Tl;dr: Use this trick if you need to save space and don't need more than one decimal precision. Save >60% more logs using the same EEPROM.

Intro

Okay, using a 512Kbit EEPROM isn't exactly the reason to save space. After all it stores 3120 data entries which consist of timestamp and 4 float values. Reminder: the internal EEPROM of the Atmega328P is 1024 bytes only. Using that, the following might come in handy.

But can we get more out of it? Yep:

It's a small trick which works if we need only precision to one decimal e.g. 25.3°C or 67.9% RH.

Floats use 4 bytes (but are much more precise of course), integers do need 2 bytes in EEPROM.

Let's assume we just shift the decimal point of 25.3(°C) to the right. We get 253, which is easily stored as an integer. Same with negative values. I use the following function to store my floats as integers (and convert them back to float):

#define floatToInt(x) (int)(x*10)
#define intToFloat(x) (float)(x/10.0)

 Here's my data structure I use to save log entries:

typedef struct
{
  uint8_t tag;
  uint8_t monat;
  uint8_t jahr;

  uint8_t stunde;
  uint8_t minuten;

  int temp1;
  int temp2;
  int temp3;
  uint16_t feucht1;
} logEintrag;

logEintrag LOG;

 When I get my values from the sensors I just write them to the struct like this:

LOG.temp1 = floatToInt(25.3);

Of course 25.3 is some variable I access directly.

When reading back the logs to write them to the SD as a CSV string I j´convert them back on the fly:

intToFloat(readLOG.temp1)

 Works like a charm, with negative values as well. It takes a few processor cycles more but now the EEPROM holds 5041 instead of 3120 logs, which is 61,5% more!

Want to save even more logs?

Other things to optimize would include:

1. saving the year-value only once and then again, when it changes
   1. savings: 1 byte
2. same with months
3. (...)

This would need more code to handle that when creating the CSV entries which are saved to SD. Doing this with year and month would get you close to 6000 entries or over two months of data in EEPROM (and thus not switching on the power hungry SD in two months).