Why is my chassis battery discharging?

[Monacoach]

I am baffled as to why my chassis battery is discharging.
Here is my situation. ... the 12 volt chassiss battery removed from the coach put on a charger fully charged . I unplug the electric plug of the charger from the wall leaving charger clamps on battery. 4 days later I plug charger back in and it shows over 13 volts and 100 % charge still. Great.....

Now when hooked to coach fully charged but overnight the voltage drops to 12 and 40 % charge. Not good.... The only difference is ground cable connecteted and positive cable connected to battery but this cable runs to the batt disconnect which is turned off. I confirmed that the disconnect is working with test light. No power beyond disconnect...

So how is it possible for the battery to drain so quickly in this casr yet hold a full charge while physically disconnected?

[Old Scout]

First--the steady state [after 3 hours] voltage level for a fully charged 12v battery should be around 12.7 or 12.8. "Over 13s after 4 days" is too high -- bad volt meter???? Second, most coaches have a variety of cables that are hooked directly to the bat--by-passing the "kill switch." Sounds like you have something shorting to cause that much discharge. Cant explain how discharge occurs with only one cable attached.

[Monacoach]

Ive often seen the surface charge remain for days while on disconnect.
We are camped at an rv park for the summer and have no need to start the coach so at the moment I do not have the extra batt cables attached so in theory it should be the same as being physically disconnected ? Perhaps the battery is finicky and discharges itself when ever it feels like it. It seems that having the ground attached is putting a big drain on the battery. There is more to the story but my post would be 3 pages long.....

[nothermark]

FWIW - aside from other possible issues wet cell batteries start self discharging as soon as the power is applied. The rate varies with other variables but the battery is running down. If you have a long story it probably includes other variables getting manipulated.

[Monacoach]

Part of the long story is that this battery was part of a duel battery system in which the second battery was defenatley bad. So it is quite likely that the bad battery infected its twin but I am still surprised that the battery keeps a charge when completely physically disconnected but does not hold a charge when hooked up but disconnected via the switch.

[YC1]

This is very common. Often just the slightest draw on a tired battery will give you the exact symptoms you have. A hydrometer would be a good test but based on your symptoms a new battery should solve the mystery. Strange battery symptoms can drive people crazy. I watched one battery seem to have a built in clock and at 20 minutes with the headlights on the voltage would suddenly drop to 8 volts. It passed all normal load testing that was possible at the time. Some sealed batteries were difficult to measure specific gravity.

[Monacoach]

Ok I think your right. Trying to figure out a battery is like trying to read a woman's mind.
No offence ladies thanks for the help

[Monacoach]

Ok I just finished charging the battery and I took some more amp readings. First of all by reversing the leads the negative symbol is gone ( ty) and with battery disconnect on the draw is 2.18 amps and with the disc open the draw is 2.45 amps.

I wonder if im taking these readings to soon after charging. Maybe I should wait for the battery to get to its resting voltage before using amp meter?

[bill06447]

Remove the negative cable from the battery bank and check there, once again with disconnect open and closed. See if you get the same readings. You may have a short somewhere between the battery and switch (or a bad switch) this assumes the batteries are clean-dirty or corroded battery cases and posts will draw down a battery quick.

[Monacoach]

These amp readings were with the negative cable off. Is there a way to check the drain of a detached battery ( no batt cables attached). The disconnect switch defenatley works. Pretty sure its a bad battery but I want to be sure that I dont have a big draw that will drain the new batteries. I will check again when the batteries are at rest just in case the surface charge is altering the readings. Guess I will clean the battery to.

[nothermark]

FWIW I'm trying to figure out how you are measuring a current draw with the cables disconnected...

[lonfu]

Originally Posted by Monacoach

I am baffled as to why my chassis battery is discharging.
Here is my situation. ... the 12 volt chassiss battery removed from the coach put on a charger fully charged . I unplug the electric plug of the charger from the wall leaving charger clamps on battery. 4 days later I plug charger back in and it shows over 13 volts and 100 % charge still. Great.....

Now when hooked to coach fully charged but overnight the voltage drops to 12 and 40 % charge. Not good.... The only difference is ground cable connecteted and positive cable connected to battery but this cable runs to the batt disconnect which is turned off. I confirmed that the disconnect is working with test light. No power beyond disconnect...

So how is it possible for the battery to drain so quickly in this casr yet hold a full charge while physically disconnected?

So, I'm a bit confused. usually there are coach batteries and then there are engine starting batteries. what is a chassis battery? Is it the one that runs your coach 12v system? Usually starting batteries are a single battery that is connected to the starter. Coach batteries are usually in banks of 2 or more batteries connected to the alternator through a high amp 12v relay. (through some use boat Boston switches for this) Assuming you are getting no readings from ground to the open/load side of the relay/switch then you must have a bad battery in your bank.

However I must point out that, "voltage does not a battery make!" unless you are using a "battery load tester" you may be getting a high voltage reading from the batt but it can't support any amperage draw. I would suggest that you test each battery with a "battery load tester", it will give you an accurate reading of the condition of the battery you are charging. I have, in the past, gotten fine voltage readings from batteries but when tested under load they quickly dropped both voltage and amperage delivery.

As said above, 12v batts have a resting charge of 12.7 to 12.8 volts. (or 2 6v's) But, just after pulling off a charger, flooded cell batts will read higher until the surface charge is removed by sitting or putting a small load on it. A charger doesn't count as they employee diodes to prevent discharge.

I suspect you have 2 battery system with at least one bad battery. Until you test them both with a "battery load tester" you are wasting your time fooling around with them.

Batter load testers can be had at Harbor Freight or most auto supply stores. It is what the auto repair shops use to determine the condition of a battery. They start out at about $25.

[Monacoach]

Lonfu when I say chassis I am referring to the engine battery. I normally have two but I have removed a bad battery that would not accept a charge which leaves me with one engine battery. I will be buying two new batteries but im not in a hurry for two reasons. 1 we wont need to start the engine until Sept and 2 I want to do some testing to see if something on the coach is draining the batteries. And to answer the previous poster I have detached the negative cable from the battery, and using an amp meter I touch one lead to the negative cable and the other lead of the amp meter to the negative post on the battery. It shows a 2.5 amp draw. Then I switch the disconnect on and the draw drops to 2.18 amps which is strange for a battery that is cut off from the coach. I took these amp readings immediately after charging the battery. .im guessing this was a mistake so my next test will be to take the amp readings on the battery after the surface charge disappears.

[YC1]

If you are measuring current with the switch off then there is one or more wires on the battery side of the switch. Things like steps and co detectors often are setup to not allow their disconnection with the switch.

It is electrically impossible for current to flow without the positive being connected to something.