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Old 04-26-2022, 10:25 AM   #1
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Chassis Batteries 6.2 volts (dead)

So this is a first for me.

Sat for 4 months at an RV park plugged into 50a power.

House batteries stay fully charged (charge off shore power using converter), but my chassis batteries are now dead (found when trying to leave) at 6.2v

Used Aux switch to get the rig started. Drove 5 hours and saw charge while driving at 13+ volts from alternator.

This morning back down to 6.2v

Always thought my generator or shore power was charging the chassis batteries too.

Going to replace them today but concerned I have some weird phantom draw that has just shown up.

Any thoughts on wiring to make sure the converter charges chassis batteries at same time as house batteries? Concerns?

House batts only a year old & doing fine. Chassis batts at least 6 years old - there when I bought the rig, but were always strong

Thanks
Jim
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Old 04-26-2022, 10:55 AM   #2
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My Monaco doesn't charge my engine battery when plugged into shore power. If I'm sitting for more than a week I disconnect it, otherwise it dies.

I'd suggest using a voltmeter and measure the voltage at your engine batteries before you plug into shore power and then measure again after plugging into shore power. It should increase.

Not all motorhomes charge the engine batteries when plugged into shore power.
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Old 04-26-2022, 11:07 AM   #3
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Yes, it is not uncommon for shore power/generator to NOT charge the chassis batteries.


Several good "work-arounds" including small smart 120 VAC charger/maintainers for the chassis bank and "smart thief devices" like the Xantrex Echo Charger) that take power from the house bank when voltage indicates the house bank is charged.
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Old 04-26-2022, 11:24 AM   #4
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Quote:
Originally Posted by Arch Hoagland View Post
My Monaco doesn't charge my engine battery when plugged into shore power. If I'm sitting for more than a week I disconnect it, otherwise it dies.

I'd suggest using a voltmeter and measure the voltage at your engine batteries before you plug into shore power and then measure again after plugging into shore power. It should increase.

Not all motorhomes charge the engine batteries when plugged into shore power.
Yeah, I am not seeing any increase in charge on chassi batts when genny is running. I have removed them and will go get some new ones. I will also start using the disconnect when on shore power / parking for a while

Thanks
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Old 04-26-2022, 11:26 AM   #5
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Quote:
Originally Posted by wolfe10 View Post
Yes, it is not uncommon for shore power/generator to NOT charge the chassis batteries.


Several good "work-arounds" including small smart 120 VAC charger/maintainers for the chassis bank and "smart thief devices" like the Xantrex Echo Charger) that take power from the house bank when voltage indicates the house bank is charged.
Thank you. Since I have shore power when parked, I think I will get a smart maintainer to plug into while parked. I also like thief idea
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Old 04-26-2022, 11:29 AM   #6
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Quote:
Originally Posted by RV-Writer View Post
Thank you. Since I have shore power when parked, I think I will get a smart maintainer to plug into while parked. I also like thief idea
Little surprised no one has already installed one in your '98 coach.
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Old 04-26-2022, 11:36 AM   #7
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End of Chassis Battery Life

The chassis battery may have reached end of life. Engine start batteries often appear to fail suddenly.

Sat for 4 months at an RV park plugged into 50a power.

House batteries stay fully charged (charge off shore power using converter), but my chassis batteries are now dead (found when trying to leave) at 6.2v


If the chassis batteries were at 6.2 volts for 4 months, they are toast. If they were below 12 volts for even a month, they would probably be severely compromised.

Sulfate on discharged battery plates slowly crystalizes. The crystals are hard to re-dissolve (charge).

Used Aux switch to get the rig started. Drove 5 hours and saw charge while driving at 13+ volts from alternator.

This morning back down to 6.2v


Was the RV plugged into shore power when voltage dropped? Dropping that much when not on charge means the chassis batteries are severely compromised. They are self discharging and probably not holding much charge to begin with.

If it was plugged in, battery terminal voltage should have stayed higher even with a defective battery.

In this case bridge circuit is not working properly. It could be a fuse or circuit breaker. It could be a failed relay or solid state device.

Always thought my generator or shore power was charging the chassis batteries too.

Different RV's are wired differently. Even if your RV at one time charged chassis battery from it's converter/charger, the bridge circuit may have failed. This is a common problem.

An alternative is that the chassis batteries just failed. Measuring chassis battery terminal voltage would tell. Measure without charging (static for an hour or two). You got 6.2 volts before.

Plug into shore power. Voltage should slowly rise to 13.6 or 14.4 volts. Charge for 14 to 18 hours. Unchanged voltage means bridge circuit is not working.

For safety, monitor the battery temperature. Some battery failure modes can cause high temperatures while charging. Just use your hand to test temperature. Uncomfortable temperature mean batteries are bad.

Voltage on good batteries will slowly drop to 12.7 volts. A self discharging battery will continue to slowly drop, possibly to 6 volts.

Going to replace them today but concerned I have some weird phantom draw that has just shown up.

Any thoughts on wiring to make sure the converter charges chassis batteries at same time as house batteries? Concerns?


Install new chassis battery. Make sure new chassis battery is charged. Voltage should be above 12.4 volts. They may test at 12.7 volts if they come fully charged.

If new battery voltage is above 13.2 volts, wait a while. After 1 to 4 hours the voltage should drop to less than 13.0 volts, possibly down to 12.7 volts.

Plug into shore power or run generator. Voltage should slowly rise to 13.6 volts or 14.4 volts. If voltage does not rise, bridge circuit is not working. If it does no reach 13.6 volts. Bridge circuit is not working properly.

It may take a while to start rising. Some bridge circuits would not start charging the chassis battery until house batteries are above a certain voltage. Some just have a built in delay. Some bridge circuits have different programmed behavior.

House batts only a year old & doing fine. Chassis batts at least 6 years old - there when I bought the rig, but were always strong

6 years is a long time for a daily drive vehicle. The RV situation depends on how often the vehicle has been started and run.

It also depends on how low the terminal voltage got during storage. 12.4 volts is the minimum for long term storage. It is the same for house batteries.

I wish you good luck and happy trails ahead!
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Old 04-26-2022, 02:52 PM   #8
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Thank you Paul - Great info - Answers inline

Quote:
Originally Posted by Persistent View Post
The chassis battery may have reached end of life. Engine start batteries often appear to fail suddenly.

** THEY WERE ACTUALLY 8 YEARS OLD

Sat for 4 months at an RV park plugged into 50a power.

House batteries stay fully charged (charge off shore power using converter), but my chassis batteries are now dead (found when trying to leave) at 6.2v


If the chassis batteries were at 6.2 volts for 4 months, they are toast. If they were below 12 volts for even a month, they would probably be severely compromised.

Sulfate on discharged battery plates slowly crystalizes. The crystals are hard to re-dissolve (charge).

** YEP, NOT SURE HOW LONG THEY WERE DOWN, WASNT WATCHING THEM LIKE A DUMMY

Used Aux switch to get the rig started. Drove 5 hours and saw charge while driving at 13+ volts from alternator.

This morning back down to 6.2v


Was the RV plugged into shore power when voltage dropped? Dropping that much when not on charge means the chassis batteries are severely compromised. They are self discharging and probably not holding much charge to begin with.

** NOPE, DRY CAMPING AT END OF 5 HOUR DRIVE

If it was plugged in, battery terminal voltage should have stayed higher even with a defective battery.

In this case bridge circuit is not working properly. It could be a fuse or circuit breaker. It could be a failed relay or solid state device.

Always thought my generator or shore power was charging the chassis batteries too.

Different RV's are wired differently. Even if your RV at one time charged chassis battery from it's converter/charger, the bridge circuit may have failed. This is a common problem.

**ANY WAY TO FIGURE OUT IF THERE IS A BRIDGE CIRCUIT? TRACE WIRING OR ?

An alternative is that the chassis batteries just failed. Measuring chassis battery terminal voltage would tell. Measure without charging (static for an hour or two). You got 6.2 volts before.

Plug into shore power. Voltage should slowly rise to 13.6 or 14.4 volts. Charge for 14 to 18 hours. Unchanged voltage means bridge circuit is not working.

For safety, monitor the battery temperature. Some battery failure modes can cause high temperatures while charging. Just use your hand to test temperature. Uncomfortable temperature mean batteries are bad.

Voltage on good batteries will slowly drop to 12.7 volts. A self discharging battery will continue to slowly drop, possibly to 6 volts.

** GREAT INFO - I AM WATCHING NEW BATTERIES & WILL CHECK THESE OUT OVER NEXT FEW DAYS

Going to replace them today but concerned I have some weird phantom draw that has just shown up.

Any thoughts on wiring to make sure the converter charges chassis batteries at same time as house batteries? Concerns?


Install new chassis battery. Make sure new chassis battery is charged. Voltage should be above 12.4 volts. They may test at 12.7 volts if they come fully charged.

** NEW BATTERIES WERE AT 12.7v & 12.8v at purchase

If new battery voltage is above 13.2 volts, wait a while. After 1 to 4 hours the voltage should drop to less than 13.0 volts, possibly down to 12.7 volts.


** INSTALLED 2 NEW BATTERIES TODAY. I NEGLECTED TO SAY I HAVE A SMARTBMV I INSTALLED LAST YEAR TO WATCH MY NEW HOUSE BATTERIES AND I DID INSTALL THE AUX LINE TO MY CHASSIS BATTERY, JUST WASNT WATCHING THEM

Plug into shore power or run generator. Voltage should slowly rise to 13.6 volts or 14.4 volts. If voltage does not rise, bridge circuit is not working. If it does no reach 13.6 volts. Bridge circuit is not working properly.

** OK WILL LET THEM REST FOR ABOUT 6 HOURS THEN RUN MY GEN & SEE WHETHER IT TRIES TO CHARGE OR NOT

It may take a while to start rising. Some bridge circuits would not start charging the chassis battery until house batteries are above a certain voltage. Some just have a built in delay. Some bridge circuits have different programmed behavior.

** UNDERSTOOD. I WILL RUN FOR 4 HOURS TO TOP IFF HOUSE BATTERIES & SEE IF CHASSIS BATTS START CHARGING OR NOT

House batts only a year old & doing fine. Chassis batts at least 6 years old - there when I bought the rig, but were always strong

6 years is a long time for a daily drive vehicle. The RV situation depends on how often the vehicle has been started and run.

** WE SIT FOR ABOUT 4 - 5 MONTHS A YEAR IN A RESORT WITH 50amp POWER. I JUST WASNT SCRUTINIZING MY CHASSIS BATTS - WE LIVE IN IT FULL TIME SO NOT IN STORAGE. JUST A GOOF ON MY PART I THINK

It also depends on how low the terminal voltage got during storage. 12.4 volts is the minimum for long term storage. It is the same for house batteries.

I wish you good luck and happy trails ahead!
** THANK YOU - I ALSO BOUGHT A BATTERY MAINTAINER IN CASE MY CONVERTER DOES NOT CHARGE MY CHASSIS BATTS. I WOULD LOVE TO SEE IF THERE IS A BRIDGE CIRCUIT INSTALLED THAT WENT BAD. DO YOU HAVE A PICTURE OF ONE? WOULD IT BE IN THE BATTERY COMPARTMENT?

Again thank you - sorry for all caps, used it to differentiate my replies
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Old 04-26-2022, 03:03 PM   #9
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ALSO

So, if it turns out I need to use the new batt maintainer:

in a 12v bank of 2, would you run the wiring for a battery maintainer to hot post on 1 batt in the bank & negative wire to negative post on 2nd battery? Or just to hot/neg on 1 battery of the 2?

Thanks
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Old 04-26-2022, 03:18 PM   #10
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Some believe that opposite corners is more effective but with a trickle charger and the huge starter cables between the batteries, it wouldn't mater.

You could hook your charger to any point in the 12 volt chassis electrical system and chassis ground.

I would look for a spot near the boost solenoid, to keep the connections away from battery acids.
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Old 04-26-2022, 03:41 PM   #11
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You should have a battery isolation relay that allows the converter to charge the chassis battery once the house batteries are charged. I have had this relay fail and have seen it fail in other coaches.
On my Holliday Rambler Vacationer (previous coach) the relay was in the one of the 2 front passenger side compartments. can't remember which one but there were 2 relays, one was for the charging circuit and the other was for the aux start relay.
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Old 04-26-2022, 05:15 PM   #12
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Xantrex Echo Charger

AMP-L-START
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Old 04-26-2022, 05:59 PM   #13
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If the batteries will not hold a charge. It sounds like new batteries.
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Old 04-27-2022, 07:52 AM   #14
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Quote:
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** THANK YOU - I ALSO BOUGHT A BATTERY MAINTAINER IN CASE MY CONVERTER DOES NOT CHARGE MY CHASSIS BATTS. I WOULD LOVE TO SEE IF THERE IS A BRIDGE CIRCUIT INSTALLED THAT WENT BAD. DO YOU HAVE A PICTURE OF ONE? WOULD IT BE IN THE BATTERY COMPARTMENT?

Again thank you - sorry for all caps, used it to differentiate my replies
Bridge circuits come in many different configurations. In old simple designs. It may have been a simple relay. Turning the ignition "on" would bridge and charge house batteries. With ignition "off", the two battery banks are disconnected. Chassis battery will not be drawn down while camping.

There are many flavors of newer designs. Some sense voltage on each bank and use a microprocessor to decide when to connect.

Most have a relay with high capacity contacts to connect the two banks. The contacts can get pitted over the years and stop making contact.

You said you could start your engine using a boost switch, so that relay is OK. Other components of more complicated systems can also fail. Solid state modules have been known to fail. Control wires have loosened or corroded.
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