question

solarcharger320 avatar image
solarcharger320 asked

Smart Solar MPPT 100/20 wrong charging current evaluation

I am using a small 12 V 7,2Ah lead acid batterie as buffer as I want directly charge into load. Due to small battery I have to limit charging current to 2.8 A.

If I setup 2 A as max charging current and I have a load of 1,3 A the battery is charged with 0,6 A only.

If I set 1 A charging limit the load still gets its 1,3 A but the battery is discharged with - 0,2 A. It seems the charging current value within the software is calculated by load current plus max battery charging current.

If a set a higher current limit like 4A the load still gets its 1,3 A but the battery is charged by 2,7 A.

It is not possible to set this 4 A limit as if the load drops out the charging current limit of the battery will be exceeded.

It seems there is a bug that charging current limit in the battery Setup considers battery current plus load current instead of battery current only.

It would be interesting what current limits would be considered for battery (charge/discharge) current and load current. I think both would be 20 A.


Setting up a higher charging current limit results in increased solar power.

I want to push solar power into load (USB charger for power banks) instead of the battery, but limit charging current the lead battery will be discharged and the solar power will be Limited.


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MPPT Controllers
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2 Answers
wkirby avatar image
wkirby answered ·

I understand what you are aiming for, but the charge controller not wrong and it's not a bug, it is working as intended for the majority of cases.
It is following the normal convention where the charge current setting is a global maximum, the loads are connected to the battery and the charger produces the requested charge current. Load current is never considered when setting up the charge controller. Also the battery is usually sized enough to absorb the full charge current from the charge controller.

So for your situation, I think there can be a way for it to work. If I understand correctly, then your battery is just a buffer, you don't want to cycle it really, it could just sit in a lower Voltage float stage?
If so, why not reduce the Absorb Voltage closer to 13.8 and then it will reach that limit quite quickly and the current to the battery will also reduce. Then also the battery will not be cycled over a wide range the whole time and will remain close to its Float Voltage. Load current could go higher but when it drops there may be a very brief period of more than 2.8A to the battery but this will quickly settle since the target Voltage is quite low. A brief surge would not cause an issue I don't think.

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solarcharger320 avatar image
solarcharger320 answered ·

Thank you very much for your immediate answer. I understand that within a certain application naming conventions could be used to make things simpler but not correct By the meaning of the word.

Your description fits to a system where you connect your load directly to the battery without additional load current shunt. The battery charging limit that is mentioned in your software parameter setup is rather an MPPT stage output current limit than a charging current limit. Even if you have connected your load directly to the battery this MPPT stage output current is not a battery charging current because your resulting battery charging current is the balance between MPPT stagebouput current and current draw,

So I think this simple attempt or naming conventions are wrong. I am also asking myself what is the sense of this parameter? Is this for output current Limitation to protect the wires? For this reason you use fuses. For big batteries there is no reason to limit the output current, the limit is the max. Output current of the device. For small batteries this parameter is of no sense because the output current is not evaluated to divide into battery charging and load.

In my opinion, if I would connect my Load directly to battery I would use a shunt for load measurement. So one could divide into load and charging current. Than you could get most out of your solar panel. It is a little bit unclear that you have two current measurements available but do not use them in the best possible way. It seems your definition of charging current is used because of simplification of software and not to make Setup more complicated but this simplification leaves available and valuable sensor values (current measurement) unused. The idea is that the software fits to the most applications and use cases but it leaves opportunities unused. Of course you develop a product for the mass market and not for niche requirements but in my opinion there is still unused potential for system improvement. The naming max battery charge current is wrong and misleading even if you consider that this naming is best practice. Sorry I am working within electronic development on the application side and I often have discussions with software and hardware developers that something that you give a certain name should have the designated function or the naming is wrong.

Perhaps you can explain more clearly the sense of the parameter max charging current if it is not what it says. It is nit helpful to limit the battery charging current and as protection of the internal power Stage there should be a fixed value that is linked to the devices output current capabilities that is getting into action to protect the device automatically.

So this parameter is only of use to reduce the MPPT stage output current, for example if you want to reduce heat dissipation if mounted in a small cabinet? I really have no idea what this value is good for.

Your workaround sounds like it could work with a little drawback when putting a high charging current on the battery, what should be possible to avoid if the parameter max. Battery charging current would work as it is named.

I will take a look how other manufacturers are handling this and hope that Victron would reconsider the value of sensor data for their control. Your device has all current measurements available but makes no use to evaluate them in the best way only to fulfil commonly used definitions of charging current. For me this is missed opportunity for improvement.

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