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Old 10-13-2014, 05:20 PM   #1
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Battery charging below 13 volts

I think my chassis batteries are being discharged
by a defective step motor. Usually while camping
I use the dissconnect to prevent the
drain but I forgot this time and they seemed to
have ran low according to the battery charger they
were at around 11 volts. Put it in charge mode
but the charger is only reading 12.5 volts. Usually
it charges at over 13 volts so im wondering if
its normal to start charging at below 13 volts when
the batteries are discharged and
then maybe after a while increase to over 13? Oh I
interupted the charging after about ten minutes
to see if the coach would start and it did.
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Old 10-13-2014, 05:30 PM   #2
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11 volts is a very low battery, in the area of causing degradation. May take it awhile to ramp up the charging voltage.
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Old 10-13-2014, 05:56 PM   #3
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I've brought batteries back up from as low as 3 VDC. There are several problems. Part is battery chemistry and part is the chargers in common use.

The short answer to the chemistry is to use a low current to bring up the cells then one can increase the current to charge the battery. I do that by putting a large 12 VDC lamp in series with the battery and charger. When the lamp dims or I check the voltage and it is up in the 12 VDC range I know the cells are working and take out the lamp.

The charger problem is that most chargers are constant voltage (not the 3 stage maintainers) so the current depends on the voltage difference, the internal impedance of the battery and the internal impedance of the charger circuit. If the current goes too high there may be overload protection in the form of a breaker or fuse.

In your case the battery was at a very low state of charge but the chemistry probably was still normal. The circuit impedances limited the charging current to a range the charger could handle. The voltage reflected the battery's share of the voltage divided between the battery and charger. As the battery charged the voltage probably increased. I'm mildly surprised the coach started. I suspect a rather hefty charger. ;-)
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Old 10-13-2014, 06:18 PM   #4
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I too have brought DEEP CYCLE batteries back from far too low, Not such good luck with Marine/Deep cycle or starting batteries but recovery is possible.

To the O/P. if you are reading the voltage at the batteries, Give 'em time, the converter may not be able to push amps fast enough to put 13 volts on the terminals when they are that low. Try again in like 2 hours.
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Old 10-13-2014, 07:26 PM   #5
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After a couple of hours the charger is reading
12.8 to 12.9. Will check again later to see if it
surpasses 13 volts.
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Old 10-13-2014, 07:37 PM   #6
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Typically if the battery is in a low state and 11 volts is a low state the charger will put out 13.6 or greater and gradually lower the voltage as the battery charges. With batteries it's always better to know to the tenth of a volt how things are going. When you say 11 volts is that exactly 11 volts or 11.3, or 11.8 etc. The D (digital) VOM's are so much better at reading battery voltage and it does help when diagnosing charging issues.

My brother retired from the AF and worked on all kinds of stuff electronic in his 40 years. He hates the DVOM's because he was used to the standard style meter. I love the DVOM's. They are accurate and very rugged.

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Old 10-13-2014, 07:39 PM   #7
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Your monitor should be reading actual battery voltage. If they were at 11 volts it will take some time to come back up to full charge. Be patient and give it a few more hours.
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Old 10-13-2014, 08:28 PM   #8
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Not sure if it was exactly 11 I think the original
reading may have been 10.9....it jumped around a bit. the fact that
the coach started made me suspect of the charger. Usually this charger
charges upwards of 15 volts then eventually down to a maintenance charge
of 13.2. I was also wondering if bad battery cables would alter the
charging voltage. I tested the charger on the car battery ( about an hours rest of a 1 hour highway trip) it showed 75 percent. ...which I thought 95 percent would have been more acurate... anyways when I get home and see over 13 volts I will be happy. The batteries are less then a year old.
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Old 10-13-2014, 08:44 PM   #9
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Oh and I connected the house batteries
with the boost switch and
put charger on both to read voltages. House batteries
read 13.2 and chassiss 12.5 house batteries are getting
charged from shore power via the converter. If all was well I believe they all should have read the same voltage.
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Old 10-13-2014, 10:35 PM   #10
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Update..several hours later its charging at 13.5
volts.... I think charger was doing its thing and
by morning batteries should be fully charged. Now
step 2 what is discharging them so quickly.
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Old 10-13-2014, 11:12 PM   #11
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One last question and the answer could solve
the battery draining problem. I turn battery cut off
switch to disconnect while charging the chassis
batteries with an external charger. Would it be safe to
leave the disconnect open while charging? Its a pita
to walk back to the battery compartment to open
the dissconnect every time I want to use the
electric awning.
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Old 10-14-2014, 02:47 AM   #12
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Leaving the switch on is fine. Check for any compartment lights that may have been left on for a serious drain issue.

When you hit the boost button and saw two different voltages on the batteries that is an indication the boost solenoid is not doing its job or you have poor connections. Unfortunately with fully charged batteries you probably will not be able to check it confidently. In any case, lock the boost button on and test the voltage on each side of the large solenoid that goes clunk when you hit the button. If you see any difference then replace the solenoid. That is if it went clunk when hitting the button.
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Old 10-14-2014, 06:33 AM   #13
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Actually the boost is working because before
turning boost on the chassis batteries read 11.something
and around 30 percent charged and after applying boost
that changed to 12.5 and 95 percent charged. As someone mentioned
maybe the batteries were to week to accept a higher voltage or poor
connections. I removed cables and cleaned them all. With cables off I put
the charger directly on one battery ( no voltage change).
Compartment lights
are off ( master switch). Good news this
morning batteries are fully charged and
voltage reads 13.7 I will leave the charger
on the chassis batteries while camped from now on.
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Old 10-14-2014, 07:42 AM   #14
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You kind of misunderstood my earlier post

It is not that the BATTERIES were too weak to accept the higher charging voltage, it is that they were sucking all the current the converter could put out..

Let's say you have a 45 amp converter and a pair of GC-2 batteries,, they can easily suck 60-70 amps when they are hungry, MORE if really hungry.. Your 45 amp converter can not output that kind of current, so it reduces the voltage.

Then, as the batteries fill up,, it increases the voltage Till eventually it regulates the voltage (usually around 14.5) as the current drops below 45 amps, then it drops to 13.6 eventually. (Full charge).

Batteries are not charged by voltage, they are charged by CURRENT.

To think of it.. If you have a water line with 100 PSI and on the end of this line is a hydrant,, You have a 5 gallon bucket,, How long will it take that 100 PSI line to fill the bucket? (Sadly you can not answer because I failed to tell you if it was a quarter inch Ice Maker line, a half inch line, a 3/4 inch line or a 2 inch fire hose line)

The bigger the line, the faster the fill.

Likewise even if you have a two inch Fire line, if the pump is only 1 GPM (Gallon per minute) then its going to take five minutes to fill that bucket.

Same with your batteries,, They are the bucket, the wires connecting them to the converter are the lines and the converter is the pump.
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