How many amps do I need to start my car?

October 2, 2009 10:21 AM

Please educate me on car battery technology.

I was looking for a new battery for my car, but the company I've decided to get it from has three different batteries available that will suit my car - a standard lead acid battery, a calcium battery and a silver battery.

I've been looking at the various different specifications, and it seems that the silvery battery is best. Money isn't a problem. However, I'm concerned about spending money on something that's not worth it.

Is having 160 extra startup amps (between the standard and silver batteries) really going to make a difference? What about in cold, damp weather? I think that the temperature of a battery makes a difference to how much energy it can produce - would buying the stronger battery mean that it can output more electricity at lower temperatures?

I'm pretty clueless about this kind of thing, so please feel free to educate me.

posted by Solomon to travel & transportation (14 comments total)

If you run your accessories a lot with your engine off, the silver battery might be better, but honestly, in a mild climate, the starting amps aren't all that important.

Once the engine is running, the alternator supplies both the recharging current and the power for your accessories. The battery is only for starting and for running things when th engine is off.

I'd go cheap, personally; the extra cost is mostly emotional in this case, not tech.
posted by FauxScot at 10:31 AM on October 2

Standard lead-acid batteries are totally fine for 99.9% of driving situations. The only time you really need extra startup amps is if you live in an area where it gets bitterly cold, which is not the UK, which is where your profile says you live.
posted by zsazsa at 10:32 AM on October 2

The only time you really need extra startup amps is if you live in an area where it gets bitterly cold, which is not the UK, which is where your profile says you live.

To some extent, I'd agree. But the particular cold and wet the UK gets n winter is pretty demanding starting conditions for cars further into their lifespan.

The extra cranking volts could be considered insurance. Your engine will spin over faster and the battery will perform better in all conditions. I always buy the largest battery I can physically fit into the space provided, as the amount of times I have been screwed by the battery not being man enough outweighs the cost, for me.

It's not strictly necessary to over-spec your battery, but it certainly does no harm and can prove useful. If money is no object, over-spec I say.
posted by Brockles at 11:01 AM on October 2

The three types of batteries you're contemplating differ---for the most part---in the metallic composition of the cathodes and anodes within the battery itself. Effectively, the more expensive batteries have less internal resistance.

The question is, does this matter? If you'll indulge me in some electronics, we can find out together!

The amount of power your starter motor pushes out is a function of the voltage and the current rushing through it (P = VI). That being said, the current flowing through the motor is a function of the resistance along the conducting path (Ohm's law, I = V/R). Your battery's voltage is always going to be around 12.5 V, and the starter motor's resistance is just about 0.12 Ω (for a small car). Thus, if we assume negligible resistance in the starter motor leads (painfully false assumption, but I'm trying to be reductionist here), the current going through your motor is I = 12.5 V / (0.12 Ω + R_batt) where R_batt is the internal resistance of the battery. A more expensive battery means smaller internal resistance, a smaller internal resistance translates directly into higher current, and a higher current translates into a faster cranking speed.

But does it matter? In modern cars, the role of the starter motor is to turn the engine over twice (anything more and you fail emissions tests!) so that the ECU can sync and then start pumping gas and igniting at the right time. A typical cranking speed for a passenger car would be around 350 rpm, so that means, in a modern car, your battery spends about half a second working each time you start it up, and then proceeds to do practically nothing for the rest of the trip, as your magneto takes over.

This would make it seem that the battery is almost a useless consideration, and you should just go for the cheapest shit imaginable. Not so! During cranking, you need to power a plethora of other electrical components: engine sensors, the ECU, spark plugs, fuel injectors, fuel pumps... If these components don't get enough power because the starter motor is too much of a drain on power, then your car is simply never going to start, or it'll behave very erratically during cranking. Naturally, if you have less internal resistance at the battery, you have less power losses, and thus you have more available power.

But all this worry about internal resistance is almost a moot point if your battery is fully charged: its voltage is high enough to neatly overcome its poor internal resistance, and it'll start like a charm. It's when your voltage is significantly below its happy 12.6 V zone, at around 12.2 V, or even when it's nearly completely discharged at 11.9 V that you start worrying about internal resistance. A high quality battery might be able to start your car even though it's almost completely discharged, whereas a lower quality one might not be able to start it below 80% charge.

So, in summary:

• Buy a better battery if your car is older. You'll have more problems cranking, so you can use the extra magic that comes from a zombie battery that starts even when dead.
• If your car is relatively new, don't bother buying anything that's significantly better than the OEM battery unless you've been having problems with the OEM battery. New cars generally don't have trouble cranking, even in mercilessly cold weather.
• You can often emulate the effect of a better battery just by changing the wires leading up to your starter motor. Swap the OEM ones with thicker gauge (and sand the contacts between the casing and the engine block, while you're at it!).

Also, I'd like to point out that the only reason people say you need a strong battery in cold weather is because the engine itself is usually just harder to start when it's cold, meaning that it has to turn over several times before it actually goes. Temperature doesn't really affect the battery as long as the solution inside remains liquid. If you've got a modern car, then you'll note that, even at -40*C, it can usually start within three or four cranks.
posted by Mons Veneris at 11:06 AM on October 2

I probably should add that the car is 22 years old. it's one of these. It's not modern by any standards whatsoever.
posted by Solomon at 11:15 AM on October 2

Mons Veneris writes "I'd like to point out that the only reason people say you need a strong battery in cold weather is because the engine itself is usually just harder to start when it's cold, meaning that it has to turn over several times before it actually goes. Temperature doesn't really affect the battery as long as the solution inside remains liquid. If you've got a modern car, then you'll note that, even at -40*C, it can usually start within three or four cranks."

Ah, no. Batteries are a chemical reaction. That reaction takes place at significantly reduced vigour at reduced temperatures. Amps available to crank an engine can drop to as low as 25% at -32F compared to the amps available at room temperature. This is not to say a new manufacturer rated battery in a well tuned engine isn't up to the job but extra capacity is good to have as the battery sulfates with use and age.
posted by Mitheral at 11:22 AM on October 2

No, it does not matter. There's theory, and then there's practice.

Yes, theory says the fancy ones might help.

In practice, well....does the car start ok with a standard battery? If yes, change nothing. If no, FIX THE CAR. Seriously, if it can't start on a regular OEM battery, it's got problems more than the battery.

I live in Michigan and I've never had a battery fail to start a car because of cold, in spite of some pretty damn cold nights. That's in 15+ years of driving nothing but (well-maintained) cars with 100K-300K miles on them. They fail, but only because they're old.

I spend a lot of money on premium parts (O how I love the, my Bridgestone RE-01Rs, Koni adjustable shocks, H&R springs, super expensive oils and fancy automotive waxes), but I wouldn't waste it on anything more than a decent quality lead acid battery.
posted by paanta at 11:28 AM on October 2 [1 favorite]

Ah, no. Batteries are a chemical reaction. That reaction takes place at significantly reduced vigour at reduced temperatures. Amps available to crank an engine can drop to as low as 25% at -32F compared to the amps available at room temperature. This is not to say a new manufacturer rated battery in a well tuned engine isn't up to the job but extra capacity is good to have as the battery sulfates with use and age.

OK then, consider my last point false!

I did run a test last winter with the OEM battery on a Can-am DS450 ATV, and it seemed to me that the cranking amps we were getting were only weakly dependent on the temperature (~110 A at 20*C vs. ~105 A at -15*C). I didn't see an effect as massive as in your link, Mitheral, but I guess it is there. Probably because the OEM battery that comes with the Can-am is pretty beefy for the size of the engine, which makes sense, considering it's a Canadian ATV.
posted by Mons Veneris at 11:40 AM on October 2

A two cylinder car? There's no way that you need the extra cranking amps. The only thing the more expensive battery gets you is a 5 year warranty versus a 3 year warranty on the cheap one. You'd have to read the fine print to see how they honor a warranty claim but I would assume it's at the very least pro-rated which means it would probably be difficult to make back the 18 pound difference.
posted by Rhomboid at 12:41 PM on October 2

For your 2CV, the cheapest battery will be just fine. The original battery probably provided far less starting current than any of those batteries.
posted by ssg at 4:07 PM on October 2

There's a significant difference in starting ability between warm and colder weather, using the standard battery. In cold weather, I can try to start the car for up to twenty minutes before it will kick in and turn over. In warmer weather, it always starts first time, hence this question.

Sadly, parking the car on the main road is the only option. I live on the side of a large hill, and the way the houses are laid out seems to form some kind of wind tunnel. I guess I should buy a thermometer or something, and check to see what temperature it is that the car has trouble at. The battery on the car is a direct replacement for the previous one, which I got exchanged under warranty. It's behaving the same way that the previous one did.
posted by Solomon at 1:09 AM on October 3

"Better" i.e. more expensive batteries may also be made to better mechanical tolerances, and have better seals at electrical posts, thicker plastic cores, better (more rugged) plate separators, and more advanced manufacturing techniques. One of the main sources of battery post corrosion in standard battery applications, is simply sulphuric acid fumes emanating from battery post seals in poorly manufactured battery cases. Battery discharge produces gases, and if they can't vent from cell caps normally, they find their way out, and cause corrosion, at whatever weak points in the battery case physical design, actually exist. Too often, these are the battery post connections.

The cost difference between the best grade of automotive battery, and the lowest, is largely made up by warranty exposure of the manufacturerer. The "silver" grade battery doesn't really include the element "silver" (Ag on the chemical element chart) as a chemistry. Whatever increase in capacity is offered, is due to additional plate surface geometry, cell volume, and lower cell connect resistance, than the low end battery design offers.

More cranking amps in a cold weather climate are nearly always worth the cost of getting them, sooner or later. But inspect your battery mounting hardware, too. A corroded mount plate, bad cables, bad cable connectors, and battery hold downs all deserve a thorough inspection, and proactive replacement strategy, if you are already on your 2nd or greater battery.
posted by paulsc at 1:29 AM on October 3

Solomon writes "In cold weather, I can try to start the car for up to twenty minutes before it will kick in and turn over."

Are you cranking for 20 minutes? If so you don't have a battery problem. Your poor cold weather start is something else. I don't know 2CVs at all but on a north american spec car with a carburetor I'd suspect a malfunctioning choke (is you choke manual or automatic); a mostly wore out fuel pump or fuel line vacuum leak; or some spark problem commonly a weak coil, incorrect gap or wet and or damaged ignition wires.

Heavy cranking like that is very hard on starting type batteries. If you can't resolve the excessive crank issue I'd recommend a Marine type battery. They are half way between a starting battery which is designed to deliver lots of amps for a short time and a deep cycle battery which is designed to deliver fewer amps but to do so for much longer periods.
posted by Mitheral at 8:32 AM on October 3

A two cylinder car? There's no way that you need the extra cranking amps.

The number of cylinders is completely irrelevant. The compression of these cylinders is however, relevant. Two, high compression, cylinders can produce just as much of a cranking load as four lower compression ones.

If you have issues with a 2CV with cold starting issues, I think you may also have an issue with choke or carburettor issues, but it is possible (2CV's being as agricultural in design as they are) that they just start like crap in cold weather. A bigger battery will help with this as higher cranking speed will help.

I suggest still going for the bigger battery - a smaller one will only have worse performance for the issue you have at present. Then address the other starting issue.

More cranking amps in a cold weather climate are nearly always worth the cost of getting them, sooner or later.

Absolutely.
posted by Brockles at 9:11 AM on October 3