What the Pylontech manuals say
The US5000 and US2000C manuals contain the same short paragraph on expansion: a new module can be added onto an existing system at any time; make sure the new battery acts as the master; because of its higher state of health, the new module may show a different state of charge from the others, and this does not affect how the parallel system works.
So a different percentage on the new battery during the first days is expected. It is not a fault, and it is not a reason to wait.
Which battery becomes the master
When models are mixed, Pylontech gives an order of priority for the master position. From the US5000 manual:
| Position | Allowed models |
|---|---|
| Master (1st) | Highest in this order: US5000 > UP5000 / US3000C / US2000C > US3000 / US2000. Between two batteries of the same model, the most recently produced one. |
| Slaves 2 to 8 | US5000, UP5000, US3000C, US2000C, US3000, US2000 |
| Slaves 9 to 16 | US5000, UP5000, US3000C, US2000C only |
In practice, adding a new battery usually means it takes over as master. Move to it the inverter communication cable (CAN or RS485) and, if you use one, the Pylon-Monitor cable on the Console port. Then chain the link cables from Link Port 1 of the master to Link Port 0 of the next battery, and so on. Set the new master's DIP switches as its manual requires for your inverter. Our parallel installation guide covers cabling and power cables in detail.
A battery that has been in storage
A new battery is not necessarily a fresh one. Pylontech asks for a battery in storage to be recharged at least every six months, to more than 90 %. A module bought a few years ago and left in its box at one LED has spent a long time at a low charge.
- Before connecting it to the stack, switch it on alone and watch the LEDs: the ALM (alarm) LED must stay off.
- If it raises an alarm or does not start, do not connect it. Contact the seller first.
- If it starts normally, you can proceed.
For a second-hand module, our used battery checklist goes further: cycles, state of health, cell spread and BMS firmware.
Do you need to empty or fill the batteries first?
No. The manuals only ask that, after connecting modules at different states of charge or voltages, you keep the system idle (no charge, no discharge) for at least 15 minutes, or until the charge LEDs of all batteries differ by one LED at most, before normal use.
The closer the levels are when you connect, the shorter this wait. If the new battery shows one LED and the stack shows four, you can let the stack discharge a little in the evening and connect then, or simply connect and wait for the LEDs to come together.
Old and new batteries together: what to expect
- Different percentages for a few days. Expected, as the manual says. They converge as the stack goes through full charges, and each full charge also lets the cells balance (how balancing works).
- Different BMS firmware versions. A new module often ships with a newer BMS than the old ones. See BMS firmware mismatch for when it matters.
- Older slaves that never report their health. Behind a new master, some first-generation modules (for example US3000A) report their voltage, current, temperatures and cells but never pass on their state of health or cycle count. That is the battery, not a fault (a real example).
- Older models with a small 4-pin Console socket (US2000B, US2000 Plus, US2500). Their Console port is RJ11, not RJ45. Once the new battery is the master, a monitor plugs into its RJ45 Console port as usual; only if an older model stays master is an RJ11 to RJ45 adapter needed.
- More than 16 batteries means several groups joined by an LV-HUB (LV-HUB guide).
Checking that the new battery has joined properly
The inverter usually shows one figure for the whole bank. To see that the new module is really working with the others, you need each battery separately: its state of charge, voltage, current, temperature and cells.
Pylon-Monitor reads them from the Console port of the master and shows every battery side by side, with the spread between the highest and lowest cell. On the free online dashboard, the first days after the expansion show whether the new battery's percentage is converging with the others, and whether one module lags behind after a full charge. If the spread stays high, a guided Force Balance session brings the whole stack to a long full charge and reports every cell before and after.
For the meaning of each value, see Pylontech diagnostics.
Frequently asked questions
- Can I add a new battery to an existing Pylontech stack?
- Yes. The US5000 and US2000C manuals say a new module can be added to an existing system at any time, and that the new battery should act as the master.
- Do I have to discharge all batteries before adding a new one?
- No. After connecting, keep the system idle for at least 15 minutes, or until the charge LEDs of all batteries differ by one LED at most, before normal use.
- Which battery should be the master in a mixed Pylontech stack?
- The highest in Pylontech’s order: US5000, then UP5000 / US3000C / US2000C, then US3000 / US2000. For two batteries of the same model, the most recently produced one.
- My new Pylontech shows a different percentage from the others. Is it faulty?
- No. The manual says a new module, with its higher state of health, may show a different state of charge without affecting the system. The values converge over the following full charges.
- What should I do with a Pylontech that has been stored for years?
- Pylontech asks for a recharge to over 90 % at least every six months in storage. Switch it on alone first: if the ALM LED lights or it does not start, contact the seller before connecting it.
See every battery of the stack side by side, before and after the expansion: state of charge, voltage, current, temperatures and every cell, with three years of history.
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