Why is bulk/absorption voltage used?

[SeaGal]

LOL... my mind has no concept of float/balance/absorb/capsize/sink. All that lead-acid terminology was before my (LFP) time.

I'm not claiming this is good or right or wrong, but the reason I implemented the algorithm that I did was to:-

a) Get the cells reasonably charged up without stressing them too much. In winter I charge at cheap rate (7 hours) at night, so need to get enough in them to last the next 17 hours.

b) To give my JBD passive balancer enough time to actually do something. Without reducing the charge current, there was only a few minutes of charging from when balancing started at 3.4V until the 3.45V cut off was reached. So my control system reduces the charge at various SOC's and again when one cell gets to 3.4V., to then trickle charge up to 3.45V. At that point, charging is stopped.

So, a typical overnight charge would give a max cell voltage graph like this... (x axis is time (24h clock), y axis is mV). The 4 dips in cell voltage from just before 2am to after 3am are when the charge current is reduced.

 

[hwy17]

Beware of PV charge controllers that continually re-bulk battery to absorb level charging voltage.

This strikes me as a relieving simplification of a single voltage charging scheme, that you wouldn't have to worry about a charge controller rebulking or absorption termination logic.

As I understand it there is a bit of a catch 22 with open loop systems where the charge controller can't use amp based absorption termination because it can't know how many amps are going to loads vs. battery. So then you have this messy inaccurate business of timer based absorption termination.

 

[hwy17]

LOL... my mind has no concept of float/balance/absorb/capsize/sink. All that lead-acid terminology was before my (LFP) time.

This is the attitude I'm here for!

I'm gonna try and understand your voltage chart there more, but I think you're controls system is more sophisticated than anything I will run.

 

[SeaGal]

From what I saw of the charge profiles posted on here of Pylontech, it looks like they have a similar algorithm in their packs.

 

[hwy17]

The 4 dips in cell voltage from just before 2am to after 3am are when the charge current is reduced

Ok, so voltage dipping based on current implies that this is in a Constant Current mode, right?

What is the CC voltage ceiling during 02:00-04:00 or 01:00-04:00? 3.45 the whole time?

So it's a nearly all or entirely all CC charge with amperage backoffs to stay in CC and not hit a CV mode?

 

[SeaGal]

Ok, so voltage dipping based on current implies that this is in a Constant Current mode, right?

Yes. My hybrid Solis inverter is controlled from the CANBus data sent from the battery system - once a second, the battery will tell the inverter what to do (I think that's how a lot of closed loop systems work).

CANBus Message ID 0x351 is for Charge and Discharge parameters containing max charge voltage, charge current, discharge current, discharge voltage (the latter is not used by Solis).

What is the CC voltage ceiling during 02:00-04:00 or 01:00-04:00? 3.45 the whole time?

I set the max charge voltage is to 56V IIRC - that never changes. Obviously that would be the max voltage that the inverter would produce on its output to achieve the requested charge current, taking into account battery voltage and resistance of DC wiring and cells. In practice the inverter will obviously just adjust its output voltage to charge at the specified rate.

So it's a nearly all or entirely all CC charge with amperage backoffs to stay in CC and not hit a CV mode?

Yes - entirely. In most cases, my system is either charging or discharging - I don't see any gain (or even opportunity) in having a CV mode. In the case of charging up at night in Winter, I don't start discharging until 7am, when the cheap rate end - so in the graph you can see that once charging stops around 3:30am, the cell voltages naturally sink to about 3.35V before they get used again. Had the cells been at a lower SOC at midnight, the charging may not be complete until 5am or 6am. Also, note that my JBD is only configured to balance when cells are above 3.4V and there is some charge current, so I fail to see the benefit of what CV mode would actually achieve.