or terminal lug resistance, or a combination. But it's definitely common.

If that is the case, you are you then suggesting that the resetting clock is an issue with the clock/radio or the power cable from the battery to the clock?

I didn't see anything about a clock, only that he said "the radio blanks out". Like I said in my first post, it's common for car audio to have undervoltage protect, so the amp is powered down during engine cranking. The result is a "blanking out" of the sound when cranking the starter.

If it is a clock resetting issue that is a fault inside the car radio. The clock memory etc will be on a 5v rail and that should still be fine even when cranking. Usually they have a separate wire for the clock power, that bypasses the ignition switch. So there is a switched power wire, and a always-on power wire.