Battery connections

A single lead-acid cell ≈ 2 V — too low for most loads. Manufacturers weld six cells for 12 V; data centers stack 24 V and 48 V racks. When you field-assemble strings from blocks or 2 V cells, how you jumper them sets system voltage and capacity.

Two numbers to track:

Connections change V, Ah, or both — never confuse them.

Series: positive of one battery → negative of next; load across total string ends.

Example: four 12 V / 60 Ah blocks in series:

• Voltage: 12 + 12 + 12 + 12 = 48 V
• Capacity: still 60 Ah (same plate area per string path)

Think of series as stacking cells inside one big battery — more pressure, same tank size.

Critical rules:

• All units should be same Ah and same state of charge when building a new string.
• One weak cell in series limits the whole string under load.
• Fuse and wire for total voltage, not single-block voltage.

Parallel: all positives together, all negatives together; load across common rails.

Same four 12 V / 60 Ah blocks in parallel:

• Voltage: 12 V (unchanged)
• Capacity: 60 + 60 + 60 + 60 = 240 Ah

Think of parallel as widening the plate area — same pressure, bigger tank.

Never parallel batteries of different nominal voltages. The higher-voltage unit forces current into the lower — overheating, venting, explosion risk.

Even same voltage: avoid paralleling old and new or different chemistries without engineering review — unequal internal resistance causes circulating current.

Split blocks into matched groups, series each group for target voltage, then parallel the groups for capacity.

Example: four 12 V / 60 Ah → two strings of two:

• Each string: 12 + 12 = 24 V / 60 Ah
• Parallel the two strings: 24 V / 120 Ah

Solar and UPS designs often use series strings to reach charge voltage, then parallel strings for current. Blocking diodes or fuses per string may be required so a failed string does not back-feed others — follow one-line diagram.

Starting point: four identical 12 V, 60 Ah blocks.

Math check: total stored energy (idealized) scales with V × Ah. Series quadruples V; parallel quadruples Ah; 2S2P doubles both → 4× energy vs one block.

• Equal-length parallel cables where possible so each string shares load.
• Torque terminals to spec; use proper lugs — high Ah strings need fat copper.
• After assembly, verify polarity before first close — reverse on series strings can damage chargers and loads.
• Monitor mid-string voltage during load on long series packs to catch weak cells early.
• Label the bank: total V, Ah, date, chemistry.

For lead-acid, recharge the whole bank with a charger rated for total voltage and bank Ah, not one block at a time unless intentionally isolated.

Photovoltaic strings follow the same rules: cells in series raise voltage to charge a 12 V battery (~14 V needed); parallel strings raise charge current. A 0.5 V silicon cell needs many in series; duplicate strings in parallel hit amp target — same logic as battery banks (detailed in the next lesson on other sources).

Situation. Technician parallels a new 12 V AGM with three-year-old floor-scrubber batteries “to get through the week.” Within hours the new battery is hot and bulging.

Cause. Lower internal resistance on the new pack accepted charge current from the older paralleled units and acted as a load for the weak ones — uncontrolled circulating current.

Applied lesson. Parallel only matched age, chemistry, and state of charge. Replace the set or isolate strings with fuses — don’t mix fresh with tired.