The whole point of these relays is to charge a secondary deep cycle battery while protecting your primary battery. Not just from discharge but from damage is what i'd say.
As soon as you let an SLI battery drop below 12.7 you're entering discharge and "killing" the battery. You might well get away with 10 %DOD a thousand times yet only get away with an 80 %DOD twice.
This is all a bit worst case scenario for effect but you get the idea.
After all, 12.4v is maybe 35% DOD but that's at least 30% more than i'd be happy with.
I could shift an extra pallet of batteries a month if i got involved with cars that smart charge, BMW X5's being a favourite. (Can't shut it's moon roof and then restart FFS)
but i REALLY don't want to get involved with anything like that, makes me look bad when a good battery lasts just 2 years
Keep a good battery topped up and it'll see 6 years plus and still be a good starter
If you do intend to semi-discharge it on a regular basis then get a semi-traction battery as your primary, not massively dearer but will take 10x the abuse.
I am honestly not trying to be argumentative here, but I also don't like it when people aren't told the full implications of something.
This thing works and is fit for purpose, it will also lead to the early demise of your starter battery when used as intended for the sake of 3/10ths of a volt in it's setting.
Sorry, just hacks me off and it's been a bad week.
