# Alternator current when charging flat battery

I've found some voltage-current-rpm graphs for car alternators and did not quite understand one thing. Basically, my alternator is rated at 75A (small car), which it should output above 2000 RPM or so. Below this RPM, alternator will output less current (due to lower input power (rpm*torque)). In any case it will try to maintain a voltage between 14.0-14.4V unless overloaded, when voltage will drop.

Now, lead-acid batteries should be charged at 0.1C rate (10 hours) to maintain their health. Anything quicker and you will be buying a new one soon. So, what happens, if I install a healthy (low internal resistance), but only barely charged battery? If I connect 14V to it, it will draw 50 amps easily. However, to maintain battery health, the current should be limited to 5A or so (for 50Ah battery).

The question is: is there any kind of current-limiting mechanism for this scenario, to not kill the battery? Or should we all assume that we should buy new batteries every year, if we don't use the car often and let it discharge to 11V?

• I think, what Charles is saying, is that you would normally install a mostly-charged battery, and never encounter this situation. You can charge a battery from something like a lab power supply, current limited to 5A, or a purpose built trickle charger, before installing. If you have some other use case, make that clear in the question. There are "deep cycle" batteries and other charging arrangements for other use cases. – Brian Drummond Jan 12 at 16:43
• From all of answers it looks like my question is unclear somehow. To rephrase: what will happen if my battery is in good health (internal resistance Ri=0.05Ω) and nearly fully discharged (Vbat=11V)? This is basic Ohm's law: if alternator would give it 14V, current is (14-11)/0.05=60A. Depending on alternator and RPM, this would be limited down to 30-60A, which is still wayyyy over rated charge current of 5A for these batteries. So the question is: is there some kind of mechanism in car's 12V circuit to limit charging current to, say, 5A, but still provide 60+ amps if other systems need it? – elektrinis Jan 12 at 17:14
• Yes there is. A trickle charger. – Brian Drummond Jan 12 at 17:53
• Installed in a car? You're just playing smart? I know all about external chargers. My question is about standard equipment onboard of a car. – elektrinis Jan 12 at 17:55
• Seems like everyone is saying, don't do that. If your use case is a car only used every few months, a trickle charger is cheaper than new batteries. It's not a use case that car makers really worry about, so of course they won't install one for you. – Brian Drummond Jan 12 at 18:19

You already obviously have a good answer from Charles, but I feel like writing so here goes =).

Doing this will cause high charge currents and some degree of battery wear and outgassing. Lead acid batteries, however, are one of the most tolerant battery chemistries to mistreatment of this and other types, so that helps. You could minimise battery wear by using a trickle charger when this happens, obviously, but compared to many of the other forms of battery damage, doing this a few times a year when you need your car boosted is marginal. Just storing a battery in different states of charge can greatly affect it's life. People would be a lot more reluctant to give each other a boost if they had to wait around and trickle charge each other's cars. Instead, we often use a running car to start the other car, no doubt causing high charge currents, disconnect and allow the now running car to charge itself. Automotive batteries are designed with a reasonable amount of this being considered regular use and as a result are designed to be reasonably tolerant. Just charging it at different rates within the battery's spec will affect it's overall life, so absolutely charging it at rates outside it's spec will cause wear.

You may be aware that one of the best ways to damage a lead acid battery is to allow it to reach extreme low temperatures when fully discharged, as this situation raises the freezing point of the liquid enough that it will actually turn solid, expanding and bending/damaging the plates in the process. This often kills the battery in one shot. It is greatly preferable to suffer a small amount of battery wear due to high charging currents rather than allow it to freeze while I find someone to trickle charge it. If you happened to be at home however and you have the spare time, by all means extend your battery life a tiny tiny bit by popping it out, bringing it inside and trickle charging.

Lead acid batteries are also one of the easiest types to recycle(although this is not always done well) and that helps keep their cost low for what they are. Plates can be cleaned/straightened/replaced and acid can be refreshed, meaning that if you had a system wherein repeated over aggressive charging cycles were necessary due to space limits, the right equipment and a regular maintenance cycle would mitigate and minimise the cost of the damage.

The low cost, ease of designing a durable, tolerant, safeish, high current battery with lead acid is a huge part of the reason they have not been supplanted by lithium ion drop-ins or super capacitor arrays.

You may also find this article of interest. A few interesting quotes from it:

"For most all lead acid based batteries—Gell, AGM, Conventional—you can safely select a charger with a maximum charge current that is no greater than 20 to 25% of the batteries capacity. I know this article is about fast charging but I should also mention that you do not want to use a charge current of less than 3% of capacity (think trickle chargers and maintainers).

Try and Remember This Safe Maximum Charge Current Jingle: Rule-of-Thumb is 5 to 1"

This is an indication of how tolerant even the less favorable types of lead acid batteries are to fast charging. Starting batteries in cars tend to be thin plated types with even greater tolerance.

We also have:

"The premium pure lead AGM batteries such as NorthStar and Odyssey that use thin plates and are highly compressed can actually accept a charge rate equal to their stated amp hour capacity! That’s right they can be charged at 100% of their amp hour rating!"

So on some starting batteries they are able to keep the internal resistance so low as to charge in one hour.

So the conclusion we can reach is that while for some battery types especially a trickle charge will be favorable to their lifespan, in general lead acid batteries are much more tolerant to this than you originally expected, with a 10A charge or more being fine for the ~50Ah-ish batteries of even smaller cars. While a higher draw for very short periods of time while batteries equalize or during initial charging phase may cause some wear, it is not likely to be of great impact due to the rarity of the event.

Another thing to bear in mind is that the 75A rating of your alternator describes the capability of it's output, and it regulates voltage, not current. Chances are your battery was chosen for the car to have reasonable draw in these situations.

Actually saying "alternator does not have current limiting and will hit it with max amps" would have been enough. :)

To say it would "Hit it with max amps" would be grossly misleading and I would not say that. Some current limiting will be provided by the impedance of the internal connections and alternator windings and regulation lag, just not much, and not intentional. During a very brief initial phase the battery will draw a current spike but for a very short period of time due to the fact that the great majority of the power the battery stores is in a moderate voltage range. Due to this, the higher the current spike, the shorter it will be. Some months ago I believe one of the engineers brought up an article during a question about battery balancing, where testing had been done for damage due to the brief high current spikes caused by connecting imbalanced lead acid batteries in parallel. It's bedtime so I won't attempt to dig it up, but you're welcome to search for it.

So in theory, if one-way current limiting device is connected in series to the battery, it would prolong it's life, right?

In theory it might prolong the battery life a very tiny amount, unless you plan to repeat the event many hundreds of times, or perhaps dozens of times if your battery has very low CCA rating (indicating possibly thick plates) or if you had an unusual battery type in your car, like a deep cycle. If you were going to mess with it at all, some testing would be in order and you would probably want to use a full BMS capable of the actual maximum charging current of your particular battery rather than just a current controller so as not to cripple the function of your car. If you had to run your car unnecessarily for 30 mins to bring it up to charge, the cost of gas would greatly exceed the cost of damage to the battery from the brief initial charge pulse, would be more damaging to the trees and animals or whatever, so you would want to avoid that situation. You would also have to figure out how to change the high current wiring of your car and provide power connection posts that bypassed the battery for the situation where you wanted to boost start off a running car.

• Thank you for detailed answer. Actually saying "alternator does not have current limiting and will hit it with max amps" would have been enough. :) So in theory, if one-way current limiting device is connected in series to the battery, it would prolong it's life, right? – elektrinis Jan 14 at 8:51
• Edited to cover additional questions. – K H Jan 14 at 9:22
• So to sum it up, if battery voltage it low, charging current would only be limited by series resistance. That's what I wanted to hear. Thank you. – elektrinis Jan 14 at 13:09
• You've heard what you wanted to hear, but this site is for everyone who comes looking for this answer in the future, and that makes the other information crucial. I would feel bad if I thought that people stumbled across this and thought a reputable source had told him that this circuit was a necessary or useful addition to most/many cars. You have the money to spend so it's fine if you'd like to arbitrarily do this project based on that cherry picked piece of information, but it's important that anyone else who sees this understand how little there is to gain and what they might lose. – K H Jan 14 at 20:33
• K H, I am not going to buy or sell any extra circuit. My project is different entirely and this was just a part of information I was missing - how is current being limited during a battery charge in a car. If you are interested, I have an idea to make a DIY head unit based on raspberry, but hate to see it boot each time, so was going to leave it idling, which consumes quite some energy (50% of battery in few days). I am aware cycling of lead-acid batteries is not healthy, however I also know that hitting it with high current charge is much worse. So I will look for different solution. – elektrinis Jan 15 at 9:06

Automotive batteries are designed to crank the engine to start the car. For the longest battery life, the car should be maintained sufficiently to prevent the need for more than a very brief cranking time. The battery is designed to supply the very brief but high cranking current and then be immediately recharged. If it is used to supply loads when the car is not running, the life will be shortened. If it is completely discharged even occasionally, the life will be shortened considerably. However, the charging system should also be designed to limit the charging current.

• The question is about charging, not discharging. – elektrinis Jan 12 at 17:08
• My point is that even if you control the charging rate carefully, the battery will fail prematurely if you let it discharge too low. Car batteries should not be used to power things until the battery discharges and then charge it. For that you need a deep discharge battery. If your car stands unused so long that the battery discharges too low, the battery will fail prematurely. Depending on climate, you may have more problems than just the battery. – Charles Cowie Jan 12 at 17:36
• Sure, however even cheapest car batteries can do couple hundreds of 100% DOD cycles if done at 0.1C current. But if you discharge once and hit it with 50A of charge, then this will be your last cycle. – elektrinis Jan 12 at 17:53
• Yes, car batteries can do some number of deep discharge cycles. There is probably data available about percent and cycles. Car charging systems are designed to avoid destroying the battery after one deep discharge, but perhaps not 100%. The design is adequate for recommended use, whatever the actual numbers are. I don't think letting a car stand unused for a month will be too risky. If the battery discharges very much in that time, there is a problem in the electrical system. If you live where it is cold in the winter, you may have both electrical and mechanical problems with no use for 3 mo. – Charles Cowie Jan 12 at 18:19

Alternators nearly always have a regulator, which adjusts the current through the field coils to keep the voltage constant,so that within the range of voltages a healthy battery should operate the charging current will be sensible.

If you're fitting a discharged but healthy battery, usual practice would be to charge it slowly using a battery charger before attempting to start the car, because

1) Pulling the 100A+ starter current will likely kill the battery, and not start the care

2) The large current draw will likely stress the alternator

3) And, as you identify, the battery will be charged to fast.

• Alternators control the current through the rotor coil, the stator coils produce the current... How will the starting current of 100A stress the alternator which is not working yet as the engine is not turning fast enough? Car batteries will supply 300 to 800A for cars depending on engine size. Large diesel engines can demand 1000A or so... – Solar Mike Jan 12 at 14:14
• If your battery is flat, and you want to charge it using the alternator, you'll need to start the engine first. Doing so will draw the cranking current for the starter motor from your depleted battery. – james Jan 12 at 21:54
• so, how does the depleted battery supply the starting current - what do you think depleted means... Most of what I put in my first comment was to find out if you really know how an alternator works.... and the evidence is there... – Solar Mike Jan 12 at 21:58
• This appears to have descended into ad-hominem territory. I can only assume you've failed to understand what I said above. You can start a car with a depleted battery, the maximum current that a car battery can supply doesn't decrease that much with voltage. However doing so will usually drastically shorten the life of the battery as the surface area of the electrodes will be reduced when the lead comes out of solution, after you've done that charging a battery from completely flat, will put more stress on the car's alternator as it's usually regulated for constant voltage. – james Jan 13 at 0:32