Just wondering if someone can give some advice here. Have 4 Rolls 6 V batteries in series for a 24 V system. Capacity of batteries is 235 Ah C20 rate. I have my smart shunt configured conservatively for 200Ah capacity. My discharge floor is 50%. The batteries are very infrequently cycled below 90% capacity. Recently used them a bit heavier with a 5 A load overnight and the smart shunt still shows 80% capacity, and the voltage was slowly decreasing until it suddenly tanked (span of 10 minutes) from 24.5 V to 22.3 V with no change in load before the sun came out and began charging. Any idea what’s causing it? Sulfation?
You can get ageing for any number of reasons.
DOD is just one of them. How old are they (from date of manufacture)? How long after manufacture did you purchase them? Have they been moved or shaken up?
Do the Rolls batteries have maintenance recommendations? What ones do you have exactly? Flooded, or mat?
Sorry, they are FLA, and I have been topping up the water as per manufacturer recommendations, beyond that, no other specific maintenance other than being careful to not go below 50% DOD. The batteries are just coming up on 2 years from purchase, don’t know the manufacturing date at the moment. They have not been moved or shaken since installed.
The smart shunt is showing what the depth of discharge should be but it seems the batteries are dead. Not a very good prospect for 2 years old but that does seem to be the problem.
As you have 4 batteries in series, have you ever checked that they are all balanced, i.e. when charged that they all charge to the full voltage and in use stay at the same voltage. When the voltage dipped were all batteries dropping in voltage or just one of them. Do you only take off 24V load, or do you have any 12V load taken from the mid point. Taking 12V off is not a good idea as the batteries get out of balance.
How do you charge them and are you sure you are getting them full. For Rolls batteries to fail after 2 years of easy use suggests undercharging to me. Are you getting them up to 28.8V and less that 2A charge current regularly. Are you stopping charging when the shunt states 100% because it can say that prematurely if you have not set it right and you use solar charging. What absorption voltage do you use and what charged voltage do you have on your shunt.
See my FAQ in the DIY section as this may be the issue. https://community.victronenergy.com/t/shunt-battery-monitor-jumping-to-100-or-reporting-a-high-soc-when-the-volts-are-low-and-or-the-bms-has-disconnected/88
I have not checked the voltage of each of the batteries, but I can check that in a couple days as I am just monitoring remotely at the moment. I do not know if it is just one battery dropping voltage. It is just strange to me that the batteries seem to handle the load and then drop off a cliff. Stranger yet, this morning they dropped off the same cliff, but then seemed to stabilize at the lower 22.5 V for awhile. I would think if they were all shot they the voltage would continue to drop, so maybe it is one bad cell/battery?
The critical SmartShunt setting with such a low power charger is the charged voltage which should be 0.2-0.4V below the absorption voltage so say 29.1V to avoid the SOC going to 100% early.
So ended up being that one of the batteries had a bad cell. After only 40 Ah discharged from the bank, the cell would drop to 0 V, pulling the rest of the bank down to 22.4 V. Specific gravity in the bad cell was around 1.300 instead of around 1.280. Replaced the battery with the bad cell (made sure the specific gravity of all the other cells were within spec at full charge) so hopefully get at least another couple years out of this bank.
