Chases battery always reads 13.5 volts

[Mtn Charlie]

I have a 2012 Itasca Suncruiser 37F on a Ford chassis. After sitting 3 months, it would not start. I hit the battery boost switch and it started right up. Turned it off and checked the battery monitor. Chassis read 13.5 and house read 13.6. Removed shore power and got same readings. Put a charger on chasis battery and it was putting out under 2 amps. I disconnected the chassis battery and it read only 6.5 volts and would not take a charge. It is over 4 years old, so no surprise there. With the chassis battery disconnected, the battery monitor still showed the chassis battery at 13.5 volts. It isn't a stuck boost relay, because it functioned properly when the RV wouldn't start and the RV wouldn't start with the chassis battery removed. I am at a loss as to where to begin to troubleshoot the erroneous chassis battery readings. Open to any suggestions.

[eyecando]

Sometimes a battery will be fully charged, but will have a high internal resistance. If this happens, the battery voltage may read normal with no load on it but will collapse when loaded. Sometimes the resistance may change and go lower or higher for a time. This could occur due to vibration while working on the battery. I have seen it before.

If your your chassis meter reads 13.6 volts with no battery installed, you may have a device installed that is charging the chassis battery possibly from the house battery. I would confirm those readings with a multimeter. If you have solar installed, which battery is connected to the charge controller? Or you may have some wiring tying the two battery systems together. Small wiring, say 14 gauge tying systems together would not support starting the vehicle without using your boost system.

[Morich]

First thought it that the timeing of the readings is too close to when charging has been happening.
Surface charge is a term used to describe when we read the voltage as we are charging or soon after. You don't mention how long you waited after the engine was running to test the voltage, so this is pure guess but easy to think it true.
Step one is to know that voltage is an easy one to fool us and not very reliable for saying if a battery is good or not.
But some more info may be needed. Are these lead acid batteries? This is vital as the chemicals are different and we get different reactions for different type batteries.
But if they are lead acid, the readings of over 13 are certain to not be correct for batteries that have had time to become stable after charging. Lead acid will only hold a readling around 12.8 if given time, so that was the first clue.

The reading of 13+ even when there is no chassis battery does sound like a problem in the wiring, though.
But before jumping too high on the idea there is a problem, do consider if solar might be adding to the question? Any possible confusion and actually seeing a charge voltage from solar?

[Mtn Charlie]

The battery was definitely bad, because it wouldn't take a charge after being removed. It read 6.5 volts after being on a charger for a half hour. It is possible that I was reading the output of the solar trickle charger, when the battery was removed. I never even thought of that being a posability.

[creativepart]

Any time a lead acid battery has a voltage reading above 12.7ish volts you are just seeing a charge current from some charger somewhere. If the charge current has just been removed you are seeing the surface charge left by the charger.

Fully charged lead acid batteries are 12.6 to 12.8 volts. Usually 12.7 volts.

So any time one sees 13+ volts at their battery they should be aware that they are not seeing their battery’s voltage.

[Bobn1957]

In theory, a fully charged battery should be 13.2 volts, so any reading higher than that is from an outside input. Is there a solar panel connected? I have a voltage sensing relay connected between my house and chassis batteries that charges my chassis battery from the house battery/converter/charger when I'm plugged in or running the genset.

[creativepart]

Originally Posted by Bobn1957

In theory, a fully charged battery should be 13.2 volts

I think you should recheck your info. A fully charged 12 volt flooded lead acid battery should be 12.7 - 12.8v. Some but not all AGM batteries charge to 13.0v. Lithium Iron Phosphate batteries charge to 14+ volts.

[Morich]

This is where we may be getting into a "language speak" area and it gets a bit messy as different folks think of "fully charged " as different.

Fully charged and stable after charging we get a different answer than when we speak of fully charged and on float voltage to cheat just a bit on how long the charge may last.

This is true of all lead acid that I know of and use. In industry or solar where we may use 24 or 48 volt, it works out the same.

If we are running a 48 volt system we use 24 cells that float at 52.08 volts. So if we devide that 52.08 by 24 we gets each cell being accepted as being 2.17 volts each?

Dropping down to a 12 Volt with 6 cells, we can figure the float voltage of 2.17 times 6 as being 13.02 volts.
So both these statements are correct in their own way! the 13.02 is correct float level and a theoretical good point to keep batteries fully charged. But if we are speaking of stable batteries that are not being charged, the level it maintains is somewhat lower at near 12.7-12.8.

But other than knowing that over 13 shows recent charge, it really gets into meaningless for most of our batteries and meters!
That theory of each cell being 2.17 volts? That is really more theory than most of our batteries will be if we look closer. We can SAY that all cells are the same but that is not true when we get down to building and using them.

If we REALLY want to know more about the charge, we need to get into specific gravity and even then it varies over time and different conditions.

Sometimes it is not really worth the time to get too picky, unless we want to move into lab situations!

[Ray,IN]

That is a very good explanation Richard.

[Morich]

The thing I learned about batteries is that much of what we know is based on lab specs and that often just doesn't work when we get into real battery use.
The lab spec may say something exact but we also know that once a battery gets into the real world, it starts to change!
The best lead and best acid will have a certain amount of impurities that change them in small ways. The best controlled heating and cooling changes the way a battery performs. Even the differences in the water we use to top them off will be different with different levels of minerals.
I started out using analog meters and really never heard of reading voltage down to two decimal points until we got into digital meters! You get a different answer if you use a better meter and calibrate it more often!

One of the reasons I retired from a long term job was the small points on batteries. We were getting new supervisors who had been in accounting or the business office for years and suddenly got jobs in the field to watch techs.

I had twenty year old rectifiers that had adjustment pots that must have been built with baling wire and I could not keep the batteries floating at 52.08 as the specs said! Getting called in for batteries reading 51.87 was enough to send me down the road!~
In their accounting background they saw that as something really wrong!

Sometimes it's all about what is really important, more than the actual ideal!

With all the abuse an RV battery gets, I try to follow the specs and try to get lab results but don't really expect them!

[Ray,IN]

Even a 10° temperature change will affect battery state of charge. Right now my 3 Renogy AGM batteries are maintaining float-charge at 13.3VDC, which is what their spec sheet states. But I had to unplug the temp.sensor because it is faulty and telling the charger the battery temp. was 103°.