2013~2016 My 2016 would not start this morning

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2019 CX5 Reserve AWD
New battery in August. Drove it about 35 miles each way on Sat. Sunday short hop maybe 7 miles round trip, 2 stop/starts. Monday about 8 miles, 2 stop/starts. Tue sat in garage all day. Went to go out this morning, green light on, no starter action but the infotainment system booted up.
Right now I have a charger on the battery and it's only at 50% charged after 1.5 hours.
No problems from August until today.

Bad cell? Alternator? If I'm able to start I'll take it for a long drive then throw a multimeter on it and see what the voltage is.
 
.... Bad cell? Alternator? ....
Maybe, or maybe something else. No point in guessing, because only diagnostic testing will get you to what the actual problem is. Part hangers would disagree, but it doesn't sound like you're in that group. Monitoring the battery voltage after the recharge, as you mentioned in your OP, is the way to start the process on this one.
 
How many miles on the 2016? Exactly what new battery? It could be either the battery or charging system, hard to diagnose from hundreds of mikes away.
 
Maybe, or maybe something else. No point in guessing, because only diagnostic testing will get you to what the actual problem is. Part hangers would disagree, but it doesn't sound like you're in that group. Monitoring the battery voltage after the recharge, as you mentioned in your OP, is the way to start the process on this one.
One thing I've noticed is I've had a charger on it for the last two hours. It started at the bottom meaning it was < 25% and it worked it's way up to 50% fairly fast but it's been at 50% for the last 1.5 hours. It's like the battery will not take a full charge. Hopefully it's a defective battery and will be replaced under warranty since it's only 3 months old.
Time will tell.
 
One thing I've noticed is I've had a charger on it for the last two hours. It started at the bottom meaning it was < 25% and it worked it's way up to 50% fairly fast but it's been at 50% for the last 1.5 hours. It's like the battery will not take a full charge. Hopefully it's a defective battery and will be replaced under warranty since it's only 3 months old.
Time will tell.
Sounds like a bad cell in the battery, these 2016 CX-5 are not known for electrical issues.
 
One thing I've noticed is I've had a charger on it for the last two hours. It started at the bottom meaning it was < 25% and it worked it's way up to 50% fairly fast but it's been at 50% for the last 1.5 hours. It's like the battery will not take a full charge. Hopefully it's a defective battery and will be replaced under warranty since it's only 3 months old.
Time will tell.
Most likely it’s bad battery even though it’s only 3 months old based on your description. What kind of battery is it?
 
What is the voltage now? (that it stops taking more charges)
There are 6 cells in the battery and each one should be about 2.1V when full.
Hence, 12.6V total is the norm.
If you see 10.5V, then, possible 1 cell is bad and so forth.

When alternator is on, voltage between 13.6V to 14.6V is norm.
 
How many miles on the 2016? Exactly what new battery? It could be either the battery or charging system, hard to diagnose from hundreds of mikes away.
34,000 miles. It's a battery that the dealer installed in Aug. $150.00
Zero problems until this morning.

I change the battery out every three years, regardless. Don't like to get stranded although that reasoning may be flawed. Luckily I was stranded at home.
 
Installed by dealer. Not sure of brand, will check.
So you have Mazda OEM “High-Performance” batteries sold in US Mazda dealers and made by Interstate.

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Finally finished charging. Tested voltage and battery was around 12.5v. I have an older multimeter which isn't digital so you have to intererpt where the reading is. Anywas it was about half way past the 12 tic. Car started right up, took it for a 9 mile ride and returned.
Check voltage while the engine was still running, looked like 12.5v. Turned the car off and tested again, 12.5v. This is the scenario I dread, untrustworthy battery. I think I will wait a few days and see how it goes then take it back to dealer for a load test on the battery.
 
Also, check for current draw on the battery. My Daughter had a new battery fail to charge completely after only 3-4 months. Bad batteries are infrequent but not impossible to find.
 
The thing I don't like is I'm only getting 12.5v when the engine is running. I would expect somewhere around 14v. I tried testing the alternator but couldn't find a connection point to measure voltage.
 
The thing I don't like is I'm only getting 12.5v when the engine is running. I would expect somewhere around 14v. I tried testing the alternator but couldn't find a connection point to measure voltage.
Try to find time stopping by the AutoZone and have them to load-test the battery and check the charging system.
 
You would think the car would throw a code or light or CEL for a 12.5 volt while running.
 
not on this car unfortunately.
Indeed 12.5v while running is too low.
could it be a loose connection on the battery cables?
 
The thing I don't like is I'm only getting 12.5v when the engine is running. ...
Is the CEL on? The ECM will have a target running battery voltage of somewhere around 13.5V, and will increase alt output as necessary in order to keep it there. Whenever that target voltage cannot be maintained, the computer should immediately turn the charging light on, set a DTC, and turn the CEL on.

I recommend finding a parts store or shop that has a real time data stream reader, in order to find out what the battery OBD values (SOC, target voltage, ...) from the ECM are, and also what codes (including pending) might be set. Something is definitely not right with your vehicle's charging system, and IMO it's likely to be an issue beyond a bad battery (although the battery might very well be trashed by now as well).
 
13.5v threshold for CEL is not true ... at least on my 2018.
I have had times when it was 13.4 (very hot summer time) and there were no CEL. Also sometimes when the car starts it takes at least few seconds to ramp up from 12 to 13 then to 14.
PCM will increase the Amperage when needed though but not always voltage which is also temperature dependant.
 
13.5v threshold for CEL is not true ... at least on my 2018.
I have had times when it was 13.4 (very hot summer time) and there were no CEL....
I wrote 'somewhere around 13.5V', not 13.5V. A reading of13.4V would be totally normal, and I'd never expect to see a CEL for that voltage, or even slightly lower.

Also sometimes when the car starts it takes at least few seconds to ramp up from 12 to 13 then to 14.
Again, nothing unusual or surprising there. What's important is the voltage once the vehicle has been running a while (and should no longer have large fluctuations).

The point I was making is that a continuous running voltage of 12.5V is abnormal and should be quickly detected and alerted as such by the ECM.
 
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