Need Your Expert Electrical Analysis on SOC

[DaveP]

I have spent about two hours on the phone with two different Renogy techs and they cannot figure this out. I have the Renogy smart lithium batteries. The four of them are networked together with the Renogy battery monitor. I also have a Magnum MS 3012M charger/inverter with the Magnum battery monitor/shunt. I use a Victron Energy SmartSolar MPPT 100V 30 amp when charging with 400 watts of Renogy solar panels. When I charge the batteries with the Magnum or the solar, the SOC often comes out different on the two systems. It may be just a few percentage points as seen in the below picture but I have also seen higher differences (of 10 to 20 percentage points) when charging partially with solar (like a cloudy afternoon). The Renogy system usually shows a lower SOC than the Magnum. Do any of you have any ideas what might cause these discrepancies? Should I be worried about it? It seems to me that they are measuring power in and power out. Everything else with the system seems to be OK.

Just as reference, the Victron Smartsolar is set to lithium. Magnum advised me to set their charger to CC/CV. I have the voltage max-out at 14.4V per there advice. I also turn the charger off at about 98% SOC as Renogy recommends not holding their lithium batteries at 100% SOC all the time.

Thanks, Dave

[DaveP]

I could really use some help here. I would like to understand what is going on so I know if sometime needs to be fixed or if this is just the way it is. Uncle Bob? GMFL? Lewster?

[uncle_bob]

Hi

How do you have each of them programmed? ( or can you even get at the programming ....).

If you take a look at a Victron BMV 712 there are roughly a dozen settings that impact what it thinks is "100%" change any of them and you change the results. Much of it is lead acid specific vs Lithium specific ( lead acid only uses 90 something % or the charge input, lithium uses > 99% .... lead acid only is "full" after charging for hours, lithium is "full" much faster ...).

So what settings do you have access to?

Bob

[oldenavy]

Bob nailed it. SOC is a calculated value, so unless both units are doing the calculations exactly the same from the same measured inputs, you are not likely to see the exact same value. It's the old "man with two watches" problem.

[uncle_bob]

Hi

So, how would you set up a charge controller for lithium?

1) It's nearly 100% efficient so cancel out all the lead acid this and that effect stuff

2) When it gets to max voltage (14.4 or whatever ) it's full. Let the current come down some and then let it reset to 100%. No need to wait 4 hours looking at this or that.

3) The thing that always gets SOC reset on these gizmos is current going here or there. Often with lead acid this gets touchy and the settings are pretty low. No need for this with lithiums.

4) SOC is based on some input for "what's full" with lead acid, you might enter 100 AH and the gizmo decides 50AH is "full". This isn't right for lithium.

Yes this does go on and on. Thus the question of what settings you can actually get at.

Bob

[DaveP]

Bob,

Thanks for your inputs. I am starting to see the "light" for this issue. I was not thinking about what was going on in the right way.

The Renogy batteries have the SOC calculations built into their BMSs. I have four batteries networked together into their battery monitor. There are no settings I can adjust. I have to assume circuitry and algorithms are correct.

The Magnum ME-BMK is built for lead acid chemistry. I got the whole system five years ago when I had AGM batteries. The controller and charger do not have a lithium setting. I just re-looked at the instructions for the shunt and the only thing I can choose to control is charging efficiency. They recommended, I and choose, to set this setting to "automatic". It looks at the voltage a couple of hours after a complete charge cycle to recalculate "100% SOC". I can see how charging the batteries partially with solar then finishing with a generator (because of afternoon thunderstorms) would really mess up the calculations and cause the bigger discrepancies.

Thanks for getting me back on track. I think I will just ignore the Magnum SOC readings and use the Renogy readings for my charging decisions.

Thanks, Dave

[uncle_bob]

Originally Posted by DaveP

Bob,

Thanks for your inputs. I am starting to see the "light" for this issue. I was not thinking about what was going on in the right way.

The Renogy batteries have the SOC calculations built into their BMSs. I have four batteries networked together into their battery monitor. There are no settings I can adjust. I have to assume circuitry and algorithms are correct.

The Magnum ME-BMK is built for lead acid chemistry. I got the whole system five years ago when I had AGM batteries. The controller and charger do not have a lithium setting. I just re-looked at the instructions for the shunt and the only thing I can choose to control is charging efficiency. They recommended, I and choose, to set this setting to "automatic". It looks at the voltage a couple of hours after a complete charge cycle to recalculate "100% SOC". I can see how charging the batteries partially with solar then finishing with a generator (because of afternoon thunderstorms) would really mess up the calculations and cause the bigger discrepancies.

Thanks for getting me back on track. I think I will just ignore the Magnum SOC readings and use the Renogy readings for my charging decisions.

Thanks, Dave

Hi

No problem ....

Set the charge efficiency on your Magnum to 99 or 100%

Bob

[DaveP]

Bob,

Good idea on the charge efficiency. At least it will give me a number without trying to recalculate.

Thanks, Dave

[DaveP]

After 3 days camping (mostly overcast), some solar, lots of generator, the SOC from both monitors has been tracking within one percentage point. Good call on the charge efficiency reset! [emoji3]

[uncle_bob]

Hi

Even with two of the same model, staying within 1% over a couple of days would be doing well. Sounds like you have it running right.

Bob