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  1. #1
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    17MKE Battery & Charging question

    Hi,
    We are brand new RV trailer users, and I'm trying to understand the battery charging system. We have a Grand Design Imagine XLS 17MKE trailer. It comes with a 165 watt solar panel and the MPPT 25A Solar Charging Controller. The dealer put two 12VDC Interstate HD24-DP Marine lead acid batteries in it in parallel, each with a 64 Amp Hour rating. Our system also has the Furrion Battery Indicator LCD gauge to tell me the state of the battery. With two 64 aHr batteries, I expected to have a max capacity of 128 AHr, but my Furrion Battery Indicator never gets that high. At first it could charge up to about 75 AHrs and then seemed to almost stop charging, very slow, even when connected to shore power (my house). Today it is up to 80.06 AHrs, but it is still charging super slow. Shouldn't I be able to charge up close to 128 AHrs before the charging slows down this much? At this rate I will never get to 128 AHrs. What am I missing?
    Craig

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    Site Sponsor NB Canada's Avatar
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    Don’t forget you can only deplete a lead acid battery 50 percent before you ruin it. You have only half of the amp hours
    2021 Imagine 2400 BH
    2018 GMC Sierra 4x4 Crew 1840lb payload

  3. #3
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    50% of what

    Quote Originally Posted by NB Canada View Post
    Don’t forget you can only deplete a lead acid battery 50 percent before you ruin it. You have only half of the amp hours
    NB Canada,
    I understand the 50% idea, but that is why I'm concerned about getting my max charged AmpHours as high as possible. I assume the best I could hope for would be a max of 128 AHrs which drops to 64 (50%), at which point I then charge it up again. But it looks like the system just looks at the voltage, not the amp hours. And not being able to pump up my amp hours concerns me, even when I am connected to shore power. For a while my max was about 75 AHrs, and the charger was trickling at the point. It should be charging at gang busters rate until it gets closer to the max, shouldn't it?
    Craig

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    Somewhere in your stack of component manuals there will be a tiny user manual for the Furrion battery monitor. The monitor is calibrated by depleting the battery to the lowest desired state of charge and then calibrating that as the 0 percent point. Then charge the battery and the monitor will count up the charge going in. For lead-acid batteries, the minimum voltage recommended to avoid damaging the battery is 10.5 volts. So the monitor counts up from this point. When the battery stops taking charge, then it is fully charged. Assuming it was calibrated correctly, the monitor will show the amount of usable amp-hours. It will not show the total amp-hours on a lead-acid battery setup unless you calibrated it with the battery fully depleted, which you would not want to do.
    2023 Imagine XLS 22MLE

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    Quote Originally Posted by tptNbone View Post
    NB Canada,
    I understand the 50% idea, but that is why I'm concerned about getting my max charged AmpHours as high as possible. I assume the best I could hope for would be a max of 128 AHrs which drops to 64 (50%), at which point I then charge it up again. But it looks like the system just looks at the voltage, not the amp hours. And not being able to pump up my amp hours concerns me, even when I am connected to shore power. For a while my max was about 75 AHrs, and the charger was trickling at the point. It should be charging at gang busters rate until it gets closer to the max, shouldn't it?
    Craig
    The 50% idea is a complete myth. Not even a shred of truth to it.
    John & Kathy
    2014 F250 Lariat FX4 6.2L SBCC
    2014 Reflection 303RLS
    SW Indiana

  6. #6
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    Quote Originally Posted by Harvestmoon View Post
    Somewhere in your stack of component manuals there will be a tiny user manual for the Furrion battery monitor. The monitor is calibrated by depleting the battery to the lowest desired state of charge and then calibrating that as the 0 percent point. Then charge the battery and the monitor will count up the charge going in. For lead-acid batteries, the minimum voltage recommended to avoid damaging the battery is 10.5 volts. So the monitor counts up from this point. When the battery stops taking charge, then it is fully charged. Assuming it was calibrated correctly, the monitor will show the amount of usable amp-hours. It will not show the total amp-hours on a lead-acid battery setup unless you calibrated it with the battery fully depleted, which you would not want to do.

    HarvestMoon,
    (For a laugh, try Googling "Doris Day Harvest Moon")
    As it happens, I do have that Furrion "component manual" and I did read the procedure given there. And I thought at the time that it was really bad to discharge (and damage) the battery at 0 VDC in order to "set up" the system. But you have indicated that "when the battery stops taking charge, then it is fully charged." But the MPPT 25A Solar Charging Controller is a three stage charger. And it is my understanding that one has to set it up so it knows when to switch from the fast "bulk" charge to the "absorption" charge (80%?) and finally to the "maintenance" or trickle charge (95%?). I have not done the steps in the Battery Indicator manual to set it up. Do I need to do that to get the correct readings on the indicator. If so, one part of the setup is to max out the charge and then "teach" the indicator at that point. And I have not really gotten close to the 128 AHrs, if that is even possible, although it was still charging today when I last looked. But it is insanely slow, and this is with shore (house) power. I just would like to know the best way to fully charge the batteries. I've even considered hooking up my 6A/2A automobile charger to give it a boost. Would that be bad?
    Craig

  7. #7
    Site Sponsor SolarPoweredRV's Avatar
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    Quote Originally Posted by tptNbone View Post
    HarvestMoon,
    (For a laugh, try Googling "Doris Day Harvest Moon")
    As it happens, I do have that Furrion "component manual" and I did read the procedure given there. And I thought at the time that it was really bad to discharge (and damage) the battery at 0 VDC in order to "set up" the system. But you have indicated that "when the battery stops taking charge, then it is fully charged." But the MPPT 25A Solar Charging Controller is a three stage charger. And it is my understanding that one has to set it up so it knows when to switch from the fast "bulk" charge to the "absorption" charge (80%?) and finally to the "maintenance" or trickle charge (95%?). I have not done the steps in the Battery Indicator manual to set it up. Do I need to do that to get the correct readings on the indicator. If so, one part of the setup is to max out the charge and then "teach" the indicator at that point. And I have not really gotten close to the 128 AHrs, if that is even possible, although it was still charging today when I last looked. But it is insanely slow, and this is with shore (house) power. I just would like to know the best way to fully charge the batteries. I've even considered hooking up my 6A/2A automobile charger to give it a boost. Would that be bad?
    Craig
    Here's the bad news..

    Voltage is a very poor way to calculate/determine your Amp Hours.

    The only accurate method is to install a Shunt style battery monitor (like the Victron BMV 712 or Victron Smart Shunt). These Shunts monitor the actual energy going into and out of the battery and can give you an accurate amps calculation (based on energy flow and not on voltage).

    The other bad news is that 50% discharge on a 12v Flooded Lead Acid battery is 12 volts (at rest).

    Zero percent State of Charge (SOC) is 11.59 volts. By the way, 100% SOC is only 12.64 volts. This illustrates why Voltage is such a poor indicator of amp hours.

    Data Source: lead-acid-battery-voltage-chart

    To check if your batteries are fully charged, disconnect the positive terminal from the battery and check the battery voltage in about an hour. If the voltage is at 12.64v, then your batteries are fully charged.

    If you want to try to set your existing controller, you could drain your batteries to 12.07v (after an hours rest) and set the controller at zero amps and recharge the batteries, you should wind up with ~64 amps showing, however, I doubt that the controller will prove to be a reliable indicator of your available amp hours. My guess is the controller will continue to give you readings that move up and down, just like the readings you are getting now. Better to install a Shunt style battery monitor for accurate information on your battery's State of Charge.
    Last edited by SolarPoweredRV; 04-02-2023 at 10:55 AM.
    David and Peggy
    2019 Ford F350 Lariat, 6.7L Diesel, Dually, Long Bed
    Running with 20k Reese Goosebox (Love It) and Ford Factory "Puck" system.
    Stopping with 8,000 lb Disc Brakes and Titan Hydraulic over Electric Brakes system.
    Powering all this fun with 1200 Watts of Solar, two Tesla, Model S, battery modules, 24 volt Victron Inverter.
    2018 Solitude 310 GK

  8. #8
    Site Sponsor SolarPoweredRV's Avatar
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    Quote Originally Posted by NB Canada View Post
    Don’t forget you can only deplete a lead acid battery 50 percent before you ruin it. You have only half of the amp hours
    Some people disagree with this 50% limitation. And it is generally believed that AGM batteries can be safely discharged beyond the 50% State of Charge (SOC), my experience with Lead Acid batteries that have been drained below the 50% SOC (~12 volts) is that they soon fail after the deep discharge event.

    However, due to their more robust design, 6 volt flooded lead acid batteries can be discharged below the 50% regularly without much harm (I don't recommend deep discharges on a regular basis).

    Here is a helpful chart that shows various types of batteries and their voltages (at rest) for various SOC levels:

    lead-acid-battery-voltage-chart

    In general, I recommend upgrading to Lithium batteries as soon as practical and getting rid of the Lead Acid batteries altogether.

    Here is a post that compares Lead Acid batteries to Lithium (LiFePo4) batteries and explains some of the benefits of LiFePo4 batteries:

    Lithium-over-Lead-Acid-Batteries
    Last edited by SolarPoweredRV; 04-02-2023 at 06:35 AM.
    David and Peggy
    2019 Ford F350 Lariat, 6.7L Diesel, Dually, Long Bed
    Running with 20k Reese Goosebox (Love It) and Ford Factory "Puck" system.
    Stopping with 8,000 lb Disc Brakes and Titan Hydraulic over Electric Brakes system.
    Powering all this fun with 1200 Watts of Solar, two Tesla, Model S, battery modules, 24 volt Victron Inverter.
    2018 Solitude 310 GK

  9. #9
    Site Sponsor NB Canada's Avatar
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    Quote Originally Posted by SolarPoweredRV View Post
    Some people disagree with this 50% limitation. And it is generally believed that AGM batteries can be safely discharged beyond the 50% State of Charge (SOC), my experience with Lead Acid batteries that have been drained below the 50% SOC (~12 volts) is that they soon fail after the deep discharge event.

    However, due to their more robust design, 6 volt flooded lead acid batteries can be discharged below the 50% regularly without much harm (I don't recommend deep discharges on a regular basis).

    [/URL]
    Unsure why you tagged me. Must have been in error
    The OP has lead acid 12 volts
    2021 Imagine 2400 BH
    2018 GMC Sierra 4x4 Crew 1840lb payload

  10. #10
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    Quote Originally Posted by SolarPoweredRV View Post
    The only accurate method is to install a Shunt style battery monitor (like the Victron BMV 712 or Victron Smart Shunt). These Shunts monitor the actual energy going into and out of the battery and can give you an accurate amps calculation (based on energy flow and not on voltage).
    I agree 100 percent and apparently so does GD. The 2023's are being shipped from the factory with a Furrion shunt type battery monitor. The OP can correct me if I'm wrong, but I expect his 2023 17MKE has the same Furrion FBM08A10C shunt battery monitor that is in my 2023 22MLE.
    2023 Imagine XLS 22MLE

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