What Ah means: energy stored, runtime between charges
Amp hours, written as Ah, measure how much energy a battery can store and deliver. Think of it like the size of a fuel tank. A 500 Ah battery holds roughly twice the usable energy of a 250 Ah battery at the same voltage, so all else being equal it runs the truck about twice as long between charges. The rating is stamped on the battery label, usually next to the voltage, and it is one of the first numbers a dealer asks for when quoting a replacement.
The key phrase is “all else being equal.” Runtime also depends on how hard the truck works. A truck lifting heavy loads to high racks all shift drains a battery far faster than a truck doing light putaway runs. Two identical batteries can last a full shift in one application and die at lunch in another. That is why Ah is a sizing starting point, not a promise of a fixed number of hours.
Ah is measured at a stated discharge rate, usually over six hours for industrial batteries. That detail matters when you compare two batteries: a rating quoted at a different discharge rate is not directly comparable. When a dealer quotes you a replacement, ask that the Ah figures be compared on the same basis so you are not comparing a six-hour rating against a twenty-hour one.
Ah vs voltage: voltage is power class, Ah is runtime
Voltage and amp hours describe two different things, and mixing them up is the most common sizing mistake. Voltage is the power class of the system. It has to match the truck exactly — a 36V truck takes a 36V battery, full stop. Voltage decides whether the battery works in the truck at all.
Amp hours decide how long it works once it is in. You can often choose between several Ah ratings at the same voltage, as long as the battery fits the compartment and stays within the truck's weight requirements. The technical specs guide covers the standard voltages and what each one is used for, if you need a refresher on which voltage class your trucks fall into.
A useful way to remember the split: voltage is the size of the engine, Ah is the size of the tank. A bigger engine does not mean longer runtime, and a bigger tank does not make the truck stronger. When a battery dies mid-shift, the fix is almost always more Ah, not more volts — and more volts is not an option anyway.
How to size Ah for your shifts
Sizing starts with an honest look at how the truck actually works. Three questions drive the answer: how long is the shift, how hard does the truck work during it, and is there a chance to charge mid-shift? A single eight-hour shift of moderate work is the easiest case. A dealer can usually match the Ah rating of the battery that is already in the truck, because that battery was sized for the job when the truck was sold.
- Single shift, moderate work: match the original Ah rating, or the rating of a battery that has been lasting the full shift with charge to spare. No need to oversize.
- Single shift, heavy work: constant lifting, high racks, ramps, and heavy loads all burn energy faster. If the current battery regularly ends the shift nearly empty, step up to the next Ah rating that fits the compartment.
- Multi-shift operation: one battery rarely covers two or three shifts on a single charge. The usual answers are a spare battery per truck with a swap at shift change, or opportunity charging during breaks — but only with batteries and chargers rated for it.
- Opportunity to top up: if trucks sit idle during lunch or between jobs, a higher Ah rating matters less, because the battery gets partial charges during the day. That only works with the right charger setup, so confirm it with your dealer before counting on it.
The most reliable sizing data you already own is the battery you are replacing. If it lasted the shift comfortably for years, its Ah rating is proven for your application. If it died early every day, you need more Ah, a second battery, or a look at whether the truck is working harder than it was originally sized for. Tell your dealer which of those is true — it changes the recommendation.
Bigger Ah trade-offs: heavier, larger, longer charge
More amp hours sounds like free insurance, but it is not free. Higher Ah means more plates and more acid inside the same voltage, which means a physically larger and heavier battery. Three trade-offs come with that, and all three are worth weighing before you upsize.
- Weight: in a counterbalance truck, the battery is part of the counterweight. A heavier battery is usually fine, but a lighter one can change the truck's stability rating. Any replacement has to stay within the weight range the truck was built for, in both directions.
- Fit: a bigger battery may simply not fit the compartment. Compartment length, width, and height are fixed, and a battery that is a half inch too tall is a battery you cannot use. Measure before you order, or have the dealer measure.
- Charge time: a larger battery takes longer to recharge. If your charging window is a fixed overnight period, an oversized battery can still be charging when the shift starts. Your charger also has to be rated for the larger capacity, or charge times stretch even further.
The right Ah rating is the smallest one that covers the shift with a comfortable reserve. Oversizing buys runtime you do not use and pays for it in weight, space, charge time, and cost.
The 80% discharge rule
For lead-acid batteries, there is one rule that matters more than any other for lifespan: do not regularly discharge past 80% depth of discharge. In plain terms, leave at least 20% of the charge in the battery at the end of the shift. A battery that is routinely run flat, or close to it, sulfates faster and loses capacity years early.
This is why sizing with a reserve matters. If your shift uses 70% of the battery, you are in good shape. If it uses 95% on a normal day, any busy day pushes you past the line, and the battery pays for it. A battery gauge that drops into the red before the shift ends is not a battery problem — it is a sizing signal.
Lithium-ion batteries handle deep discharge better than lead-acid, which is one of the reasons they are popular in multi-shift work, but the habit of leaving a reserve is still good practice. And on lead-acid, the rule is not optional: a battery that is drained past 80% and then left sitting empty sulfates fastest of all, so plug in at the end of the shift, every shift.
If your current battery keeps hitting the 80% line before the work is done, that is the time to talk to a dealer about a higher Ah rating, a spare battery, or replacement with a battery sized for the job as it is today — not the job as it was when the truck was new.