What is the terminal voltage of the battery while the engine is cranking? If it's below 4.5 volts you either have a bad battery, a bad starter, or there's trouble inside the engine that's causing too much drag. If a battery load tester is available, load the battery down to the same voltage as when the engine is cranking and see what the amperage load is. Try cranking the engine with the spark plugs removed and compare the cranking speed with the compression relieved.
Making wild guesses about electrical problems without adequate troubleshooting is a good way to spend a lot of money needlessly. Like Sun Electric Co. used to say, "We Test, Not Guess!"
Jerry
