Setting the low-voltage disconnect for a lithium retrofit

This setting decides which device opens first when a site runs the bank down. Get the order wrong and the BMS becomes your load-shedding mechanism, which is not what it is for.

VOLT48 engineering desk · Published 24 Aug 2026

Key figures at a glance

Parameter	Value	Where it comes from
Plant low-voltage disconnect	≥ battery LVD + 0.5 V	Plant maker's lithium application guidance
Practical floor	≈38 V on a monostable contactor	Plant maker's lithium application guidance
Guaranteed service range on a −48 V bus	−40.5 V to −57.0 V	ETSI EN 300 132-2
If a generator is installed	Plant LVD must also sit below its start threshold	Site design
Who owns cell protection	The BMS — not the plant controller	Plant maker's lithium application guidance

Two disconnects, and they must not be equal

A lithium retrofit puts two low-voltage disconnects in series on the same bus. The battery's own, enforced by the BMS, protects the cells. The plant's, enforced by the controller and its contactor, sheds the load to protect the battery from a deep discharge. If the two are set to the same value — or worse, if the battery's is the higher of the two — then the BMS opens first, the load drops without the plant having managed anything, and the plant cannot tell you why. Set the plant disconnect at least 0.5 V above the battery's own.

The practical floor, and why it is not zero

The published guidance puts a practical floor around 38 V where a monostable contactor is involved: below that the contactor may not hold reliably, so a disconnect set lower is a number on a screen rather than a behaviour in the field. This also sits well below the −40.5 V bottom of the range telecom equipment is specified for, which is the point — by the time the bus is that low, the load has already been outside its guaranteed window for a while.

The generator case, which people find last

If the site has a generator, its start threshold is a third number in the same sequence, and the plant disconnect has to sit below it. Otherwise the plant sheds load before the generator is asked to start, which turns a survivable outage into an unnecessary one. This is the constraint that most often gets discovered after commissioning, because the pilot site did not have a generator and the second one did.

What this does to a mixed VRLA and lithium estate

The disconnect that was correct for a VRLA string is not automatically correct for a lithium one, because the discharge curves end in different places — a 15S LiFePO4 pack finishes discharging around 40.5 V, right at the bottom of the specified window, while a lead-acid string sags gradually toward its own end point. If you are migrating site by site, the disconnect is a per-site setting during the transition, not a fleet standard you set once.

What to establish before you order anything

The battery's own disconnect voltage, from its datasheet, not from the family brochure. The plant's current disconnect setting and whether the controller lets you change it — some generations do not, and that is a firmware question. The generator start threshold if there is one. And the contactor type, because the practical floor depends on it. Four facts; they are four of the twelve on the sheet our compatibility audit hands over.

Equipment this guide covers

Common questions about this family

Can I just leave the plant disconnect where the VRLA had it?

Only if you have checked it against the new battery's own disconnect and confirmed the 0.5 V separation still holds in the right direction. The VRLA setting was chosen against a different discharge curve and a different protection device, so it being unchanged is not evidence that it is right.

What happens if the BMS opens first?

The load drops, and it drops without warning from the plant's point of view — the controller sees the bus disappear rather than a managed disconnect it initiated. Recovery can also be awkward, because some BMS units need a charge source present to reconnect, and the charge source is on the other side of the contactor that just opened.