I have some actual data on this.
I have started my 5.2 liter v8 engine on only an 18AH AGM. My starter is rated at 1400 watts, that is its output, its input was measure at nearly 1800 watts.
1800 watts at 10 volts is 180 amps.
How much a jumper pack will assist the engine starting battery depends on how discharged the engine battery really is/ and its condition/health, and the cables paralleling the batteries (their length and gauge) as well as the size and condition and state of charge of the jumper battery. The jumper pack's rested full charge voltage also plays a part.
I can easily hook my 18Ah AGM battery semi permanently in parallel, to assist my engine battery, with my meter inline to record surge amps.
My engine starting battery is a group 27 northstar AGM. rated at 90Ah, 930CCA, 5.5 years old and ~1100 deep cycles and double that amount of shallow cycles, and many thousand engine starts which I do not consider a cycle..... It is nearing the end of its ueful lifespan.
My fully charged 18AH AGM battery will assist the Northstar with 53.5 amps of starter current, over~ 4 feet of 8awg, when the engine battery is close to fully charged, and it provides more amperage the lower the state of charge of the engine battery, I've seen as high as 72 amps assist with the engine battery around 65% charged. I've not had to hook my 18AH jumper battery to assist my overdepleted engine battery, yet. but I suspect the surge amperage measured would be higher if I instantly parallelled and started, compared to parallelling then letting the jumper partially charge the jumpee battery.
With both batteries in parallel the engine cranks noticeably faster and voltage falls much less than the engine battery alone. I have digital ammeters and voltmeters on my dash and view every engine start as a mini load test, and have been watching the voltage farther and farther as the battery ages.
Theory and equations and math are great, but proper tools and temperments to collect actual data, for the win.
When jumpstarting, the thinner the jumpercables conductors, the longer one should leave the batteries in parallel before cranking the dead engine. BIG fat all copper jumper cables with quality parrot clamps can allow the jumpee to turn the key immediately. The thicker and shorter the copper parallelling the battreies, the more they act like one bigger battery.
Be aware that many jumper cables have 10AWG conductors in really thick insulation. 10AWG is rated for ~30 amps continuous. this 10AWG might be copper clad aluminum wire, which is raed for less amperage continuous. Lots of clamps are intended to dig into the soft lead of battery posts and cable clamps. They do not dig into steel very well and the actual surface area touching between clamp and battery might be pitifully small. Some jumper cables will only have one 'live' side of the clamp's jaws.
Remember a healthy battery depleted to less than 80% charged, can get to back upto 80% charged realtively quickly with a high amperage charging source seeking mid 14 volts, but 80% to 100% cannot be accomplished in less than 3.5 hours, and those 3.5 hours assume the battery is being held at 14.5ish vlts for that duration. Lesser electrical pressure(voltage) slow down battery charging greatly.
Any car requiring a jumpstart should ideally be plugged in to a grid powered charger, as ALL lead acid batteries ideally want to be kept at 100% state of charg always, and also kept cool. A battery hovinging in the 80 to 85% SOC range will live a fraction the amount of time as one kept in the 99%+ charged range.
So If one needed a jumpstart of a discharged but still healthy battery, and then drove for an hour, the battery is likely well less than but certainly no more than 85% charged, and 3+ more hours are required to approach 100% charged, assumig the vehcile is holding voltage in the 14.5v range, which very few to none actually do.
A relatively healthy battery will accept about half to 1/3 the amount of current with 13.7v at battery terminals, compared to 14.7v.
Unhealthy batteries will likely never reach full charge at 13.7v, no matter how long 13.7v is applied.
This is proveable with a hydrometer, but the 'trickle charge it overnight' mafia is alive and loud, and delusional, if they believe that 12 hours held at 13.7v is some magcal battery restorative. Any charging is better than no charging but a lead acid battery at any state of charge less than 100%, craves a true full charge .
The alternator and its voltage regulator are not magical instant battery chargers, nor are they intended to be, though the general motoring public seems to believe so.
They in fact are quite poor at bringing a battery back upto 100% State of charge, and the lower their average state of charge and the higher the average battery temperature, the shorter they can be expected to live.
Defects aside, The worst lead acid battery kept fully charged, will outlast the best battery chronically undercharged. regardless of the price paid, the warranty, or the font and colors on the sticker.
However the motoring public and even skilled mechanics seems to believe the exact opposite, that somehow a better marketed, more expensive battery is immune to living chronically undercharged. It is NOT.
Most every lead acid battery on the planet at this moment is less than 100% charged. Exceptional battery life can be achieved when it is regularly returned to 100% charged, and kept as near there as possible.
Voltage is ONLY indicative of state of charge on a battery which has not seen any discharging loads or charging sources for many many hours, and full charge resting voltage can range from 12.6 to 13.2v. Judging battery state of charge, or state of health, by voltage alone is like throwing a rock at a rubber band and guessing how heavy the rock is, by how much the rubber band stretched. You are unlikely to even hit the rubber band, much less determine how far it stretched.
