1998 Isuzu Bighorn Diesel. The car just won't start. My local dealer can start it using a cold start spray. But he has not been able to solve the problem. Please could somebody help

I have a 93 bighorn and once in a while the battery is dead and won't start. I carry around a charger and a set of jumper cables. It has done it since I got it two years ago and the mechanic cannot find the problem. I have had the alternator rebuilt and put in a new super heavy duty battery when I got it. It doesn't happen all the time but I try not to use anything electrical unless I absolutely have to. I never use the heated seats. I am so frustrated. It may only happen once a month or two or 4 and then happen every second day for two weeks. My son had a Bighorn a year older and it did exactly the same thing. I have a 93 bighorn and once in a while the battery is dead and won't start. I carry around a charger and a set of jumper cables. It has done it since I got it two years ago and the mechanic cannot find the problem. I have had the alternator rebuilt and put in a new super heavy duty battery when I got it. It doesn't happen all the time but I try not to use anything electrical unless I absolutely have to. I never use the heated seats. I am so frustrated. It may only happen once a month or two or 4 and then happen every second day for two weeks. My son had a Bighorn a year older and it did exactly the same thing.

get a bigger AMP HR. battery , CCA.?
diesels suck big amps from starter and glow plugs. if starter sounds slow, it is , fix that first. get a bigger AMP HR. battery , CCA.? diesels suck big amps from starter and glow plugs. if starter sounds slow, it is , fix that first.