Bit of help needed with my maths

[Bonnox]

Hi guys just needing a bit of help, I'm in the process of building my off grid shed, I'm going to be using PVC panels and deep cycle batteries but currently can't afford the control unit to switch off the solar charging and not over charge the battery, and probably won't be able to till after Christmas now, so was going to use the two batteries I have and use one while one is charging and swap them around, just need the help with the maths to make sure I don't over charge them, and I'm not massively up on my technology and current conversion etc so.....

If I use for example a 75Ah battery I can use something that uses up 1 amp per hour for 75 hours,
And if the solar panel puts out/charges at 2Ah then it would take 37.5 hours to charge from flat till full, am I right in this ?

Many thanks for your guys help