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Old 08-09-2019, 05:07 PM   #1
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Magnum Energy BMK SOC issue

I don’t seem to get consistent and expected readings with my Magnum Energy Battery Monitor system for the State of Charge (SOC). I use it to monitor by Sportsmobile AGM battery bank with my Sprinter second alternator. Has anyone else had similiar issues.

For example, I will be at 100% SOC at home after a full charge. I can then be out camping and run the batteries down to 70%. I then may drive for 8 hours expecting to be fully charged again but the reading may actually be down to 60%. I can then just wait for 6-8 hours and see a reading of 80% or often 99% by the next morning with absolutely no further driving or charging. The problem seems to a significant delay of many hours in correctly reading a higher reading after I charge with my alternator. I had assumed the SOC would be close to a real time reading. Thanks for any ideas.
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Old 08-10-2019, 07:08 AM   #2
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I would suspect a problem with the unit. There really should not be a delay. I have to battery monitors on board, one being the BMK. There usually are the same. I have seen both of them claim 100% charged to early when on solar and the sun slowly drops mimicking a charger completing. That is a rarity, but can happen.

I would check and see how much current you are reading when charging off of the alternator, amp hours and such and see if they correlate.

Also give Magmum a call.

And I think user Fitz just mentioned he had a bad BMK, might want to touch base with him.
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Old 08-10-2019, 08:38 AM   #3
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Correct, I just replaced the unit. Very similar symptoms, the one symptom that convinced me (and the helpful Magnum rep) that it was bad was the Amp reading. It was obviously wrong when in use and it was holding a constant 26 Amp reading with everything disconnected. I changed out the unit and all is now well. Suggest you perform the same test, disconnect all loads and see what the Amperage reading is.
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Old 08-22-2019, 08:05 PM   #4
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I figured out my problem with my battery monitor. Sportsmobile wired it wrong at the BMK shunt. Changing the cables at the shunt solved the problem. The Magnum tech suspected incorrect wiring and sure enough he was right. Thanks.
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Old 08-22-2019, 09:35 PM   #5
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Good to hear that you found it!
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Old 04-24-2020, 12:49 AM   #6
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@jrobe Resurrecting this old thread. I had SMB-W install a Magnum BMK and all indications are that they got it right (after a return trip). But I'm puzzled by the SOC values that I see. After fully charging from shore power, the SOC will be at 100%. As I discharge the battery, I would expect SOC to drop to 50% when voltage reaches 12.2 on my 210ah AGM battery. Instead, I've seen 75% once and other percentages at other times. In addition, I've seen that charging does not bring SOC to 100% when the a/h out is negative and has to charge long enough for this to reach zero and go positive.

All of this makes me wonder about the Magnum remote and BMK settings. I've seen some mentions that SMB leaves things set at default values (and I recently found the battery size set at 200aH instead of 210).

Would you share the settings of your working system so I can check mine?
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Old 04-24-2020, 07:59 AM   #7
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When I was getting incorrect SOC readings, I found that SMB had wired the BMK DC shunt incorrectly. If you look at the diagrams in the manual, you can see that the DC shunt looks like it is the same on the right and left side. In fact, the load side and the battery side are different and it won't give correct readings if it is wired wrong. This would be an easy mistake to make because it is not marked well. I simply flipped the wires and it has worked perfect ever since.

I don't think I made any significant changes to the settings on the remote panel. I would review the settings though to make sure they at least entered the correct battery type and the size of the battery bank. Everything else on mine is probably set at the defaults.

https://www.magnum-dimensions.com/me...-64-0013-rev-b

https://www.magnum-dimensions.com/si...-ME-RC_Web.pdf
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Old 04-24-2020, 03:01 PM   #8
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BMK Shunt Wiring

I went through many of these same issues with the unit wired incorrectly at the beginning.
Here is what my shunt looks like with everything working correctly:


The fancy labeling is by me. The Battery side is on the left and the Load side is on the right. The three small diameter wires (orange, blue and black) all go back the BMK module. On the left side is the lead coming from the house battery, on the right is the lead going to the Magnum charger/inverter and the lead going to chassis ground.
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Old 04-27-2020, 03:30 AM   #9
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I found that SMB left the inverter at default values. I rcvd the correct charge profile from Lithionics to reset the inverter. Nations supplied the Balmar regulator along with the 2nd alternator and programmed the regulator correctly.
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Old 10-04-2020, 01:01 PM   #10
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Fitz, I do not have the BMK on my SMB.
Do you find this to be a worthwhile option or would you prefer a different option.
I have not had any problems but have been thinking of a BMS lately since changing over to Lithium.
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