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Old Nov 04, 2010, 03:47 PM
Stuart
srnet's Avatar
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Simple NiMH Charger ?

I am after a simple automatic charger that will cope with a circa 1A charge for 8 cell 2200mah NiMh pack, its the battery pack for a 35Mhz RC transmitter. Cant find a ready made charger that is small enough for my needs.

Must run from 12v input, which obviously needs a step up circuit.

Dimension engineering do a small and neat step up regulator;

http://www.dimensionengineering.com/AnyVoltMicro.htm

If there was something similar that could charge my NiMh I would be happy.

Or an IC that only needs 1 or two components ?
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Old Nov 05, 2010, 04:06 AM
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Quote:
Originally Posted by srnet View Post
I am after a simple automatic charger that will cope with a circa 1A charge for 8 cell 2200mah NiMh pack, its the battery pack for a 35Mhz RC transmitter. Cant find a ready made charger that is small enough for my needs.

Must run from 12v input, which obviously needs a step up circuit.

Dimension engineering do a small and neat step up regulator;

http://www.dimensionengineering.com/AnyVoltMicro.htm

If there was something similar that could charge my NiMh I would be happy.

Or an IC that only needs 1 or two components ?
You can use an LM317 (voltage regulator) in constant-current mode. It needs a resistor and small heatsink.

A quick google came across this page.
To calculate the current output = 1.25V / 27Ohms = 0.046A = 46mA

I have made a 4 cell charger with 4 of these Ic's however I think it's possible to charge 2 or more batteries in series.
The trick is to set the charge current low (slow-charge) so that when the battery is fully charged, it won't overheat and destroy itself.

ps only other issue is that 12v input is quite high.
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Old Nov 05, 2010, 05:09 AM
Stuart
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UK, Cardiff
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Problem is I do want it to auto cut off.

There a chage rate for NiMh which will not cause problems if the overchage is for a shaort while, c/10 maybe.

But I want to charge the pack in 2-3 hours, so auto cut off is essential, the cells will overheat if overcharged at this rate.
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Old Nov 05, 2010, 07:28 AM
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http://www.lmgtfy.com/?q=nimh+charger+circuit
..a
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Old Nov 05, 2010, 08:19 AM
Stuart
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Originally Posted by Andy W View Post
Which one of those fits the criteria in my original post ?
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Old Nov 05, 2010, 09:16 AM
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Quote:
Originally Posted by srnet View Post
Cant find a ready made charger that is small enough for my needs.
You must have a need for a very small charger, then.

Most any of the cheap four button chargers will run off external 12V, be just larger than a deck of playing cards, and have up to 14 cell NiMH programed charging with auto cutoff when the battery is full.

Comb the Batteries and Chargers For Sale/Wanted listing for a used one if you don't want to spend the money on a new one. (Full disclosure: I have one for sale there myself right now. I'd be willing to sell just the charger without the 12V power supply...)
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Old Nov 05, 2010, 10:20 AM
Stuart
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UK, Cardiff
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I already have an iCharger 106 and and one of the 4 button chargers you describe.

For Lips you can get tiny chargers like this;

http://www.giantcod.co.uk/23cell-min...-p-405125.html

These have a switched mode step up to provide the 14v or so needed to charge a 3S LiPo.

Something of similar size would be ideal.

Considering that the Dimensions Enginnering up convertor modules have been made so tiny, not that much bigger than T0220 regulator, I was wondering if anyone had made a similar sized module for NiMh charging, perhaps to imbed into a bit of equipment.
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Old Nov 05, 2010, 11:19 AM
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That cheap charger you pointed to does not have a switched mode step up circuit. It charges ONLY through the balance plug, and charges each LiPo cell sequentially, not in series. Thus the maximum charger output voltage is under 5V. (That's what makes it cheap...)

If you have a four button charger, then you have the best solution already.

Nobody I know of is building a NiMH only charger that is smaller in dimension than the little 4 button jobs while still having intelligent peak detection and automatic cutoff. Not much call for it round here. Either go with the overnight trickle charge, or use the four button jobbie with peak detection and backup thermal probe cutoff.
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Old Nov 05, 2010, 12:00 PM
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Quote:
Originally Posted by srnet View Post
Problem is I do want it to auto cut off.

There a chage rate for NiMh which will not cause problems if the overchage is for a shaort while, c/10 maybe.

But I want to charge the pack in 2-3 hours, so auto cut off is essential, the cells will overheat if overcharged at this rate.
If charging at a higher rate, I believe the only way to accertain the charge status is to monitor the cell temperature.

The slow-charge method is a convenient set-charging & forget-about but obviously will take longer.

I suppose it would be possible to make a charger which, if the initial cell voltage is below a certain point, performs a high-charge rate and then switches to a trickle charge when the cell voltage is near full-charge voltage.
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Old Nov 05, 2010, 12:41 PM
Stuart
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Originally Posted by Mike Barton View Post
If charging at a higher rate, I believe the only way to accertain the charge status is to monitor the cell temperature.
Cell temperature is used in some chargers, or as a backup, just in case.

The common chargers, such as the iCharger series or the B6 style, will fast charge NiMh using delta V detection alone. When NiMh is at full charge the terminal voltage stops rising. You need to pick this up fairly quickly as an NiMh will start to heat up rapidly. Not such a problem with NiCd.
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Old Nov 05, 2010, 02:20 PM
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A simple LM317 circuit:
http://www.sentex.ca/~mec1995/gadgets/sccs.htm
Another circuit with peak detection,but mor work!
http://www.sentex.ca/~mec1995/gadgets/peakchrg.htm
I use the above circuit to quick charge Ni-cad or Nimh Rx packs
Here is the pdf for the Maxxim chip:
http://pdfserv.maxim-ic.com/en/ds/1666.pdf
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Last edited by coriolan; Nov 05, 2010 at 02:27 PM. Reason: Add Maxxim link
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