How to save "big" data.

So I'm building this throwing app and I have option, that you can record every throws acceleration data, if you want. The record is 2 s long and every axis has a 200 points.
This is not yet a problem, the problem is how to save this data. In one game you can have 20-30 throws, so I can not store all the throws in a array. If I just push all throw data to a array, I will run out of memory. So that's not an option.

Now I'm trying to save every throw to json file and at the end of the game, the throw app will collect all the json files and write that to the one csv file.
But I will have the same problem: if I read all the json files, I need to temporarily hold the data somewhere, before I write the csv file. So thats not good option also.

Now I'm trying to out this "offset" option on the write file, maybe that is my solution. I will past my code in here. I know it doesn't work yet, because I misunderstood "offset" option in write command.
But before I continue, is this the right ( only?) solution or is there a better way?

var HZ = 100;
var SAMPLES = 2 * HZ;
var SCALE = 2000;
var accelx = new Int16Array(SAMPLES);
var accely = new Int16Array(SAMPLES);
var accelz = new Int16Array(SAMPLES);
var timestep = new Int16Array(SAMPLES);
var accelId = 0;
var throws_acc = [];



function SaveThrowJson(json_n){
  require("Storage").writeJSON("KyRec_" + (json_n + 1) + ".json", [accelx, accely, accelz, timestep]);
}

function RemoveOldJson(json_files){
  //let json_files = require("Storage").list(/^KyRec_.*.json$/);
  json_files.forEach(f_json => {
    require("Storage").erase(f_json);
  });
}

function SaveFile(){
  let storage = require("Storage");
  let csv_files_N = storage.list(/^kyAc_.*.csv$/).length;
  let json_files = storage.list(/^KyRec_.*.json$/);
  let csv = "";
  let date = new Date();
  let offset = 0;
  let offset_old = 0;
  let fn = date.getHours() + "-" + date.getMinutes() + "_" + date.getDate() + "-" + (date.getMonth() +1) + "-" + date.getFullYear().toString().substr(-2);
  fn = "kyAc_" + fn + "_" + (csv_files_N + 1) + ".csv";
  let json_data = "";
  //print(throws_acc);
  json_files.forEach(f_json => {
    json_data = storage.readJSON(f_json);
    csv = "";
    for (var i = 0; i < json_data[0].length; i++){
      csv += `${json_data[3][i]},${json_data[0][i]/SCALE},${json_data[1][i]/SCALE},${json_data[2][i]/SCALE}\n`;
      offset++;
    }
    csv += `${0},${0},${0},${0}\n`;
    offset++;
    if (offset_old == 0)
      storage.write(fn, csv);
    else
      storage.write(fn, csv, offset_old);
    print(offset_old);
    offset_old = offset;
  });
  //if (csv != "") storage.write(fn, csv);
  //    storage.write(file.name,txt,file.offset);
  //  file.offset += l;
  RemoveOldJson(json_files);
}


function recordStart() {"ram"
  console.log("start settings");
	accelId = 0;
	Bangle.accelWr(0x18, 0b01110100);
	Bangle.accelWr(0x1B, 0x03 | 0x40);
	Bangle.accelWr(0x18, 0b11110100);
	Bangle.setPollInterval(10);
  startRecord();
}

function recordStop() {"ram"
  console.log("end settings");
  Bangle.setPollInterval(80);
	Bangle.accelWr(0x18, 0b01101100);
	Bangle.accelWr(0x1B, 0x0);
	Bangle.accelWr(0x18, 0b11101100);
  if (settings.save_record) SaveFile();
  //print(accelx);
  //print(throws_acc);
}

function startRecord() {
	var stopped = false;
	var round_n = 1;
	g.clear(1);

	var start_time = getTime();
	var Throws_n = 0;
	var maxMag = 0;
	var aX = 0;
	var t = 0;
  var t_old = 0;
  var throw_time_limit = 0;
  var write_record = 0;
  var wrtie_time = getTime();
  let date = new Date();
  function add_round(){
    round_n++;
    total_throws = total_throws + Throws_n;
    Throws_n = 0;
    if (settings.save_record){
      let date = new Date();
      if (settings.save_record) SaveFile();
    }
  }
	function accelHandler(accel) {
		t = ~~(getTime() - start_time);
    aX = Math.abs(accel.x * 2);
    //console.log(aX);
    //print(getTime() - start_time);
    if (t != t_old){
      //print((t - t_old));
      t_old = t;
      render_layout();
    }
    //print(t -throw_time_limit, t, throw_time_limit);
		if (aX > settings.throw_g_lim && (t -throw_time_limit) > 3 ) {
      //console.log(aX);
      throw_time_limit = t;
			Throws_n++;
      render_layout();
      write_record = 1;
		}
    if (settings.save_record){
      if (write_record == 1 && (t -throw_time_limit) > 1) {
        SaveThrowJson(Throws_n);
        //json_n++;
        //print(Throws_n);
        write_record = 0;
        wrtie_time = getTime();
        accelx.fill(0);
        accely.fill(0);
        accelz.fill(0);
        timestep.fill(0);
      }
      accelx[accelId] = accel.x * SCALE * 2;
      accely[accelId] = accel.y * SCALE * 2;
      accelz[accelId] = accel.z * SCALE * 2;
      timestep[accelId] = (getTime() - wrtie_time)*1000;
      accelId++;
      //print(accelId);
      if (accelId == SAMPLES) accelId = 0;
    }
	}

	Bangle.on("accel", accelHandler);
}