Its a Titan solar generator but I don't don't want to buy a proprietary battery from them I just want something like what big battery use to sell that will plug into the expansion port of the Titan and I would like to stick with NMC , I tried to use my 4 Lion energy batteries hooked in 2S2P set up , but when I charge the batteries up within a .5 volt of the Titan's 29.2 max voltage as recommended by point zero and let the batteries rest overnight the voltage drops way below the .5 volt window and I am afraid I will damage both .