How should you connect a 12V load battery to charge a 74V solar battery?

To appropriately connect a 12V load battery for charging from a 74V solar battery system, using three batteries in parallel or three batteries in series is the correct approach.

Connecting batteries in series increases the voltage while maintaining the same capacity (amp-hour rating). If you take three 12V batteries in series, you get a total voltage of 36V, which is still not enough to charge a 74V battery. However, in the context of parallel configurations, you can connect multiple instances of this setup to build a higher capacity.

By utilizing three batteries in parallel, you can create a broader storage system that can accommodate greater amp-hour ratings, allowing for a more substantial load or longer use. However, since the voltage will not increase in this arrangement, connecting a single bank of three batteries in series becomes necessary to ensure you're working within the capacity required.

The potential combination of configurations might lead to complexities in ensuring compatibility with a 74V system and achieving maximum efficiency. Thus, while it's essential first to think about voltage matching and capacity needs, the outlined options provide a clear understanding of how to tie into a higher voltage system effectively.