Does a solar charge controller really know the SOC of my FLA batteries?

[Vigilant24]

I'm learning about the "art" of determining the true charge state of flooded lead acid batteries.
- Read the voltage?
"That only works if they've had no charge or discharge for 5 hours (or maybe 12 hours)"
" They can have a surface charge, so you need to put a load on them and then check. Still, it's iffy"
- Read the specific gravity with an hydrometer?
" That doesn't work well immediately after charging or discharge due to electrolyte stratification."
" Gotta sir up the electrolyte (bulb squirt or equalization charge) for this to be accurate."

Given all that, how does a SCC (e.g. the EPEVER 3210AN that I'm looking at) ever determine the true SOC and when to go from bulk charge to absorption charge to float charge? Is it via something to do with the rate at which the battery voltage itself changes as the charging process occurs? If so, how the heck does it even measure battery voltage given the issues cited above?

Also, some SCC's have a display for "% state of charge" that is ostensibly there to tell you how full the battery is. Some folks who know more thn I do (virtually everyone! ) say this isn't very accurate--at all. Still, again, wouldn't the SCC need to know the SOC to know intelligently change charging modes?

Thanks for any insight on this.

Mark

 

[sunshine_eggo]

No.

Any voltage-based SoC measurement is completely worthless in an active system. If you turn everything off and let the battery voltage settle for 20+ hours with no charge or discharge, it's mostly accurate.

 

[mikefitz]

Epever and similar low cost solar chargers, just use voltage as an indicator of state of charge, a very poor indicator of SOC that confuses many.
The 3210AN charges durring the bulk stage to a target voltage called ' boost voltage '. It holds this voltage constant for the 'boost duration' ( default 2 hours), then drops to float voltage. It has no idea of state of charge. The process is voltage and time based.
It sort of works but the default fixed boost period, absorbtion duration, is a compromise that can, in some applications, leave the battery undercharged, or alternatively overcharged.
A better technique to establish full charge is to monitor charge current and terminate the absorbtion period when the current falls to a pre determined value. This may be 1 amp or lower for a 100 Ah lead acid battery.
Some chargers, Victron for example , monitor the battery voltage in the morning when it 'wakes up', makes a guess at state of charge and adjusts the absorbtion duration accordingly.

Mike

 

[sunshine_eggo]

forgot to mention... Get a battery monitor. First three are viable:

Off-grid Solar Battery Monitors

Building a vehicle mounted solar power system? Let me help.

www.mobile-solarpower.com

 

[Vigilant24]

No.

Any voltage-based SoC measurement is completely worthless in an active system. If you turn everything off and let the battery voltage settle for 20+ hours with no charge or discharge, it's mostly accurate.

Epever and similar low cost solar chargers, just use voltage as an indicator of state of charge, a very poor indicator of SOC that confuses many.

Thanks for that. I figured that was probably the case.

I don't mind if my setup requires a bit of "hands-on." It's strictly for emergency use, so if I have to
manually set the duration of the absorption charging period each morning, that will be okay.

Just to check:
1) The SCC's changeover from "bulk" (constant amperage) to "absorption" (may be called "boost", constant voltage) occurs when the charging voltage eventually reaches a preset point, but this may not be a solid indication that the battery is really at 80% SOC?

2) Then, the SCC stays in absorption mode (constant voltage) for a preset period of time (I can set this), but there's no way for it to directly measure battery SOC to know that the battery is "full" when absorbtion ends and it goes into "float" mode?

sunshine_eggo,
Thanks for the recommendations on the battery monitor. The Victron is probably out of my budget, but a cheapie option that can tell me the running IN vs OUT of the previous day (or three) should give me a good handle on how much charging needs to occur. Obviously, my goal is to get back up to "FULL" on these FLA batteries as soon as practical. So, I'd think that it would be preferable for me to err a bit by leaving the batteries in absorption mode a little beyond "full" rather than cut off charging leaving the battery at 90% overnight. This is especially true since there won't be any float charging overnight and I may not be sure if the next day will be sunny at all. So, charge while I can, but don't go crazy and boil away the electrolyte.

Thanks again, any comments are very welcome.

Mark

 

[mikefitz]

Note with a solar charger, the current in the bulk stage will not be constant, it will be dependent solar yield. When the target is reached the controller enters constant voltage, with lead acid this could be anywhere between 50% and over 90% SOC, depending on the charge current. To estimate 'full charge', monitor the current Into the battery whilst in the absorbtion period. If it drops lower than 1 amp per 100Ah, its near full, ( this assumes the available current is well in excess of 1 amp from the charger).
The low cost battery monitors don't take charge efficiency into account , this efficiency factor could be in the range 80% to 95% for lead acid.

 

[sunshine_eggo]

Also, with FLA, you can always check SG of the cells to confirm state of charge once they've been in float for 2+ hours.