Using MPPT GT inverter with a battery

In preparations for installing PV system sometime next spring I have been looking at various options. One has been runing a GTI directly from 4S3P 40Ah LifePO4 bank. While testing this needed to understand the MPPT algorithm and have ability to tweak a GTI for battery use as usually those are optimized for PV voltages.

Today connected an ACS578 (current sensor) to an Arduino test board and was somewhat surprised of the logging data of running a GTI from a battery bank. Looks like the MPPT algorith is constantly hunting for best output as it should but did not realize how big peaks that generates to the battery.

The attached logfile is 250ms interval logging showing how the battery voltage and current is going up and down. Even though my Solar30 shows a steady 12A load in real life the current is changing between 2-30A. No wonder that the battery terminals get a bit warm. I also destroyed one 3P lifepo4 pack before now realized this, initially was planning to run 8S.

Any similar or different experiences from logging the battery output current when using a GTI ?



JBecker's picture

Re: Using MPPT GT inverter with a battery

What do you think where the MPPT should stabilize if the GTI is connected to a battery (a source with very low impedance)?

One possibility would be that it stops increasing its input power at maximum input current or power threshold (if there is one). Or it stops at minimum input voltage. Or it resets the MPPT at minimum input voltage. Or ....

Can you give information about the type of GTI you use?

BR, Jörg.

Petrik's picture

Re: Using MPPT GT inverter with a battery

Good comments...

The GTI is Green Power 500W, 14-28V that is what I settled for trialling. Tried also some others like 350W 10-30V and 500W 24-52V. Tried also connecting 5S3P lifepo4 but then the amp goes up to 30+ amps (average), way too much for the battery pack so settled for 4S3P Lifepo4, 36Ah.

With 12V battery (4S3P 13.8V Lifepo4) the 14-28V does not even start. So first modification is to lower the threshold of voltage divider R1/R2. The resistor values of R1/R12 normally outputs 1/10th of the actual volts so its easy to modify the minimum voltage threshold. I assume this also reduces the maximum voltage threshold. 

So to get the output to a decent working working I needed to modify the reference that gives OPamp outputting voltage to PIC for the duty cycle. So this reference voltage adjustment makes it possible to set a desired average output power (amps drawn from battery). Now the GTI keeps the average output (12-13A) from 13V to 11V. It takes several minutes to ramp/learn with the MPPT to the maximum output.

The new thing to me was that even amps are at average on e.g. level 12A the input amps vary between 2-30A. While writing this message came to my mind that as this happens in a relatively fast cycle or so its also possible that the GTI just takes the input peak power varying the input need with the AC sine curve. If this is the case a cap in input could help the situation ??

Dont know if this cycling happens also with PV cells or with other type of MPPT inverters so would be curious to hear more - either measurement data or alternatively speculation ??

JBecker's picture

Re: Using MPPT GT inverter with a battery

There is an alternative firmware (as C source) for Chinese GTIs like the PowerJack series and similar ones. Most of these use an ATmega88 and in most cases the pcb has the holes for mounting an ISP header (6 pins one row). 

With this alternative firmware it would be possible to modify the behavior of the GTI. Changing the voltage divider to decrease minimum input voltage is not always helpful, because the fixed transformer winding ratio will then give an output voltage that is too low for mains feeding. At least the output current will no longer be sinusoidal.

Using an MPPT with battery feeding is also not a good idea. Due to the low impedance of the battery the MPPT will always go to the lowest battery voltage as this will give the highest input power.

With the alternative firmware you could disable the MPPT and just set a maximum (and/or minimum) input power. 


Petrik's picture

Re: Using MPPT GT inverter with a battery

Excellent, thank you. I was thinking its atmega. Now just need to find the alternate neue fw - any hints ?

(auch auf lensun)

JBecker's picture

Re: Using MPPT GT inverter with a battery

There is a thread about this firmware here (in german):

My GTI is also from lensun, it is a 28-52V 350W, the attached file is for this model. Defines for other models are possibly not correct (I only have this one)!

Seems I am not allowed to make an attachment ???????????? Please send your email address.



Robert Wall's picture

Re: Using MPPT GT inverter with a battery

Seems I am not allowed to make an attachment ???????????? Please send your email address.

I am - so it looks like a permissions problem. I'll tell Trystan.

Petrik's picture

Re: Using MPPT GT inverter with a battery

Yes, Jörg is correct - after a bit of testing agree its much better to control the maximum current as that seems to be possible by todays technology ;-) Many thanks for bringing this information available !!!  Really good stuff, also when reading the code noticed about the ability to define Dumpload voltage to start diverting excess power to other sources !!!

stuart's picture

Re: Using MPPT GT inverter with a battery

Petrik - how are you charging the battery to power the inverter ?

Did you find the source code for the MPPT controller ?

Petrik's picture

Re: Using MPPT GT inverter with a battery

The solar cell is connected by using a standard 30A solar controller which has adjustable voltage limits for charger. Between the controller and lifepo4 battery there is a BMS which controls the max current and balances the battery. The Solar controller takes care of minimum and maximum voltage which is handy. Basically its wired like this: PV or charger -> Solar controller -> Battery management system -> Lifepo4 battery

Yes, on the german board that Jörg referred there is indeed a source code for a Grid Tie Inverter firmware. What I understand is that the GTI hardware design is very similar between different brands. E.g. I got the generic firmware v1.00 working straight out and just needed to adjust some voltages as my is Green Power brand even though original sourcecode was for Power Jack brand. Looks like there is somekind of reference design for GTI:s. The firmware v1.04 did not work with a couple of tries so left that there.


A lot of information on the German board, e.g. including how to wire up an LCD to the GTI to see the voltages and currents on line on the LCD. Based on what I read about connecting the LCD to the GTI, it should be fairly easy to hook up the RF12 board to the SPI port of the Grid tie controller to hook it up wirelessly with emoncms.  To do that I would appreciate some support to understand how to implement the RF12 board in native Atmega8 mode to interact with emoncms (without jeelabs library) and without an interrupt port connected. Any hints for this ???



Petrik's picture

Re: Using MPPT GT inverter with a battery


Thanks to Jörg & Co. I have now been able to modify the GTI for battery usage by modifying the MPPT algorithm for constant current with power limiter. Looks like the GTI has a reference design with port addresses and just a modification of some voltages are needed. The processor is Atmega8 so anyone involved with emon project should be able to do the needed tweaking.

Attached is the code that is publicly avail on german board referred earlier. This code is now modified for my own purpose of using 7xLifePO4, 36Ah outputting max 10A / 200W constantly.

Btw - .c files are disabled to be attached ?



Petrik's picture

Re: Using MPPT GT inverter with a battery

The lessons learned part continue.... Have had some problems as the battery terminals seem be heating up, battery itself is not even warm. Today relized that the copper joints between batteries was a bit dark on the surface and some battery terminals needed a spacer to really tighten the bolts. Simple thing that i had forgotten is that all the connecting surfaces must be extra clean for this high currents. Now the 7S3P lfepo4 36Ah battery terminals are running about the same temperature as the 500W GTI (with firmware limiting the output to 300W / 16A max as a precaution).

fraggle42's picture

Re: Using MPPT GT inverter with a battery

Hi guys, sorry for digging up and old thread by the information in this one is excellent so far.

(my German is zero so the linked forum, which sounds perfect, is of limited use (via auto translate))

I have two little 500W GTIs that use a PIC controller - they no use basically as the MPPT says 26V DC from my 5KWh battery pack is too low to start.

So I need to buy something that will work.

The GTIs that use the ATMegas that allow us to adapt the firmware to our our use are perfect, but I know that PowerJack make their GTIs under a few brand names, and also update them every year.

I found Powerjacks blog here

With this being an entry about their 2013 600W GTI

A couple of things they mention in that post:-

10.    Signal manage chip : the old version adopt general signal manage chip , and new version adopt 2 signal manage chip from Microchip
11.    New version adopt import manage power IC ,CPU , Microchip from USA

I've not really looked at PICs or AT Megas (Windows developer myself), but from reading a bit about PICs it's very difficult to extract the code from a PIC.

i see that Microchip have produced arduino clones, which would be easy to program, but I imagine that again, they're a a pain to extract the code from.

Anyone done any work on these 2013 GTIs?


OR does anyone have the exact model numbers of GTIs that use the AT mega CPUs that the above projects code is written for / works with?





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