The battery bank in my motorhome consists of four 100Ah AGM batteries that are 5 years old.
On a recent trip, I noticed that the battery bank output dropped to 10.5v after a few hours of a mild 250W load. I guess that means my batteries are getting a bit tired and probably in need of desulfation.
But, then I noticed that the Victron SoC reading is 79%. How can that be?
Do I need to recalibrate the Victron?
4 in parallel ?
They’re not just a little tired … they are apparently almost dead
Clamp them all apart and let them stand for 1 hour … then measure the voltage to make a statement. Charge them all individually and then measure again. If you can already see dents, they are broken.
I understand all that, but…
How can the SoC be 79%? Shouldn’t it be much lower?
I like your idea of unstrapping all four batteries and measuring the voltages. That might tell me that one battery is hurting more than the others. I will do that tomorrow.
The SOC says nothing about the health of 4 batteries. Only the charges that have gone in and out. This is all calculated, but the chemistry of the battery is not taken into account here.
So how is SoC calculated?
Does it assume that a 100aH battery will always be able to deliver 100 amp-hours?
How does it compensate for older batteries that are no longer able to do that?
The SOC is not called SOC for nothing
It is ONLY the State of Charge
When the batteries are “full” and the charging voltage and the tail current reach the parameterized values, the shunt or BMV THINKS that the battery is full.
The BMV cannot know anything about physical values such as plate sulphate, sludge or normal ageing.
For this, you would have to run specific charge/discharge cycles in order to then redetermine the values via the CAPACITY.
A new battery, as in your case, would have 100Ah. However, since you can only use 50% with lead, the measured voltage at this low load would almost be a dead battery.
Therefore, measure them all individually to get an idea.
I think we’ve identified a flaw in the way the battery monitor calculates SoC.
All batteries deteriorate over time. Let’s imagine that my four aging 100ah batteries are now barely capable of storing 60ah each. How do we re-educate the Victron about the actual capacity of the batteries?
As it has not been specifically answered, the Victron battery monitors start at 100% when the charge cycle is complete. They are an Ah counter (or Coulomb counter) and measure the current in and out. They do not allow for ageing, you can do this yourself by reducing the battery capacity if you do a capacity test or look at resting voltage.
That is the question I’m asking.
What is the procedure for testing/calculating the capacity of a battery?
I’d like to avoid taking it to zero for obvious reasons.
The correct way is to apply a moderate load around C20 load (in your case 400/20=20A) to the battery until it reaches 10.5V, counting the Ah discharged until it reaches 10.5V. That is your current capacity. Immediately recharge.
If you do not want to do this, discharge until you get to around 11.8V and then stop the discharge. Note the Ah consumed. Leave the batteries to recover for 24hrs so that the voltage recovers to resting voltage, hopefully around 12V. Then lookup one of the voltage vs SOC tables to see what SOC the batteries are at. From this you can do the simple math from Ah discharged and SOC from resting voltage. It will be an approximation.
In your case, if the batteries went from full to empty (10.5V) after a few hours of 250W = 20A, leys be generous and say 5 hours = 100Ah, your batteries have lost 75% of capacity. It is time for new ones. AGM do not typically respond to desulphation, the voltage is too aggressive and water loss can not be replenished.
Thank you for explaining that.
The people who sold me those 4 AGM batteries recommend a desulfation procedure of 16v for 6 hours. The charger I’m using is a Beleeb C40 with a pulsed desulfate mode. Many are skeptical of pulsed desulfation, but I haven’t heard any reason not to use it. Although my charger can deliver up to 16A, I’ve chosen to limit the current to 10A. Once I’ve fully charged all 4 batteries, I’d like to perform the drawdown test you describe, but I will have to overcome a few obstacles first, such as finding a 20A load, how to stop the test at a certain point without human monitoring, and how to read the results the next morning. Does the load need to be 20A or could it be 10 or 30 or more?
I’m thinking of performing a real world test. If I connect one battery at a time to my 3000W inverter, it will automatically shut down when the input voltage drops to 10.5. Then the problem becomes how to know how long it ran before it shut down. Maybe an old analog clock connected to the output of the inverter? Then, is there a way to read the Victron to find the number of amps, or amp-hours, that were used?
I’m thinking of
10 or 30 A will work, it is just that C20 is a standard and base capacity is different at different currents. Best of luck with the desulfation, I know a few brands of expensive AGM allow an equilisation charge. As they sound dead there is little to loose.
For a real world test pass the discharge through the SmartShunt, its history function should count the Ah and time. I have not used history to be 100% sure as I have a full system with Cerbo GX for monitoring.
OK, except that I have a BMV-700. How can I read history from that?
OK, except that I have a BMV-700. How can I read history from that?
Chapter 4.3.2 in the manual covers connecting to the VictronConnect app on your phone and 5.4 and 5.5 the trends and history. If you have a GX device then this can save data to VRM, the Victron cloud service.
https://www.victronenergy.com/media/pg/BMV-700/en/index-en.html
I have owned my BMV-700 for 5 years. and, yes, I’ve read the manual.
I just have two questions:
The BMV counts the Ah of energy used and divides that by the battery capacity that you input in the settings. The downside as you have found is as the batteries age. The way around this is to reduce the battery capacity input in the BMV from in your case 400Ah to perhaps 100Ah for example. You can do a capacity test to determine this, or look at your battery voltage to estimate the SOC and vary the battery capacity to match the SOC. The latter is trial and error so may take a few attempts to get right. The manual should state how to update the battery capacity.