BMV700 Settings with Solar and Alternator Charging

[cobrien22]

Hi,

After reading a couple threads about this, I still had a clarifying question about the settings for the BMV700. I have a 200AH Renogy Deep cycle AGM, 180W of Solar, and a Kisae DMT-30 charge controller which acts as the charge controller for the solar and draws power off the alternator. Currently, the SOC jumps around quite a bit - if we are camping and not driving, the battery monitor will basically show the charge as consistently dropping from day to day, but the Solar charger will show it moving from bulk to absorb to Float. When we would drive for 20 or so minutes, the BMV would often jump to 100% SOC.

So I essentially have used the Kisae's display as an SOC approximation, knowing that I needed to change the settings at some point. We have pretty modest power use, with an Espar being the biggest draw, but on a sunny day, the solar has been enough to fully recharge on a sunny day. On cloudy days, we've just planned a day that involves a short drive to make sure we recharge. The van is my daily driver, all though Covid etc has me driving less.

After a few years, I finally picked up the Bluetooth dongle to make changing the settings not a total bear. Currently, I have my settings at:
Capacity: 200AH
Charged Voltage: 13.3 (float voltage is 13.6 for my battery, but the solar is reading float at 12.8)
Discharge floor: 50%
Tail Current: 4%
Charge detection: 3m
Peukert: 1.25
Charge efficiency: 95%
Current threshold: .1A
Time-to-Go: 3m

My sense is that I should change the Charged Voltage up to 14.1 because we are often charging off the Solar, and adjust the tail current down to closer to 1%.

It'd be great to have the BMV showing a more realistic charge. Any thoughts?

Thanks!

 

[DavidEM]

For those non cognoscentes here, a BMV700 is a Victron battery monitor that uses a shunt to measure and integrate current over time, in and out of the batteries. I would depend on it for SOC rather than the KIsae's display as the Kisae is calculating it indirectly from charging current and voltage. The Victron does it directly. But you do have to reset it from time to time to a known 100% SOC.

David

 

[borabora]

In your list of values, I am not sure if you are referring to the charger or the monitor and I especially confused by the "Charged Voltage" line.
AGM batteries are charged to 14.4V (it varies a bit by brand but that's a good average) and should float around 13.7V. 13.3V is not close to fully charged during the bulk phase. It seems that your solar charger is undercharging the battery.
If you reduce the tail current then the absorption phase will be longer which will help but I think that the core problem is that your charger switches from bulk to absorption too early.

 

[cobrien22]

Thanks for the replies so far. For clarification: the list of values comes from the Battery Monitor (victron BMV700), not the charger. I've attached a screenshot.

I don't believe the issue is the charger (KisaeDMT) is undercharging off of solar. I can see why you would say that, but the float voltage of 12.8 I see on the charger is when it is absolutely full - no power usage for a few days other than fan/lights, and a long (3+hours) of driving to fully charge it, and full days in the sun. The kisae manual also indicates that this is the correct float voltage when the battery is totally full. When I draw down the power after running the heater for a while, say, and let the solar charge it up to full so that the charger reads "float", and then drive, the charger will not flip into bulk or absorb when charging off the alternator at a different voltage. But at the alternator's voltage, it will cause the battery monitor to jump around. The two different currents coming when the charger draws solar vs. alternator power causes the Battery monitor to not provide accurate reasons.

My fix, I think, is to adjust the "Charged Voltage" line on the BMV up to 14.1, as it is in "absorb" on solar at 14.4. Because most of the charging happens from solar, which is charging at 14.4, I think the monitor isn't reading the SOC. When the battery charges at 13.4 off the alternator, it reads correctly (I think). Should I adjust the tail current, though? if so... to what?

I read the long post on battery monitoring in the stickied post, and it seems like something I really need to sit down with rather than skim to understand the monitoring process of the BMV.

 

[gltrimble]

Victron states that the “Charged Voltage” should be 0.2-0.3 volts below the Float voltage. Do not confuse “Charged Voltage with “Bulk Charge”. Victron recommends 13.2-13.3 volts for the charged voltage on an AGM battery. I have found the Victron settings very accurate. Your current Victron default settings are all correct, I would not change them. I recently adjusted my Balmar voltage regulator to match the Victron MPPT lithium settings.

 

[cobrien22]

Gltrimble,

my understanding is that Victron also recommends having the charged voltage at -3 the absorption voltage in systems that use solar as a charging source.

When I discharge a lot the bmv won’t recognize the discharge until the engine starts, even when it’s been under full sun all day.

I have no doubt that the BMV is accurate! It’s either something in my settings or install that is the problem.

 

[gltrimble]

I have had good luck with my Victron MPPT 150/35 solar charger? My Victron MPPT defaults to 14.2 volts for absorption and 13.5 volts for float for AGM batteries. I have a Nations 280 amp auxiliary alternator that feeds directly into the house batteries. My BMV-702 has been spot on in measuring voltage, current, and SOC. Occasionally it will leap from 95%+ SOC to 100. For measuring power draw it is accurate to 1-2 watts most of the time.