Weird Miata Intermittent No Power / Start Issue | 2001 NB Miata

2001 Mazda Miata (NB)

Went to drive home last night and the car wouldn't start. This morning I tried jump starting it and got nothing. Thought maybe my boost pack was not charged enough. Came back later and after a minute it decided to start and I was able to get it home. Now this evening I go try to do some diagnosing and I am stumped. 

Battery shows 12.4v sitting. It's a tiny lawnmower battery that the PO put in before I got the car. Cranking is a little slow with it generally, but with 12.4v, that should be enough to have interior lights. 

When the issue is happening, I have no electricity anywhere, no interior lights, no radio, no crank, no door open beeps. 

But then sometimes if you wait a couple minutes and then open the door, the interior light will come on and the door open beep will sound. Sometimes it's enough juice to crank and start, sometimes when you go to start something clicks and then nothing has power again. 

I checked the battery terminals, and they are solid. At some point I'll put a proper battery in, but for now it's ok. 

Fuses in the engine bay seem ok, including the 100A main fuse.

Any ideas?

The only things that have happened recently that may give some sort of clue or cause, is yesterday I scraped going into my buddy's driveway pretty decent, but I don't notice anything horrible underneath off the bat. Or I have noticed that the last few days sometimes it's taken a little longer to start up when cranking and sometimes needing a little throttle to help it. It has gotten colder the last few day though. 

In reply to AWSX1686 :

I’d guess either a bad ground connection, either from the battery to the PPF, or in the engine bay. Second guess would be a bad battery cable, either negative or positive. 

Swap with known good battery, see if problems go away.

Check where the ground cable bolts to the body in front of the battery, and where the ground cable bolts to the PPF near the rear diff.  Generally what you're saying would be a battery terminal problem, but since you say they're good I'm thinking it's a problem with one of the main ground leads. 

I have had an Optima battery with internal wiring problems that would act like this. Basically, an intermittent internal connection on one terminal. 

how did you check the terminals? i only ask because the same thing happened in my truck last year. Turned out the ground terminal end was cracked and it seemed tight but caused a lot of weird electrical issues/no starts. One sign that lead me to look closer into the terminal was after a few attempts to start it and no luck and all the electrical stuff freaking out i touched the ground cable and it was warm/hot. but it does definitely sound like a grounding issue

Might be worth checking the ign switch.

Good advice above, check the ground first. Then try a good battery. 

As an aside, my 01 Miata got me home running on just the battery a few weeks ago. One hundred miles, and two hours without an alternator. Still running with 7.5 volts showing on the ScanGauge.

how did you check the terminals? i only ask because the same thing happened in my truck last year. Turned out the ground terminal end was cracked and it seemed tight but caused a lot of weird electrical issues/no starts. One sign that lead me to look closer into the terminal was after a few attempts to start it and no luck and all the electrical stuff freaking out i touched the ground cable and it was warm/hot. but it does definitely sound like a grounding issue

The main way is just 'mess with them' and see if the symptom changes. When you have a total loss of power one easy thing you can do is turn the headlights on and then go mess with the terminals. The headlights don't depend on the ignition switch in any way and if they light up you get visible confirmation and you dont have to go back into the car and turn the ignition to figure out if you accomplished anything. You can also check voltage between the terminal and the post itself with a multimeter. There should always be 0 v between a terminal and its post unless something is wrong. The hot cable/terminal is also a good clue. If you had done something like crank the car for an abnormally long time you could expect the terminals/cables to be warm. But, since the same amount of current flows out of a battery as back into it, BOTH sides should be warm. Any time one side is a lot hotter than the other, it's a sign of a bad connection.  When you take the same amount of current and force it through a smaller space in the wire, it will heat that area of the wire more than the rest of the wire. So, you can narrow down the location of a bad connection somewhat based on temperature.  Generally only the starter will flow enough current to make a lot of heat, though. 

I do believe I found the issue, and a number of you were correct. It appears that it was the terminals.

I replaced the terminals and hooked up to the new (used) battery and all seemed well. I am now just waiting to get my drill back from a friend so I can adjust my battery mounting to accommodate a proper (not lawnmower) battery that I recently acquired. 

Old terminals: One of these things is not like the other...

I do feel slightly dumb for it being such a simple fix, but at the same time, I had not seen that type of battery terminal before to know how they likely didn't have a good connection inside.

Thanks to all for the input!