backchannel: 1/3rd rule for sizing DC DC chargers
the matter at hand
In a discussion about sizing a DC-DC charger to an alternator one clueful commenter mentioned “the 1/3rd” rule. IOW, for a 100A alt it would be reasonable to run a ≤33A DC-DC.
Another commenter1 replied:
The “1/3 rule” is not a Rule. The recommendation by all the marine ABYC certified experts is “No more that 1/2 the rated alternator output” for the size of the DC2DC charger.
factors to consider
[Note: at this writing I have not found the ABYC recommendation that matches that description. If I do find it I will update and quote it here.]
marine vs. road vehicles
It is common for marine alternator-based charging systems to run the alternator right at the edge of max for present conditions. This is done with an external regulator that can safely derate output based on current, voltage, engine/alt RMP, alternator temperature observations,
Some house bank charging is expected for boats, especially with cruisers. This is rarely the case for road vehicles.
DC-DC chargers can pull more than their rated output
If we used the 1/2 rule to put a 50A DC-DC on a 100A alternator it is extremely likely that the 50A charger will pull >50A. See the sizing link above for details.
the audience
The real problem isn’t technical, it’s the perils of providing advice in public when we don’t know the competence of OP or onlookers.
my approach
I use two rules of thumb when speaking to an unknown audience:
- 1/3rd rule if the counterparty seems to have at least a minimal clue (e.g. can be trusted not to idle on a black parking lot in 110°F ambients to charge a 400Ah bank for hours)
- 1/4 rule if the counterparty appears lost or cluless. Even at idle it’s unlikely they could fsck up using 25% of the alt’s rated capacity.
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a reliably abrasive fellow ↩