How to Size an AGV Battery: Shift and Charge Window
How to Size an AGV Battery: Shift and Charge Window
Automated guided vehicles live on a predictable rhythm: they work a shift, they pause to charge, they work again. Sizing their battery is therefore not about peak power alone, though that matters; it is about matching stored energy to the shift and matching the charge window to the available downtime. Get either wrong and you either strand a vehicle mid-shift or you buy a pack twice as large as necessary. This guide sets out the arithmetic and the procurement discipline to size an AGV battery to its real duty cycle, with margin you can defend.
What an AGV duty cycle actually looks like
Before any amp-hour number, map the day. How many hours does the vehicle run? What is the average draw, and what are the spikes when it accelerates, climbs a ramp, or lifts a load? How long is the longest uninterrupted work block, and what downtime exists for charging – a meal break, a changeover, an overnight dock? A lithium battery manufacturer needs this duty profile to size honestly; a bare “make it last 8 hours” is not enough because 8 hours of light tow is a different pack than 8 hours of frequent heavy lifts. The duty cycle is the input; everything else is derived from it, and skipping this step is the root of most AGV battery over- and under-sizing.
Defining the shift and the charge window
The shift is the longest continuous work block you must cover on one charge. The charge window is the downtime you can actually use – not the theoretical gap, but the gap where the vehicle is truly idle and near a charger. If your shift is 6 hours and your only charge window is a 30-minute changeover, you need either a pack that survives 6 hours or an opportunity-charging scheme that tops up in those 30 minutes. These two numbers frame the whole design. Be realistic about the charge window: a window that exists only on paper produces a vehicle that never finishes its shift. Write both numbers into the requirement with the actual observed schedule, not the planned one, because the plan and the floor rarely agree.
Throughput math: amp-hours per shift
Convert the duty into energy. Take the average current in amps across the shift, multiply by shift hours to get amp-hours, then add a 20-30% margin for acceleration spikes, aging and the fact that you should not discharge to empty. If the vehicle averages 20 A over a 6-hour shift, that is 120 Ah of throughput; with 25% margin you need about 150 Ah of usable capacity, which at an 80% depth-of-discharge cap means a roughly 190 Ah nameplate pack. This simple multiplication beats guesswork and gives the supplier a number they can match to a cell. A battery application solutions engagement that starts here, with measured current rather than hoped-for endurance, produces a pack that actually finishes the shift.
Peak versus continuous current for drive motors
An AGV motor draws modest current while cruising but spikes when it accelerates from stop, climbs a gradient, or lifts a pallet. The BMS, connectors and busbars must survive the peak, not just the average, or you get voltage sag that trips the pack or welds a lug. Specify both: continuous current for sizing energy, peak current for sizing the plumbing. A pack correctly sized for 20 A average but only rated for 30 A peak will fail a 60 A acceleration surge. Ask the supplier to rate the pack and its interfaces for your measured peak with margin, and confirm the BMS trips only above that peak, not within normal operation, because a pack that trips on every acceleration is a pack you cannot use.
Opportunity charging versus full-shift packs
| Strategy | Pack size | Infrastructure | Best when |
|---|---|---|---|
| Full-shift pack | Large | Simple, one dock | Long continuous shifts, rare downtime |
| Opportunity charge | Smaller | Multiple fast docks | Frequent short breaks, 24/7 ops |
| Swap pack | Medium | Spare packs + swap station | Zero downtime tolerance |
The table shows the trade. Opportunity charging lets you buy a smaller, cheaper pack and top it during breaks, but it demands chargers at the break points and a BMS that accepts frequent partial cycles without complaint. A sodium-ion battery is not ideal here because its lower C-rate makes fast opportunity charging harder; LFP is the natural fit for AGV duty because it is cheap, safe and cycle-friendly. Choose the strategy from your floor’s actual rhythm, then size the pack to that strategy, because the two decisions are coupled and cannot be made independently.
Chemistry choice for AGV duty
Most AGVs run LFP, and for good reason: it is safe, long-lived and inexpensive per cycle, and it handles the moderate C-rates an AGV asks for. Sodium is attractive on material cost for very large fleets but its lower specific energy and slower charge make it a poor fit for opportunity charging, so reserve it for slow, fixed, weight-irrelevant carts. A semi-solid-state battery could add energy density for a premium autonomous platform where every kilogram of payload matters, but its cost rarely pays on a ground vehicle where weight is free, so it stays a niche choice. A home energy storage system and an AGV fleet share the LFP preference for opposite reasons – the storage bank wants cheap cycles, the AGV wants safe, durable, moderately fast ones – but the chemistry choice lands in the same place. Pick LFP for the typical AGV, size it to the shift, and spend your engineering budget on the charge window rather than exotic cells.
BMS and fleet telemetry
An AGV fleet is a managed asset, and the BMS is what makes it manageable. Per-vehicle state-of-charge, cycle count and temperature let you predict which vehicle needs service before it fails mid-shift, and fleet-level telemetry tells you whether your charge windows are actually being used. A BMS that reports over a simple bus to your fleet controller turns battery health from a mystery into a dashboard. Specify state-of-health reporting and a log you control, not one locked in a vendor cloud, because the data must survive the supplier relationship. The telemetry is what lets you prove a pack was used within rated conditions when a warranty claim arises, and it is what separates a planned swap from an unexpected stranded vehicle on the production line.
A practical AGV sizing worksheet
Turn the theory into a one-page worksheet before you talk to any supplier. List shift length, average current, peak current and its duration, available charge windows and their lengths, and your target depth-of-discharge. Compute usable amp-hours from the shift, add margin, divide by your depth-of-discharge cap to get nameplate, and check that your charge window can refill that energy at your charger’s rate. If the window is too short, either add a faster charger, add a charge point, or accept a larger pack. Hand the sheet to the supplier and ask for a pack rated at your peak with margin. A ten-minute worksheet exposes mismatches early – a “200 Ah” pack that only delivers 160 Ah at your 80% cap is a 160 Ah pack in practice, and pricing it as 200 Ah is where margins quietly leak on a fleet of fifty vehicles.
Common AGV sizing errors
- Sizing to planned downtime instead of observed downtime.
- Forgetting peak acceleration current when rating the busbars.
- Assuming opportunity charging works without chargers at the break points.
- Choosing a slow-charge chemistry for a fast-charge strategy.
- Ignoring depth-of-discharge so the pack is effectively smaller than named.
Frequently asked questions
Charger sizing for the charge window
The charge window only works if the charger can actually refill the energy in the time available. Compute the energy to replace – usable amp-hours times pack voltage – and divide by your real downtime to get the required charge power, then add margin because chargers derate as they warm and as the pack approaches full. A 30-minute window that must replace 80 Ah at 48 V is about 6.4 kWh, demanding a charger near 13 kW, which is a serious piece of infrastructure, not a wall brick. Undersized chargers quietly extend the effective charge window and defeat the whole opportunity-charging plan. Specify the charger current and confirm it matches the pack’s maximum charge rate and the BMS limits, because a charger that overwhelms the BMS will be blocked, and you will wonder why the vehicle never finishes charging. The charger is part of the battery system, not an afterthought, and it deserves the same specification discipline as the cells.
Do I need a huge pack for a long shift? Only if you cannot charge during the shift; opportunity charging lets a smaller pack cover long days via short top-ups.
Why specify peak current separately? Acceleration and lift spikes exceed average many times; the BMS and connectors must survive the peak or the pack trips or fails.
Is sodium good for AGVs? Usually not – its slower charge and lower specific energy fight opportunity charging; LFP is the standard fit.
How do I know my charge window is enough? Divide the energy you must replace by your charger rate; if it exceeds the real downtime, add chargers or a bigger pack.
Written by Karl at China Battery Technology. Share your shift and charge-window schedule and we will size the pack—request a quote for an AGV battery matched to your floor.
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