It depends on the battery, with a flooded battery slight over charging does not matter, and I would charge for 1 day a month, and top up after charging, with a sealed lead acid or a AGM/VRLA then over charging is more of a problem.
If you look on Halfords web site you will see for that type of charger
Not suitable for vehicles with START/STOP technology, or for long-term connection.
Lidi and Aldi do from time to time do chargers like this
Ctek do a range but the price is a lot higher, seem to remember last one I got from Lidi was around £15 the Ctek MXS 3.8 is just under £40 from internet, looking at £44 for the Ring 6V/12V Smart Car Battery charger from Halfords.
How long a battery needs charging varies, and I have used one of these
to monitor the way the charger works, and putting the batteries off caravan on charge at just 0.8 amp using the under 12 Ah setting, and it has dropped to the 0.1 amp setting in well under an hour. Typical graph for a discharged battery
the 3.8 amp charge rate is very short lived, the 3 amp is not much longer, but the 0.8 amp can last for days, before it drops to 0.1 amp, this is because the lead acid battery simply can't recharge fast, it takes time to absorb the charge, before the smart charger I would use a fixed voltage power supply at around 13.2 volt.
A typical car charges at 13.8 volt, but the rate is higher to allow for the short charge time, so fit and forget looking at 13.2 to 13.4 volt, a standard battery charger is around 15 volt open circuit voltage and the old dynamo with a RB106 or RB108 regulator was 16 volt open circuit, but when we used dynamos we had batteries we could top up.
I came to charge some stair lift batteries which had been left flat for some time, I had to put it in parallel with a good battery to get the charger to work, and I left them to see if they would charge, after around 12 days it was as if I had flicked a switch, the battery started to take charge and fully recharged, I have repeated this now a few times, sulphated plates take time to recover, this includes part sulphated, time on charge matters.
If I talk about the Lidi charger, the rest do some thing similar, I have two selections, under 12 Ah and over 12 Ah (plus 6 volt but that does not interest you) in under 12 Ah it will charge at 0.8 amp until volts hit 14.4 volt then it will drop to 0.1 amp and when at that charge rate it hits 14.4 volt it switches off, and it will stay off until the volts drop to 12.8 volt. In the over 12 Ah mode it starts at 3.8 amp, and steps down to 3 amp and 0.8 amp and finally when again it hits 14.4 volt drops to 0.1 amp and when the voltage drops to 12.8 volt will return to 0.8 amp charge rate.
So if I monitor it the voltage seems to remain at 12.9 volt 98% of the time, when it turns back to 0.8 amp charge rate it only stays at that rate for a few minutes, and if I did not have a monitor I would not know it had gone to the 0.8 amp rate, even with a large 90 Ah battery in good condition it only returns to 0.8 amp for around 5 minutes.
The Lidi charger will only return to 0.8 amp, and will not restart charging after a power cut until you press a button, this means if a cell should go short circuit, then only 0.8 amp so lot explosion risk, the Ctek will return to the 3.8 amp output, and does not need you to press a button to start it charging, so is liked by caravanner as will work as stand-by charger if built in one fails, but should a cell go short circuit then more risk is involved, this
was the result when one cell when short circuit with a pair of mobility scooter batteries 35 Ah AGM with a 3.5 amp charger, they were left on charge in the garage as done many times before, and normally the stage charger would have switched off once charged, but shorted cell resulted in it did not switch off, this is why I do not use the caravans built in charger when it is stored, can you think what a 35 amp charger would do? Cells don't often go short circuit, and the smell soon tells you when they do, bad eggs smell, so when your around the battery you will know, but if not visited you don't know, and you can see the result.
Warning hydrogen burns with a blue flame very rapid, and if a spark does ignite it, it will expand faster than battery vents can handle, so the casing fails shooting bits of plastic and acid out, i.e. it explodes, wear glasses, don't smoke, and have water available, and don't work on your own. Some one pouring a bucket of water over you could save your eyes, so always have some one else around when working on batteries.
