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I was posed the question of whether or not you would be able to start a car with 8 small 1.5V batteries in series (thereby creating the required 12V most cars run on). I immediately answered "no", with my reasoning being that the small AA batteries would not be able to supply enough current for the car.

Although I know this to be true, I have been thinking more and more about the reason behind this. Although there are many factors involved, would it be correct to say that the main determining factor for a battery's "current pushing ability" is its internal resistance?

My reasoning behind this is that, when the internal resistance of the battery is relatively high, and due to ohms law where I = V / R, the resistance of the battery will play a greater role in determining the current pull at low load resistances, but not much of a role at high load resistances?

This also makes sense to me, as it seems like car batteries tend to have internal resistances of many magnitudes smaller than tiny AA batteries.

pipe
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Gary Allen
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  • I have started a car with a laptop battery, but it had 9x 18650. It depends on the size of the car's starter and the drain level of the lead battery. If your battery is very flat but still clicking over, you can start it with 8AA batteries of the highest spec, i.e. lithium ones, but if it is flatter than that or without a lead batter in it at all, the lithium AA batteries will probably lose peak amper output after about 2 seconds, before the car has revved, but they might just start a hot car. ... resistance goes up with battery wear, so ya. – bandybabboon Sep 19 '20 at 04:36

4 Answers4

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Short answer, yes you are right. The Ri of a battery limits the current it can supply, but the Ri is not the real cause, more a symptom. The design and characteristics of the electrodes, chemical processes, temperature, etc. all kinds of internal and external parameters interact when current is "requested" and Ri is just your way to put all these influences into one handy value. And we need to mention: Ri changes while the battery operates.

For example, my 50km/h RC car does not move a mm when the LiPos are 0°C or below. Temperature is a huge influence on Ri. In the summer 2 of these packs should start my car easily.

JRE
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schnedan
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  • Thats true, but small Cells can deliver very high currents also. a 12V Lipo which can deliver 100A and more compared with a lead-acid Car Battery, which will deliver 5-8 times more current is much smaller. if you fill the volume of a lead-acid Car Battery with lipo cells you will get a battery which can deliver more current than a lead-acid Car Battery. Well a lead-acid Car Battery will still deliver some 30% of its capacitiy below 0°C while a Lipo will not. So it also depends on the enviroment – schnedan Sep 20 '20 at 09:43
  • hmm, seems one has deleted his/her comment. I will leave my answer to that on the page... – schnedan Sep 20 '20 at 09:46
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Well, there's no little resistor inside the battery – instead, "internal resistance" is our model for the proportional-to-current draw voltage drop you'll see with any real-world power source in some current region.

So, as schnedan said (go and upvote their answer!), "internal resistance" is the symptom, not the cause. Of course, actual ohmic resistances in conductors and electrodes contribute to that – but so do limited reaction speeds and limited speeds of ion transfer.

Marcus Müller
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It is the other way round.

For the sake of simplicity, we model the battery ability to deliver high power with something called "internal resistance".

The model is pretty much basic and in particular really bad for high currents.

The "internal resistance" of an electrochemical cell is non-linear.

It consist of more-or-less ohmic behavior of the electrolyte, the exponential V/A behavior of the chemical reaction itself and has a high current limit because of the limited diffusion of the substances to the reaction zone. And don't even get me started on AC-related (transient) behavior of a battery.

All of these also depend in complex way on the temperature, state of charge and the past discharge history of the cell.

That's why, for example, car batteries' ability do deliver power is rated in CCA (cold cranking amperes) and not by their internal resistance.

... and AA cells are not the best bet when trying to start a car, unless the car is manual, you are strong enough to push it (or have a helping hand or a favorable slope) and you know what you are doing.

A typical car needs ~150A @ 9V in order to start reliably (the battery is 12V when idle and sags 2-3V when starting)

There are ~3Ah, 30C or 50C rated LiFePO4 cells that are pretty much able to do so when used in series of 4, without being much more in volume or mass compared to 8 AA cells.

fraxinus
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For some kinds of battery chemistry, such as lithium polymer, other important limiting factors would be either the presence of protective circuitry which will prevent the delivery of excessive currents, or the exothermic response of the batteries to excessive current draw in the absence of such protective circuitry. If a 10-amp load would only cause an immediate 25% drop in a battery's output voltage, but sustaining that current draw for two seconds would cause the battery to burst into flames within a few seconds, the limiting factor for current would not be the battery's internal resistance, but rather the need to prevent thermal runaway.

supercat
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