What physically happens on lowering
When lowering the load, gravity drives the rope drum. The drive motor becomes a generator and feeds current back into the DC link. The recoverable energy follows directly from E = m · g · h: 500 kg over 200 m is around 0.27 kWh mechanically, and with realistic efficiencies of gearbox, generator and inverter about 60–75 % of that arrives in the battery. That sounds like little — until you multiply it per working day by dozens of runs and see the effect: every kilowatt-hour fed back is one the diesel does not have to deliver.
What matters is the power, not the amount of energy. A log that is braked in a controlled way within a few seconds briefly generates around 6–12 kW of feedback power. With a small pack of 2–4 kWh those are momentary charge rates of about 2C–4C — in pulses, irregular, in the middle of operation. It is exactly these short charge pulses that the cell selection and BMS have to absorb without the charge acceptance collapsing.
Why the SoC window must not sit at 100 %
A battery at 100 % SoC has no free charge capacity. The cell voltage stands at the upper limit (around 3.65 V for LFP), the BMS has to open the charge path, otherwise the weakest cell overcharges. If it opens, the inverter is stuck with the feedback energy and switches to the braking resistor — regenerative braking is dead. That is why the pack is deliberately not run full: the operating window typically lies at 20–80 % SoC, starting at around 75–80 %, so that the first lowering operation can absorb energy immediately.
On top of that, charge acceptance drops anyway with rising SoC. Above about 90 % SoC the BMS goes into voltage limiting and throttles the permissible charge current noticeably. A battery at 60 % fully accepts a regenerative-braking pulse, the same battery at 92 % only a fraction. The reduced window is therefore not a compromise, but the precondition for regenerative braking to work reliably at all — and it incidentally spares the cells, because high SoC values are the strongest single driver of capacity ageing.
Why lead-acid is ruled out here and LFP fits
Lead-acid batteries accept high charge currents poorly. Their charge acceptance collapses sharply with rising SoC, and pulse charging at 1C–2C leads to gassing and accelerated wear — an impulse-like regenerative-braking profile ruins them within months. LFP cells, by contrast, tolerate 1C–2C continuous charge rate, and more in short pulses depending on the cell type, and age barely under partial cycling in the middle SoC window. Their flat voltage plateau also makes the charge-current control predictable, and the thermal-runaway onset at around 250 °C is relevant in steep terrain, where a cell fault is neither quickly reachable nor extinguishable.
The project with MM Forsttechnik
In the cable crane project with MM Forsttechnik the load profile was the starting point of every design: fluctuating loads, steep alpine terrain, many lifting and lowering operations per working day. The critical figure was not the capacity, but the ability to handle high currents in both directions in rapid succession — lift powerfully, then swallow a regenerative-braking pulse seconds later. We chose LFP cells with sufficient continuous charge rate, designed our own BMS for millisecond reaction on overcurrent and deliberately kept the SoC window below 100 %, so that there is always buffer room for the next pulse. The BMS continuously reports to the inverter over CAN how much charge power the pack can currently accept; the inverter regulates its feed-in current accordingly. If no buffer is available, the braking resistor takes over — safety before efficiency, in that order.
Checklist for regeneration-capable cable crane batteries
- ✓SoC window deliberately limited (e.g. 20–80 %), start charge not above ~80 %.
- ✓Charge acceptance of the cell checked: continuous charge rate ≥ 1C, pulse charge rate for the real descent times.
- ✓BMS CAN interface: reports the currently permissible charge power in real time to the inverter.
- ✓Fallback braking resistor has to remain — it absorbs what the full or cold pack does not take.
- ✓Cell chemistry: LFP for safety and cycle life in hard-to-reach terrain; lead-acid is ruled out for pulse charging.
- ✓Temperature: below ~5 °C charge acceptance drops sharply — include cell temperature in the regenerative-braking management.
Once you know the feedback profile of your cable crane — load weights, lowering heights, cycles per day — the pack and charge window can be designed for it concretely. These numbers are exactly the starting point, not a datasheet nominal capacity.