Automotive charge current limiting

Thread Starter

Sergio34

Joined Nov 10, 2018
52
Hi, as usually I experiment on my guinea pig motorcycle to make things, I have that dream now of making a robust regulator/rectifier that will never die. As I understand so far generally in voltage regulators a low resistance has to be at the output, for ex. the datasheet of LM317 shows a 0.2Ω resistance at the output to protect the cells of the battery. It makes sense that any charging circuit should have such resistance as the internal resistance of the battery is very low, correct? But as I see in the bike’s wire diagram there is not such resistance which again makes sense as it would burden the alternator with more load during operation. At any given moment I believe the current draw is anything around 10A (headlight only needs 4.5A) so even if the reg/rect had a 0.2Ω at the output that would be 20W loss, plus a bulky resistor. Is it assumed that this low resistance is provided by the current path (wire, connectors etc)? If yes, is this a reliable practice? It seems like my tendency to be hypochondriac when I do electric repairs and make the crimp connections as low resistive as possible using both crimp and solder is not good here…
 

dl324

Joined Mar 30, 2015
10,985
A wall of text and no description of your application. Twenty views and no bites. I wonder why...

Automotive current limiting is typically done with fuses.

Ideal voltage sources will have 0 ohms internal resistance.

A bare LM317 will limit maximum current to around an amp.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
52
A wall of text and no description of your application. Twenty views and no bites. I wonder why...

Automotive current limiting is typically done with fuses.

Ideal voltage sources will have 0 ohms internal resistance.

A bare LM317 will limit maximum current to around an amp.
You bit though, do you always use fuses instead of resistors? And what is wrong with wall of texts? You do not have to read but if you do, do it properly.
 

dl324

Joined Mar 30, 2015
10,985
You bit though, do you always use fuses instead of resistors?
No. For automotive applications, fuses and fusible links are commonly used. Some test equipment manufacturers used carbon composition resistors as a protection device, but PTC self resetting fuses would make more sense these days.
And what is wrong with wall of texts?
It's a poor reflection on your education. We were taught as children to use paragraphs and proper punctuation to express and organize our thoughts.
You do not have to read but if you do, do it properly.
I'm trying to be helpful. If you want to be snotty, that's your choice. One more dig from you and you'll be on my ignore list.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
52
No. For automotive applications, fuses and fusible links are commonly used. Some test equipment manufacturers used carbon composition resistors as a protection device, but PTC self resetting fuses would make more sense these days.
It's a poor reflection on your education. We were taught as children to use paragraphs and proper punctuation to express and organize our thoughts.
I'm trying to be helpful. If you want to be snotty, that's your choice. One more dig from you and you'll be on my ignore list.
I am not asking for short circuit protection, the regulator’s output is 14.5 (I thought this is implied since I am talking about automotive) this is 2.5V more than a 12 battery. Even if the battery has 0.5Ω resistance the charging current would be 5A which is too much for small motorcycle battery of 6Ah, even for larger batteries. It should be around 600 mA to 1A. In fact the service manual states that the battery charges with 900mA when engine is on. So I wonder how the current going into the battery is reduced.
 

shortbus

Joined Sep 30, 2009
7,761
So I wonder how the current going into the battery is reduced.
Most motorcycles alternators are pemanent magnet aternators, while car/truck alternators are wound field(in the rotor). The wound field ones use a regulation based on the electromagnetism of the field, the voltage applied to it, to control the output. Motorcycle permanent magnet alternators can't do that. so instead as they reach the regulator set point the extra electrical energy is sent to a large resistor and dissipated as heat.

Short answer is car and motorcycle alternators are different and aren't regulated the same way.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
52
Most motorcycles alternators are pemanent magnet aternators, while car/truck alternators are wound field(in the rotor). The wound field ones use a regulation based on the electromagnetism of the field, the voltage applied to it, to control the output. Motorcycle permanent magnet alternators can't do that. so instead as they reach the regulator set point the extra electrical energy is sent to a large resistor and dissipated as heat.

Short answer is car and motorcycle alternators are different and aren't regulated the same way.
Thanks, I know that, my question though is not what happens between the alternator and regulator but what happens between regulator and battery. I measured and I saw with my own eyes that voltage when (motorcycle) engine is running is 14.5V and there are two wires from regulator to the battery one to positive and one to negative, nothing else between, there are two assumptions I can make:

No 1: the battery charges with more than 3A which could reduce the life of the battery but instead it lasts for 4-5 years.

No 2: motorcycle batteries have high internal resistance which contradicts the other fact that these batteries have a resistance in the mΩ range.

So it is still a mystery for me
 

geekoftheweek

Joined Oct 6, 2013
436
The battery will only charge as much as it needs to for a given voltage. It's all about converting chemicals into other chemicals to shed electrons (or attract them in the case of charging). Once the reaction is in equilibrium it will stop until something else happens. It takes a little more voltage to get it going and keep it going so your battery is not actually at 14.5 volts (should be somewhere between 12.5 - 13 for the most part). What your battery voltage is with the engine off it the actual voltage your battery is charging to.

Edit...

Unfortunately I am no expert, but I think the internal resistance has something to do with the overall charge state of the battery and how much current it can deliver and not so much likr a normal resistor.
 
Last edited:

geekoftheweek

Joined Oct 6, 2013
436
In fact the service manual states that the battery charges with 900mA when engine is on. So I wonder how the current going into the battery is reduced.
At first I thought a shunt resistor would work to measure current and work it out, but then you still have the variables of whether or not your lights are on and such since they would draw power after the regulator when charging. Maybe a typical battery is chemically limited to around 900 mA.
 
One more thought... I work regularly with four 12 volt batteries in parallel (semi trucks). At a dead state using a 16 volt 100 amp charger the batteries will only draw around 20 amps (according to the charger gauge). I have had the charger apart and can tell you there is nothing in there to limit the current. By the time they are charged enough to get a couple good cranks and hope for the best the charger shows the batteries are taking a 60 amp charge. The only time the gauge gets close to 100 amps is if you try to start the engine... then it pegs the gauge and the thermal overload will kick out if you try too long.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
52
One more thought... I work regularly with four 12 volt batteries in parallel (semi trucks). At a dead state using a 16 volt 100 amp charger the batteries will only draw around 20 amps (according to the charger gauge). I have had the charger apart and can tell you there is nothing in there to limit the current. By the time they are charged enough to get a couple good cranks and hope for the best the charger shows the batteries are taking a 60 amp charge. The only time the gauge gets close to 100 amps is if you try to start the engine... then it pegs the gauge and the thermal overload will kick out if you try too long.
Thanks, probably it s a combination of the things you said.
 

shortbus

Joined Sep 30, 2009
7,761
No 1: the battery charges with more than 3A which could reduce the life of the battery but instead it lasts for 4-5 years.
Voltage at that point is what is being regulated, not current. The excess current is being shunted to ground. And to another view, when measuring the voltage with the battery connected, how do you know what is being measured is just the battery voltage?
 
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