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.
Key figures at a glance
| Parameter | Value | Where it comes from |
|---|---|---|
| Flatpack2 48/2000 HE output range | ≈43.5 – 57.6 V | Manufacturer datasheet |
| Factory default setting | 53.5 V — a lead-acid value | Manufacturer datasheet |
| Wide-range variant 48-60/2000 HE | ≈39.9 – 72 V | Manufacturer datasheet |
| Lithium charge setpoint | 52 – 55 V region · float = equalize | Plant maker's lithium application guidance |
| Automatic equalize | Off — lithium is not equalized | Plant maker's lithium application guidance |
| Temperature compensation sensor | None | Plant maker's lithium application guidance |
| Charge current limit | 0.1C – 0.5C · 20 A on a 100 Ah pack = 0.2C | Manufacturer datasheet |
| Limit above 0.25C | Requires a minimum controller firmware version | Controller firmware condition |
| Plant low-voltage disconnect | ≥ battery LVD + 0.5 V · practical floor ≈ 38 V | Plant maker's lithium application guidance |
| Walk-in (soft start) | Enabled · module default voltage set | Plant maker's lithium application guidance |
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.
How it is calculated
- Read the charge voltage the battery datasheet asks for (cells × volts per cell at full charge).
- Confirm the module's adjustable output range covers that value.
- Set float and equalize to the same datasheet value — lithium is not equalized.
- Confirm the result stays at or below the −57.0 V upper limit of the service range.
Equipment this guide covers
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.
Limitations
Setpoints are taken from the battery datasheet, never from this guide. Menu names differ between Smartpack generations and firmware versions; the exact path is confirmed on the installed controller.
Sources
- Delta Electronics — standard telecom power solutions (Eltek product range)
- ETSI EN 300 132-2 — Power supply interface at the input of ICT equipment; Part 2: −48 V DC (normal service range −40.5 V to −57.0 V)
- Datasheet of the specific rectifier module and of the specific battery model quoted (no public URL — cite by title)
- User manual of the installed controller, for the firmware version actually running (no public URL — cite by title)
Editorial and technical methodology →
Other guides
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- Can I put a 51.2 V rack battery in my −48 V telecom plant?
- How do I disable temperature compensation on a NetSure controller?
- Is the Eltek Flatpack2 discontinued?
- Where do I set the low-voltage disconnect on a lithium retrofit?
- What paperwork does a telecom lithium battery need to ship?
- How do I identify my 48 V DC plant?
- Does my controller need a firmware update before a lithium retrofit?
- How does a lithium BMS talk to a telecom DC plant?
- How do I size a lithium bank to replace my VRLA string?
- Does the grounding scheme change when I retrofit a −48V plant to lithium?
- Can I reuse the old breaker when I swap VRLA for lithium?
- Can a VRLA string and a lithium string run in parallel during a migration?