Safety first: home battery DC can deliver high current. Wrong cable size, mixed battery ages, reverse polarity, or live work can cause fire, arcing, or equipment damage. This page explains series vs parallel at a high level so you can plan capacity and talk to a competent person. It is not a wiring procedure for live systems.
If a single battery is short on capacity, voltage, or peak power, you can expand with more modules. Most UK residential hybrid systems stay on ~48V/51.2V and add modules in parallel. Series is mainly for higher-voltage battery architectures. For sizing context see the battery overview and DIY home battery boundary.
High-level layout (what connects to what)
panels / strings
MPPT + charger
BMS + modules
consumer unit
Concept only: protection devices, isolators, earthing, and exact cable routes are design-specific and must follow manufacturer instructions and BS 7671 work by a competent person where required.
What is DIY-adjacent vs electrician-only
| Task | Typically DIY-adjacent (planning / unpowered prep) | Electrician / competent installer territory |
|---|---|---|
| Choosing series vs parallel and target kWh | Yes – research and sizing | Final design sign-off if grid-connected |
| Mechanical rack assembly (unpowered modules) | Often yes, following manufacturer manual | If structural or fire strategy is complex |
| DC battery cables, busbars, torque, polarity | Only if competent and modules isolated | Recommended for most homeowners |
| Inverter battery port live connection | No | Yes |
| AC coupling, consumer unit, earthing, isolation | No | Yes (Part P / competent person) |
| DNO G98/G99 and export metering | You can prepare paperwork | Commissioning evidence often needs pro involvement |
How to Connect Multiple Batteries?
You can connect batteries in series or parallel, with each option offering different tradeoffs. Much like connecting solar panels, it is a matter of what you are solving for, increasing the voltage or current. With batteries, though, there are a few basics you need to keep in mind before you proceed:
- Batteries use higher currents.
- Connecting batteries with different specifications is not advisable and can even be dangerous.
| Configuration | Voltage | Max current | Amp-hours | Total power |
|---|---|---|---|---|
| One module (the baseline) | 50V | 100A | 100Ah | 5kW |
| Two in series | 100V | 100A | 100Ah | 10kW |
| Two in parallel | 50V | 200A | 200Ah | 10kW |
| Four in parallel-series | 100V | 200A | 200Ah | 20kW |
Read the bold cells and the whole decision is there. Two modules give you 10kW either way, and 10kWh of stored energy either way, because energy is voltage times amp-hours and the two arrangements just trade one for the other. What differs is what that 10kW is useful for. Series raises voltage so the same power travels as less current, which matters when the batteries sit a long cable run from the inverter or when the inverter needs a higher voltage to start. Parallel raises current and amp-hours at a fixed voltage, which is what almost every UK home wants, because it lets you add another module later without redesigning anything.
Select the Correct Cables Size
Ensure the cables leading the positive and negative pole from the battery to the inverter are equal in length and cross-section area. The same principle applies for cables connecting a battery to the next one.
Due to the high currents at which your battery operates you need to male sure you chose the cables accordingly. Special cables are required that can handle the peak discharge current of your batteries and potentially the new current after connecting more batteries.
For example, my home battery is rated at 100A and 48V. I have connected two such batteries in parallel to a 3.6kW inverter. At 48V, the inverter cannot draw more than 75A. So, I have opted for a 16mm2 (AWG 6) cables.
Connecting Batteries in Series
Connecting batteries in series increases the voltage and keeps the current constant. The voltage of the connected battery is equal to the sum of the voltage of each battery, and the current is equal to the current of the battery with the smallest current in the series. The Ah capacity of the batteries remains constant.
Take the two modules from the table. In series they read 100V and still 100A, so the pair can deliver 10kW, and the amp-hours stay at 100Ah because the same charge now sits at twice the voltage.
Understanding battery capacity is essential. While charge capacity is gauged in Ah, energy capacity is quantified in Wh . To determine the energy capacity, you multiply the voltage by the charge capacity: Energy Capacity (Wh)=Voltage (V)×Ah
The increased voltage of a series of batteries can be particularly useful when:
- Your inverter requires a voltage threshold that a single battery cannot meet.
- Your batteries are far from the inverter, and longer cables are required. Battery cables are thick and costly because they carry large currents. Increasing the voltage allows you to drop the current and thus reduce the required cross-section area of the required cables.
Connecting Batteries in Parallel
Connecting batteries in parallel increases the current and keeps the voltage constant. The current of the connected batteries is equal to the sum of the current of each battery, while the voltage remains equal to the voltage of a single battery in the parallel setup. The Ah capacity of the battery is added up.
The same two modules in parallel read 50V and 200A, and their amp-hours add to 200Ah. That is the same 10kW and the same stored energy as the series pair, reached the other way round. The practical gain is that the inverter can now draw more current at the voltage it already expects.
The increased current of a parallel setup of batteries can be useful in cases:
- When the system needs to be easily scalable, allowing for the addition of more batteries to increase capacity.
Practical Aspects of Connecting Batteries in Parallel
Some battery manufacturers offer server racks designed to fit the batteries and host additional conveniences such as busbars and safety doors.
A rack in domestic solar energy systems offers better safety for pets and young children around the batteries. Some racks come with door locks for extra safety. The busbars come with wiring utilities which is a significant improvement given that battery connectors are large and require tight fitting for safety reasons.
When connected, batteries can suffer from Imbalance in State of Charge (SOC): Over time, individual batteries in parallel might not share the charge equally, leading to some batteries being overcharged while others remain undercharged. This is a function of how the battery nearest to the consumer tends to provide a larger current than the rest of the batteries. The effect is emphasised when large currents are drawn.
One of the ways to mitigate this is to use a diagonal connection. This is where you connect one of the poles at the top of the busbar and the other pole at the bottom. This will reduce the proximity effect of the battery closest to the consumer connection, but still, over time, imbalances will appear.
To reconcile differences over time, batteries need to be charged one by one up to 100% SOC. Having the connected in parallel can be a useful way to achieve this without having to manage any wiring if the batteries have a local shut-off switch.
Connecting Batteries in a Parallel-Series
Connecting batteries in a parallel-series configuration combines the characteristics of both series and parallel configurations. This means you’ll increase both the voltage and the current.
You build it in two steps. Pair the modules in series first, which gives you two 100V sets, then wire those two sets in parallel. The result is the last row of the table: 100V, 200A, 200Ah and 20kW from four modules.
One caution the arithmetic hides. Every module in a series set carries the same current, so a weak or mismatched pack limits its whole set, and the two sets must be built identically or they will not share load evenly. That is why the matching rule at the top of this page matters more here than anywhere else.
The parallel series is a useful method where we benefit from the strengths of each of the other methods and limit their drawbacks as much as possible.
