glenn-matthiesen avatar image
glenn-matthiesen asked

Balanced Parallel Charging

Hi Margreet,

Thank you for this excellent resource. An essential reference for anyone planning a renewable energy system.

I noticed in Section 3.3 Parallel Battery Wiring, page 19, the diagrams of "correct" parallel wiring, appear not to be the best option.

IOTA Engineering published an article on the subject of balanced charging.

Here is an excerpt;

Also of note, with regard to the installation of an RCD(s) within a system. No mention is made of the NEC Code requirement that an RCD must be installed between a PV array and the MPPT.

This information should be included within 4.10 Solar panels and page 56, Where to mount a RCD.

Also, on page 61, the statement;

Isolation of MPPT solar chargers

No isolation between PV input and DC output

Basic isolation between input/output and chassis.

This statement is rather brief and unclear to me. Can you elaborate?

Best regards,


battery charging
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1 Answer
ben avatar image
ben answered ·

Regarding wiring, actually the diagram in 3.3 on the left is better than either of the IOTA suggestions, although IOTA's first suggestion is the same as the center diagram.

In the real world, other practical considerations come into play and can affect the results: chemistries' fundamental Ri varies, cells vary, fuses matter, and so on. Neither the Victron book nor the IOTA document capture all the nuance; one could write a whole text just on this topic.

Regarding NEC specifications, this is a great tip, but please keep in mind that:

a) NEC is a USA spec but this document is written by a European company for a global audience,
b) even within the USA, NEC does not govern for non-residential installs, like boats or RVs

It's probably impractical for Victron to devote the space or wherewithal to incorporate all the constantly-evolving worldwide codes and specs for various electrical installations in this overview document.

That said, an RCD might be a great choice in some installations, so thanks for mentioning it.

Regarding isolation on MPPT chargers, could you please post this question separately and we will take a crack at answering it?

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