I have this circuit and it almost works as expected. But I am wondering if I am damaging the rechargeable 12 V battery used to power it. I have also attached some voltage and current data for the circuit. These measurements are made using an ADC (not shown in the schematic).

The issue is the Vcontinuity voltage dropping so low, which means the drain voltage is also well below 12 V, the supply voltage from the 12 V battery. Am I damaging the battery by, in essence, shorting the +12 V terminal to ground through the MOSFET and tiny current sense resistor?

I can correct the continuity data (reading False when current is still flowing) by changing how I measure it. I can pulse the MOSFET gate for around 300 ms, then turn it off, measure the continuity, and then pulse the gate again with a higher voltage. But that does not address the issue of the MOSFET shorting the +12 V battery to ground.

Should I be be worried, or am I seeing ghosts?

Thanks!

 

Is it stable? Each section of a TLV2462 has a 90° phase shift above 1kHz, and the capacitance of the MOSFET gate will contribute a few degrees more. That’s enough to make it into an oscillator.

 

Do you mean stable as originally designed or with the change to the software to cycle the MOSFET off to measure the continuity, and then on again to light the igniter? Either way. I have tested both and it is stable.

 

You never said the chemistry and size of your 12V battery. A lead-acid car battery?
A powerful car battery can destroy your switch, wiring and Mosfet.
The datasheet for your Mosfet shows that some of them need a gate voltage that is at least 4V for it to turn on and get destroyed in your circuit.

Why do you want extremely high current?

 

What are the units in the column for the desired current, microamps, milliamps or amps?

 

What are the units in the column for the desired current, microamps, milliamps or amps?

All currents are in mA, all V_* are in volts. The battery is a FlashFish E300 330W Portable Power Station. I have attached all the info I have on it.

I have run the circuit both with a 12V car light bilb as a load and with an actual igniter, and it works as designed. The igniter bursts into flame, and the light bulb tuns on. Now that I am shutting off the MOSFET to read the continuity, the light bulb turns on and off as expected. On for 400 ms and off for 100 msec.

When I say "run the circuit" I mean around 15-25 times so far. Nothing has happended to the FlashFish but it shows a loss in % charged. It displays how many watts it is consuming and a % charged. The circuit consumes about 2-3 watts when "idling", and around 4-6 watts when igniting. However, there is a bit of a lag for the "watts" display to change, so the "instantaneous watts out" is not very accurate nor very "instantaneous".

 

The unknown battery you are using to power the FlashFish is overloaded since its voltage drops low. The overloading could damage your unknown battery.

 

The unknown battery you are using to power the FlashFish is overloaded since its voltage drops low. The overloading could damage your unknown battery.

Any suggestions on how to modify the circuit to the batter is not damaged over time?

 

I think you are doing right, so do not be afraid. But overloading can be harmful, so don't take the risk and make sure you use the safety precautions.

 

Please tell us the chemistry and post the datasheet of your battery. If it is Lithium-Ion then overloading it can cause it to explode and catch on fire.

 

Please tell us the chemistry and post the datasheet of your battery. If it is Lithium-Ion then overloading it can cause it to explode and catch on fire.

The user manual says Lithium-ion battery, 18650, 300 Wh. I am using the DC output 12-16.8 V 10A (Max). The max current I am drawing is 3 A for 400 msec. Is there a different battery that I should use?

 

The specs for the Power Station says it safely shuts down the 12V to 16.8V from its internal Lithium-Ion battery if the output current is higher than 10A.

The battery voltage drops when used maybe because the battery or its charging circuit is defectlve. Maybe the battery has never been fully charged. Maybe the battery is old and needs to be replaced.

Your ignitor circuit might draw a current much higher than the 3A you said, if the Mosfet is very sensitive.

 

Does it really take almost half a second for the igniter to go open? That seems long to me. Can you get an oscilloscope capture of one igniting?

 

The specs for the Power Station says it safely shuts down the 12V to 16.8V from its internal Lithium-Ion battery if the output current is higher than 10A.

The battery voltage drops when used maybe because the battery or its charging circuit is defectlve. Maybe the battery has never been fully charged. Maybe the battery is old and needs to be replaced.

Your ignitor circuit might draw a current much higher than the 3A you said, if the Mosfet is very sensitive.

The FlashFish is about 2 years old, but has hardly been used. Probably only 4 discharge/recharge cycles in that time. The battery charges without any indication of a problem, and the output voltage is around 15.8V. I don't think the battery is defective.

 

Does it really take almost half a second for the igniter to go open? That seems long to me. Can you get an oscilloscope capture of one igniting?

In that particular test run, the "igniter" was a 12V car light bulb, and not a true igniter, so it never "opened". There is no data available on the characteristics of the current StarTech igniters, and I was hoping to get some data if this rocket launcher ever "gets off the ground"! The old Estes wire igniters took about 30 ms and 12A from a 12V car battery to break (see attached).

Unfortunatley, I don't have the test gear needed to repeat the experiments in the attached screenshot. It is from a very NAR old white paper on igniters. Since I am using light bulbs as stand in igniters for most of the testing, I need the current for a little more time so I can actually see the light bulb glow. When I have attached actual igniters and tested them, they do glow, produce a flame, and 2 out of 6 have actually broken.

 

Your Lithium-Ion battery has four 4.2V cells in series so it is 16.8V with its charging current dropping low when it is fully charged.
My attached graph from batteryuniversity.com shows that your 15.8V charged voltage is charged to only 50% of its capacity.
The graph shows a battery that is fully charged in 3 hours. But its voltage is the same as yours (15.8V for 4 cells) in only half an hour.