EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: FenderBender on February 29, 2012, 09:55:06 pm
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Hello,
I made a thread before regarding battery chargers except with Li-ion that time. I've decided that I would like to use NiMh in my project, specifically a 9.6V (8-cell) "9V" battery.
Problem is: There seems to be a lot of ways to charge these things and I'm not sure what's good or not. I was thinking of having a simple constant current charger. Perhaps a ~12V input into an LM317 constant current. C/10 charging I would guess.
Any other interesting ideas?
Thanks.
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With a series string or pack of NiMH the important thing is to keep all the cells balanced.
The easiest way to accomplish a balanced charge is to use C/10 for about 12-14 hours as the charging method.
When a pack is known to be in good balance you can get away with a C/2 charge with minus delta-V termination, but even when doing this you should periodically re-balance the pack with a timed C/10 charge.
When discharging, try to put the low battery cut-off at 1.0 V per cell or higher, maybe even 1.05 V per cell. This is to avoid damaging any weaker or unbalanced cells in the pack by reversing their polarity.
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Interesting. Thank you.
I'm not so sure about the low voltage cuttoff. I think that might be harder than it seems, since I don't plan on using any micros and I'm trying to keep it as small as possible.
Is there a crude way to do that?
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I'm not at all an expert in low battery management circuits or solutions, but I think simple cheap solutions might be tricky unless you can find a dedicated chip for the job.
Possibly the simplest solution of all is to arrange that the circuit being powered stops working when the supply voltage gets below 8 V or so, so that users are encouraged to stop and replace/recharge the battery.
If the device power consumption is low the risk of damage to the battery is lower, but 9.6 V NiMH batteries are notorious for short life and early failure due to excessive discharge during use. It's a risk you take with multi-cell NiMH solutions.
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Interesting. Thank you.
I'm not so sure about the low voltage cuttoff. I think that might be harder than it seems, since I don't plan on using any micros and I'm trying to keep it as small as possible.
Is there a crude way to do that?
Try search for MAX713 IC from Maxim. It's a dedicated charger IC specific for NiMH battery. Free samples are available thou. ;D
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Max713 has been out of production for a while. Not saying that it's bad.
And Ian thanks. I might try looking at something like a shunt regulator to get a cutoff for the battery.
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Max713 has been out of production for a while.
The DS2715 is similar and active.
Apart from NiMH and 9.6v you have given zero information about your requirement so it is hard to offer any useful advice.