12v Battery Is Discharging, Re-Charge Now !

Testing:
4/24: 12.46V @5:30pm. Outside weather: 91F
4/25: 12.58V @4pm Outside weather: 77F (I'm really not sure how my voltage went up 0.12V. My car was left untouched. I triple-checked all measurements. Maybe my multimeter is too cheap)
4/26: 12.53V @10pmOutside weather: 66F

 

but the tipping point is under load, and may not correspond to an exact static value.

 

this seems very good so far, albeit a bit low for a newer battery

 

Weirdly mine was up from 12.31 to 12.32 from yesterday to today. Probably measurement error by my cheap tester. Anyway, took a 40 minute drive and it went up to 12.55.

 

So at my first 12v test, I was getting a quite wide range of reading depending on what device I used.
12v battery health observation | Page 2 | PriusChat

I wanted a very accurate voltmeter, but the $395 price tag on a top of a line Fluke 87V multimeter with +/- 0.05% accuracy was totally out of my reach. After searching for a net, I found this video. Apparently, even el cheapo china made $6 multimeter can be very accurate.

I looked for the same multimeter, but most are shipping from China. Didn't want to wait for it, so I ended up buying a different el cheapo china made $6 multimeter model.


Well, using this multimeter on our other car's 12v battery today.

B-951 cheap multimeter $5.99: 12.67v
Craftsman model 82141 digital multimeter $20: 12.47v

Oh well, I still have no idea which device is giving me the most accurate reading.

 

I work with lot's of test equipment that gets Cal'd yearly with traceability back to the National Standards.
Here's what I would do if I had a bunch of cheapO meters.
Bring a dozen quality donuts to your favorite auto shop or electronics repair shop, if you know of one.
Compare your meters to what their Flukes (hopefully). Be ready to write down the results.
Since your only dealing with 12 VDC systems, you only need to connect to one battery and start comparing.

If you want to test the Ohm meter functions bring a light bulb or fuses, but don't wear out your donut welcome.

 

Thanks for the pointer. Well, that's an idea, but won't work while I am under lock-down. I was thinking buying something like this. http://www.amazon.com/Comparator-Automotive-Modification-Industrial-Application/dp/B07HGY9G5Q/ref=sr_1_5?dchild=1&keywords=12v+voltage+reference&qid=1588172148&sr=8-5
Although, I have no idea how such reference module works. Any thought?

By the way, are those cheapO meters if calibrated, would stay calibrated for a long time? Or do they deviate very quickly. How about those expensive meters??? Do they stay accurate forever??? I think not but I don't know enough about those gadgets.

 

While my Milwaukee meter was on the fritz*, I picked up an inexpensive analogue (needle style) meter, to tide me over. It's somewhat analogous to CD's vs records: it doesn't have n'th degree accuracy, but it's a lot less fiddly. I get the feeling they'll be hard to come by, in future?

* What toasted my Milwaukee meter: it has all this heavy-duty sealing, and then inside, the batteries (two AA's IIRC) reside in a semi-open cradle, directly over the circuit board, if you place the meter with the display facing down. The batteries are a pain to access too, screws to remove, lids to pry off. Batteries barfed all over the circuit board, bye-bye Milwaukee. Luckily this happened before the warranty ran out.

 

I have couple of those in my drawer somewhere. They are old, but they were more expensive than the $6 digital multi-meter I just bought. They were probably not made in China. In a pinch, it maybe better than digital ones for there is no need to place a battery inside. Searching through YouTube, I just learned that low battery in a digital multi meter can cause inaccuracy in reading, as much as 15-20% off.

 

Hooked up the new charger (Batter Tender 4 amp) last evening. Disconnected this morning, turned high beams on for 30 seconds and off, and then tested battery. 12.62 volts. Checked battery 4 hours later, 12.92 volts.

 

looking good!

 

Thanks, yeah not sure why the voltage came up after 4 hours.

 

maybe 30 seconds of headlights brought it down too far, then it stabilized

 

I think that's normal behavior right after you finish charging. Did you open the driver's side door open in order to turn on and off the headlight? If you did, that would have caused enough loads to pull the SOC down from fully charged 12v battery to temporarily show lower SOC of 12.62v which later stabilized to the full charge SOC of 12.92v.

 

Maybe, but I thought 12.6 was normal for a full charge. Maybe it still wasn’t stabilized after the 4 hours. Does the voltage bounce up and down during the stabilization process?

 

Look at my chart on 12v Battery Is Discharging, Re-Charge Now ! | PriusChat. The voltage was 13.06v right after the charger was removed. and the voltage stayed well above 12.6v for a long time. I did not open the door or turned on the light during that time. If I did, I am sure the voltage would have come down temporarily, but most likely bounced back to the 12.9v level.