Battery Discharge

That sure sounds like a battery to me. Mine was very weak after many years of service. Off a charger would crank great, then the next day weaker, then weaker, then wouldn't crank. I'd keep it going by charging it every day until eventually I shut the car off after a long drive and it would not start immediately after. Replaced the battery and all good.

Certainly sounds like a week battery. Replace it and if possible consider an automatic trickle charger to prevent completely draining it and then making the charging system work even harder to drive the car and charge up a completely drained battery.

I mentioned it to Dev a week ago but a few of the new Ford vehicles actually have two batteries now. One is an AGM for the Stop/Start functions and a second is in the trunk just for all the 12v electronic crap in them.

Makes me so glad the Spyders were so relatively low tech.

If its a wet cell, it could be low on water in one or more cells.  A fully charged lead-acid wet cell battery should be ~ 12.7 V and yes, the 200 mV difference means its discharged to about 90% capacity.  There is always a parasitic current draw on the battery unless you fully charge it and leave the negative terminal disconnected.  That is the best way to store it, or as stated above, keep it on a trickle charger.

Sometimes batteries develop an internal short that drains the voltage when idle.

try as Neomr2 has suggested and disconnect the battery after a full charge then monitor the static voltage over days or weeks.

also be sure that you have clean connections at the battery and your cables are in good order.

Or, Maybe your old gal is jealous that you are ignoring her.

It can still be any number of things especially for an old car.

1. The alternator needs to be load tested.

2. The battery could just be weakened from continuous discharge.

3. The starter could be on its way out. 

4.  The wiring including the grounds might be corroded and the battery posts are not tight enough.  This is usually the cause of this kind of situation in an old car that is not driven much compared to a newer car. You need to inspect the grounds and need to see if you can rotate the battery clamp on the posts. If its loose you should buy some battery post shims and retighten the clamps so it has some bite to it.  

 I have the same issue with my old Honda and a new ground wire and shims made it a whole lot better when the car sits but Im afraid that if it sits for over two weeks it barely turns over.  

I bought one of those “load testers” that has a piece of coat hanger inside and monitors voltage under load. They’re pretty cheap. Plus I have to maintain a fleet of old jalopies that my children and spouses drive.

Posted by: @wilcomr-s

Certainly sounds like a week battery. Replace it and if possible consider an automatic trickle charger to prevent completely draining it and then making the charging system work even harder to drive the car and charge up a completely drained battery.

 

I mentioned it to Dev a week ago but a few of the new Ford vehicles actually have two batteries now. One is an AGM for the Stop/Start functions and a second is in the trunk just for all the 12v electronic crap in them.

Makes me so glad the Spyders were so relatively low tech.

 I was watching some commentary on Youtube and they say that a lot of these newer cars will end up totaled and completely unsalvageable in the near future because of all this tech is going to be difficult to reproduce and replace.  

 I think the Spyder is the last series of cars of this type where it can be restored as vintage. If they make a new MR2 its not going to be the case at all as the industry as a whole has run into a crisis that Ted Kaczynski wrote about. 

Even though Spyders have depreciated horribly there might be a time when they will start appreciating just based on the practicality of it all. When no one wants to touch a 5 year old  $100k+ German luxury car that is a good indication something is wrong with consumption. 

Posted by: @oldman

I have retired and no longer drive my Spyder much. It often sits unused for weeks at a time. The battery will not crank the engine after an extended period of idleness. I have never before let one of my cars go unused for more that a few days, so I'm wondering if my battery discharge situation should be expected.

I recently started the car by "rolling-it-off" and drove for about 20 miles. At the end of the trip, the battery could barely turn the engine over. I put the battery on slow charge for 24 hours. After that, it cranked normally. The next day (or two) it cranked less vigorously. The cranking was even less energetic a few days later.

I thought the alternator might be failing, so I did the cheap-&-dirty test: I measured the battery terminal voltage with the car not running and then again with the engine running at 2000+ RPM. All seemed well. Non-running voltage was ~12.5v and running voltage was ~14.5v, so I figured the alternator was OK.

I tried to start the Spyder again yesterday, and again I had to "roll-it-off". Only a few days ago, the car started, albiet somewhat sluggishly. I'm now suspecting the battery: It is less than two years old, so I am reluctant to blame it. None-the-less, it seems that it can't hold a charge.  I don't want to replace a good battery, but that is the next step in my investigation.

 

Ideas/Opinions/Suggestions?

First of all, you must realize that the life of a lead-acid battery is inversely proportional to the time it spends less than fully charged, and really loses a lot of life every time it gets nearly fully-discharged.  So if you want the next battery to last for a long time, you will need to keep it on a float charger between drives.  I keep my occasional-use cars on a float charger whenever I expect them to be stationary for more than a few days.  Every modern car has slow power drains that occur in the engine and body ECU's as well as a little bit of leakage through the alternator diodes.

Some float chargers have quick-connect kits that allow easy hookup and removal.