Yeah tony, i can see your point, the RAPS36 should be at least able to "maintain" itself when plugged in to car, and under laod from the fridge.
I'd think though that the fridge shouldnt be drawing 5 full amps all the time though? UNless its onl super freeze constantly?
So whilst youre "losing charge" when it is running, it should again win and gain charge when the fridge compressor is idling.
I run mine at about 2 or 3c, on say a 32degree camping trip (in the shade) ... and it kicks in maybe 30% of the day ... the rest of the day its idle and not drawing. At night it barely kicks in at all. At the start of a trip the fridge will drop to bloody -5 anyway as I've put a bunch of frozen meats in!
(compressor won't even run)
My RAPS36 very rarely ever went off "fully charged" 13.7v ... even with fridge sucking 5amps, battery charged, and the 6amp charger running in float mode ... it'd never tap into the battery.
Agreed though mate, the 4amp and generally "slow" charge rate of the RAPS36 builtin charge system is less than ideal. Perhaps try and chase the issue with Waeco? They may say what you're doing is not what RAPS36 is intended for, i.e paralell and long term inline use.
I have the RAPS36 so I can run independantly of the car altogether (and take charge from 240volt which is intermittently available)
If i was running solely off a car, I'd probably not have a RAPS at all ... and go dual batts.
Bottom line : the charging system builtin to the RAPS36 is not great other than a bit of topup. I'd think of the builtin charger as non existant, and address your charging considerations in lieu of that. I'm sure it cant be too hard to find a decent 6amp charger, that can run off 12v input ... so you can get proper oompf from your alternator & car batteries, into the RAPS. (but again, if you already have dual batts ... id remove the RAPS from equation altogether.).
my 6amp pulse & float charger (AGM/Gel compatible) cost about 70 bucks from supercheat auto ... cheapy, but works, and won't screw the battery, and will charge faster than I can draw.
12 Volt Battery Chargers
some of this might help?
Thjis place has cool stuff : The 12 Volt Shop
Im no sparky!