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Hi, and sorry for so many words...

Wanted some opinions on alternator loads when DC-DC charging a lithium battery at a high current...

Wafty Mad Plan: I'm getting a 100ah Lithium battery for my van. One of its uses is possibly powering an infra-red heat lamp (~1000W, pub-garden style), or electric heater (12v 50A cab heater) for short periods of time. I go in the sea a lot and crave a short blast of heat (10 minutes or so), to warm up when changing out of a wetsuit on a cold day, or at 6am when I need a reason to get out of bed. Doing the math and hooking up a delay timer to a relay, I could get ~6-10 10-minute blasts from about 70% of the battery capacity with these heaters. This is ample for a day's activities.
Problem is, I'd need a hefty DC-DC charger to get this back the next day. I never stay in one place for more than a night, and will always drive about 1-2 hours a day on such trips. A 50A or 60A charger could get me charged back up in just over an hour of driving. Great! Except 60A, plus potentially hi-beams & stereo on pushes this up to a 100A requirement from the 150A alternator (maybe more?).

Is this pushing it a bit hard?

I'd be doing this two weekends a month. Some mention a strain on the engine, but is that not about 1.4kW drawn from an engine that has 80kW max output, maybe uses 20kW cruising (vaguely based on eNV-200 datasheet), so the alternator is adding ~7% here??. I do understand however that an alternator wouldn't like a heavy load while idling with no airflow.

Am I completely mad? Ignore my strange requirement of an electric heater - I'll get a diesel heater eventually (!). I was just curious to know if my assumptions of power draw were accurate, and if it could technically 'work' without things melting. My van is probably still at the 1425kg kerb weight after removing 5 seats and building a wooden bed and cupboard, so it is always lightly-laden, so I'm not worried about pushing the engine itself too hard. Also I'm aware of shortening the battery life from draining it this hard - but even 2000 cycles would be 10 years of doing this.

More of a thought experiment at this point.
 

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2018, Nissan, NV200, Hillside Dalbury
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Not really an answer to your question, but just interested as I'm doing something semi-similar with my NV200 conversion.

I've currently got a 70Ah AGM leisure battery that's charged by a split charger - I'm switching to a 100Ah lithium battery, and so a DC-DC charger (together with a swap of the onboard/EHU charger to one with a lithium compatible profile).

Anyway, I plumped for a CTEK D250SE as my DC-DC charger, mostly because I don't think I'll have any issues with it being mounted under the bonnet (where the leisure battery lives). That'll charge the leisure battery at 20A from the alternator, so well below what you're looking for. However, it can be paired with a CTEK Smartpass to boost that to potentially 120A, in a controlled/profiled fashion, for the leisure battery. Not a cheap combination though.
 
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