Battery? Alternator? Something else?

OK this has me stumped.

Battery is an optima yellow top, replaced almost exactly one year ago 5/16/16 according to my sharpie on the battery.

Truck is on the original alternator, 234k miles.

I put a small load (~4 amps) on the battery, when I got home and parked it was at 12.8, after 10 mins wiht said load it was down to 11.8. Load cuts at 11.9, so it turned itself off. Was at 11.8 this morning. Starts fine, 13.6-14 ish volts when driving in this morning, then same thing battery is under 12v in minutes.

Thoughts? Is the alternator simply putting out the correct voltage but not enough amps?

Posted this awhile ago. But it hasn't been dipping like that lately.

https://www.youtube.com/embed/LDd4Rn4lpFU

Started fine at 11.8? Or did you charge it?

I'm thinking your measurement device may be in question.

In reply to Robbie:

Starts fine at 11.8 (always has, I have a auto disconnect set for just below that and it still starts fine). Thats at the OBD port.

The fridge has auto shut off at 12V and its wired straight to the fuse block. (it shuts itself off as noted in ~8 mins)

So what exactly is the problem? Don't like that it drops to 11.8V?

How much does it get driven? Even if the alt is working fine, short trips won't be enough to fully recharge.

I highly recommend a complete battery/charging test. Voltage only tells part of the story. Need to know what amperage the alt is putting out, especially under load. Also need to know what the cold cranking amps the battery is capable of. Auto parts stores will run this test for free.

In reply to NEALSMO:

Fridge turns off under 12v, beer gets warm

Went out with the DVM it reads 12 at the battery, no smaller digits on this unit... reads 12 at the fridge plug, and Scangauge says 11.8.

Drive is 12 miles to and from work, plus errands. Yes I know voltage is only part of it. Sadly I dont have a DC amp meter. I will have to swing by the Ovancezone I guess.

I went out a set the LV shut off on the fridge lower, we will see what happens....

Warm beer? No expense should be held back. New battery AND alternator is in order!

In reply to NEALSMO:

Ha we will see. Id like to figure this out.... Never issues starting, but it just seems like I am not getting enough amperage out of the alt. I have heard brushes could be replaced on these.

Sadly the alternator is going to be a huge PITA to get out if it needs to come out.

With that mileage, the brushes are probably worn out an lose contact with the slip ring.

Voltage should be 14+ when driving own the road.

Volts are the force that push the amps through the wires

So this is a pickup truck? A motorhome? It has a refrigerator? You want a single battery to run the refrigerator all day while you are at work? You are concerned about the voltage dancing around on some sort of digital doodad?

In reply to Streetwiseguy:

It's A 4runner, yes I do have a fridge I toss in it occasionally, and yes I do think it should run it for a few hours. (dual battery OS just about done....) it's a 12v fridge designed for car use and efficiency is one of its big factors.

The dancing voltage had me concerned before its being read by a scan gauge.

FWIW, the Engel fridge I have will run for 1-3 days (depends on how cold it is outside) without draining the Trooper or van's battery. You definitely have a problem somewhere.

In reply to Tom Suddard:

Exactly. It should run for a day easy. I feel like I'm not getting th amperage needed.

Voltage is not really a good indicator of charge rate. If it's higher when the car is running than the voltage the battery floats at then it's charging and if it's lower then it's discharging but that's all you can really tell. While it's not unheard of it's rare for an alternator to charge but to do it at the wrong rate so I suspect that your battery is defective. However, to determine that for sure you need to test one or the other. I'd charge the battery with an external charger and see if it'll run the fridge overnight.

Sounds like your battery's capacity is just degrading. Just as a reference point, your battery probably has a reserve capacity (RC) rating on it of something like 60-120 mins. Thats 60-120 minutes that it should be able to put out 25(!) amps before going from 12.65 to 10.5v. If you're going from 12.6 to 11.8 in 10 minutes off a 4 amp load, that certainly suggests the battery is failing. Starting the car is sort of a separate issue. Cranking amps are sort of a burst or instaneous measure of the battery. RC is about endurance. I'm a somewhat muscular 6'0 man who doesn't work out. If you only measured my 'instaneous max' strength i may look like a decent athlete, but if you measured my endurance you'd realize i'm on the way out (32yo you know..) and hire yourself a youngin'.

In reply to APEowner:

Thats an easy test, I'll do an overnight charge on the charger and see how it does.