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homeless · 06-26-2014, 09:09 AM

Trying to understand how to use my "millimeter" and how my house electrical systems works. I have found that both my dash cigarette liters, the wire that hooks the small TV inverter to the batteries, the cigarette liter plug near the bedroom TV and the bed lamp I taped to run my Wife breathing machine seem to all show charging "13.? volts" when the main engine is running. When the generator is running they also show same rate of charge! When not running the alternator or generator they all show the same about "12.? volts".

Now under the hood if I turn on the main engine the chaise batteries shows charging "13.? volts" and the house batteries seem to show idle at "12.? volts"

When I run only the generator all batteries seem to show idle "12.?"!! No batteries seem to show the charging rate of "13.? volts"!!

I have never had any issue with running the breathing machine and an over head fan all night. But I do notice the lights seem to show a little dimming in the morning.

I don't use campground hook up so generator and actuator is my only source of power.

My Questions is!!! I would have assumed before I started trying to understand things! Is that the chaise battery is only used for the Engine?

I would also expect to see house batteries showing a charging voltage when the generator is running "13.? volts"?

Like I say I have never had an issue in three years with low water in any battery. And never had an electrical issue other then the generator over voltage until I had the generator regulator changed and tuned up.

I understand if it aint broke don't fix it!! I am just trying to understand how things work....

I am the kind of guy that is looking for worse case issues, until I started trying to understand all this, I would have thought if the engine is not on and I run my power to the limit "dead". That I would still have a chaise battery that is able to start the generator and the engine??

Any thoughts would be appreciated!!

Muddypaws · 06-26-2014, 09:30 AM

What we've called a 12v automotive/marine/RV battery is not 12v. 13.2 is about right.

6v golf cart batteries are about 6.2 each.

Charging voltage varies but is always > 12v depending on the device doing the charging and how 'smart' it is. I'm plugged in now and my entire 12v system reads 13.5v.

frankdamp · 06-26-2014, 09:41 AM

I would have expected the dashboard 12V outlet to be on the chassis battery, since it's a standard item on the truck dashboard. The remaining ones in the "house" section would be on the house batteries.

The generator should keep the house batteries charged up. I'd suggest putting an ammeter in the line from the generator so you can see if there's current going through to the battery bank. Maybe a permanent ammeter installation with a remote display somewhere convenient inside the house would be a good idea. You can't be sure the batteries are being charged just by reading voltage, but if it reads more than 12.0, there's a good chance it's putting something in

Another thing to check is whether you have a switch thet connects house and chassis electrics together. Most RVs have one, intended primarily to allow the house system to help the engine starter and it's usually a spring-loaded push button or toggle switch that only stays on while you're holding it. That may be stuck.

You don't say how big a generator you have, but just a couple of lights and a ceilng vent fan and your wife's CPAP sholud be no problem if the generator is working properly and is 3 KVA or more. I don't know how much current a typical 12VDC CPAP machine draws.

Also, what total capacity (ampere-hours) does the house battery system have?

Clay L · 06-26-2014, 11:24 AM

After being charged for a while a battery will read close to the charging voltage and over time or with a load will drop to the actual battery voltage of 12.6 to 12.8 volts.

Voltage State of Charge
This is under some load
12.6+ 100%
12.5 90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.9 40%
11.75 30%
11.58 20%
11.31 10%
10.5 0%

Kurmudgeon · 06-26-2014, 01:54 PM

First, make sure your tester is reading accurately.

A 12 volt battery is not really 12 volts. Each lead acid cell is actually approximately 2.1 volts when fully charged. That's why a fully charged, good condition battery will read more than 12.6 volts

Charging voltage to a 12 volt battery should be 13.2 to 14.4 volts. That is the accepted industry standard.

Once a battery is charged, you must let it stabilize for several hours before checking its voltage. Otherwise you will get an incorrect reading. Immediately after charging, it would not be unusual to see a reading of 13+ volts. What you are seeing is the residual surface charge from the higher charging voltage. Once stabilized, it should read about 12.7 volts.

A battery gradually loses capacity as it ages, so a several year old battery may show less than 12.7, no matter how long you charge it. A fairly new battery that cannot be charged to full capacity is either defective or you are not charging it at high enough voltage. If your charger is putting out less than 13 volts, it's not likely you will ever fully charge a battery.

A charger/converter must have a load on it to accurately measure its output voltage. Without a load, it will show a higher voltage than what it would actually be feeding to a battery when it is charging. That is called "open circuit" voltage. Always check charger output when it connected to a load.