Confusion re. charging rates of NiMH's.

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BrendanP

Senior Member
I've got a MAX712 charging 3X 3000 milliamp hour NiMH AA 3/4 batteries in series. The batteries power my 40X1/GSM combo.

Im a little confused on the charge rate that I should be using/seeing going into them.

Im charging them at C4 rate. My power supply shows that there is 300 odd milliamps being drawn on fast charge, this drops back to 80 on trickle.

This probabaly sounds dumb but I though that I should see at C4 a quarter of 9000 milliamps per hour ie. 2500 being drawn.

But I now think this is incorrect because I look on pg. 14 of the data sheet

http://datasheets.maxim-ic.com/en/ds/MAX712-MAX713.pdf

and it has a working example of 3X1000 mAh batteries being charged at a 1A rate being charged in 1 hour. I had thought at that rate they'd take 3. hours?
 
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ylp88

Senior Member
The batteries would be connected in series, therefore the charge current flowing though them would all the be the same at 1A. Thus at 1C rate, I think the batteries should charge in approximately 1 hour (minues losses). The voltage across the set of cells is, of course, proportionally larger, however the nominal cell voltage should remain the same regardless of the number of cells charged.

As for charge current, 2.5A soulds like a rather high charge rate... Aside, what is the value of your resistor between "BATT-" and "GND"? What transistor are you using to buffer the "DRV" signal, and what is the value of the resitor you are using to pull the "DRV" pin high?

ylp88
 
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BrendanP

Senior Member
Im using a R100 as a current sense R. (R1 on typical opperating ciruit on page 1. of data sheet )

Im using a 2N6109 transistor as per the data sheet.

"...what is the value of the resitor you are using to pull the "DRV" pin high...."
I think you're referring to R2 on the data sheet. Im using a 150R as in the typical opperating ciruit on page 1.

I think I have a grasp of what is going on now. Voltage increases as the number of cells to be charged in series increases but current remains the same.
 
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BeanieBots

Moderator
Indeed, the current of a "constant current" charger should remain the same irrespective of number of cells and/or cell voltage.
When you say "C4", do you mean 4C or C/4?
I can't recommend 4C (charge in 15 minutes) if you want any sort of life span from your batteries. Premature cut-off (maybe about 70%) will help if you don't mind the reduced capacity.

Also, be careful about what you are measuring. A delta-peak charger switches the current on and off. Also, that chip has the ability to work in switchmode mode which would see reduced input current for increased input voltage despite the same net average charge current.
(but I don't see any inductors in their circuit so I don't see how that can work).
I didn't read everything so probably missed something in the text.
 
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BrendanP

Senior Member
Thanks BB.
Yep, I meant C/4. There's a schematic depicting a switchmode charger based on the MAX712 a bit further into the data sheet.

My intention was to charge the batteries over a four hour period which is the slowest rate the part allows because the fast charge time out period is 264 minutes.

Im getting a handle for what is going on now.
 
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BrendanP

Senior Member
I got the modules direct from a manufacturer. It's been a long, hard 18 month slog learning to use them. If I'd known how hard it was going to be I probably wouldn't of started.

I know sparkfun have modules mounted on break out boards. The device I'm using isnt intended for the hobbyist. I don't say that out of hubris. Ive sunk probably several thousand hours into this. I've spent $5,000 at my friendly pcb maker on protos.

Just sourcing the parts to use with the modules was/is a struggle. SIM card holder, antenna, connectors. Sure we'll sell them to you, just so long as you take a reel that has 5,000 parts on it.

As I say have a look at what sparkfun have.
 
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