I would like to design a product that uses a NiMH battery (3.6v, 700mAH). But I do not want to include the proper NiMH fast-charging circuitry, partly because of cost, but mostly because of board space. I am willing to give up fast and medium speed charging in order to be able to use a simple series resistor. The question is, what charging rate is safe for a full-time trickle charge? I was thinking that about C/15.
You should refer to the battery handbook from your selected manufacturer for this information. Sanyo has some pretty good info. The good news is that you're unlikely to cause the battery to go into thermal runaway spraying caustic all over the place with C/15. ;-)
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
AFAIK NiMH batteries like to be trickle charged in pulses. Panasonic has some excellent information on charging NiMH.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
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Even if it does not go into thermal runaway, is it likely to shorten the life of the battery to use full-time C/15?
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According to Powerstream, full-time (i.e. standby) charging should be around .03 to .05 C, so C/15 is a little high.
I have charged _NiCd_ successfully at .031 C. It takes a lot of time, but it works. That does not mean it is the best way to charge them, but they do eventually come up to terminal voltage which is 1.43 volts per cell. That said, I have not done that with NiMh. So, try it and see if it works for you. If you can get a good charge at between .03 and .05 C, you can leave your cells on the charger full time per Powerstream.
Ed
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IIRC, neither Sanyo nor Panas The overcharging of nickel-metal hydride batteries, even by trickle charging, causes a deterioration in the characteristics of the batteries. To prevent overcharging by trickle charging or any other charging method, the provision of a timer to regulate the total charging time is recommended.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Basically, if you're going to have to work with anything people put in, then you need to not do trickle. If you control the cells, then you can get away with long term low rate charging, IF you use cells specifically designed for it.
OK, I'll see what I can do with a timer. Thanks.
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I know that is what they say, but the shelf life of charged NiMH is not exactly stellar and my camera is a bit tetchy about batteries that are even slightly off their peak fully charged voltage. I have been giving some thought to having a solar powered trickle charger that keeps a set of 4 NiMH cells properly charged countering their comparatively high self discharge rate. The recommended method of low duty cycle high current top up is not so attractive on a solar powered device.
ISTR you can leave them almost forever on a C/300 charge rate. (though the manufacturers do not approve)
Regards, Martin Brown
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Why? Can't you use a capacitor to supply the peak current?
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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Do the calc's. Q, delta Q, Delta V for batt and cap.
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Hmmm. Makes me wonder how they would respond to a C/10 "burp" 1 second discharge once every 5 minutes with a C/30 trickle charge. Might be an interesting experiment.
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There's no numbers provided, but assuming a 286us pulse (bq2002) and C/2 on a 2.2Ah per cell pack, and 1V delta V, you'd need 315uF. So a
470uF cap would be more than enough. No big deal. What "cal's" did you have in mind?Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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OP specified 3.6 V, 700 mAh and asked about C/15 charge rate. It is still there.
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Why don't you do it for me, based on the 286us pulse width I found..
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Cause i don't think that short of pulse will do much charging, won't be anywhere near square enough (i), and i have no idea of what rep rate you expect to achieve. Nor what final charge rate you expect.
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