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Old 07-12-2018, 06:10 AM   #1
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How to tell when battery 50%

2 questions: When boon docking, can I leave my tow vehicle power cord connected to 5th wheel, or will this drain my tow vehicle battery?

and: On my Lippert monitor (the one I use to raise and lower jacks, and has battery voltage monitor) how low can the voltage go before I have to recharge with my generator? It now reads 12.2. When I recharged earlier, it read 13.0.

Thank you, in advance.
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Old 07-12-2018, 06:38 AM   #2
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batteries are a subject of conjecture with many folks, as far as 'how low' they can go, safely... with most quoting the typical 'never below 50%!' rule...but once in a while is not going to damage them.
Your typical RV deep cycle 6v batteries can drop to 11.9 to 12, which is around the 50% rule, but mine regularly drop to 11.5 or so before my inverter cuts out(LBCO setting). My AGS usually kicks on at around 11.8 or so, if I have it enabled.

I would use the general rule that while you are 'using' the batteries, such as to raise/lower the levelers, that dropping to 11.5 is not doing any damage. Once you stop the leveling process, you'll see the numbers actually rise again, because now the system is reading them 'at rest', without a load.
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Old 07-12-2018, 07:13 AM   #3
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Quote:
Originally Posted by Scheinin View Post
2 questions: When boon docking, can I leave my tow vehicle power cord connected to 5th wheel, or will this drain my tow vehicle battery?

and: On my Lippert monitor (the one I use to raise and lower jacks, and has battery voltage monitor) how low can the voltage go before I have to recharge with my generator? It now reads 12.2. When I recharged earlier, it read 13.0.

Thank you, in advance.
Depends what TV you have Some/Most allow TV to be Connected and NO Batt Drain! You are at the Lower Voltage at 12.2,Lower Voltage is BAD for High Load items such as "Slides and Jacks"! Short answer is "Keep Your Batt Full Charge"! Youroo!!
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Old 07-12-2018, 07:18 AM   #4
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I never leave my truck hooked to the rig because my rig doesn't have an isolating device (one way flow) in the truck to rig charge line of the 7 pin connector that I know of. Here is a site that explains the 12V system and has a great battery charge chart. hope this helps
The 12volt Side of Life (Part 1)
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Old 07-12-2018, 07:46 AM   #5
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Some TV have a relay in the fuse panel ( along with the fuse) for the 12V aux line. The relay is only activated when the ignition is on. In others the 12V aux line is always active. You need to check the 12V aux pin on the Bargman connector on the TV when the ignition is off. If not sure disconnect.
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Old 07-12-2018, 03:27 PM   #6
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Ballpark estimate of 50% level is when you do not use any 12 volt electrical loads for 30 minutes (i.e. battery at rest) and your voltage is 12.0 volts.
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Old 07-12-2018, 05:37 PM   #7
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Depends what TV you have Some/Most allow TV to be Connected and NO Batt Drain! You are at the Lower Voltage at 12.2,Lower Voltage is BAD for High Load items such as "Slides and Jacks"! Short answer is "Keep Your Batt Full Charge"! Youroo!!
With my trailer I have no need to operate the slide or jack when the tow vehicle is unavailable. I back the Tow Vehicle in place then attach the umbilical, leaving the engine running. While the slide is being retracted and jack operated to facilitate setting up or hitching up, I have the battery power supplemented by power coming from the truck.


If one is worried about how to tell when a battery is truly ready to be charged, invest in a Batttery Monitor like the Victron BMV-7xx series.

There will be an easy to understand answer on the meter expressing "percent of charge" and "Time Remaining" along with voltage, current, etc. No more guessing off bad information from the OEM power panels.
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Old 07-12-2018, 07:09 PM   #8
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Quote:
Originally Posted by Scheinin View Post
2 questions: When boon docking, can I leave my tow vehicle power cord connected to 5th wheel, or will this drain my tow vehicle battery?

and: On my Lippert monitor (the one I use to raise and lower jacks, and has battery voltage monitor) how low can the voltage go before I have to recharge with my generator? It now reads 12.2. When I recharged earlier, it read 13.0.

Thank you, in advance.
I left my rig hooked up for an overnight and woke to find both house and vehicle batteries dead. This was in the early days of ownership and didn't realise how many devices still use the 12v power when everything appears turned-off.
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Old 07-12-2018, 07:27 PM   #9
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You CANNOT tell battery state of charge while using it! Period.
You CANNOT tell battery state of charge with voltage readings unless you FULLY charge overnight...disconnect negative post and WAIT 24 hours.(or put on a dummy load).
You need to recharge when your battery hits 50% AND you need to recharge fully to 100% or lose capacity.
You need a TRUE battery monitor ... not a voltmeter. A Victron or Trimetric
What will they tell you?
Battery state of charge. Battery Full. Amps being used NOW, Amps of Charging NOW, Time left at current use rate, Amp draw of individual items.
If you boondock //you need one. In the end it will pay for itself in saved battery life cycles AND saved gas in recharging from generator. And saved life from chronic undercharging...the quickest way to murder a battery.
Well worth the investment.
Don't bother if you ALWAYS plug in.
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Old 07-13-2018, 08:33 AM   #10
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As mentioned, the only real way to tell is with a proper battery monitor. I’m partial to the Victron BMV:
Victron Battery Monitor | Learn To RV
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