High power automotive current limiter

[edrean]

Hi there!

I am trying to design a system to charge an aux AGM battery from a vehicles alternator. When the AGM battery is flat it will draw a large current when connected to the running alternator. To protect the alternator from damage, I would like to limit the current draw from the AGM battery to around 60A or so. The alternator's output is regulated at 14.2VDC.

What would the easiest/best way be to implement such a current limitation?

I have looked at a circuit involving 2 transistors and 2 resistors (**broken link removed**), but I'm worried about the power dissipation that might be involved in the sense resistor (0.6V x 60A = 36W).

If you clever people could just give me some direction it would be great!

 

[Diver300]

That current limiting circuit will always have quite a big voltage drop, at least 0.6 V, and there could be loads of heat generated in the transistor.

For currents that big, you need some serious power transistors. You should also have a circuit with a much lower value sensing resistor, and an amplifier to control the current. You should also have a connection to ground, so that you can have a supply to run the amplifier and to turn on the transistor. That will mean that the voltage drop will be very small when you don't need to limit the current.

 

[edrean]

I can get a transistor that looks as if it could work (https://www.electro-tech-online.com/custompdfs/2012/02/0900766b8002db6f.pdf). What I'm worried about is the power being dissipated in the sense resistor when current is being limited at 60A (voltage drop of 0.6V with 60A going through it). Or am I wrong?

Is that circuit even the right thing to be looking at?

 

[crutschow]

Alternators are self protected from high current. You don't need to add an external current limit unless you are trying to protect the battery. In that case just add a low value power resistor in series. You don't need any transistors.

 

[ronsimpson]

I am trying to design a system to charge an aux AGM battery from a vehicles alternator.

I think that you have two batteries in the system. The "aux battery" is connected / dis-connected from time to time.

If the alternator is only charging the aux battery I don't see the problem. If the normal car battery is fully charged and you add in a discharged aux battery I see a huge current flow from one battery to another.

 

[edrean]

Yes, the AGM battery is a second battery in addition to the starter battery. When the vehicle is off the AGM will be isolated from the starter battery. When the vehicle is running the AGM will be connected to the alternator and therefore to the starter battery as well. If the AGM is flat enough it might draw more than 100A. Depending on the alternator this might be more than the alternator is capable of supplying, so it will draw the rest from the starter battery. I would like to avoid that.

If I go the series resistor route, how do I calculate the value I would need?

 

[crutschow]

.............................

If I go the series resistor route, how do I calculate the value I would need?

Divide the voltage difference between the alternator charge voltage (perhaps 14V) and the minimum AGM discharged voltage and divide that by the maximum current you want. For example if the discharged voltage were 10V and you wanted a maximum current is 60A, the resistor value would be (14-10) / 60 = 0.066Ω. If you can't buy that value you can parallel some larger values to get what you need. If you used a commonly available 0.1Ω value, the maximum current would be 4/0.1 = 40A. The peak resistor power would be high but it's likely the current will rapidly diminish since the AGM voltage should rise very quickly with that amount of charging current.

 

[charlie_r]

I don't know about your location, but try looking for a commercial product called a "battery isolator". We use them here on medium trucks to run aux equipment like winches and snowplows.

 

[ronsimpson]

I think this is a common problem in motor homes. I think there is a answer there.

What isolates your AGM battery?

In motor homes they add a diode to isolate the two batteries. It allows the AGM battery to power lights and not allow the light to get power from the starter battery.

I don't think you have a problem of the alternator being too small. If you have a diode isolator, then the alternator will send current into the battery that is lowest. In your case it will sent current into the AGM battery first. In about 15 minutes it will have some charge in the AGM battery and it will start charging both batteries. (more current to the AGM and less to the starter battery at first)

If your isolator is a switch then there is a problem. If the isolator is a diode or some part that comes in a motor home then I don't see the problem. Ask a motor home repair expert.