Running VRLA and lithium side by side during a phased migration

"Keep the old string as backup" and "keep both strings electrically connected" sound like the same plan. They are not, and the difference is the float voltage.

VOLT48 engineering desk · Published 24 Aug 2026

Key figures at a glance

Parameter	Value	Where it comes from
What a reversible pilot actually keeps	The old string physically present and chargeable, kept as a fast fallback	Site design
What it normally does NOT do	Hold VRLA and lithium on the same bus at the same float setpoint simultaneously	DC protection design
Why a shared setpoint is a problem	VRLA and lithium want different float behaviour; one chemistry ends up outside its ideal range	Manufacturer datasheet
What to confirm before assuming dual operation is possible	Whether the controller supports a per-string setpoint or only one global value for the shelf	Controller firmware condition
Where this belongs in the plan	The pilot-then-rollout sequence, not a permanent mixed-chemistry bus	VOLT48 compatibility audit

What a '30-day reversible pilot' is actually describing

A phased migration usually starts with one non-critical site, the old VRLA string kept on hand for a set window so the change can be undone if something goes wrong. That description is easy to hear as "both strings stay connected" — but in practice the reversible version almost always means the old string is disconnected from the live bus and kept charged and ready nearby, not left hanging on the same rail as the new lithium string while both draw float current together.

Why a shared float setpoint hurts one chemistry or the other

VRLA float is typically set higher, often with temperature compensation raising it further in a cool cabinet. A lithium pack's BMS expects a float value inside a narrower window and will limit or reject charging outside it, while a VRLA string left below its designed float undercharges and shortens its own life. A plant with a single global float setpoint for the whole shelf cannot serve both chemistries correctly at the same time — one of them is always outside the range it was designed for, which is a slow, invisible failure rather than a dramatic one.

What genuinely parallel operation requires, if it is ever needed

On the rare site where both chemistries must be electrically present at once — not just physically on hand — the two strings need to be isolated onto separate distribution with their own breakers, so each can be held at its own appropriate setpoint rather than sharing one. That is a bigger change than swapping a battery, and it is worth confirming it is actually necessary before designing around it: most reversible pilots do not need it, because the old string only has to be ready, not live.

The one thing to confirm before assuming your plant can run two setpoints

Some controllers support a per-string or per-branch charge setpoint; a good number only expose a single global float and charge-current value for the entire shelf. Which of the two your controller does is a fact you determine by checking the menu structure, the same way you would check its firmware generation before a lithium retrofit — not something to assume from the model number. If the plant only has one global setpoint, a genuinely parallel dual-chemistry bus is not available to you, and the pilot has to be sequenced as a disconnect-and-swap instead.

Equipment this guide covers

Common questions about this family

Can I just leave the old VRLA connected as backup while I test the lithium string?

Usually not on the same bus at the same float value — one of the two chemistries ends up charged outside its designed range, which degrades it quietly rather than failing it outright. The reversible version keeps the old string disconnected but charged and close at hand, so it can go back in quickly if the pilot doesn't work out.

What if my plant only has one float and charge-current setpoint for the whole shelf?

Then a genuinely parallel VRLA-and-lithium bus isn't something that plant can do correctly, and the pilot needs to run as a clean disconnect-and-swap rather than a side-by-side test. Check the controller's menu structure for a per-string setpoint before assuming either way.