Simple but Smart, a Mix/Match AAA/AA 4-Cell NiMH Charger?

[SuzyC]

Is this design viable/practical?  Any suggestions?

 (see schematic attached)

I have an idea for a 4-Cell NiMH battery charger that will allow intellegent, smart charging of any mix of AA or AAA batteries.

My circuit uses just one 500mA CC source to charge from 1 to 4 cells at once.
In the circuit shown, B1 and B4 are the cells being charged.

The idea here is to use MOSFETs to shunt control the charging current of each AAA/AA cell from 0 to 500mA.

If any MOSFET is off, the full 500mA charging current charges that cell, if the MOSFET is PWM Duty-Cycle controlled, 100% Duty Cycle would shunt all the current around any battery in the string and that battery would be disconnected from charging.

The goal here is:

Limit each cell's charging voltage to a safe 1.6V limit.
Limit AA cells inserted in charger to 500mA max CC charging rate and limit AAA at 250mA.
Monitor each cells temperature and terminate charging when a 5-10 Deg rise in temperature is detected during charging.
Insert any one to four cells and cells can be inserted as a combination of AA or AAA cells.

I've already have a 12V 4-amp wallwart power supply and I have made a MCU programmable voltage buck regulator to supply the 500mA constant current to the string of batteries to be charged.

Note: The attached circuit only shows 2 of the 4 series connected charging circuits, imagine there are 4 instead of the two shown.

The question I have is:

Is my circuit goal achievable using the attached circuit and control scheme?

What  capacitor value(C1,C2), (using a a 64-MHz Fosc PIC MCU to generate four 10-bit PWM shunt-control signals) would allow me to accurately control the shunting of half the charge current around AAA cells (using the MOSFETs) to limit the charging current to 250mA and yet allow 500mA for AA cells and allow me to measure and adj. the charging voltage for each cell during charging? 

[MazeFrame]

For simple charge circuits, I usually just go with one of the example circuits given with the LM317. Depending on the cells in use, I change values around a bit.
May not be the most efficient way, but it is compact.

[SuzyC]

Thanks, but no thanks MazeFrame. I already had a dumb charger to start out with!  Your circuit is simple, but even dumber.(no, not you, your circuit!)

What I am after is making a SMART CHARGER, something that charges accurately, automatically, safely, doesn't damage a battery, yet gives feedback on charge, charge progress, etc., a charger that uses a MCU for sophisticated control and performance.

What I am attempting here is to be the HW backbone of a well-designed SMART, yet SIMPLE CHARGING.circuit.

[BradC]

Is my circuit goal achievable using the attached circuit and control scheme?

Probably provided you never encounter a shorted cell or excessively high resistance cell.
At the end of the day, I can't see it being much simpler than 4 independently switched CC sources.

[SuzyC]

BradC, checking for a shorted cell or excessively high-resistance cell is quite easy with my circuit which allows easy voltage monitoring of each cell and PWM current control, so I can't see any problem there. Bad cells need to be detected and discarded.

In my design, the parts count and cost and PCB area required for individual cell charging and current management is quite small compared to the parts needed for  four separate CC charging sources, ( likely by a factor of two or more). Each CC source would require a SMPS IC and a fairly expensive (in size and cost) inductor that is needed to make four buck convertors, and either circuit design requires a master control MCU to manage each buck convertor charge circuit and provide an user interface for operation.

[BradC]

Not sure why you need 4 separate bucks for 4 cells of the same chemistry. Anyway, sounds like you have it under control.

[MazeFrame]

Fair point.

I would likely stack 4 LM317s next to each other with their respective current limiter resistor and monitor voltage across those resistor. That way a uController could figure current (or lack thereof) without increasing circuit complexity too much.

Seems you are pretty much sorted with your design though 

[SuzyC]

BradC,
Any of the four cells could be in a different state of charge and either AA or AAA, so each require there own circuit to not underchage/overcharge any in the string of batteries being charged.

[SuzyC]

MazeFrame,

Simple doesn't mean really dumb charging. 

I have seen some major brand home appliances i.e., HandVacs that take simple to the extreme and only use a series 5-watt resistor to charge a many-cell battery pack. A LED indicated charging.

This simplest of charging methods ensures battery failure after a few months of use, and then the owner needs to buy and pay someone to install a new battery pack, otherwise throw the whole appliance away and buy another.

Your idea would require maybe large heatsinks for each LM317 and by itself would not automatically start or terminate charging at an optimal charge point or battery temperature and easily could lead to undercharging/overcharging/battery damage.

Yes, my goal is certainly fixed upon automatic MCU controlled smart charging.

[BradC]

BradC,
Any of the four cells could be in a different state of charge and either AA or AAA, so each require there own circuit to not underchage/overcharge any in the string of batteries being charged.

Correct. And you're already planning on terminating the charge on Delta-T.

[SuzyC]

BradC,

Now you've got it!