Anyone else running a 30A DC-DC charger from a 140A alternator — is it actually enough?

Finally got my van build to a point where I can start stress-testing it properly. Running a Renogy 30A DC-DC (B2B) charger off the standard 140A alternator in my Transit Custom, feeding into a 200Ah lithium (LiFePO4) battery. On paper it all looks fine, but I'm starting to wonder whether I'm leaving performance on the table.

The issue is that on longer motorway runs I'm seeing the leisure battery climb to maybe 80–85% by the time I arrive, which is decent enough, but on shorter 45-minute hops around town it barely shifts past 60%. I've read that the B2B charger pulls a steady 30A regardless of what the alternator is doing, so theoretically there shouldn't be a problem — but I've also noticed the engine feels slightly sluggish on cold starts when everything kicks in together.

Has anyone swapped up to a 40A or even 60A unit on a similar-sized alternator? I'm a bit wary of overloading the alternator, especially with the van's own electrical load on top — heated seats, headlights, the lot. Wondered if there's a rule of thumb people actually use in practice rather than just "don't exceed 50% of alternator capacity," because that would put me at 70A max and I feel like I could push it a bit further safely.

TimGreen | 847 posts | ⭐ Regular

@DustyWanderer good setup choice overall. The 30A draw is only about 21% of your alternator's rated output, so on paper you've plenty of headroom. The Transit Custom alternator does throttle back at idle and lower revs though, so if you're doing a lot of slow urban driving or idling, you might not always see full charging current in practice.

Worth keeping an eye on your alternator casing temperature on longer runs — they don't love sustained high loads in warm weather. A cheap IR thermometer sorted that concern for me.

The B2B itself will naturally taper as your leisure battery approaches full, so peak demand is fairly short-lived anyway. What capacity is your leisure battery? You mentioned 20 before cutting off — 200Ah? That'll determine how long you're actually pulling hard from the alternator.

RayWatson81 | 1,203 posts | ⭐ Regular

30A from a 140A alternator is absolutely fine — the Transit Custom's alternator isn't rated for sustained 140A in real-world conditions anyway. Factor in engine management, HVAC, lights, and you're looking at maybe 60-70A actually available. Your 30A B2B sits comfortably within that.

The smarter concern is alternator temperature on long idles or urban stop-start driving. That's where B2B chargers earn their money over split-charge relays — the 30A cap protects the alternator far better than a direct connection would.

I ran a similar setup on my van conversion before upgrading. 200Ah of Fogstar lithium charged perfectly well from motorway runs. 45-60 mins of decent driving and you're pulling meaningful charge.

If you're doing a lot of short urban hops though, that's where solar earns its keep as a complement.