DC Solar, DC Battery and String Wiring

Introduction

The DC side of a PV system is where the raw solar energy lives — and where many mistakes happen. Understanding how strings are wired, how series vs parallel changes voltage and current, and how DC batteries connect is essential for a safe, efficient installation.

What Is a String?

A string is a series connection of multiple panels. Panels in a string are connected positive-to-negative, one after another. Series wiring adds up the voltage while the current stays the same as a single panel.

  Panel 1     Panel 2     Panel 3
  (+)--[+]--(-)  (+)--[+]--(-)  (+)--[+]--(-)
          \_________/     \_________/
      Series: voltages add, current the same
  e.g. 3 x 400 W panels (Vmp 37V, Imp 10.8A)
  = 111 V string, 10.8 A

Series vs Parallel

The choice of wiring determines your DC system's characteristics.

Series (string)

• Voltages add, current unchanged
• Raises voltage — good for reducing cable losses and matching inverter input
• A shaded panel in the string limits the whole string's current

Parallel

• Currents add, voltage unchanged
• Used to combine strings or increase current capacity
• Requires proper fusing to protect against back-feed

Series (voltages add):   [+p1-][+p2-][+p3-]  -> V = V1+V2+V3, I = same
Parallel (currents add): [   p1   ]  \
                         [   p2   ]  |-> I = I1+I2, V = same
                         [   p3   ]  /

Most residential systems use series wiring within a string, then combine multiple strings in parallel at the combiner box.

The DC Combiner Box

When multiple strings meet, a DC combiner box safely joins them. It provides:

• Fusing for each string (protects against overcurrent and back-feed)
• A common positive and negative bus
• Isolation points for maintenance

   String A ---[fuse]--+
   String B ---[fuse]--+--> +DC bus
   String C ---[fuse]--+
                              |
                          to inverter

DC Battery Connections

In battery systems, the battery bank connects to the DC side, typically through a charge controller or a hybrid inverter's DC input. Correct wiring matters:

• Match battery bank voltage to the system design (e.g., 48 V bank)
• Use proper cable sizing for the DC currents involved
• Include a disconnect so the battery can be isolated safely
• Watch polarity — a reversed battery connection is dangerous

   PV strings --> [MPPT/Charge controller] --> Battery bank
                                                 |
                                        [Battery disconnect]

Cable Sizing

DC cable sizing is a real safety factor:

• Undersized cables overheat and can start fires
• Use the panel's Isc and string current to size conductors
• Account for voltage drop over distance
• Use PV-rated, UV-resistant cables on the roof (pairs of red/black)

Fuses and Overcurrent Protection

Each string's fuse must be sized to protect the cable and the panel's maximum series fuse rating. Too big a fuse fails to protect; too small a fuse trips unnecessarily.

Real-World Example

You have 8 panels with Voc 45 V and Isc 11.5 A, arranged as two strings of 4. Each string is 4 × 45 = 180 Voc. Both strings connect at the combiner, where each gets a fuse sized for the cable and panels. Combined current at the inverter input is handled by the inverter's DC rating.

Summary

• A string is a series connection of panels; series adds voltage, parallel adds current
• The combiner box joins strings safely with fusing
• DC batteries connect to the DC side via a charge controller or hybrid inverter
• Cable sizing and fusing are critical for safety and efficiency

Next Lesson

Why do some systems have batteries and others don't? Let's define the role of batteries in PV systems.