Rated output is a bench figure
An alternator rating is measured with the rotor spinning fast. Engine idle spins it far slower, and output follows rotor speed steeply at the bottom of the range, so a unit rated at 140 amps may be delivering something in the region of half that with the engine idling — sometimes less, occasionally more, depending entirely on the pulley ratio and the idle speed of that particular vehicle. Output also falls as the unit heats up, so the figure at the end of an hour in traffic is not the figure from the first two minutes.
The percentage in this calculator is a placeholder for a measurement. If the answer matters — if you are adding equipment, or diagnosing a battery that will not stay charged — clamp a meter on the output lead at idle with the load switched on and use the real number. Everything downstream depends on it, and no assumption substitutes for it.
The stoplight discharge
This is the failure the arithmetic is for. Total load higher than idle output means that every time the vehicle stops, the battery is supplying the difference. Pull away and the alternator catches up, partly. Over a winter of short trips with headlights, blower, defrost and heated seats all running, the battery never quite gets back to where it started, and the pattern ends with a car that will not crank on the coldest morning of the year.
The reason it gets misdiagnosed is that everything tests fine in the shop, on a warm engine, at fast idle, with nothing switched on. The battery gets replaced, the new one lasts a while because it starts full, and then does the same thing. The load audit is what finds it, and adding up the amps takes ten minutes.
Reserve capacity, amp-hours and CCA are three different things
Reserve capacity is defined as the number of minutes a fully charged battery can deliver 25 amps before its voltage falls to the cutoff. It describes how long the vehicle keeps running with the charging system dead. Amp-hours describe stored energy at some specified discharge rate, usually a far gentler one. Cold cranking amps describe how much current the battery can push for a few seconds at low temperature — a completely different property, governed by internal resistance rather than stored energy.
| Rating | What it measures | What it does not tell you |
|---|---|---|
| Reserve capacity | Minutes at 25 A to cutoff | Behaviour at any other current |
| Amp-hours | Stored charge at a stated rate | Cranking ability |
| CCA | Short high-current delivery when cold | How long anything runs |
Converting between them is a trap. Multiplying reserve capacity by 25 and dividing by 60 gives an amp-hour figure that is only meaningful near a 25 amp draw, because effective capacity shrinks as the discharge rate rises. High CCA does not mean long runtime, and a deep-cycle battery with excellent runtime may crank poorly in the cold. This page uses the reserve capacity conversion because it is the honest one for a vehicle load, and flags that it is an upper bound.
Adding equipment to a vehicle
Every added load lands on the same charging system, and the arithmetic above is the first check. It is not the only one. A circuit sized for the current, protected at the right place, with a ground path that can carry what the load draws, is the difference between an accessory and a fire. High-current additions can also exceed what the original output cable and its terminations were built for, which is not obvious until something gets hot. Ratings, wire sizing and fusing come from the equipment and from the wiring rules that apply, not from a load total.
Loads that run with the engine off — a fridge, a radio, an inverter at a campsite — are a different calculation again, because there is no alternator in the picture at all. The battery runtime calculator covers that case, and the off-grid load audit covers building the load list. For a vehicle that spends real time parked and powered, see the RV and van solar calculator. Nothing on this page touches hybrid or electric drive systems; those voltages are lethal and are not a place for arithmetic from a website.
Questions people ask
How do I find out what my alternator actually makes at idle?
Clamp an ammeter around the alternator output lead with the engine idling and the loads you care about switched on, once the engine is warm. That gives you the real figure for your pulley ratio and idle speed, which is the only figure worth planning from. Watching battery voltage is a cruder proxy: if system voltage sags towards battery voltage at idle with everything on and climbs when you raise the revs, the alternator is not keeping up at idle regardless of what any table says.
Will a bigger alternator solve a deficit?
Sometimes, and it brings work with it. A higher-output unit needs a belt and pulley arrangement that can drive it, an output cable and terminations rated for the current, and a ground path to match. Fitting one and leaving the original cable in place moves the bottleneck rather than removing it, and undersized cable carrying high current gets hot. It is also worth asking first whether the load is the thing that should change — a large continuous draw that only exists because of something added is cheaper to switch off than to feed.
My battery keeps dying but it tests good. What is going on?
Two common explanations, and this page addresses one of them. The first is exactly the case modelled here: total load higher than idle output, with a driving pattern of short trips that never lets the battery recover. The second is a parasitic draw with the vehicle switched off, something staying awake and pulling current overnight, which this calculator does not cover and which is found with a meter in series and patience. Both look identical from the driver seat and both leave a good battery flat.
Does converting reserve capacity to amp-hours work?
Only near the rate it was measured at. Reserve capacity is minutes at 25 amps, so multiplying by 25 and dividing by 60 gives the amp-hours delivered at 25 amps. Draw 60 amps instead and you get meaningfully fewer amp-hours out of the same battery, because effective capacity falls as current rises. The conversion is useful for a rough answer and dangerous as a specification. Where the number matters, use manufacturer data at the actual discharge rate.
Should I include the starter in the load total?
No. Starting is a very short, very high current event that the battery supplies and the alternator then replaces over the following minutes. It belongs in the cranking discussion, which is what CCA is for, not in a continuous load audit. What this page adds up is the steady draw with the engine running, which is what determines whether the system gains or loses ground over a drive.