Sizing a VRLA to lithium retrofit without copying the old number

The most expensive mistake in a lithium retrofit is the cheapest to make: ordering the same number of amp-hours that is written on the old string.

VOLT48 engineering desk · Published 24 Aug 2026

Key figures at a glance

Parameter	Value	Where it comes from
What does NOT transfer	Nameplate Ah between chemistries	Cell arithmetic
What does transfer	The site's load in amps and its required autonomy in hours	Site design
What to request	The discharge table at your actual current, not the nameplate figure	Manufacturer datasheet
Discharge rate ceiling	The C-rate the pack is specified for	Manufacturer datasheet
Telecom retrofit architecture	15S — a 16S rack pack is a different application	ETSI EN 300 132-2
Enter in the controller as	Ah × number of modules	Plant maker's lithium application guidance

Why the old amp-hour number is the wrong starting point

Amp-hours on a label are a nameplate figure measured under stated conditions, and those conditions differ by chemistry. What a VRLA string actually delivers on your site depends on how deeply you are willing to discharge it, how fast you draw, and how warm the cabinet is — and the margin the original designer built in for all three is invisible on the label. Copying the number across means you are also copying somebody else's assumptions about a chemistry you are no longer using.

What does carry across: load and autonomy

Two things about your site are chemistry-independent and they are the two that should drive the order. The load in amps — measured, not estimated from equipment nameplates, because the difference is routinely large. And the autonomy you actually need in hours, which is a business decision about how long a site must survive before someone arrives or a generator starts, not a technical constant. Start from those two and the required capacity is an output, not an inherited input.

The document to ask for, and the one not to accept

Ask for the discharge table at your actual current — how long the pack holds at the amps you will really draw, to the end-of-discharge voltage that applies to your plant. Do not accept a single nameplate Ah figure as the answer to a sizing question; it describes a test, not your site. And note the C-rate ceiling the pack is specified for: it caps how fast you may draw, which matters on sites with a large step load and is the constraint people find after ordering.

The architecture question sits underneath all of it

None of this matters if the pack is the wrong architecture for a −48 V plant. A telecom retrofit is 15S; a 51.2 V rack pack is 16S and asks for a full-charge voltage above the range telecom equipment is specified for. Settle that first — capacity arithmetic on a pack that cannot go in the plant is wasted work. Then, when the pack is chosen, the string capacity has to be entered in the controller as Ah × number of modules, because several of the plant's own limits are computed from it.

Equipment this guide covers

48V LiFePO4 Telecom Lithium Batteries

Common questions about this family

Can I put in fewer amp-hours of lithium than the VRLA I am removing?

Frequently yes, and that is often where the economics of a retrofit come from — but it is a conclusion you reach from the discharge table at your load, not a rule of thumb to order against. Ask for the table, apply your own measured load and your own required autonomy, and let the number fall out of that.

Does temperature change the sizing?

It changes what a bank delivers and how long it lasts, and cabinets in the field are rarely at datasheet conditions. The practical step is to state the temperature range your site actually sees when you request the discharge data, so the figures you are quoted describe your cabinet rather than a laboratory.