NiMH charger using MAXIM 712

Without downloading the data sheet and studying it, the following is based on my experiences with another MAXIM charge controller and may be way off the mark.

What conditions does the MAX712 cite for charge termination? Terminal voltage, delta V, cell temp, delta T? Preumably it is one of these that is causing the early termination.

Temp - are you sensing temp at all, and if so, how?

Voltage - are the cells tab types with soldered interconnections, or tab-less and placed in a holder? Any change in connection impedance will reflect in terminal volts as seen by the charger (under constant current conditions), and may trip the charge termination.

My experience with applying a MAXIM charge controller on Li-Ion packs was that the device lived up to my expectations and the claims/specifications of the data sheet. I would suggest the same would apply to their products generally, and suspect that there is something about your configuration causing the termination. Review the mechanisms which MAXIM list for charge termination, and you should find a reason.

For MAXIM 712, the first condition for charging termination is dV/dT, then the cell temp. And I would say that MAX712 doesn't check terminal voltage unless cell temp check failed. I bought a holder for 4 AA cells and also put a thermalmeter in contact with cells already, but this problem (stop charging too early) still exists. I am checking the ckt configuration. . . I used the MAX712 sampling interval of 168sec, not sure whether it is too long or not.

How much voltage drop are you getting in the battery holder?

Ed

Em, before charging , 4 AA cells have approx 4V, 0.95-1V per cell, after 2 hrs fast charging, it reaches 1.5-1.55V per cell. But since the problem is that it terminates charging too early, sometimes when cell voltage reaches 1.3-1.4V. I think may be the sampling interval 168sec is quite a short interval for the battery, it should have a longer sampling interval. I mean the dV/dt termination method cannot work very well.

I just did a new test, in which I changed the dV/dt sampling interval from 168sec to 84sec. During 2 hrs designed charging time, it stopped twice as many as before :-( I have to resume to fast charging mode manually. . . Can anyone help me? Or did you know any similar case?

The question was: "How much voltage drop are you getting in the battery holder?"

Sorry, I didn't make my question clear.

Charger----[Rholder]----battery

How much voltage drop is created by the resistance of the battery holder (Rholder) ?

The point being that a poor connection between batteries and the contacts in the battery holder may be a factor contributing to your problem. And that factor is *often* overlooked.

Ed

Thank for that. I just checked. The holder has almost not voltage drop

Have same problems with 712, usually poor matching of new - or older - cells. as soon as the delta V does not raise anymore the process terminates.

In a future project I would be tempted to use the 713, although this is a NICd variety, backed up by limit timer and temp check.

Can I know the detail of your problem? Mine is like that I used a switch-mode charger with 712 in it to charge

I decide to change the V limit value to meet my requirement. Previously it set to V ref = 2V, now I changed it hope that I can use it to control the max voltage for charging process.

I am facing the same problem as you. What is your new Vlimit? Thinking of setting it to 1.65V.

Wot kind of secondary battery r u using? i am using 2000mah one, now the V limit is 1.55V, i think below this batteries cannot be fully charged. U can set it a bit higher, depends on the situation. The biggest prob is that even with limit=1.55V, the charging process cannot be terminated. I think MAX 712 really dunt have MAX voltage control, if dV/dt and temp both fail, there is no proper charging.

I am using 2000mAh batteires also. Have tried using 1.55V Vlimit, and no success. I have more than 10 chargers, not even a single one of them has a predictable behavior.

Wrong, I should say they are predictable - they will terminate the fast charge prematurely!

Wonderingly if u know of any NiMH chargers that performs intelligently, and correctly? What termination criterias are needed? Negative slope detection + Timer + High & Low temperature? What if u turn off the charger after battery fully charged, and turn it on again?