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Author Topic: How do NiCad and NiMh chargers detect -deltaV to stop charging? (SOLVED)  (Read 2788 times)
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With that last clarification i think i understand what you are saying mr. Lefty. Please correct me if i am wrong:

You are basically saying that as the battery charges up, its voltage increases. A constant current charger will try to maintain a constant charging current and therefore will also increase its charging voltage more or less in the same manner as the battery itself increases its own voltage (since it charges up). When the battery is at full charge, its voltage (aka EMF) will start dropping. The charger will keep the charging current constant and will therefore have to decrease its battery charging voltage in a similar manner, following the voltage drop (-deltaV) of the battery. By measuring the drop in voltage of the constant current charger we can identify a drop in the voltage and therefore stop the charging process since the battery is full.

Am i correct?
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Am i correct?

Yes. And that charging voltage and it's little peak shift have nothing to do with the actual terminal voltage of the battery after you disconnect it. It's a small dynamic voltage change during the charging process to signal that it's time to terminate the charge process. Other means of stopping a constant current charge cycle is to have the temperature sensor inside the battery pack as the temp will start a rapid rise upon full charge being reached and that is how a lot of the early 'rapid' chargers worked. But of course that worked with only higher priced battery packs that had the proprietary temp sensor (compatible with the charger) in them.

Lefty
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Retro,

Would you be so kind as to explain for us how that litte magic black box that you are calling a "constant current charger" works internally? Based on the OP's question, I believe THAT was what he was asking. I would assume that if he already HAD a constant current *charger* he wouldn't need to know what he is asking.

Silly me assumed that he wanted to know how that part actually worked because he wanted to build that little magic box himself.

Yes, if you are using a constant current charger (little magic black box) you just measure across the battery and the voltage will change. But the entire reason that works is because of what is happening inside that little magic black box and that is exactly as I explained.

I am immensely curious how a constant current is maintained without feedback.
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I am immensely curious how a constant current is maintained without feedback.

No magic required.
Any constant current regulator does have and uses current feedback internal in it's own circuitry, so it does indeed know what the current is at any specific time and if the load resistance changes causing a change of current the regulator will raise or lower the loop voltage to keep the current at the desired constant current value desired. That part is standard for any charger that uses a constant current charging method. What the OP was asking is how to detect when to stop charging using the -deltaV detection method, which I think I answered satisfactory according to the OP's last posting?

Lefty
« Last Edit: February 18, 2013, 06:27:57 pm by retrolefty » Logged

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I do indeed have a battery charger (Turnigy Accucel 6). I was asking this question because i did not know how it would charge NiCads or NiMhs using the -deltaV method and i was planning on using such technology to charge NiCads or NiMhs with a microcontroller. Thank you very much Retro for explaining what's basically inside the charger and thank you Lefty for answering the question.

Again, thanks both of you. This case is closed smiley
Have a nice day. Thanx.
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Something that may be of interest (very loosely) is that I was able to revive some Li-on cells that had dropped down to 0.2 volts and wouldn't charge.
http://ajb2k3.wordpress.com/2012/11/18/psp-110-battery-revive/ Not very informative but it did work.

I think Boeing tried that already smiley-wink
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Something that may be of interest (very loosely) is that I was able to revive some Li-on cells that had dropped down to 0.2 volts and wouldn't charge.
http://ajb2k3.wordpress.com/2012/11/18/psp-110-battery-revive/ Not very informative but it did work.

I think Boeing tried that already smiley-wink

LOL  But I don't think Boeing is laughing at the moment, it's costing them a fortune in downtime and whatever rework falls out is bound to be pretty costly as well. Li-po cells are great from a energy density and current capacity point of view, but they do bring along a lot of care and feeding rules to be as safe as past battery types.

Lefty
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