I have a friend who recently purchased for their 24V system four 12V 200AH batteries. In theory, that should be 24V, 400AH, right? These batteries are going to replace their failed Lead-acid batteries. Can any of you recommend a meter that could be connected with a shunt to monitor the charging / discharging current and inform the user as to the current state of charge of the batteries (as i understand voltage isn't helpful with LiFePO4 batteries since their discharge voltage as pretty flat).

I looked on Lazada, and I can't tell whether there are any gauges that would meet this need. (Seems like the gauge would need to be programmed or else "learn" the full capacity of the batteries and then report the current capacity based on current in and out. 

 

Hope this all makes sense.

These type of batteries aren't suitable for purpose.

You need 3.2v cells x 8 and a 24v 8 cell BMS.