Setting the charge voltage for lithium on an Eltek Flatpack2 plant
The parameter list is short. Getting it wrong is what turns a lithium retrofit into a warranty argument.
First: what the module can actually deliver
The Flatpack2 48/2000 HE has an adjustable output range of roughly 43.5 V to 57.6 V, and its factory default setting is 53.5 V. That default is a lead-acid number and it is where most plants are still sitting. The wide-range variant (48-60/2000 HE) covers about 39.9 V to 72 V, which is a different conversation — check which one is actually in your shelf before assuming the range.
Float equals equalize — lithium is not equalized
On a VRLA plant, float and equalize are deliberately different values and the plant periodically raises the bus to equalize the string. LiFePO4 does not want that. The plant manufacturer's own lithium application guidance puts the setpoint in the region of 52–55 V with float and equalize set to the same value, and the exact number comes from the battery datasheet, not from a rule of thumb. Automatic equalize should be disabled.
Temperature compensation goes off
Temperature compensation is lead-acid behaviour: the plant raises or lowers the bus as the battery room heats or cools. Lithium does not want to be chased around by ambient temperature — the BMS manages the cells. Set the compensation sensor to None. Leaving it enabled is one of the two most common reasons a lithium retrofit misbehaves in the field.
Charge current has to be limited
A plant sized for VRLA will happily push more current into a lithium string than the battery wants. The charge current limit has to be enabled and set — typically somewhere between 0.1C and 0.5C depending on the battery, and one widely used 100 Ah telecom pack specifies 20 A, which is 0.2C. Note that on some controllers a limit above 0.25C requires a minimum firmware version; if the controller is older than that, the correct answer is to quote the controller upgrade as part of the job rather than to promise the setting.
Low-voltage disconnect and soft start
The plant's low-voltage disconnect must sit at least 0.5 V above the battery's own low-voltage disconnect, so the BMS is not the thing that opens first, with a practical floor around 38 V on a monostable contactor. If the site has a generator, the disconnect also has to sit below its start threshold. Finally, enable the rectifier walk-in (soft start) and set the module default voltage, so a plant that comes back after a long outage does not slam full current into a cold bank.
The parameter list, in one place
Charge setpoint from the battery datasheet, in the 52–55 V region · float = equalize · automatic equalize off · temperature compensation sensor = None · thermal runaway management (BTRM) off, because the BMS owns that · string capacity entered as Ah × number of modules · charge current limit enabled and set · low-voltage disconnect ≥ battery LVD + 0.5 V · walk-in enabled · module default voltage set. On a NetSure plant these are the NCU parameter names; on an Eltek Smartpack the equivalents exist but are named differently and have to be mapped before you quote, not during a commissioning with the site down.
Common questions about this family
Can I just leave the Flatpack2 at its 53.5 V default?
It depends entirely on the pack. On a 15-cell LiFePO4 string, 53.5 V works out to about 3.57 V per cell, which is high on the charge plateau and leaves the pack essentially full — but 'essentially full at the default' is a conclusion you reach by reading the battery datasheet and the plant configuration, not an assumption to start from. Temperature compensation and the charge current limit still have to be dealt with either way.
Does the controller need a firmware update?
Sometimes, and it is a condition of entry rather than a detail. Some controllers require a minimum firmware version before you can set a charge current limit above 0.25C, and a later version again before the default voltage and walk-in are adjustable. If the plant is below the minimum, the honest quotation includes the controller upgrade.