battery bank awareness

11.17.2011 – Thursday

My tale of trouble shooting an errant battery monitor readout. Is the PV charger bulk and float voltage set too low? Is the electrolyte not mixing under charge current? Is the battery monitor being tricked into thinking the batteries are fully charged when they are not? Are the batteries going bad after just 2 years?

…I’m up for the Michigan gun deer season and it seemed like a good time to check the health of the batteries. Here’s the data I have to work with:

  • Last time to 100% SOC (source: battery monitor): 5 days ago
  • At that time the MS3000 was charging in absorption mode
  • Specific gravity readings from 3 cells (42°F): 1.180, 1.180, 1.180
  • Temperature corrected SG readings: 1.165
  • SOC (source: SG readings): 47% est.
  • SOC (source: battery monitor): 83.7%
  • Voltage (source: battery monitor): 11.78 V
  • Load at above voltage: 250-300W

Here’s what stands out in my mind as odd: 11.8V, 83.7% SOC from battery monitor, 1.165 corrected SG.

  • 11.8V @ 42°F should be ~ 65% SOC (according to past measurements)
  • 83.7% SOC should be ~1.240 SG (temp. corrected), 12V
  • 1.165 SG should be ~ 47% SOC

Hmm… something isn’t right here. Five possible explanations come to mind:

  • I have acid stratification due from the low charge current from the solar (charge rate typically is <3% battery bank capacity)
  • I entered the wrong charge efficiency factor (CEF) on the battery monitor
  • The Voltage is not high enough on the PV charge controller, when the PV controller goes to float the battery monitor syncs to 100% SOC
  • The batteries have less capacity than stated
  • The battery monitor is being tricked into thinking the system is on float and syncs to 100% SOC when the system is actually at 70%, 80%… any value less than 100%.

Some tests to rule each out:

  • Acid stratification/battery capacity: equalize and discharge batteries about 15-30% (normally 200-400A), test SG and see if it makes sense. If it makes sense then acid stratification was the problem. If not, we lost battery capacity.
  • CEF is tricky. Drift from an overly optimistic CEF set point could account for the errant reading. In other words, if the battery monitor thinks it takes 105A to put 100A into the batteries when it actually requires 110A to put 100A into the batteries then slowly over time a stated 100% SOC will actually be 96%, 95%, 94%… decreasing with time/charge cycles. However, this can only occur if the batteries never return to full charge. The battery monitor syncs to 100% when a charge controller enters float.
  • Charge controller voltage set too low: test battery SG when charge controller indicates full charge (after the batteries reach indicated full charge under PV power).
  • Incorrectly syncing to 100%: record the time when the battery monitor reads 100% SOC. Then record each hour the charge current being put into the battery bank. From these data, estimate the total number of additional amps required to reach 100% SOC and calculate the percent SOC the battery monitor was off by. Hopefully it agrees with the SG reading.

The short term plan:

As a quick check of battery monitor accuracy, I’m going to have the ol’ man check a few things while I’m gone. Using some quick math; 50% of our battery bank at the 24h discharge rate should be 770A; or about 150A / 10% SOC. So… when taking winter temperatures into account, this comes to about 130A / 10% SOC (worst case scenario). This is a rough estimation; the should be capacity to 50% will vary under really light and especially under really heavy loads due to characteristics (internal resistance) inherent to all FLA batteries. The quick check will be based on past data recorded on 1-12-2011 which was as follows:

12.1V, 300W AC load, 85% SOC, 41°F battery temp.

11.9V, 300W AC load, 60% SOC, 36°F battery temp.

The long term plan:

I knocked down the charge efficiency factor (CEF) to 90%, down from 95% to [possibly] more accurately reflect internal battery losses during charging (102-110% charge current is required to charge a FLA to full). I also checked the C60 PV charge controller set points for bulk and float to make sure they match the MS3000. The C60 is set 14.5V bulk, 13.5V float, and has a temperature sensor to correct for the cold. Also, as a precaution I restored the factory default of 1.25 for the Peukert exponent (was 1.15).

The final change I made was to set Vc to 15.0V. This setting, along with It (Tail Current), is used to determine when the batteries are at full charge. In order for the Victron to assume the batteries are at full charge under my new settings the batteries have to have a current of less than 0.5% of the 1540Ah battery bank and the voltage needs to be above 15V. This should prevent the Victron from being fooled into thinking the batteries are fully charged on a cloudy day; on an overcast day the PV may only trickle charge at 13.3V at 5A – and just like that the Victron assumes the batteries are under a float charge and declares 100% SOC. But not any more. By changing the Vc to a voltage only attainable under equalization, I can guarantee that a false high SOC cannot happen. I can still end up with a false low SOC.

11.18.2011 – Friday

I made some progress. The Honda EU3000 generator has gotten a workout today and the system still isn’t back to 100% SOC. Based off of the charge current over time I can estimate that the system was about 400-500A below what the Victron stated. This is roughly a 25-35% difference. My SG reading gave an SOC 47% while the Victron claimed 83.7% SOC. This is close enough for me to rule out acid stratification and loss of battery capacity. With that figured out I can now focus on CEF and the PV charger set points. The plan now is to:

  1. Equalize the batteries
  2. The Victron will automatically reset to 100% SOC
  3. Draw off 100-200A
  4. Allow the PV panels to fully recharge the batteries
  5. Observe and adjust either the CEF of the Victron battery monitor to match the charge efficiency of the batteries, or the bulk and float set points of the Xantrex C60 charge controller. Worst case scenario now is that the Victron will fall out of sync and drift downward to 98%, 97%, 95%… and so on when the system is fully charged. This is easily remedied by holding two buttons on the Victron to manually sync – an option preferable to thinking you’re at 85% SOC when you’re actually below 50%.

4 thoughts on “battery bank awareness

  1. Jason

    I think that your problem has more to do with the number of parallel battery strings you have in your system. You should always try to construct a battery bank from 1 series string if possible. Example:

    Lets say you need a 24V 600aH bank. You should look for a 2 – 6v battery with a capacity as close to 600aH as you can find. this way you have 4 x 6V or 6 x 4V batteries in series and do not have to worry about balancing parallel strings.

    In your case (looking at your wiring diagram), you have multiple small series strings of batteries in parallel to bump up the capacity. The absolute MAX number of parallel strings you should have is 4 – it looks like you have about 7 strings. This is a nightmare to keep every string properly balanced. If you have one weak battery in a parallel string, you drag all the other healthy strings down to pick up the slack . Your SG reading will be all over the chart and you will never be able to keep you bank healthy.

    Ideally, you should get new batteries so that you have only one string of cells connected in series. If you do not have the $$ to do this, I would try the following:

    Connect only 1 string of batteries at a time and charge/equalize them until you get the SG readings up. Once all the strings are back up, connect HALF of the strings to your inverter and run off of them – and connect the REST to the charge controller and charge them up. Just keep swapping them back and forth otherwise you will keep having these issues.

    Hope this helps – if you have any questions, feel free to email me:\


  2. Bobby

    “…..This should prevent the Victron from being fooled into thinking the batteries are fully charged on a cloudy day; on an overcast day the PV may only trickle charge at 13.3V at 5A – and just like that the Victron assumes the batteries are under a float charge and declares 100% SOC. ..” This is incorrect. During overcast both Vc and It will be low if the batteries are not full. A voltage of 13.3v on Victron and a very low current will certainly mean the battery are charged up no matter what the conditions of the are outside. If the battery is flat, the voltage is around 12.1v, when it starts charging the voltage increase slowly up to 14.4v before it slows down to floating voltage of bettwen 13.3 -13.6v with very low current. Even if its an overcast day, such voltage and current only means one thing – the batteries are on float charging.

    1. offgridcabin Post author

      The false 100% SOC issue was most likely to occur when clouds roll in. It’s relatively easy to exceed 13.3V early in the charge process when the batteries are around between 60-70% – introduce a long cloud and the amps will drop but the voltage will stay high for a bit. This is a bit of a problem in winter when all too often the morning is sunny and the afternoon transitions to clouds.


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