Chassis Battery

[Charger1204]

Okay, here's one for ya....chassis battery tests good...but wont start engine without hitting the boost switch, all connections are tight and clean. It will start WITHOUT boosting up to 2 hours later, but beyond that, shes a no go. Has me completely stumped. Thanks for any and all help. I have a 2010 Winnebago vista 26p

 

[Matt_C]

Charger,
I hate to say this to you, but it is clear that all your connections are A:Correct, B:Clean & C:Tight. A common issue is that a cable is also bad at the connector. Pull on the cables and see if any come apart. If one does come apart, then it was not clean and tight....
Matt_C

 

[Mark_K5LXP]

A few checks with a voltmeter will reveal the current bottleneck. Start at the battery to see if that's where the voltage is taking a dive, then along the cable path to the starter. Grounds should be checked too.

Mark B.
Albuquerque, NM

 

[DonTom]

..chassis battery tests good..

Tests good in what way? How are you testing it? If not testing it under load, you're wasting your time testing it.

How old is the chassis battery?

-Don- Auburn, CA

 

[Tulecreeper]

I'm going to make an educated guess that the battery is not "good". Many people think if the battery is reading 12+ volts it is fully charged. Here is a chart that gives a better idea of the State of Charge (SOC) of a battery. There are others, but they all are pretty close to each other in their estimates.

 

[DonTom]

I'm going to make an educated guess that the battery is not "good". Many people think if the battery is reading 12+ volts it is fully charged.

It can even read perfect fully charged and be bad when there is no load. High internal resistance inside a battery is just like a high value resistor in series with it. With a decent voltmeter, it can read perfect when the battery is as dead as a doornail under a load.

They must be tested under load. At least turn on the headlights when checking the chassis battery voltage. There are many ways for batteries to fail and can all test differently when there is no load.

-Don- Auburn, CA

 

[DonTom]

Many people think if the battery is reading 12+ volts it is fully charged.

From what I can tell, all batteries are usually rated at their lowest usable voltage. For an example, a new 1.5 volt battery will read around 1.6 volts. At 1.5 volts, it is near dead for most purposes.

Same with the 12V battery. At 12 volts, it is near the end of its charge (the end of the green in your chart).

If we rated them when new, and fully charged, the 12V battery would be called a 13 Volt battery, etc.

-Don- Auburn, CA

 

[WILDEBILL308]

I think you are right. The fact it won't start the coach after testing good is a good indicator. I would also recommend getting a "start" battery for the chassis and not a "deep cycle"
Bill

 

[Tulecreeper]

I think you are right. The fact it won't start the coach after testing good is a good indicator. I would also recommend getting a "start" battery for the chassis and not a "deep cycle"
Bill

I didn't read where the OP said they had a deep cycle battery.

 

[John From Detroit]

There are multiple ways to test a battery One is a carbon pile load tester.. If a battery has high internal resistance. What you see is what you will get... Exactly that.
Also cables have two ends.. The end at the battery is clean what about the other end. Again for starter cables the load tester is a good tool.. Hook to the Battery lead (Big) on the starter solenoid and a good ENGINE ground. dial it up to about 100 amps and see what the voltage does.
Or just leave it at zero amps and hit the starter switch and see what the voltage does. If the battery tests good.. but the 2nd test goes to like 8 volts.. You have a cable problem.> Damp finger touch all connectors one will be hot.

 

[John Canfield]

Only one thing to add to very good replies. Using voltage measurements to determine state of charge is done with the battery at rest with zero load (battery disconnected) for 24 hours. If the battery has some bad cells, its internal resistance will be high and will cause the battery to self discharge.

 

[TheBar]

The majority of car batteries that I've replaced will test 12.5v or better for several days after charging. If you have LED headlights testing with them on won't tell you anything. But if it sits a couple of weeks and the engine turns over slow is my first sign of sulfation. Or if it turns over slow when the engine is hot. Pulse charging can make them last another 6 months but if the battery is > 7 years old or the pulse charging only lasts a couple weeks I just replace it. A battery and the engine will last longer if started and run up to operating temperature at least once a month.

I'm old as dirt so I like an old fashioned ammeter. A volt meter will show 14+ volts when the engine is running period. But when an ammeter shows 30+ amps for a longer and longer time that means the battery is slowly dying. Also after shutting off the engine when the ammeter goes below zero tells you the dome light is on etc. An ammeter gives you an immediate warning if it goes to zero when running which means either the alternator quit, the battery posts are loose or corroded, or you have a dead cell.

I've even had batteries develop a dead cell even when just stopping for gas. That leaves you stranded. If you pull the caps off and shine a flashlight down it and one cell looks different or white that's the bad cell. An old time emergency fix is drive a nail in the good cells on each side of it and wrap several turns of any metal wire between them. Or if an end cell from the next good cell to the post. Won't last long but might get you to the nearest Walmart or auto parts store.

 

[WILDEBILL308]

I didn't read where the OP said they had a deep cycle battery.

Good for you, I never said he did and I never read where the OP said he did. I simple warned not to make that mistake. Many places display start and deep cycle together. I hope I was able to clear that up for you.
Bill

 

[RNS]

Had one that tested good static over 12.6 but watching it during start would drop to 6 on meter. replaced battery all is fine.

 

[Ray-IN]

I imagine charger1204 is quite busy testing all the recommended remedies. This is another important test. How to test battery cables for voltage drop.
I've seen many people focus on the positive side of the wiring and totally ignore the negative side. Each side is 50% of the circuit and rates 50% of the testing and attention.
Nearly 80% of 12V issues are due to a poor/missing ground. I did have the reference for that statement, but lost it when a HDD crashed several years ago.

 

[DonTom]

, its internal resistance will be high and will cause the battery to self discharge.

Nope. High resistance will not load a battery until you put a load on it.

12V in series with a 10 trillion-ohm resistor (that's high!) is still 12 volts.

If you can find a meter that will not load it down to prove it is another question.

-Don- Auburn, CA

 

[Ray-IN]

Nope. High resistance will not load a battery until you put a load on it.

12V in series with a 10 trillion-ohm resistor (that's high!) is still 12 volts.

If you can find a meter that will not load it down to prove it is another question.

-Don- Auburn, CA

Fluke says that would require a high-impedance DMM.

 

[DonTom]

Fluke says that would require a high-impedance DMM.

With my sample of ten trillion ohms, you will not be able to find a meter that will work. The meter would have to be near infinite resistance/impedance. In fact, ten trillion ohms means it's a great insulator! I doubt if a piece of dry wood would even be that high.

But to test a battery, you want the opposite. A very heavy load (very low resistance) is what you want to put a load on the battery. So turn on the headlights to get that low resistance load because no meters will load it enough. Meters are considered high quality when they have negligible effect on what is being tested. But to test a battery, you want the exact opposite. A large load.

-Don- Auburn, CA

 

[John Canfield]

Load testing and determining SOC at rest via voltage measurement are two very different evaluation methods.

 

[DonTom]

Load testing and determining SOC at rest via voltage measurement are two very different evaluation methods.

Yep. SOC is for known good batteries to check the state of charge. Load testing is to test the condition of the battery itself. Really should be loaded for so much time to get an accurate AH test, but usually accuracy is not needed for just finding out if the battery is the problem, so just the load is all that is needed.

-Don- Auburn, CA