I am building a NiMH charger for 2 AAA cells in series using the MAX712 chip.
I have schemed and built a layout on stripboard.
The build stalled as I was waiting for a boost regulator to arrive to boost an existing 3.7V supply to 6V (minimum voltage for operating the chip).
Regulator arrived and was connected. Circuit powered.
Major error - my scheme showed components but not the transistor heatsink location pins. I forgot to cut those tracks. Chip got hot.
Tracks cut, transistors checked for shorts, re-power. Didn't work.
Chip out, new chip installed, still didn't work.
Now plan B. Build the circuit a bit at a time on a breadboard.
6V power, 150 ohm resistor to pin 15, 0V to pin 13 gnd.
Pin 15 should shunt regulate to give 5V. I measure full 6V supply on pin 15, it is not shunt regulating. I'm assuming I have somehow cooked both chips though I don't know how I've done the second one.
I have schemed and built a layout on stripboard.
The build stalled as I was waiting for a boost regulator to arrive to boost an existing 3.7V supply to 6V (minimum voltage for operating the chip).
Regulator arrived and was connected. Circuit powered.
Major error - my scheme showed components but not the transistor heatsink location pins. I forgot to cut those tracks. Chip got hot.
Tracks cut, transistors checked for shorts, re-power. Didn't work.
Chip out, new chip installed, still didn't work.
Now plan B. Build the circuit a bit at a time on a breadboard.
6V power, 150 ohm resistor to pin 15, 0V to pin 13 gnd.
Pin 15 should shunt regulate to give 5V. I measure full 6V supply on pin 15, it is not shunt regulating. I'm assuming I have somehow cooked both chips though I don't know how I've done the second one.
the joys of diy.
best to build step at a time if you had problems.
replace both ic's.
i have been doing it 40+ years and still mess up sometimes.
best to build step at a time if you had problems.
replace both ic's.
i have been doing it 40+ years and still mess up sometimes.
Chip ordered. UK supplier.
RS only have surface mount version and list as a battery fuel gauge. It's not that smart! It's a smart charger, not battery gauge.
RS only have surface mount version and list as a battery fuel gauge. It's not that smart! It's a smart charger, not battery gauge.
Keep it simple.
Being a Minimalist, I charge my 800 maH NiMH AAA bateries from any available 5V USB supply through a resistor 😱
Value comes from using C (capacity in maH)/10 so 80mA and voltage drop, in my case 5V-2.4V so 33 ohm
Dissipation is 0.2W so any 1/4 w resistor is fine, 1/2W would have a nice safety margin.
Redo Math for your available power supply (3.7V ?)
Charge controller is my Smartphone. 😕
No, not any App at all, just put them to charge overnight and set Timer to 10 hours 😎
Works like a charm.
Just in case you are worried about overcharging, add a calibrated thermal sensor: the back of your fingers 😎
If they feel warm compared to any object at ambient temperature, they are fully charged and wasting non stored energy as heat.
No damaged caused if you forget and they take a couple hours extra, energy involved is small.
I call that an Intelligent Charger ... the Intelligence residing inside your own skull, of course 😀
Being a Minimalist, I charge my 800 maH NiMH AAA bateries from any available 5V USB supply through a resistor 😱
Value comes from using C (capacity in maH)/10 so 80mA and voltage drop, in my case 5V-2.4V so 33 ohm
Dissipation is 0.2W so any 1/4 w resistor is fine, 1/2W would have a nice safety margin.
Redo Math for your available power supply (3.7V ?)
Charge controller is my Smartphone. 😕
No, not any App at all, just put them to charge overnight and set Timer to 10 hours 😎
Works like a charm.
Just in case you are worried about overcharging, add a calibrated thermal sensor: the back of your fingers 😎
If they feel warm compared to any object at ambient temperature, they are fully charged and wasting non stored energy as heat.
No damaged caused if you forget and they take a couple hours extra, energy involved is small.
I call that an Intelligent Charger ... the Intelligence residing inside your own skull, of course 😀
Cells are embedded in hair clippers.
OEM charge system was trickle charging through a resistor. I suspect originally designed for NiCd then product updated to remove cadmium. The instruction sheet suggests to fully discharge every 6 months - again usually a NiCd recommendation to avoid the so called 'memory effect'.
1 - it is impractical to wait until cells are fully discharged before charging - you might get half a haircut! Full discharge is required for safe(ish) trickle charging as you describe.
2 - As a result of overcharging partially discharged cells, cells have a very short lifetime hence my desire to improve the charging system.
3 - As Magnus Magnusson used to say 'I've Started, So I'll Finish'. After all I will never improve my hobby skills if I don't complete a project.
OEM charge system was trickle charging through a resistor. I suspect originally designed for NiCd then product updated to remove cadmium. The instruction sheet suggests to fully discharge every 6 months - again usually a NiCd recommendation to avoid the so called 'memory effect'.
1 - it is impractical to wait until cells are fully discharged before charging - you might get half a haircut! Full discharge is required for safe(ish) trickle charging as you describe.
2 - As a result of overcharging partially discharged cells, cells have a very short lifetime hence my desire to improve the charging system.
3 - As Magnus Magnusson used to say 'I've Started, So I'll Finish'. After all I will never improve my hobby skills if I don't complete a project.
More non-success.
New chip arrived.
Connected a basic no frills circuit on breadboard.
Charging current around 900mA, not the expected 450mA.
Thought maybe it would work better soldered up as the current sense resistors are very low value (3 in parallel giving 0.56 ohms).
Soldered up & tried out on a couple of AA cells.
Charging current again around 900mA.
Fast charge pin should sink current - it doesn't.
Current sense resistors should have 0.25V across, they have 0.33V across.
After 132mins, should switch to a trickle charge - it didn't.
I have tested transistors out of circuit. I have checked for connection errors more times than I can count. I have checked for dry joints and shorted tracks.
Is it likely the chip is very sensitive to heat from soldering? I was quick, board was tinned. I could try again with a new chip & ic socket (rather than soldering the chip).
MAX712 NiCd/NiMH Battery Fast-Charge Controllers - Maxim Integrated
New chip arrived.
Connected a basic no frills circuit on breadboard.
Charging current around 900mA, not the expected 450mA.
Thought maybe it would work better soldered up as the current sense resistors are very low value (3 in parallel giving 0.56 ohms).
Soldered up & tried out on a couple of AA cells.
Charging current again around 900mA.
Fast charge pin should sink current - it doesn't.
Current sense resistors should have 0.25V across, they have 0.33V across.
After 132mins, should switch to a trickle charge - it didn't.
I have tested transistors out of circuit. I have checked for connection errors more times than I can count. I have checked for dry joints and shorted tracks.
Is it likely the chip is very sensitive to heat from soldering? I was quick, board was tinned. I could try again with a new chip & ic socket (rather than soldering the chip).
MAX712 NiCd/NiMH Battery Fast-Charge Controllers - Maxim Integrated