Battery current limiter

[James_Watson]

Hello.

I want to design a current limiter for a battery powered project.

Battery will work at 42V when is fully charged and 30V when is discharged.

I want to limit the output current of the battery in 2 or 3 different values, depending the use, at 2 amps, 4 amps and 6 amps (with a potentiometer or different resistors).

Any help about which componets or integrated to use?. I have made some tests with mosfets, but the problem is the voltage changes between 42V and 30V and the current has not a fixed value when battery volatge changes.

Thanks for your time.

 

[betwixt]

There are several ways to do this but we first need to know your expectation when the current limit is reached. Do you want it to work like a resettable fuse, just disconnecting the battery if the current is exceeded and if so, what resets it, or just set a ceiling current, in which case it becomes a constant current source when overloaded. Note that the second scenario means the voltage will vary to keep the current constant.

Brian.

 

[BradtheRad]

A plain transistor provides resistive drop. By sending a fixed current through the bias, you persuade the transistor to admit a much larger fixed current. It doesn't matter what is the load or supply voltage. (Within reason, that is.)

It's a basic principle of transistor use. Mosfets are voltage controlled and not as easy to use in current regulation.

 

[James_Watson]

Hello Brian and Brad.

First of all, thanks for your answers.

I have to explain full detais, so you can get all the info.

The complete application is for adding a second battery, working as a charger with limited current of a first battery, which power a motor,

I expect to maintain current when the limit is reached, so this second battery, will keep 2 amps (for example) charging the main battery or powering the motor, dependind the state of the motor.

I hope this schematic can help.

Thanks again for your help.

 

[betwixt]

You can do this with two transistors and two resistors. I'm not able to draw diagrams from where I am at the moment but you should be able to find a schematic if you search. One transistor carries the main current and the other is biased by current through a sense resistor. You choose the resistor value so that when he current reaches the desired limit, enough voltage is dropped across it to bias the second transistor into conduction and it shunts the bias current away from the first one. limiting its conduction.

Note that all current limiting circuits will drop some voltage. It isn't possible to keep the voltage stable and limit the current at the same time, the very act of limiting the current is achieved by dropping the voltage available to produce it. For example, if you wanted to maintain 42V at the motor, the secondary battery would need a higher voltage than 42V.

Brian.

 

[FvM]

Charging a battery from another battery of same nominal voltage (same number of cells) doesn't really work. There will be a certain equalizing current when you initially connect the batteries but no relevant charge transfer. If cross-charging is the sole purpose of your current limiter design, it's not worth the effort.

You would need a boost converter (voltage step-up) for effective charge transfer.

 

[cupoftea]

Yes, you want to limit your battery discharge current, to 2A, 4A, or 6A.....
From your top diagram, you just need a buckboost converter, with an error amplifier to regulate its input current to 2A, 4A, OR 6A.

Why not use eg a HV9901B, and put it on constant off time mode.....then add in the error amplifier, make it "look" at the output of a current monitor in the battery supply.......and move it forward like that.

If you want to charge one batt from another....then as above, i dont know why you do this...but if you do want it...then just do the same again (input current regulated buckboost) from batt to batt.

Dont forget, any converter with which you regulate its input current, will need an output overvoltage shut-off. otherwise if accidentally started on no load, then it will go badly high voltage at its output.

You may even be able to buy a finished SMPS power supply off the shelf which gives you access to its error amplifier output....and then you just have to add the current regulation opamp etc. Try recom, tdk, mornsun, murata, vicorpower etc etc