I still remember the first time a customer called me panicked because his “24V system” was reading 12V on every outlet. He had wired his two 12V lithium batteries in parallel instead of series, thinking more batteries automatically meant more voltage. It doesn’t, and the difference between these two wiring methods trips up more first-time builders than almost anything else in a lithium conversion.
The quick version
- Series wiring (positive to negative between batteries) adds voltages together and keeps amp hour capacity the same.
- Parallel wiring (positive to positive, negative to negative) adds amp hour capacity together and keeps voltage the same.
- Most RV and van 12V systems use parallel wiring because appliances, inverters, and chargers are built for 12V.
- Series wiring to 24V or 48V is common in larger boats, off-grid cabins, or systems using a 48V inverter for efficiency.
- Never mix battery brands, ages, or capacities in the same bank, in series or parallel.
What series wiring actually does
In a series connection, you run a jumper cable from the positive terminal of one battery to the negative terminal of the next. The two end terminals that are left open become your new positive and negative for the bank.
Connect two 12V, 100Ah LiFePO4 batteries in series and you get 24V at 100Ah. The voltage doubles, but the amp hour rating stays exactly where it was on a single battery. Total watt hours are the same either way, roughly 1,280Wh for a pair of 100Ah 12V batteries, series or parallel doesn’t change the energy stored, only how it’s delivered.
This matters if you’re running a system designed around a 24V or 48V inverter, which is common on boats over 40 feet or larger off-grid setups where lower current means you can use thinner, cheaper cable for the same power delivery.
What parallel wiring actually does
Parallel is the flip side. You connect positive to positive and negative to negative across all batteries, then pull your system positive and negative from a common point, ideally a busbar rather than directly off the battery terminals.
Two 12V, 100Ah batteries in parallel give you 12V at 200Ah. Voltage stays the same, capacity doubles. This is by far the more common setup in vans and truck campers because nearly every 12V accessory on the market, your Victron or Renogy solar controller, your fridge, your water pump, is designed to run on 12V without any conversion.
Tip: If you’re not sure which your rig needs, check your inverter’s input voltage rating first. A 12V inverter means you’re building a parallel bank. A 24V or 48V inverter means series, or series-parallel if you need more than one battery’s worth of capacity at that higher voltage.
A real scenario: sizing a cargo van bank
Say you’re converting a cargo van with a 3,000W inverter running a residential fridge, an induction cooktop, and occasional power tools. At 12V, that inverter can draw up to 250 amps at full load, which is a lot of current for 12V cable and lugs to handle safely.
Some builders solve this by going to a 24V battery bank (two batteries in series) paired with a 24V-input inverter. Same 3,000W output, but now you’re only pulling around 125 amps, which lets you use smaller cable and lighter fusing. It’s a legitimate strategy, but it complicates every other component: your DC-DC charger, solar controller, and monitoring shunt all need to match that 24V system too.
For most van and truck camper builds under 400Ah total, sticking with a straightforward 12V parallel bank is simpler to troubleshoot and keeps your parts compatible with the wider aftermarket. I’d only push someone toward series wiring if their inverter load genuinely demands it.
The mistake I see constantly
Mixing batteries of different ages or capacities in the same bank, series or parallel, is the single most common wiring mistake I run into. Someone adds a third 100Ah battery to an existing pair of 2-year-old units, or pairs a Battle Born with an off-brand battery because it was cheaper.
In a parallel bank, the weaker or older battery will tend to take on more of the charge and discharge cycling because its internal resistance differs slightly from the newer cells, and over time that imbalance gets worse, not better. In series, it’s even less forgiving. Each battery’s BMS manages its own cells independently, so a mismatched battery in series can trigger low-voltage cutoff on one unit while the others still have charge left, and that shuts down your entire string.
Buy batteries for a bank in a matched set, at the same time, from the same manufacturer. If you need to expand later, buy a full matched additional bank rather than one odd battery.
Fusing and cable sizing differ by configuration
Every battery in a parallel bank needs its own fuse between the battery terminal and the interconnect, sized to the battery’s maximum continuous discharge rating, not just the bank total. This protects against a fault in one battery pulling excessive current through the others. For details on picking the right fuse value, see our guide on fusing a lithium battery correctly.
Cable size follows total current, not per-battery current, on the main output run, but interconnects between batteries in parallel should be identical length and gauge to keep current sharing even. If one interconnect is a foot longer than another, that battery sees slightly more resistance and does less work, which over hundreds of cycles shows up as an unbalanced bank. Our battery cable sizing chart walks through picking gauge based on amperage and run length so you’re not guessing.
For series banks, the interconnect between the two batteries only carries the same current as the rest of the circuit, so it can typically match your main cable gauge rather than needing to be oversized.
Where the wiring lands: busbars, not battery terminals
Whichever configuration you choose, avoid running your main positive and negative cables directly off a battery terminal that also has interconnects attached. Stacking too many lugs on one terminal is a common way to loosen connections and create resistance hot spots.
Instead, bring your bank’s positive and negative out to dedicated busbars, then run your loads, charge sources, and monitoring shunt off those busbars. It keeps every connection accessible, makes future troubleshooting far easier, and it’s how ABYC-compliant marine installs are typically done. Our busbar wiring guide covers layout and sizing in more depth, and the ABYC standards are worth a look if you want the formal reference for marine and RV DC wiring practices.
Which one should you actually pick
If you’re building a typical van, truck camper, or trailer under 24V, run parallel. It’s simpler, matches nearly every off-the-shelf component, and it’s the configuration most lithium battery manufacturers, including Battle Born and Ampere Time, design their BMS units to handle without extra coordination between units.
Go series, or series-parallel, only when your inverter load or system voltage genuinely calls for it, usually above 3,000-4,000W continuous, or when you’re matching an existing 24V or 48V boat system. When in doubt, start with the pillar guide on installing a lithium battery bank safely, which covers the full install sequence these wiring decisions feed into.
Whichever way you wire it, take ten extra minutes to double check polarity with a multimeter before you connect the final cable. A reversed connection in a series string, even briefly, can send a fault current through a BMS that it was never designed to survive, and that’s a expensive way to learn the difference between series and parallel the hard way.
