Car battery 15.51V

[jabalv]

I'm restoring old tractor and for testing electricity in it I had 2 old batteries in series circuit for 24V. (old truck and old car batteries).
I had like 25-26V total, but lately I see that I had ~28V total voltage and car battery is 15.51 (seems increased)

I believe 15.5V is quite high, so after googling around this could actually be dangerous as it can explode or so.
Can you comment?

[HighVoltage]

It is normal to put two 12V batteries in series on a 24V truck system and have them charge with 28V
And if both batteries are in the same condition, they usually end up with about the same 14V across them.
But if one battery is much older then the other and internal resistance has changed, one battery can charge up to a higher voltage than the other.

There seems to be a definite imbalance between the two batteries and you might replace them with new and of the same type.
15.1V is for sure to high.

What is the charging voltage across both batteries, when the truck is running at a rpm above idle?

[RobertBG]

Just to add to the HighVoltage's post,15.1 is quite high but it may be just what is called surface charge. Put a little load on the battery and retest before condemning it. But besides that he's spot on. 

[jabalv]

I use those batteries just to test electric components. I can't launch tractor with it, cause start current is too low from both of those batteries. Just bought new ones recently with 2x 12V 1000A 180Ah. Will probably switch to those ones tomorrow.

[jabalv]

It's not charging. Those 2 batteries just standing for couple weeks in that series circuit.

[Turbochardged]

I agree with the others. It's to high, yet could be a surface charge. When working on batteries in series, always change them at the same time with identical manufacturer and model.

[Gregg]

You kind of answered your own question when you stated that the batteries couldn’t start the tractor.  Your purchasing two new identical batteries is certainly is a good solution
At least one of them is in very bad shape and continued charging of them in series will only make things worse.  The battery with the lower voltage at rest may have a shorted cell which causes all of the rest of the cells to see a higher than nominal voltage.  The one that reads 15.1 may be the better of the two and with a little more testing may be a good enough battery for testing low current 12VDC items.  Hook up a 12 volt headlight and leave it on for about 5 minutes, then read the voltage; if it drops below 10.5 volts, it is pretty much worthless.