Evaluating a charging/balancing strategy?

[Solarfun4jim]

On a mobile RV setup, with three charging sources,
1. Solar input via a SCC (constant whilst the sun shines) [0.1C charge rate]
2. A sterling Bat to Bat charger (to charge from vehicle on long drives). [0.2C charge rate].
3. Charger on a multiplus inv/charger, whose charge function would only operate on the very few occasions that i connected to campsite/grid power [0.25C charge rate].

My belief is that very little BMS balancing takes place outside of the 'knees', since voltage differences are so minimal outside of this. I have no desire however to charge up into the kness significantly, looking to terminate around 3.47v/cell. Hence i was thinking, that if i profiled the SCC & Sterling to charge to 3.47v/cell equivalent(27.8v on 24v nominal) but programmed the multiplus to charge to 3.6v/cell and had my BMS programmed to 3.65v/cell max, on the very few occasions that i'm grid connected, might this be enough to effect any necessary 'balance function' ?
I fully intend to 'Top balance' anyways, so felt this might be wholly enough to stay perfectly in balance over time?

Any opinions?

 

[Bob142]

Does your BMS allow you to configure a start voltage and cell delta for balancing? I've got mine set to start balancing cells at 3.4V with a delta of 15mV (suggested starting point from Overkill Solar - has been working well so far). That way you don't have to get up into the knee before balancing during charging takes place. If you can do that, you won't have to set different charge sources to different pack maximum voltage levels and you'd get some balancing every time you charged regardless of source.

 

[Craig]

I like your approach. Seems sound to only worry about it now and then with the randomness being determined by when you are grid connected.
And as long as you watch things you can alter your plan if things ever get out of whack.

 

[Solarfun4jim]

Does your BMS allow you to configure a start voltage and cell delta for balancing? I've got mine set to start balancing cells at 3.4V with a delta of 15mV (suggested starting point from Overkill Solar - has been working well so far). That way you don't have to get up into the knee before balancing during charging takes place. If you can do that, you won't have to set different charge sources to different pack maximum voltage levels and you'd get some balancing every time you charged regardless of source.

Thanks bob...i have a chargery unit, but haven't played about with all the functions yet, so cant say exactly what settings i can mod.

 

[Solarfun4jim]

Does these guestimate charge profiles look likely?
Red profile is full charging at 3.65v as comparitor
Light blue profile is what i'd expect aiming for a full charge at 3.47v
Dark blue is what i would be aiming for (approx 85% SOC) if terminating on hitting 3.47v/cell equivalent, 1 minute absorb time and float set below the resting voltage.
Dark green shows the current profile at 0.5C
Light green shows expected profile of current if terminating after the one minute absorb time.

Does this chart look reasonable for what happens as regards SOC and time taken in each phase???