Any potential problems with my diy home battery?

[Rory1984]

Hello,
I'm in a preparation of designing my own cost effective (and fun) home battery to charge when solar panels have over capacity and use when needed. It is a on grid systeem. The idee is to read the serial data from my home power (P1) meter with Arduino and use this data to adjust charging or discharging power realtime from battery to the grid. I want to use a charger with adjustable current using a potentiometer. I want to replace the potentiometer with an digital potentiometer in order to change the current realtime with Arduino. For discharging, I want to use a inverter with a power limiter that uses a hall sensor to measure current. I don't want to use the hall sensor because it only can measure one phase of my 3 phase connection and I have the realtime power already available from serial data. Note: if I use and supply power on different phases at the same time it will be equalized, so for my energy bill it doesn't matter that I only supply on 1 phase. I want to use the Arduino to simulate the same analog signal like the hall sensor. Like this the inverter will be real time limited in power to match my power consumption. I will also measure AC current from and too battery to provide feedback.
Planned specs:
24S Lifepo4 105ah = around 8kwh
2000w charging and discharging
Any potential mistakes or tips are welcome. Electro technique is my hobby but I'm not very experienced.

 

[Diver300]

Make sure you know what is isolated, what is grounded and what is connected to the mains.

The three active terminals of a digital potentiometer (CW, CCW and wiper) generally have to be at voltages inside the power supply range, but there certainly can't be big voltages between the power supply and those terminals.

If you need to isolate, perhaps using opto-isolators or transformers, make sure that your insulation and gaps are all rated for mains use. You may need to cut slots in your circuit board / veroboard and make sure there are big gaps with no conductive material.

Even connecting power to the home power meter could have issues. I have heard of someone reading a home power meter by getting an Arduino or something to count the pulses from a photocell stuck in front of the flashing light. I consider that a way of getting the information safely.

 

[rjenkinsgb]

A mains-synchronised inverter is not a DIY project, by any means. The slightest error in synchronisation and things get very dangerous and expensive!!

A safer (but still dangerous) and more practical way I can think of achieving something like that effect is to charge the battery pack from normal AC power, but to feed the battery DC back across the existing solar inverter input (where the solar panel array connect) via a switched-mode system, with voltage control to balance the proportion between that and the solar panel array.

It would require two external power diodes, one from the solar panels and one from the inverter, in line with the respective (eg.) positive from each source, to the inverter DC input.

Increasing the voltage slightly will allow the inverter to draw more current, with the solar panels still providing almost the same power.


Best still - you can get solar inverters with the built-in facility to either run from, or run from and charge, a battery bank. Your 24 cells works out at around the same range as a 72V lead acid system.


I strongly advise that you never disconnect the current sensors that are part of any PSU or inverter, they are also part of the safety system and an external device response would be too slow.
Some monitor current on a cycle-by-cycle basis and any delay could cause massive instability.

 

[Rory1984]

Thanks for your reply. The inverter that I'm planning to use has an optional wired hall sensor input. It can be used with our without this the sensor. Because I'm planning to put this system in my garage away from my house. (I don't want a high power hobby project indoor for safety and insurance) and because I need the power consumption of 3 phases and not one. My plan is to try to simulate an analog signal with my Arduino that is recognized by the inverter as an power input. In short: serial realtime power consumption from home meter to Arduino. Arduino proces the data and outputs a "hall" signal that is recognized by the inverter as an power (limiter) input and inverter only gives limited power. I'm not opening the inverter. Only connect simulated power limiter input.

 

[schmitt trigger]

If I understand your project CORRECTLY….you want to measure 3 phase power, then you require 3 independent voltage and current channels.

Then you require to calculate each phase’s true power, which means calculating the power factor. I ignore whether the Arduino has the computational capabilities to perform all these calculations for the three phases simultaneously.

If not, Analog devices makes ICs that will do that. The output is a variable frequency pulse train, which may be easily counted by the Arduino and converted to DC from a filtered PWM output.

 

[Rory1984]

Hello Schmitt trigger,
My home power meter installed by the energy company is measuring the total power and this data is available via a serial data connection. The Arduino only needs to simulate a signal to the inverter like the hall sensor would do.

 

[Diver300]

The Hall sensor connected to the inverter measures current. Something else measures the voltage. If you produce a current that is in-phase with the voltage, and pass that current through the Hall sensor, that will be seen by the inverter as the mains current, and therefore mains power.

You could create a current from a very low voltage, so little power would be needed. You can also run several turns of wire so that less current is needed.

Putting wires through a current transformer is usually safe as the current transformer isolates the primary from the secondary.