Yes: you can connect lithium iron phosphate (LiFePO4) batteries in series to raise the voltage and in parallel to raise the capacity, provided the manufacturer allows it for that model. Series adds voltage and keeps the amp-hours of one battery; parallel adds amp-hours and keeps the voltage. Two 12.8 V 100 Ah batteries make a 25.6 V 100 Ah bank in series or a 12.8 V 200 Ah bank in parallel, and both store 2,560 Wh.
The wiring is simple. The problems come from batteries not rated for series use, units joined at different charge levels, mixed ages and capacities, uneven cables and missing fuses. This guide covers the rule, the numbers and the habits that keep a bank balanced.
Series adds voltage, parallel adds capacity
In series, the positive terminal of one battery connects to the negative terminal of the next. In parallel, all the positives are joined together and all the negatives are joined together. The table shows the effect on the bank.
| Property | In series (S) | In parallel (P) |
|---|---|---|
| Voltage | Adds up | Same as one battery |
| Capacity (Ah) | Same as one battery | Adds up |
| Energy (Wh) | Adds up | Adds up |
| Current | Same in every battery | Shared between batteries |
Energy in watt-hours is voltage multiplied by capacity, so both connections add up the stored energy. A “12 V” LiFePO4 battery is normally four 3.2 V cells in series, which is 12.8 V, the figure used below.
Worked examples with 12.8 V 100 Ah batteries
Each battery stores 12.8 V × 100 Ah = 1,280 Wh. In the names, the number before S is the batteries per series string and the number before P is the strings in parallel, so 2S2P is two strings of two batteries.
| Layout | Batteries | Voltage | Capacity | Energy |
|---|---|---|---|---|
| 2S | 2 | 25.6 V | 100 Ah | 2,560 Wh |
| 2P | 2 | 12.8 V | 200 Ah | 2,560 Wh |
| 4P | 4 | 12.8 V | 400 Ah | 5,120 Wh |
| 2S2P | 4 | 25.6 V | 200 Ah | 5,120 Wh |
| 4S | 4 | 51.2 V | 100 Ah | 5,120 Wh |
4P, 2S2P and 4S all store 5,120 Wh; what differs is the voltage, and with it the current. A 2,000 W load draws about 156 A at 12.8 V, 78 A at 25.6 V and 39 A at 51.2 V, before inverter losses. The guide to 12 V, 24 V and 48 V system voltage helps you choose; the guide to sizing a LiFePO4 battery bank covers how many watt-hours you need.
Check the datasheet, then match the batteries
Manufacturers publish a maximum number of units in series and in parallel, and some models are not rated for series use at all. Some allow up to four in series and four in parallel, but that is specific to the model. Each battery’s BMS (battery management system) is designed around set voltage and current values.
The datasheet wins
If the datasheet does not say that series or parallel use is allowed, ask before connecting anything. Two batteries sold as “12 V” can be stacked into 24 V only if the maker approves it.
Then match the batteries:
- Same model, same capacity, ideally the same age. Unequal internal resistance means unequal current sharing, and in series the weakest unit sets the limit.
- Same state of charge. Charge each battery fully, or bring them to the same voltage, and check with a multimeter. Batteries joined in parallel at different voltages exchange current until they equalise.
- Each battery keeps its own BMS. Never bypass it. In a series string one BMS cutting out interrupts the whole string; in a parallel bank the others must carry the share of a unit that drops out, so keep each battery’s load well inside its rating.
Why equal charge matters
Current between joined batteries is roughly the voltage difference divided by the loop resistance. As an arithmetic illustration only, 0.3 V across 0.01 Ω (battery internals plus cables) would push about 30 A between them. Real values vary, so follow the manufacturer’s stated limit.
Wiring a parallel bank and a series string
In a parallel bank, every battery should carry a similar share of the current:
- Equal-length, equal-gauge cables from every battery; otherwise the battery with the shortest path carries the most and wears first.
- A busbar or the diagonal method. With a busbar, each battery connects to common positive and negative bars through identical cables. With the diagonal method, take the bank’s positive from the first battery and the bank’s negative from the last, so every current path has the same length.
- Terminals tightened to the manufacturer’s torque, then checked again after the first days of use.
- A fuse for the bank and a disconnect, close to the batteries, plus a fuse on each battery if the manufacturer asks.
- Short cable runs, sized for current and voltage drop. For a fixed installation, consult a qualified installer.
In a series string, link each positive terminal to the next battery’s negative with a short, heavy jumper and take the bank’s connections from the two free end terminals. Where strings are joined in parallel, as in 2S2P, build and check each string first.
Never take 12 V from just one battery of a 24 V string: it would discharge faster than the others and unbalance the string. Feed 12 V loads from the full bank voltage through a DC-DC converter. A 4S string is about 58.4 V when full, so have a qualified installer check a fixed installation.
Charging a series-parallel bank
The charger, any DC-DC charger and any solar controller must be set for the bank voltage, which follows the number of batteries in series, not in parallel. At the commonly used 3.65 V per cell:
- 12 V bank (one battery, or any number in parallel): 14.6 V
- 24 V bank (two in series): 29.2 V
- 48 V bank (four in series): 58.4 V
Always use the voltage your battery manufacturer specifies. Charge current limits add up across parallel strings but not along a series string: if each battery accepted 50 A (an example figure), a 2P bank would accept up to 100 A, a single 2S string 50 A and a 2S2P bank 100 A. A smaller charger just takes longer, but none should exceed the combined limit. Mains and alternator sources follow the same rules: see the guide to mains and shore-power charging and the guide to alternator charging.
Common mistakes
- Mixing old and new batteries, or brands and capacities. The weakest unit limits the bank: in series the smallest battery fills and empties first and its BMS stops the string.
- Thin jumper cables, no fuse or no disconnect. Thin cables overheat, and a short circuit across lithium batteries can release very high current.
- Joining batteries at very different charge levels, or in series when the model is not rated for it.
- Charging at the wrong voltage. A 14.6 V source cannot charge a 24 V bank, and a 29.2 V source overcharges a 12 V one. The BMS is a last line of protection, not a charge controller.
Batteries to build a bank from, and a checklist
LiFePO4 batteries to build a bank from
Open each product page and read the documented ratings, including any stated series and parallel limits, the charge voltage and the maximum charge and discharge current, before deciding how many to buy. If a page does not say, ask us before you order.
Before you connect anything:
- Read the datasheet: confirm series and parallel use are allowed and note the maximum number of units. Confirm the batteries are the same model and capacity and of similar age.
- Charge each battery fully, or equalise their voltages, and check with a multimeter.
- Check that the charger, DC-DC charger, solar controller and inverter suit the bank voltage.
- Prepare cables of equal length and gauge, and plan a busbar or diagonal layout.
- Fit a fuse and a disconnect close to the batteries, plus a fuse on each battery if the manufacturer asks.
- Tighten every terminal to the manufacturer’s torque, with loads and chargers off, and make the final connection last.
- Test with a light load first and check that the batteries share current.
Frequently asked questions
Can I mix batteries with different Ah ratings?+
It is best not to. In series, the smaller battery reaches full or empty first and its BMS stops the whole string. In parallel the current share is uncontrolled and the smaller battery may be loaded beyond its rating. Use the same model and capacity unless the manufacturer says otherwise.
Can I put 12 V batteries in series to make 24 V?+
Yes, if the manufacturer rates the model for series use. Two 12.8 V batteries give 25.6 V nominal and need a 29.2 V charge voltage, and the charger, inverter and solar controller must be 24 V models. Some models are not rated for series use at all.
How many batteries can I connect in parallel?+
As many as the manufacturer states, which is specific to the model: some datasheets allow up to four, others fewer. Beyond that, current sharing becomes hard to control. For more energy, consider a higher system voltage or larger batteries.
Do I need a balancer?+
Not by default. Each battery’s BMS balances the cells inside it, and batteries in parallel equalise through the wiring. In series nothing usually balances the units against each other, so match them and start them at the same charge level. If the manufacturer specifies a balancing device, follow it.
Do I need to fuse each battery?+
Always fuse the bank. Fusing each battery as well is common in parallel banks, because the others can feed a fault in one, and some manufacturers require it. Use a DC-rated fuse with adequate breaking capacity, close to the battery’s positive terminal.
The figures above are arithmetic examples. Ratings and limits are supplier-stated, vary by product and are not independently tested by PVShop. Follow the battery, charger and converter manufacturer’s specification for series and parallel limits, charge voltage, fusing and torque, and consult a qualified installer for fixed installations.