Max712 to use with 120w solar installation

[konstantin.neo]

Hi.
Once I used max713 to make a charger for my cordless screwdriver, the original B&D had a "class 2" charger consisting of a single transformer outputting 5vAC. For this project I used a wall adapter to get suitable output voltage for the max713 to use. That was a while ago.

This time I intend to use my small PV installation consisting of a pair of 65W panels which charge a 100AH 12v battery and power a well pump. The output voltage, while charging the battery, varies between 11.5v-13.5v depending on the charge of the battery. I need to charge the NiMh cells in packs of 4, since I use them in 2 portable radios.

Since the input voltage for the max712 would be 1.9*4+1.5=9.1v, I will have surplus of voltage here, I am not sure how to approach this. Should I set the charging rate lower and just use it like that or use another solution?
Please advice.
Thanks.

 

[Harald Kapp]

Hi Konstantin,
I'm not particularly familiar with the MAX712, but from the datasheet it looks as if this part is perfectly suitable for the job. Just use the typical operating circuit (front page of the datasheet). The MAX712 will handle the charge current and voltage limiting for the NiMh cells.
Use PGM0 and PGM1 to program the MAX712 to 4 cells.
Use PGM2 and PGM3 to limit the fast charge time (to prevent overcharging the cells)
Use Rsense to set the charge current.
Everything is well explained in the datasheet. You may also have a look at the application notes (http://www.maxim-ic.com/datasheet/index.mvp/id/1666/t/do) although I don't think they are particularly helpful in your application (unless you want to build an energy efficient switch mode charger).

By the way: The MAX712 is not recommended for new designs.

Regards,
Harald

 

[konstantin.neo]

Thanks Harald.
I am unsure about the voltage limitations, supposedly the input voltage must be at least Nx1.9 + 1.5 being N the number of cells. Is that right? If so then I need to supply at least 4*1.9+1.5=9.1v so the cells are properly charged. Im am not sure about the upper limit, should it be equal to 9.1v or could be more? Since my input voltage will be 12-13v, will it pose a problem? Should I compensate for it by choosing a slower charge rate? Will 13 volts fry my 4 cells or the circuit?

I am only familiar with max712/713 are there other NiMH charger IC's around?

 

[Harald Kapp]

Hello Konstantin,
Add 1: yes, your input supply must be higher than the highest charge voltage on your batteries since some voltage drop will occur across the charger/rregulator circuit-
Add2: dirctly placing 12-13 Vacross your batteries will destroy them. But don't you worry: that's what a charger circuit is for: regulating the charge flowing into the batteries and protecting them from overvoltage. No need to limit the charge rate as a function of input voltage, just observe the charging specs of the batteries. Once the voltage of the batteries will reach the terminal voltage for fully charged batteries, the charger will automatically turn off charging (or go into trickle charge mode, depending on what you have set it up to).
Add 3: There are a lot of other NiMH chargers available from e.g.MAXIM, Linear Technology, Texas Instruments and others, do a Google search. But your issues stay the same regardless of the charger IC.

Regards,
Harald