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
or
- 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?