tomhalfmoon avatar image
tomhalfmoon asked

BMV 702 showing declining SOC while Multi-plus is charging

Thanks in advance for any help

I have a Yacht application. MultiPlus 120, CCGX, BMV702, (6) Firefly AGM Batteries. System has been working flawlessly. We have recently been away for normal Mains / Shore Power connection. Recharging via DC Alternator and Generator / Multi. Now we are back on Shorepower connection.

With Multi / ON and charging and various DC loads ON (dcv Frig, lights, Radio etc..)

The SOC is now declining slowly while the Multi is charging (Float @ 13.4 dcv). Via the CCGX I am seeing a small (-1 amp to -4 amp) load from the Batteries and this "I assume" is causing the SOC to fall.

It appears that the Multi is covering all DC Loads and is maintaining the Batteries but the SOC is fall. I have review the BMV settings

01 BC 660

02 CV 13.2

03 TC 4%

04 CD 3 min.

05 PE 1.07

06 CE 95%

07 CT .1

Remaining set Default

Any suggestions??

Thanks Tom

BMV Battery Monitor
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Alistair Warburton avatar image Alistair Warburton commented ·

Do you have a shore power limit set and if so could you be exceeding that?

If the charger is holding the battery at float hhere should not be a battery load at all and I would agree that he load you are seeing is responcible for the decline.

I am not a Victron expert but I would assume that the SOC will have to fall below some limit for some tome before Bulk is initiated.

I am inclined to suggest you have loads that exceed the float chargers current limit or there is an instrumentation issue.

You sound like you have a handle on this stuff which leads me to hink something isnt right.

If the battery is discharging either the voltage mesurment is wrong or the current mesurment is wrong, either way its odd, at float, assuming the voltage isnt falling due to load, DV current should be positive, or at worst 0


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3 Answers
tomhalfmoon avatar image
tomhalfmoon answered ·

HI Back on the boat.

After much testing and head scratching... I have determined that the BMV was read approx. 5 DcA low. With MultiPlus OFF, I switched ON a 3 amp load. With a multimeter reading across shunt -.3Mv., with multimeter in series the Bat cable and shunt -3 amp. BMV reading -8amp.

It appears to me that BMV is reading about 5 amps below actual....

Again scroll thru the BMV settings nothing appeared wrong. As a last resort I reset BMV to Default and reset my parameters..

All seems CORRECT now....... BMV and direct Multimeter readings correspond.

I have not made any BMV setting changes in months so not sure what or why it changed.

Anyway problem solved.


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Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

@TomHalfmoon, are you certain that the shunt for the BMV is wired correctly? Specifically, that all NEGS of loads, chargers, and etc (including the Multi) are wired to the load side of the shunt, and that the ONLY negative connected to the battery is going to the battery side of the shunt?

The most common call that I receive regarding the BMV not showing accurate SOC is a result of either a load or a charger or both bypassing the shunt and being wired straight to the battery. If your Multi NEG is connected straight to the battery rather than being connected to the load side of the shunt, then the shunt will be unable to "see" it charging the batteries when connected to shore power, and thus will display a declining SOC because it can "see" the loads but not the charger.

I recommend double-checking that nothing other than the "battery" side of the shunt is connected to the battery NEG, then - if you've checked that and you're still having the problem, getting back to us with perhaps some pictures of the setup so that we can continue troubleshooting. The only other possibility I can think of offhand is that perhaps the Multi input current is set so low that when you're connected to shore power it literally isn't pulling enough amperes to be able to meet your loads and charge your batteries at the same time, but that seems unlikely to me... thus why I'm hopeful that it's just a misplaced NEG.


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tomhalfmoon avatar image
tomhalfmoon answered ·

Hi Thanks for your input.

The multi is receiving full AC input and routinely shows 100 + amp output.

I reviewed the wiring ( all new from Battery to multi to DC panel). Only connects ahead of the shunt are the multi sensor wire and the temp sensor. All is OK.

But I am suspecting the negative wiring from the start battery. The AUX/trickle charge from multi to start battery is connected . But on the Neg side it is an indirect connection. Start Battery Neg to starter and Neg on Alternator to output side of shunt. No direct Battery to output side to shunt.

Next I will disconnect the trickle charge connection and see what happens then.


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