Solid state relay PWM help

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esus
 
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Solid state relay PWM help

Post by esus »

Hi im an electrician with little electronics knowledge or experience and reached a bit of a dead end with my attempt at building a high capacity lithium ion battery tester.

I have been exploring the viability of a Solid state relay by either Crydom or Omron capable of being used in series with a suitably sized resistor and pulse width modulation on the signal input to the SSR to enable constant current discharging.

(The idea being as the voltage decreases on the battery under test the pwm would allow the load to be pulsed and ((hopefully)) the current can remain close to constant)

Its potentially a small part of a much bigger project but I wanted to see if anyone can give me some helpful information.

Failing that my alternative solution for creating a circuit with constant current discharge is an array of resistors in parallel being switched by smaller relays.

I did some research on that and I felt it was certainly an option but preferred the idea of pwm perhaps due to a more precise control of the load current.

And lastly where is the best place can I share further details about this with an aim of collaborating with other people who might have an interest
im this?

The ultimate goal is an Adafruit based battery tester which can perform the following tests:

DC series resistance measurent( According to a specific IEC standard)

Coluombic charge efficiency

AH capacity test

Peak AH output

And my particular project is a 12KW off grid solar energy project where I intend to determine the current condition of a 12 KWh battery pack which has a number of knackered cells with new Lithium Titanate chemistry 30AH cylindrical cells.

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adafruit_support_mike
 
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Re: Solid state relay PWM help

Post by adafruit_support_mike »

It's certainly possible to approximate constant-current discharge with PWM, but there are some details that matter:

First, you have to regulate your PWM to the battery voltage. The current through a switched resistor is V/R (the voltage across the resistor divided by the resistor value). As the voltage changes, so does the current.

The average current through a PWM'd resistor is (V/R)(t.on/t.off), where t.on/t.off is the PWM duty cycle: the amount of time the switch is on (currrent flows through the resistor) divided by the amount of time the switch is off (no current through the resistor).

Using that math, 4V/100(10%)=4mA and 3V/100(13.3%)=4mA. As the battery voltage falls, you have to increase the duty cycle to keep the average current the same.


Second, SSRs are most useful for AC current. They're designed to allow current to flow through in both directions. Batteries are DC devices, and battery discharge only needs current to flow in one direction. It will be more effective and easier to use power mosfets instead of SSRs.


Third, battery stack management is a major challenge because the battery's negative terminal can be many volts away from GND. Compensating for those voltage offsets tends to be difficult. It's usually easiest to use a battery-powered circuit that can be connected across a single cell.

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esus
 
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Re: Solid state relay PWM help

Post by esus »

Thank you - The maths part particularly.

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