Battery question and voltage drop

[LucasDK]

Hi,

Imagine 2 new 12V batteries:

a) 80 Ah Cold cranking 800A
b) 60 Ah Cold cranking 600A

The car of interest is rated to use the 80 Ah battery and starts the car perfectly. During start, the voltage drops from 12,6V to 10V and raises normally again to the alternator voltage of 14,5 V.

Question:

What would happen if I install a 60Ah instead (without having the option to make any measurements)?

a) Due to the lower cold cranking, the car will struggle more to deliver sufficient amperage to turn over the engine but the voltage still only drops to 10V before it raises again (like the bigger battery).

b) Due to the lower cold cranking, the car will struggle more to deliver sufficient amperage to turn over the engine, but now the voltage drops to lower value than 10V because it struggles even more to deliver the amperage.

What will happen?

Lucas

 

[BrendanC]

ohms law V=IR then power, W=VI (watts = voltage • current). lower voltage equals more amperage draw which is harder on everything in the system. oversizing batteries if possible is a good idea, undersizing is not.

 

[JavierH19]

Depends on how cold because if it's a mild winter cold it will probably still be okay but in general undersizing puts a bit more strain on everything else and oversizing reduces strain.

 

[Rand]

What are the CCA ratings of the 60AH and 80AH batteries?

 

[twX]

The smaller battery will have a lower cranking voltage, and it may also deliver less current to the starter. The starter won't have as much power to crank the engine, and so the engine's cranking speed will be lower.

 

[SC Maintenance]

The resistance in the cable is the same.

The starter most likely won’t pull the max from either battery.

I am not sure it will make any difference. The larger battery will crank longer.

 

[GSCJR]

What will happen?

A strain on the charging system.

 

[The_Jeff]

It actually stresses starter motors to have less power available to turn them than they're rated for. To a point, they use more current which makes more heat.

Realistically there's plenty of overhead. For example the starter for a GM LS v8 takes 1100 watts; at 9 volts (typical dip during cranking) it's 122 amps and you can't find a battery that small.

Don't accept this as license to get a cheezier battery than stock, though.

 

[barryh]

There will be a bigger voltage drop on the 60Ah battery because the internal resistance of the battery will be higher than the 80Ah. Simple ohms law. It's not the battery capacity that's the issue here, it's the CCA being different which implies the internal resistances are different.

It won't matter in moderate temperatures because starter motors don't pull anything like 600 Amps. It's more like 150 amps.

 

[ripcord]

In practice, nothing noticeable will happen with the weaker battery installed... For a year or two. Once the battery starts to get some age, that's when problems may show up.

What folks have already said is accurate.