Quote:
Originally Posted by redpoint5
I don't quite follow your question.
Let me give an anecdote though;
I went from charging my Prius at 120v (L1) and 12 amps to charging at 240v (L2) and 12 amps (same EVSE modified to accept the higher voltage). Doubling either the voltage or the amps doubles the wattage. That means it takes half as long to charge the Prius. The line losses (which are fairly negligible) remain exactly the same because I'm charging at the same amperage.
The only explanation for that resulting in cutting losses in half is that the vehicle spent half the time running pumps and fans and relays and computers during the charging process. It spent half the time charging because it charged at twice the power.
Furthermore, 100% of line losses exhibit as heat, so if there was major line loss, the cables themselves would get quite hot. If someone was charging at 10 kW, and there was 10% line loss, that would be 1,000 watts of heat generated in the cable, which is a huge amount of heat. The cable would burn up, because it wasn't designed to be used as a space heater.
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Yeah i forgot to add that the transformer and DC-DC converter that converts the 120 AC or 240 AC to the nominal DC voltage that the battery is adds to the inefficiency as well. The efficiency will be greater with the 240v charge as it is closer to the nominal battery voltage as well. This could explain the efficiency gain with the equal amperages. But typically yes less amps will be more efficient if ambient temp being too cold is not a factor. (same road block as wanting to use 48v electrical systems over 12v electrical systems in a car, but for a plug in prius i assume the nominal voltage is around the mid 200s and the optimal would be like the 800v system that they use on some hypercars)
My original comment was more about an entirely theoretical instance where if the whole system was 1000v ac and dc there would be a lot less losses from high current heating up the system for the same kW/h charging rate. Not entirely applicable.
But yeah somebody try differing your amps on your 240v charger and note the efficiency change. The longer charge should be more efficient and it is better for battery longevity. Fast charging reduces cycle lifes available a lot more.
*when i say losses and heat i mean in the whole system not just the wire. We would just measure at the panel to record the overall loss. The heat is loss in the batteries, the onboard charging system components, the wire, each connection etc...