Victron 702/712 battery monitor configuration with solar

[Herve]

Hi,

I got the Victron 712 battery monitor and am in the process of installing it.
I see on page 10 of the manual:
"In solar systems or other applications with fluctuating charge currents, the following measures can be taken to reduce the probability for the BMV to reset prematurely to 100% state of charge:
a) Increase the charged voltage to only slightly below the absorption charge voltage (for example: 14.2V in case of 14.4V absorption voltage). Increase the charged detection time and/or decrease the tail current to prevent an early reset due to passing clouds"

What have you guys been setting?

My setup:
- Toyota Tundra (not sure what the alternator voltage is but likely too low) and FWC wiring
- IBS-DBR dual battery relay (http://ibs-tech.ch/en/products/dual-battery-system/ibs-dbr-dualbatt-relay.html)
- Two 125 AH batteries (https://www.vmaxtanks.com/SLR125-12Volts-125AH-Deep-Cycle-Solar-AGM-Battery_p_38.html) - Charging Voltage 14.4-14.9V and Float Voltage 13.5V-13.8V
- Blue Star Solar Boost 2000e MPPT with charge voltage set to 14.5 V (http://www.blueskyenergyinc.com/uploads/pdf/Manual_BSE_SB2KE.pdf)
- 12v 180w solar panel (https://www.solarblvd.com/products/solar-cynergy-180-watt-12v-monocrystalline-solar-panel/)
- IOTA DLS-30 and IQ-4 (https://www.iotaengineering.com/pplib/IQ4manual.pdf)

The Blue Star Solar Boost 2000e MPPT is only 2 stages: bulk and absorption but no float. "The 2000E provides a two stage charging process, Bulk and Constant Voltage. The 2000E will be in Bulk charge when battery voltage is below the charge voltage setpoint. During Bulk the 2000E delivers as much charge current as possible to rapidly recharge the battery and drive battery voltage up to the charge voltage setpoint. Once the battery recovers sufficient charge for battery voltage to rise to the charge voltage setpoint, both PV current and output charge current are reduced as necessary to control battery voltage at the charge voltage setpoint."

The IQ4 has 3 stages: bulk/absorption/float at voltages 14.8V/14.2V/13.6V.

I rarely plug the camper in the AC. When not in use, the camper is parked in the sun with the solar maintaining the batteries. It's applying 14.5V during the day (as I type this message in the afternoon, it's delivering 0.7A at this voltage, I have not used the camper in a week).

How bad is it that my solar charge controller does not have float?

I'm thinking of setting the Victron battery monitor charged voltage to 14V so it's below both the solar charge controller and the IQ4 absorption voltage and not changing the other settings as I don't quite grasp the problem with passing clouds yet (and what about nighttime?).

Herve

 

[ckent323]

It seems that you will be primarily relying in your solar controller for charging the house battery. So I will just focus on that part.
Looking at the links you provided - your battery is an AGM.

In bulk charging stage the battery spec calls for 8A - 35 A
In Absorption stage the battery spec calls for 14.4 V - 14.8 V (14.6 +/- 0.2 V)
In float stage the battery spec calls for 13.5 V to 13.8 V (13.65 +/- 0.15 V)

From a technical paper on battery charging:

Overcharging - Excessively high float voltages cause a higher positive plate corrosion rate. Overcharging also causes excessive gassing. Recharging after a discharge must be controlled to 10% of the discharge current to prevent damage to the cells.
-
Undercharging - A low float voltage reduces capacity because of self-discharge. Undercharging can also result in sulfation, which can damage the plates.

I do not know what excessive means in terms of volts. is 1 V over charging excessive? My assumption is that if the charging voltage is not within the specification of your specific battery then it is either being overcharged or undercharged. that may be a very conservative assumption.

The Blue Star manual says that the default setting is 14 V but that you can adjust the Solar Controller to output a lower voltage than 14 V to say 13.8 V or 13.6 V.

It is my understanding that to maximize battery life you need to be able to bulk charge a battery in Bulk, Absorption and Float stages.

Ref: Battery University site section on charging and the Battery Stuff .com site on charging

From my Lifeline AGM Battery technical manual:

"The most efficient method of charging is to use a three stage charging profile. In the first stage a constant current is applied until the voltage reaches a pre-set limit. The first stage is often called the Bulk charging stage.

In the second stage the voltage is held constant at the same pre-set <voltage> limit until the charging current tapers to a very low level, at which point the battery is fully charged. The second stage is often called the absorption charging stage. A voltage setting of 14.3 V +/- 0.1 V should be used when the battery temperature is 77 degrees F. The battery is considered fully charged when the current drops below 0.5% of the battery's rated capacity. The absorption stage will typically last 2 - 4 hours before the battery reaches this level.

In the third stage, the voltage is reduced to a lower value that minimizes the amount of overcharge, while maintaining the battery at 100% state of charge. This third stage is often called the float charging stage. A float voltage of 13.3 +/-0.1 V should be used when the battery is 77 degrees F. "

(The charging voltages at other temperatures can be determined from a table they provide.)

If you set your controller voltage to provide the float voltage of 13.65 -/-0.15 V for your particular battery then I do not know what happens in what would normally be the second stage, maybe it just takes longer to get to the 0.5% of rated capacity.

I do not think you want the float stage voltage to be higher than recommended for your specific battery ads I have read that is a common cause of premature failure of AGM batteries.

I need to reiterate that I am no expert on this stuff and others here may be (likely are) more knowledgeable.

Also to recap there are three charging stages the first one is a constant current stage ==> Bulk charging
The other two stages use constant voltage.

I think a call to your battery manufacturer asking about only having two stage charging from your solar controller will yield the best answer.

;-)

Regards,

Craig

 

[Herve]

Thanks for the reply.
I've set the solar charge controller charging voltage to 14.5V so that it charges the battery rapidly (absorption mode). If I was to set it to something like 13.7V then it would charge too slowly when I'm actually using the camper (float mode).

I did this based on a recommendation from Vic (http://www.wanderthewest.com/forum/topic/13230-i-need-more-power-scotty/?p=180381) and from the battery company customer support.

But I did not realize until now that the charge controller had no float mode and was therefore likely overcharging my batteries. I did not mention this in the message Vic replied to and in my message to customer support.

Charging the battery at 14.5V / 0.7A from solar when the camper has been sitting in the sun for a week with nothing running in it is excessive right?

I'll contact Blue Sky and the battery company to get their opinion and will report back.

 

[Herve]

BTW, my initial question was: did anybody tweak the BMV 702/712 settings so it detects full charge correctly with a solar system? What did you do?

 

[ckent323]

For both my Victron BlueSolar 75/15 Solar controller and Vicron BVM-702 battery monitor I entered the manufacturer recommended voltages for my specific batteries (I have two Lifeline GPL-4CT 6 V 220 Ah batteries) using the Victron App on my iphone.

I see no advantage to doing anything else.

I have not zero load calibrated my battery monitor yet (as is covered in the manual).


Craig

 

[rando]

The full charge detection settings on the BMV-7XX are dependent on charging, not the battery specifications. When I had an AGM battery I set the full charge detection voltage to be 0.2V below the absorption voltage (14.0V), the tail current to be 4% and the detection time to be 3 minutes. What this means is that if the battery voltage is above 14.0V, and the charge current drops to less than 4% of the battery capacity for at least 3 minutes, the BMV sets the SOC to 100%.

Now that I use a lithium battery, the settings are a little different, but the concept holds.

 

[ckent323]

Interesting.

Lifeline recommends the following voltages for all of their 12 V batteries (6 V is half).

Bulk/Absorption - 14.3 +/- 0.1 V
Float 13.3 +/- 0.1 V

100% charged + 12.8 V or more

The BVM-7x2 assumes a 100% charged voltage of 13.2 V

From the manual:

The BMV resets to ‘fully charged’ when the voltage exceeds 13.2V and simultaneously the (tail-) charge current is less than 4.0% of the total battery capacity (e.g. 8.8A for a 220Ah battery) during 3 minutes.

Because it is closer to 13 V not 14 V I interpreted that to be the float voltage not the absorption voltage

Using the Victron App I set my battery capacity to 220 Ah in the BVM-702 and I have been using the factory default settings of 13.2 V and 4%.

For the 75/15 I used the app to set the battery to 220 Ah, the absorption to 14.3 V and the Float to 13.3 V.

I also put in the recommended Peukert constant for lifeline batteries which is 1.12


I have not calibrated the zero load voltage however.

Are there any errors here?


Craig

 

[rando]

Setting the 'fully charged' voltage near the float voltage is appropriate if you are primarily charging your battery all in one go, say from an AC charger. In that case, once you hit float voltage, the battery is full. However with the combination of solar and alternator charging in a camper, the situation is less clear. It is possible that a drop in battery voltage and charge current down to near the float voltage and tail current setting is actually due to the sun going away, or shutting off the truck engine, not the battery being full and the charger switching to float. So in this case it is better to detect the transition from bulk to absorption charge followed by a decrease in current. In this case the battery is *mostly* full, and you won't be fooled by a cloud passing in front of the sun.

You want to base these settings on the settings of your solar charge controller, as in most cases it will be the solar charger that finishes off your charging, not the alternator. For Craig's case above, set the detection voltage to ~14.1V. This is not a huge deal as it is only a periodic recalibration of the SOC number. If anything you want to be conservative with these numbers (ie synchronize less often). A little bit of extra drift over time is less of an issue than resetting to the SOC to 100% prematurely and thinking you have more capacity left than you really do.

Edit to Add: You may also want to reduce your tail current down to ~2% or so. With a small solar system, the limit to the amount of charge current is often the amount of available solar energy, not the batteries ability to accept the current. For much of the year your solar system will produce less than 8.8A even when the battery still has a ways to go to be full.

 

[ckent323]

rando,

Thanks and good points on the Solar system (or clouds) doing the tailing off.

Your explanation makes perfect sense.

The complication for me is that I am presently storing my camper outside with the solar panels active(i.e. the power is on to the batteries) and am not using it regularly so it sits for several months at a time with no loads.

I am thinking that sitting there with little use the Solar is keeping the batteries in float and it may be better to leave things the way they are but when I go on my long, many week, trips I can change the setting to your suggestion above.

Thoughts?


Craig

 

[rando]

In the storage situation, the Victron will still accurately read 100% SOC regardless of a recalibration just from the charge accounting while the battery sits on float charge. Also, you probably don't care about SOC when the camper is in storage.

 

[ckent323]

Rando,

Just to be sure I am understanding,

Presently the Victron battery monitor is set for 13.3 V as fully charged and with 4% tailing.

I think are suggesting I change the battery monitor to ~14 V for fully charged and tailing to 2%.

I don''t think you are suggesting changing the solar charger settings because that would result in overcharging the batteries since the charger will attempt to keep the batteries at 14 V even in float.

I think in the end analysis what you are saying is that the battery monitor is simply reporting SOC and has no effect on the actual charging so modifying the definition of fully charged has no real risk and may add more accurate understanding of the battery SOC given the use and charging conditions.

Is that correct?

If so that makes sense, I just just making sure.


Craig

 

[Vic Harder]

I have my tail current set at 3%.

rando is right, the BMV and the MPPT controller do not interact with each other, and each have their way of determeninig "Full".

 

[rando]

Just the settings on the battery monitor, not the charge controller.

Rando,

Just to be sure I am understanding,

Presently the Victron battery monitor is set for 13.3 V as fully charged and with 4% tailing.

I think are suggesting I change the battery monitor to ~14 V for fully charged and tailing to 2%.

I don''t think you are suggesting changing the solar charger settings because that would result in overcharging the batteries since the charger will attempt to keep the batteries at 14 V even in float.

I think in the end analysis what you are saying is that the battery monitor is simply reporting SOC and has no effect on the actual charging so modifying the definition of fully charged has no real risk and may add more accurate understanding of the battery SOC given the use and charging conditions.

Is that correct?

If so that makes sense, I just just making sure.


Craig

 

[ckent323]

Thanks for confirming and after thinking it through I appreciate that it is a good suggestion.

Regards,

Craig