onboard NiMh battery recharging

Hey guys.

I’m powering my project with these QTY: 4, NiMh cells in series.

  • low self discharge
  • 2,000 mAh capacity
  • 1.2V / cell (so 4.8V total for my project)
    I need to create a circuit that would allow me to plug in a wall-wort to my project, charging the batteries, and still be operational.

After doing some research here on charging NiMh batteries, it seems the best way to charge 'em is w/:
a relatively fast ~1C (max) current w/ charge termination on:

  • negative delta voltage (NDV) of 5 mV/cell
  • voltage plateau
  • delta Temp (if monitoring battery temp)
  • max Temp (if monitoring battery temp)
  • or timeout (a last-resort)
    I see two possible paths for me to take:
    - Incorporate a charging circuit that uses the arduino’s micro controller for charge-logic
    - Incorporate a charging circuit that uses a dedicated NiMh charging IC for charge-logic.

Since I already have a micro-controller on the bill of materials, and that micro-controller is perfectly capable of implementing the charging logic, I was hoping to bypass the need for a dedicated charging IC, thinking that would save me a buck or two on the bill of materials.

What I’ve seen thus far:

  • SOLUTION #1: It seems the most common thing to do, at least according to this thread and a few others like it, is to use the LM317 (datasheet) to trickle-charge at a constant 0.095C current with no built-in charge-termination.

This is conflicting w/ the Battery University information, which claims NiMh trickle-charging (0.05C ← their figure) is a bad idea, because it increases battery memory, and makes it nearly impossible to properly detect when to terminate the charge. So in turn, trickle-charging results in harmful over-charging of NiMh and should be avoided.

  • SOLUTION #2: comes from this Arduino-forum thread, where Tritemio is using the LM2941 (datasheet) that incorporates a nice ON/OFF pin feature for charge termination and arduino logic to charge and terminate accordingly.

  • SOLTUION #3: comes from using a dedicated NiMh charging IC. There are many out there, including the MAX713 that Tritemio mentions in his thread, or the LTC4060, which is one of the first to pop-up on google.

My initial questions:
1. which of the solutions above do you think is best?
2. Do you think I would be saving money on the bill of materials by avoiding a dedicated charging IC?
3. Is there a favorite NiMh charging IC that you would recommend?

Can't comment on which is better but I can say you won't hit 1c charging(2.2amps).

Also go on hobbykings website and find some better nimh batteries some of which can take 2c or more charging rate

I ended up purchasing the components necessary to test all 3 solutions.
I’ll keep you guys posted.

Re: Not being able to hit 1C charging: It seems all of the components I purchased cap-out at 1A or 1.5A, so I guess you are right about that. Although I imagine if I dug deeper I could find something that would allow higher current, but I am running out of time and 1.0A or 1.5A charging is fast enough for my application.