I could turn this around - if you start with a dead battery and charge with 13.5v until current reaches zero, what will the SoC be? It's not going to be 100% of the possible maximum charge.
Just some thinking out loud. No guarantee that I am right about any of the following:
I am thinking maybe too much attention is paid to SOC? I dont really know of any batteries that have SOC tied to anything directly except a reset to 100% at a known position that is semi-arbitrarily set by drop in manufacturers or user set with things like the Victron shunt, followed by a counting of current in/out.. "Charged detection criteria" is set in the Victron shunt in three places that will together reset SOC to 100%. If I remember correctly...its Charged voltage, tail current and charged detection time. Typically, Victron recommends a charged voltage of .2 volts less than absorption. Tail current is a percent of total capacity and detection time is how long the two other values are seen. Once that happens then it just resets SOC to 100%. As far as I can tell SOC in all LFP batteries was added as a convenience item to give users a half decent fuel gauge since the flat voltage no longer allowed good capacity tracking. In drop in batteries those values are set by the manufacturer and not settable of course. In addition things like "current detection threshold" of the Victron shunt or the in the case of drop ins...the internal shunt have varying levels of accuracy. Its well know the Epoch V1 had a current detection threshold of 1.3 amps. Anything less was invisible to calculate SOC. The Victron shunt is settable but comes preset to .1 amps. And they warn setting it too low can induce/pick up on noise in the system to make amp measuring inaccurate if set too low. So .1 is probably good. The current detection thresholds conspire to add drift into SOC calculation and tracking for very low draws/charges, or every time the current direction is reversed if that reversal isn't symmetrical in time spent under the thresholds at low currents in each direction. I have a theory that this is why SOC on solar tends to drift so much more than something like inverter/charger charging. But solar can also spend a very long time in a midrange SOC without ever getting to the SOC reset criteria. A few weeks on solar without ever reaching SOC reset and the drift accumulated is likely significant. Floating for extended periods as well.
Jeff, in your quote above you say charging to 13.5 will not result in 100% SOC. But what if that is only due to previously induced and accumulated drift in SOC accuracy and the fact that the system hasn't achieved the SOC reset criteria? Since SOC seems to be such a poor indicator of
actual capacity it would seem that the only way to determine the state of the battery after charging and absorbing at 13.5 volts is by experiment by doing an actual capacity test. One of the best tests I have ever seen was Ben Steins tests in this article.
What voltage should I charge my LiFePO4 batteries? That seems like a simple question likely to have a single, direct answer. But, the actual answers are often unclear. Many LiFePO4 battery manufacturers recommend 14.6 volt absorption. But, that singular recommendation doesn't account for...
panbo.com
He charges and cap tests a 100AH battery at voltages from 13.2 to 14.6. It must have taken quite some time to compile. Only at 13.4 did the battery not quite achieve full rated capacity on discharge tests. 13.5, when full absorbed essentially matched all voltage above it for capacity. But charge speed took a big hit. In actuality 13.4 did make rated capacity but all higher voltages added another 3ah or so. You can also use Bens charts to help inform for absorption time.
I don't put too much stock in SOC unless I know I have recently hit the SOC reset point AND haven't spent extended time near very low currents in and out. Boat systems that float for long periods accumulate drift in SOC. Solar systems that spend many days in a mid level SOC with lots of in/out reversals at very low currents and dont make it to SOC reset for days weeks or even months will be wildly inaccurate. But charge up a golf cart to 14.6 which resets SOC, followed by a hard drive where amp draw is hefty and constant with no reversals and SOC will be fairly accurate over the full discharge. When I had the V1s in the boat I would float for weeks at the dock. SOC would drift 4-7% from the Victron shunt and the Victron shunt was guaranteed to be less than perfect as well (but much better than the EpochV1). I could care less about the SOC because as long as I hit my voltage and absorption marks I knew the battery had 100% capacity or very very close. Hitting those marks also were guaranteed to reset the Victron SOC but not the Epoch SOC. So there was that too. So If I wanted to align them I would just ensure that on the next charge cycle I hit all the marks of the various SOC tracking meters and they all reset to 100%. Then its just a slow slide of SOC drift away from accuracy over time.
So you dont need to charge to 100% SOC. You need regular visits back to SOC
reset criteria. To further illustrate, I have seen the opposite effect where 100% SOC was reached too early at too low of a voltage to be considered full capacity. Obviously, the SOC meter wont go above 100% so it just continues to charge at a decent amp rate for long periods sitting at 100% until the programmed charge cycle is met. Again, soc was just along for the ride.
In practice I don’t see this happen. I may see a brief discharge current, then a charge current, etc. There is never an ongoing trickle discharge. The alternator covers the DC loads. I think the longest I have been underway like this is perhaps 36hrs, and SOC doesn’t budge. I think to stabilize at a lower SOC the voltage needs to be lower. I think more like 3.4 vpc.
I agree. Its still all about the voltage from what I can gather. And 3.4 vpc should give full capacity as measured by a proper capacity test. Especially for the longer runs. I am sure battery charge current had long dropped to 0. Andy at Off Grid Garage on Youtube did many cell level tests over the years similar to Ben Steins tests of a complete battery and came to the same conclusions regarding voltage, absorption time VS measured capacity. The only difference between the cell level testing and the full battery testing was the addition of an SOC meter, that in reality is a marginal tool. I am sure your all Victron system is very accurate for SOC. But I also believe it will rebulk every 7 days (or maybe s BMS managed for rebulk?) which will reset the SOC at regular intervals.
On of the biggest tip offs that SOC (especially in drop in batteries with Bluetooth) is to be viewed carefully and skeptically is dealing with many golf cart users. Many new golf cart people with their new lithium drop in batteries would charge up the packs with the simple Cc/Cv charger to max voltage of 14.4 to 14.6 volts. This would always reset SOC to 100%. They would go on the long drive and come back with SOC at 20%, but they would charge again and manually stop the charge at 80% because thats what they read they should do. After doing so 6 or 7 times they would head out with 50% remaining only to have the SOC drop to 0 all of a sudden, the cart dies a mile from home. Many BMS have a low voltage SOC reset as well. But its just prior to discharge mosfet disconnect....lol.