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Battery charger for four NIMH AAs in series while under load

Dennis Eichmann
Guru 74080 points
Other Parts Discussed in Thread: TPS61161, LM317, BQ2002, BQ2000

Hi everyone!

I'm really a non-expert in battery-management, so I will ask for help before designing something (it is hobby only, no consumer product) that will take me time and then will not function properly.

What I want to do: I got a lamp that is normally connected to a small wall-plug adaptor. It contains a hand full of LEDs driven by a TPS61161. I want this to be portable to take it out (of course while still illuminating my way) so it needs to be battery powered as well.

The goal is to charge 4 in series connected AA NIMH batteries (2200mAh). And this should be done in less than 15 hours at C/10, that's why I'm looking for something able to kind of "fast-charge" my batteries. Since the battery pack will be replaceable, I do not plan to connect a thermistor. This will limit my maximum charge current. Now the first question for you experts: What is a suitable charge-current without sensing the temperature? I was thinking about something like 500mA which results in ~C/4. Possible? Could go lower, but the batteries should be charged over night (~8hrs).

Furthermore the PMIC should detect a full battery and then go over to trickle charge mode to not damage the cells. The wall-plug adapter is connected all the time - the power will never be removed. If the wall-plug adapter is connected, the device should be powered by it, switching over to the batteries if not. I do not plan to build something rocket-science-like, but it should do it's job well.

I'm familiar to electronics - only well done battery charging is new to me. Of course I was searching the internet for different solutions and I had a detailed look at the TI components browser.

An IC that seems to fit into my application is the BQ2002 series in combination with a LM317. Second question: What do you think - is that still a good solution for charging batteries? It has -dV and peak voltage detection termination. It looks simple and I found this application note:

http://www.ti.com/lit/ug/sluu007b/sluu007b.pdf

But there isn't really much better stuff concerning this IC. Other threads like this one:

http://www.eevblog.com/forum/beginners/battery-charger/

haven't pictures in it anymore.

Third question: What are all those different types of BQ2002/F/..... I can't see the differences at first sight.

I link the schematic in:

This is quite clear to me, only a few things: Why do I need Q2? Thought the BQ has an integrated open-collector at CC, too. What is R15 for? Maybe simple, but I just don't get it at the moment. First I was wondering about the voltage regulation for the circuit, but then had a look at D1 which is a zener (wrong symbol), which then makes sense.

Could you provide me any help?

Thanks, Dennis

  • 0
    Guru 74080 points

    Hey all!

    Me again - I have switched over to the bq2000 switching charger now and I'm almost done with my design. But one thing I don't understand. I'm reading the manual for using the bq2000:

    http://www.ti.com/lit/an/slua064b/slua064b.pdf

    On page 9 there is an example for selecting the timing components. I want to charge my four 2200mAh NiMHs at a rate of C/5. This leads to a time-out value of 400 minutes. Since I'm using a timing capacitor of 330nF, my timing resistor is 36k681. OK so far, but now the text says "Verify that the trickle-pulse rate selected from the graph multiplied by the charge rate determined in step 1, is less than or equal to 1/32 for NiCd or 1/64 for NiMH, and that R < 500000". And I do not understand that.

    In the example (point 9) the pulse-trickle rate is 1/10.7 for a resistor of 444k which is greater than the recommended 1/32 for NiCd batteries, so the capacitor has to be changed. But where is this 1/10.7 from? And which graph? There is no graph in the how-to and in the datasheet where you could read 10.7 with a timing resistor of 444k. The only 10.7 is on page 4 where the hysteresis is calculated. But how do I have to calculate that according to my timing R and C?

    Any help? Dennis

  • 0 in reply to Dennis Eichmann
    TI__Guru*** 143880 points
    The graph referred to on page 9 is datasheet figure 8 on d/s page 11. At 444k, if you extend the graph you would get ~187ms with is 1/5.35 of a second. The 1/10.7 should be C/10.7 = C/2 (from step 1) * 1/5.35 = 1/10.7.
  • 0 in reply to Jeff F
    Guru 74080 points
    Thank you very much Jeff!

    Now it's all clear to me. I will calculate it with my values and see what it will give as results.

    Dennis