Problems with Battery Tester Circuit

[rjenkinsgb]

I would need at least 25V, 30V would be best

NOT with that opto driver - that gives about 8V out, those FETs are logic level devices and would be switched fully on by one of those.

That make the FET gate ratings totally independent of the overall system supply voltage and avoids any possible excess gate-source volts or drive circuit leakage to the load side.

 

[ADWSystems]

So doesn't raising VgsMax. I haven't been able to figure out how to hook up the FET driver to get a linear response. I I haven't been able to find any I have a model for, so simulation is out. Plan A. The direct route. The problem is VgsMax is too low. Solution: Find one with a higher rating. Simple solution. Now to find one.

 

[ADWSystems]

NOT with that opto driver - that gives about 8V out, those FETs are logic level devices and would be switched fully on by one of those.

That make the FET gate ratings totally independent of the overall system supply voltage and avoids any possible excess gate-source volts or drive circuit leakage to the load side.

But that wasn't the request.

I haven't been able to figure out how to hook up the FET driver to get a linear response. I I haven't been able to find any I have a model for, so simulation is out. Plan A. The direct route. The problem is VgsMax is too low. Solution: Find one with a higher rating. Simple solution. Now to find one.

 

[ronsimpson]

VgsMax >30V

You are not going to find 30 volts parts. You need to build a gate drive circuit that limits the voltage to a reasonable voltage.
There are many different ways. This one uses a 12V zener to reduce the +24 to +12. (add a capacitor from +12 to ground)
Also, use a R-R input op amp and save the -12V. Or at least use a op-amp that has a common mode input voltage range that includes the negative supply. (to save the -12V supply)

 

[rjenkinsgb]

The problem is VgsMax is too low. Solution: Find one with a higher rating.

They are not made, as far as I know.

I routinely work with industrial gear that has FETs or IGBTs on the "high side" of a 600V supply. They are still only driven at 12 - 15V and typically have a 20V gate limit, but use isolated or bootstrapped gate drivers to make the overall supply voltage irrelevant.

A bootstrap system puts current through the load circuit so is not suitable for your application. That leaves an isolated system and the opto driver is the simplest and cheapest type you can use.

Just add an appropriate load resistor across the output and you get a pretty linear response over some input current range; look at the graphs in the data sheet for whatever device.

 

[ADWSystems]

You are not going to find 30 volts parts. You need to build a gate drive circuit that limits the voltage to a reasonable voltage.
There are many different ways. This one uses a 12V zener to reduce the +24 to +12. (add a capacitor from +12 to ground)
Also, use a R-R input op amp and save the -12V. Or at least use a op-amp that has a common mode input voltage range that includes the negative supply. (to save the -12V supply)

I wouldn't actually need the zener as there is boost converter raising +12V to +24V. With Vgs of 10V, 12V power supply - 10V Vgs = 2V Vload limiting the circuit to testing a maximum of 2 cells. Not very useful.

As for parts with VgsMax>30V, they do exist.

These are only from OnSemi. I have never been very good at finding parts to meet a spec. The only thing I can think to do is to find and check each manufacturer individually then try to find one (ore more) that are for sale from a vendor that I can purchase small quantities from. This means I have (a) find ALL the manufacturers of MOSFETs, then (b) search each one to make a list, then (c) try to find a vendor that has stock and sells small quantities. The ALL part and the vendor search is where I get usually zilch out. I'm hoping by now there is a better way but I have yet to find it. So step 1, where do I find a list of MOSFET manufacturers?

 

[ronsimpson]

I have never been very good at finding parts to meet a spec.

Try going to KigiKey.com or some other supplier and search for parts. This way you can look at all types at one time.
There is a 45 volt MOSFET but these kinds of parts are hard to find, may not be made next year. Try to use parts that are common.
DigiKey FET
use a 78L12 voltage regulator.

 

[ADWSystems]

Try going to KigiKey.com or some other supplier and search for parts. This way you can look at all types at one time.
There is a 45 volt MOSFET but these kinds of parts are hard to find, may not be made next year. Try to use parts that are common.
DigiKey FET
use a 78L12 voltage regulator.
View attachment 115703

I hear you on the 12V supply to the opamp driving the FET. But if Vload is 14V then Vg must be 10V above that at 24V. How do I get that with a 12V positive supply?

View attachment 115704

I don't understand the zener in this circuit? It does not appear to be based on Vs. The zener has no way to clamp Vgs to less than 20V when Vs may range from 1V to 14V thereby requiring Vg to range from 0 to 24V. A 12V zener in this circuit is equivalent to the 12V power supply. Both would result in the same problem, Vg max =12V therefore Vs max =2V, but Vs MUST be able to reach 20V.

I misspoke earlier and screwed up a calculation. I will actually need to bump +24V to +30V. As I have been saying. Vg = Vs +10V; Vs= 1.0 * # of cells. Max number of cells is 20, therefore Vs = 20V, thus Vg must be able to reach +30V. The opamp can handle a 30V postive rail as can the boost converter on the 12V supply (currently producing +24V).

 

[ronsimpson]

But if Vload is 14V

I don't know where Vload is.
If all switches are closed. Vth_Set goes from 0 to 5V and the voltage on RLoad also goes from 0 to 5V. (assuming there is a battery that has at least 6V)

IRFP064??? 70A transistor. OK I don't understand your circuit. As drawn you made a 0 to 5mA current source.
If RLoad was 1 ohm then you would have 0 to 5A load on the battery.

Vs may range from 1V to 14V

Where is Vs? (Source of Q1?) If all switches closed Vsource can not get to 14 volts.

What is the battery voltage? Minumum battery voltage? (From post #1 you say 2.1V) The battery voltage must be more than Vth_Set or the entire thing falls apart!
What is the current in Q1?
What do you want the circuit to do? Discharge the battery at a constant current?

 

[ADWSystems]

I don't know where Vload is.

For lack of any other way to say this, Vload is the point labelled Vload.

Where is Vs? (Source of Q1?)

Which also happens to be Vs, the source of Q1.

IRFP064??? 70A transistor. OK I don't understand your circuit. As drawn you made a 0 to 5mA current source.
If RLoad was 1 ohm then you would have 0 to 5A load on the battery.
What do you want the circuit to do? Discharge the battery at a constant current?

I beg to differ. That is exactly what it does.

What is the current in Q1?

Vload/Rload. In this simplified circuit Vth_Sel milliamps.

What is the battery voltage? Minumum battery voltage? (From post #1 you say 2.1V) The battery voltage must be more than Vth_Sel or the entire thing falls apart!

That depends on the battery pack and it's condition. The final design is to test Nicad batteries and packs from 1 to 20 cells (0 to 26V) with load ratings of 100mAHr to 6300mAHr (test currents of 0.1c = 10mA-630mA).

If the battery voltage is less than Vth_Sel at the start then the test done.

This is a simplified but completely representative test circuit of the final design. The final design covers 4 pages. The test circuit as shown is the one being tested to make sure the basic concepts are working. With that said, the 5V referenced pot is actually a 5x opamp in the final design (0-25v but will need to change to 6x 0-30V). As it is it has already detected two problems. 1. VgsMax conflict and 2. The battery may read good voltage but once under load dies (this is a case for the processor to detect not the circuit).

 

[ronsimpson]

But if Vload is 14V

The circuit you posted has no chance of Vload reaching 14V. (with the switches closed)
Now I understand this is not the circuit and Vth_Set is actually 0 to 30V. TLoad is not 1K at all, etc.
Sorry for my not understanding. I hope you get it working.

 

[ADWSystems]

It IS the test circuit. I'm sure there is a bit of TLDR. I chose to use 1k load and 5V threshold to avoid smoking any parts during initial testing. I'm sure you have done something similar. And it does work, when I use a battery that has capacity left.

Two changes and it's real. 1. Connect pot to +24V (soon to become +30V), and reduce Rload.

The issue at the time the OP was written. was the fact that one of the batteries (used AA alkaline) had no capacity left. It had absolutely nothing wrong with the circuit or its operation. But during the troubleshooting process discovered the possible issue with VgsMax.

The facts are that with S1 open and S2 closed, the OpAmpOutput (Vg) will go to the upper rail (+24V) and exceed Vgs (Vs=0) and (likely) pop the (present VgsMax=20V) FET.

I'm not looking to get silly complicated. K.I.S.S. is best. Are the circuits presented using zener diode the only way to control Vg so as to limit Vgs? The best way to control Vgs?

 

[ronsimpson]

Here is how I would discharge a battery.
The idea is to keep the voltage across R1 to 0 to 2V. I normally only go to 1V. V1 must be larger than V3.
The supply is a single 12V supply. U2 must work when the inputs are at 0V. I chose a R-R input amp.
R3 stops some 1mhz oscillation. C1 slows down the amplifier to keep it stable.
The Gate voltage is restricted to 0 to 12V.

No switch on R1.

 

[ADWSystems]

The switch on R1 is not in my circuit. It was added to debug the circuit when Vs and V1 when to 0V when M1 (Q1) began to conduct. As mentioned the problem was the battery not the circuit.

My circuit works fine to do the same thing. Just change the power supply.

I don't see how this works to discharge a 20 cell battery pack (V1 starting at 26V) and not allowing Vg to drop below 1V per cell (20V).

This post was to debug a particular issue and focused on that issue. I am always game for improvement. Let me see if I can recall all of the design parameters:
-Discharge single and multi cell NiCd battery packs; 1-20 Cells with the inital charge ranging from 1.0-1.3V per cell
-Final voltage not to fall below 1V per cell
-Discharge at a constant selectable current equal to 0.1C; Cell capacity to range from 100-6600mA-Hr (discharge rate at 0.1 = 10mA to 660mA)
-Access to the battery and load voltages, preferable also the opamp output voltage, for data logging.

If I recall more I will add them here.

 

[ronsimpson]

I don't see how this works to discharge a 20 cell battery pack (V1 starting at 26V) and not allowing Vg to drop below 1V per cell (20V).

I do not understand. Maybe I don't understand what your MOSFET is doing.


My circuit: It is a constant current load. My test shows the current is exactly the same from 40V to 1.5V. That is 0.1V/cell in a 20 cell battery.
Will work with any total battery voltage from about 2V to 100s of volts. The upper voltage is only limited by the breakdown voltage of the MOSFET. The battery voltage is mostly across the MOSFET and not across R1. The R1 voltage is kept low. Really R1 is only there to measure/set current.

 

[ADWSystems]

How about one aspect at a time.

How does your circuit keep the battery voltage from falling below 1V per cell?

I ask because as you noted, the voltage across R1 is limited to 2V. This means to me that for battery pack with more than 2 cells, the circuit will drain it to 2V or less to MUCH less than 1V per cell.

 

[ronsimpson]

How does you circuit keep the battery voltage from falling below 1V per cell?

OK it is my problem. I now see you want to disconnect the load from the battery under some voltage. I thought you wanted a load that could be be set for "10mA-630mA ".

Try this. R2/R4 divides the battery down (1/3) so the voltage is easy to measure. V1=battery
Put the MOSFET on ground so it is easy to gate drive.
This circuit will want to oscillate but that I can fix.

 

[ADWSystems]

I don't think it is your problem or my problem. We just need to sync up.

I know I'm a bit scoped locked on the layout of your previous/my current design (which are VERY similar). How is this circuit working? Looks like it will load V1 until it reads 4V and is no longer a constant current load.

 

[ronsimpson]

In post #37 the load is R1. There is no constant current. The transistor will not be hot because all the power is going into the resistor. The transistor will be on hard or full off.

In post #33 the load is constant current with no low voltage limit. Here the transistor is the load (hot) and the resistor is to measure current.

If you want constant current and low voltage cutout we can push the two circuits together.

 

[ADWSystems]

The constant current is a requirement. But the 'constant' current must be adjustable to be close to but not more than 0.1C of the battery. By adjustable I mean selected for the test, not varying during the test.