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I want to use a solar panel to extend the use time of 3 NiMH AA's in series to power a microcontroller.

The solar panel is rated at 1 W with a max load voltage of 6.4 V and the product page is here.

The battery capacity is 2450 mAH with a nominal 1.2 V per cell and the datasheet is here. (I plan to use three, for ~3.6 V to power a microcontroller that needs 3.3 V)


My confusion is about trickle charging the battery vs. fast charging it. I know that the trickle charging rate for a NiMH is between C/10 and C/40, which comes out to 245 mA and 61 mA, respectively, and fast charging is between C/2 and 1.2C.

My problem is finding an IC that can do both, specifically for NiMH batteries.

For slow charging, I want the input to be solar panels (varies between 0 to 5.5 V max).

For fast charging, I want the input to be a wall charger (fixed at 5 or 9 or 12 V).

Is there an IC that can take an input voltage between 250 mV - 5 V (at least, but up to 12 would be even better) and output a current between C/10 and C (but between C/40 and 1.2 C would be perfect), with automatic charge termination and switching between the two modes I mentioned above?

If such an IC doesn't exist, how can I modify an existing one to do this?

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  • \$\begingroup\$ First I want to encourage you to keep at this project. Solar power and battery charging is very interesting and useful for many applications. But I think you have too many questions. It would take so many paragraphs to fully address this question that it is kind of daunting. Do you think you could break it down into one question at a time? \$\endgroup\$ – mkeith Feb 25 '15 at 17:17
  • \$\begingroup\$ @mkeith Sure, I could remove the last chunk of additional info. I just wanted to provide more context but you're right it is quite a bit to answer all at once. \$\endgroup\$ – Christina Feb 25 '15 at 17:23
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Well I am not sure how much my answer will help you, but I am working on similar project of NiMH batery charging for last 2 months and putting what I have learned till date.

My confusion is about trickle charging the battery vs. fast charging it. I know that the trickle charging rate for a NiMH is between C/10 and C/40, which comes out to 245 mA and 61 mA, respectively, and fast charging is between C/2 and 1.2C

Trickle charge, as you said C/10, C/40 is usually very low charging current and is good for long battery life and mainly used in applications where you are not expected to the replace battery for months.

Fast charge is can be between C, C/2, 2C also and is used to revive the batteries quickly and come into action, but with this charging the battery life is reduced.

You can look into this link for specifics. So it depends on your application what sort charging you should go for. Tricke or Fast. And there is another term also called as Top-Off charge which is basically a maintenance charging.

My problem is finding an IC that can do both, specifically for NiMH batteries.

For fast charging we tested TI part BQ2002 which will only do fast charging and is good for 5V wall supply.Here is a 12V application circuit using BQ2002.

Then comes the trickle charging we tested BQ24400 and this IC controls the charging current based on the Rsense resistor and you can manually set the charging current based on your requirement.

I understand you are actually looking for a combination or merged properties of both the ICs I suggested, but if you are okay to go with some charging current between Fast and Trickle charging current, BQ2002 may be useful for you.

Now another important point is to take care for both the ICs is there charge qualification stage, they actually scan the battery voltage and temperature regularly before deciding which charging need to go. And when to terminate the charge, will again depend on the battery voltage and temperature of the battery. If you are not willing to use the thermistor for temperature sensing(like me) make sure to fool the ICs by providing the expected voltage on the Tsense pins to qualify for the charging. We have tested the fooling option it works fine.

And do not misunderstand me as TI agent, due to availability of free quick samples, we tested their parts as of now.

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I don't know of the IC your looking for, but on the solar side of trickle charging you could get away without any IC. Just hook up the solar panel directly to battery and it will charge at approx 170 ma (according to the spec sheet) which is between the c/10 and c/40 you are looking for. Then you just need the IC for fast charging.

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  • \$\begingroup\$ Could you explain the fast charging part more? If the panel only outputs up to 170 mA, how does the fast charging IC output 2A? \$\endgroup\$ – Christina Feb 27 '15 at 7:21
  • \$\begingroup\$ I was only talking about the slow trickle charge half of the equation - just connect the panel to the battery directly. The fast charge part will need an actual IC separate from this that you use in conjunction with a plug in. \$\endgroup\$ – Filek Feb 27 '15 at 7:33
  • \$\begingroup\$ Yes I understood that part, but as far as using the fast charger I'm not understanding how it can turn 170 mA into 2A.. \$\endgroup\$ – Christina Feb 27 '15 at 7:38
  • \$\begingroup\$ And as far as the trickle charging part, how is it that sending a constant 170 mA doesn't overcharge the battery? How do I know it's actually equal to the self discharge rate? \$\endgroup\$ – Christina Feb 27 '15 at 7:38
  • \$\begingroup\$ It is unlikely that the 170 ma is exactly equal to the self discharge rate, but many people have tested Nimh batteries and they have found that a trickle charge of between c/10 and c/40 does not hurt the battery. If the 170 mA is higher than the self discharge rate, the battery can handle the extra charge and dissipate the extra charge as heat, without hurting the battery. \$\endgroup\$ – Filek Feb 27 '15 at 20:57

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