BMV Total AH Consumed, Battery Voltage, and % remaining.

[chrisski]

I am getting a battery voltage on my BMV-712 that is not matching the % remaining for my golf cart Flooded Lead Acid Batteries. My 150 AH consumed brings my percentage remaining down to 72% so that roughly matches up, but the battery voltage is 12.2, which is pretty close to the 50%.

My next step is to do some specific gravity readings to see where the batteries are at, because the only thing I see that could cause the readings I'm seeing is if my batteries are dying already. These batteries were purchased in July, Charged with a right sized 26 amp charger and kept on a float charger until I used them for about 10 days.

 

[Mex Rider]

I am getting a battery voltage on my BMV-712 that is not matching the % remaining for my golf cart Flooded Lead Acid Batteries. My 150 AH consumed brings my percentage remaining down to 72% so that roughly matches up, but the battery voltage is 12.2, which is pretty close to the 50%.

My next step is to do some specific gravity readings to see where the batteries are at, because the only thing I see that could cause the readings I'm seeing is if my batteries are dying already. These batteries were purchased in July, Charged with a right sized 26 amp charger and kept on a float charger until I used them for about 10 days.

Did you correctly synch when the batteries were at 100 percent soc?

 

[chrisski]

Apparently not. Let me get the manual And see how to do that,

Did you correctly synch when the batteries were at 100 percent soc?

 

[chrisski]

After reading the manual, I never did a manual resynchronization, which I will try.

The Battery Monitor does this automatically when first turned on, so it should have synch’d automatically the first time powered. It’s easy to resynchronize once fully charged, so I will try this.

My monitor reads 100% State of charge way early. It’s not supposed to read fully charged until it’s at 13.2 volts with a tail current of less than 4% for 3 minutes. With a 458 Ah battery bank, that comes to about 18.3 amps. Since I’m at the shortest sun day of the year, I don’t see 18.3 for very long.

Although the SOC goes from 80%+ to 100% in the course of a few minutes, I’ve never seen the BMV jump to 100% SOC, but I think I may need to dial back the tail current setting. Not uncommon during the charging process to turn on something that draws amperage so that the current to the battery is reduced from 40 amps to less than 18.3 amps for more than 3 minutes, but the battery voltage will remain above 13.2, so the BMV thinks its fully charged.

My charging amps slowly climbs form just above 0 around 8 am to 20 amps by 10 am. It will max out round 40 amps around 1pm, and then go back wards so its 20 amps around 3 pm, and 0 amps around 5 pm. If the day is sunny, the panels put back the 110 ah to 150 ah I have used during the night.

 

[Mex Rider]

After reading the manual, I never did a manual resynchronization, which I will try.

The Battery Monitor does this automatically when first turned on, so it should have synch’d automatically the first time powered. It’s easy to resynchronize once fully charged, so I will try this.

My monitor reads 100% State of charge way early. It’s not supposed to read fully charged until it’s at 13.2 volts with a tail current of less than 4% for 3 minutes. With a 458 Ah battery bank, that comes to about 18.3 amps. Since I’m at the shortest sun day of the year, I don’t see 18.3 for very long.

Although the SOC goes from 80%+ to 100% in the course of a few minutes, I’ve never seen the BMV jump to 100% SOC, but I think I may need to dial back the tail current setting. Not uncommon during the charging process to turn on something that draws amperage so that the current to the battery is reduced from 40 amps to less than 18.3 amps for more than 3 minutes, but the battery voltage will remain above 13.2, so the BMV thinks its fully charged.

My charging amps slowly climbs form just above 0 around 8 am to 20 amps by 10 am. It will max out round 40 amps around 1pm, and then go back wards so its 20 amps around 3 pm, and 0 amps around 5 pm. If the day is sunny, the panels put back the 110 ah to 150 ah I have used during the night.

When you first turn it on it "assumes" the batteries are at 100% soc whether they are or not. When I initialized my 712 my batteries were not fully charged....hence my original suggestion.

 

[snoobler]

After reading the manual, I never did a manual resynchronization, which I will try.

The Battery Monitor does this automatically when first turned on, so it should have synch’d automatically the first time powered. It’s easy to resynchronize once fully charged, so I will try this.

My monitor reads 100% State of charge way early. It’s not supposed to read fully charged until it’s at 13.2 volts with a tail current of less than 4% for 3 minutes. With a 458 Ah battery bank, that comes to about 18.3 amps. Since I’m at the shortest sun day of the year, I don’t see 18.3 for very long.

Although the SOC goes from 80%+ to 100% in the course of a few minutes, I’ve never seen the BMV jump to 100% SOC, but I think I may need to dial back the tail current setting. Not uncommon during the charging process to turn on something that draws amperage so that the current to the battery is reduced from 40 amps to less than 18.3 amps for more than 3 minutes, but the battery voltage will remain above 13.2, so the BMV thinks its fully charged.

My charging amps slowly climbs form just above 0 around 8 am to 20 amps by 10 am. It will max out round 40 amps around 1pm, and then go back wards so its 20 amps around 3 pm, and 0 amps around 5 pm. If the day is sunny, the panels put back the 110 ah to 150 ah I have used during the night.

Per Victron's instructions, for solar charging, you need to specify something just shy of absorption voltage not float voltage. You can tweak this as needed to make it work. Mine kicks to 100% about 30 minutes before it drops to float.

The tail current is a BELOW number. If you're BELOW that current and that voltage, that's the trigger. Charge profile is:

Max current to absorption voltage
Hold absorption voltage while tapering current.
Once current DROPS below tail current, go to float, 100% charged.

So your BMV absorption voltage needs to be 0.2V below your absorption voltage, and the tail current needs to be something that works and is realistic.

If you have insufficient current to maintain absorption until current DROPS to tail current, your batteries are not truly fully charged.

If these are FLA/AGM, and you're struggling to push 18.3A, you may be doing damage to the batteries. 458Ah needs 45.8A optimally, but you can often get away with a little more than half that. Failure to achieve those current values in bulk phase encourages sulfation during charge. If they are FLA, equalization charges can help restore them. If AGM, you just take your lumps.

 

[chrisski]

Per Victron's instructions, for solar charging, you need to specify something just shy of absorption voltage not float voltage. You can tweak this as needed to make it work. Mine kicks to 100% about 30 minutes before it drops to float.

I missed that. Until you made that comment, what the charged volatge was a mystery to me. I had done a google search and the few hits I got said 13.2 as the manual recommends. Sure enough in the manual, it says end of charge voltage, which in my case is 14.7.

The tail current is a BELOW number. If you're BELOW that current and that voltage, that's the trigger.

I had figured the charger was looking at for the charged voltage of 13.2 with a tail current of 4% or in my case < 18.32, and once it saw that, regardless of the state, it would read 100%. I’ve dropped the tail current to 2%, or a little above 9 amps.

BMV absorption voltage needs to be 0.2V below your absorption voltage

So since my absorption voltage is 14.7, I should set this to 14.5? I’ve set it to 14.5 for tonight.

If these are FLA/AGM, and you're struggling to push 18.3A, you may be doing damage to the batteries.

I would not say I struggle to push 18.3, but I may not see that until 9:30 or official sunrise +2:00. Provided it is sunny out, I will reach 30 amps around 1000, peak around 40 amps, and then go in reverse.

If they are FLA, equalization charges can help restore them

They are FLA. I could not do a specific gravity test, my hydrometer was contaminated with dust inside. I may try cleaning with Windex and distilled water, but may just buy a new hydrometer tomorrow.

I am alone here for a day or two and my energy needs are pretty modest, but once my family shows up, we’ll be using 100 to 150 ah after sundown, depending on the temperature. Most of that is the blower for the heater. Last week was below freezing and the blower blew a lot more than a couple weeks before when it had been in the 40s at night.

 

[snoobler]

Yes, 14.5V.

Based on your solar charge profile and 30A charge potential, you should have no problems with a typical tail current termination. Just make sure it's a little above what your actual tail current is. Unless you're using a SCC that allows you to program it in, you may be at the mercy of the SCC's built-in criteria, and it may take some staring at it to determine the appropriate tail current. Some just do a fixed absorption time.

 

[chrisski]

Thanks

Based on your solar charge profile and 30A charge potential, you should have no problems with a typical tail current termination. Just make sure it's a little above what your actual tail current is.

The tail current is the last part of the current I see in the absorb phase, or is it the current is start to see when my batteries float?

I have the Victron MPPT solar charge controller which communicates with the Battery Monitor, so I hope the absorb phase is more than just a fixed time. To me a fixed time would be something a PWM charge controller uses, and variable times based off inputs is what I hope the MPPT algorithm is based off.

 

[snoobler]

Cool. I thought you did. You may need to go into expert mode and check it out:



Adaptive may cut it short. If it does, you can pick Fixed.

 

[chrisski]

The with a couple of input tweaks, the SOC reads accurately.

Setting the Charged voltage to 14.5 (.2 volts below the Trojan Battery Absorption voltage of 14.7) and setting the tail current to 2% did the trick. The charge meter no longer jumps from 82% to 100% SOC, but climbs up gradually.

I can even notice the change from Bulk where a constant current is applied to the battery —> Absorption where the voltage level stays at 14.7 and current tapers off —> Float where there is a low voltage and barely current supplied to the batteries. Not sure if this is me setting the meter correctly or actually starting to understand what those phases mean.

I have not depleted the battery except for a few percent on this trip since it is a bit warmer and my big load, the DC heater fan for the propane heater has not kicked on like it did in the weeks prior. I have not gotten a low enough DC voltage early in the morning to go out to the battery box and do a specific gravity test with the meter reading a 72% SOC, but maybe over the next couple of days.

 

[snoobler]

Nice. A little bit of erratic behavior in the 90%+ SoC range is pretty impossible to eliminate. Charging in that range is less efficient. I've played with that to make my charge terminations look better, but it only works for my typical discharge ~80% SoC. When I go lower, it really jacks up the next charge.

Mine jumps from 98% to 100% most days, and I've just learned to live with it.

 

[chrisski]

I think the SOC is reading accurate now to within 5%. Setting the tail current, Charged voltage, and synchronizing the Battery Monitor is what I believe is giving it a more accurate SOC reading.

I actually synched my charger to read 100% when I got the Specific Gravity readings as high as I could, which was 96%. So when it reads 100% it’s at true 96% capacity which is about 4% high.

Last night, my batteries got down to 84% SOC off the Battery Monitor. This matched the Amp Hours Consumed perfectly. For the battery voltage, I measured it when there was no load for a few minutes and I got 12.48 VDC which is 79%. Since the SOC is about 4% high, the voltage reading is within 1% of what the Battery monitor is reading,

I do not want to go below 55% SOC, although I never went below 72%.

As far as me turning the generator on to charge the batteries, I will turn it on by 55%. I’m at an overcast day now, and my 1000 watts of panels are pushing 12 amps, and I’m 87% which is 78 amps from being charged. Supposed to be cloudy until 2 pm. If this forecast keeps up I will be between 95% and 100% charged by the time the sun sets.

 

[snoobler]

It's probably better than that. Resting voltages are AFTER a LONG rest. You might have seen another .05V+ bounce after 30-60 minutes, which would put you in the range.

The BMV SoC is very accurate - mathematically. You programmed a capacity, and it counted. If you used more than a 20hr rate, it's accurate as it takes Peukert into effect, BUT if you used less than 20hr rate, it's conservative. It becomes less accurate over time as your battery degrades ever so slightly with each cycle. I addressed this by reducing my actual capacity vs. rated. My 300Ah of T-1275 tested out to 225Ah on the weakest of the 8 batteries, so that's my limit, and it correlates pretty nicely with voltage.

It's surprising how accurate it is including other factors like cold. My biggest draw has been 103Ah, but that registered at 30% SoC, and it correlated very well with a lightly loaded voltage. The kicker was the very low temperature and associated capacity loss... it was 8°F in the morning. At that temp, the T-1275 only has about 50% of its rated capacity available.



Assuming an average temperature of around 25° for the discharge period (average of daily high and low), I had about 65% of capacity available.

0.65 * 225 = 146.25Ah
103/146.25 = 70.4% used

My display rounds off the decimal, but the chart shows it:



29.8% low. 0.2% off.

The 5% error I referred to is the SG vs. SoC relationship. If you have healthy batteries that meet the rating, and the BMV is programmed correctly, it's probably MORE accurate than SG, though it drifts over time and require periodic resets to 100% via the charge termination criteria.

You can trust it.