Would this work to "dumb" charge a NiMH battery? - Page 1
Forum section: electronics / general

author: XOIIO
author_rank: Super Contributor
timestamp: December 14, 2014, 07:18:57 pm
content: So, I'm not familiar with how NiMH battery chargers work, but I assume they are fairly simple, I'm just wondering, since I can't find the charger, would it work to use a a power supply at 1.2v and a multimeter and just monitor when it stops drawing current? I'm not sure if that's essentially what the actual chargers do or not.

author: Yansi
author_rank: Super Contributor
timestamp: December 14, 2014, 07:25:53 pm
content: No, take a 1.5V supply and limit the current to 0,1C (that means for a 2000mAh accucell the current is 200mA). Leave that charging for quite some time (>10h) and it will charge. This amount of current is charging the battery slower than traditional "higher tech" charger, but has the advantae of not destroying the cell, when left on the charging supply for just too much time.When NiMH or NiCd cell is fully charged, the current will still flow (unlike Lithium based accumulators) and the cell starts to dissipate the current as heat (it begins heating up). And with this small amount of current, you are not in the danger of overheating the cell. It is quite safe method of charging these. The energy supplied from the charger of these parameters (1,5V max, 200mA) is only 0.3 watts, which is safe to be dissipated on such a cell.

author: SeanB
author_rank: Super Contributor
timestamp: December 14, 2014, 07:28:44 pm
content: You generally charge with a constant current of C/10 for around 14 hours, which will charge the cell safely. Charging for more than 14 hours will however lead to capacity loss, so generally you do not want to leave it charging forever, as the NiMH cell does not recombine excess gas very fast. the normal way is to do the charge and stop charging at around 1.4V per cell or if it times out at 14 hours.

author: rob77
author_rank: Super Contributor
timestamp: December 14, 2014, 07:32:20 pm
content: as it was mentioned ... MiMh is supposed to be charged by constant current. the end of the charging cycle is indicated by a slight voltage decrease (during charging the voltage is increasing across the cell) , after that point the battery is just dissipating energy as heat.

author: IanB
author_rank: Super Contributor
timestamp: December 14, 2014, 07:39:13 pm
content: And also, supposing you have four of the cells to charge, you can put them in series and charge all four at the same time using the same method. Just set the current to the C/10 rate as before and up the maximum supply voltage appropriately.

author: XOIIO
author_rank: Super Contributor
timestamp: December 14, 2014, 09:23:51 pm
content: Ah, I wonder if that's why my battery seems to be dying really fast now though I haven't had it all that long, I have a razer ouroboros and whenever I stop using it a stick it on it's charging dock. I figured that it would stop charging it automatically but the battery seems to barely last now. That or for some reason it's not entering charging mode. Hrmm

author: IanB
author_rank: Super Contributor
timestamp: December 14, 2014, 09:54:49 pm
content: Simple charging docks typically don't have any smarts in them to stop charging when the battery is full. They simply keep charging for as long as the device is in the dock, with the inevitable result that the battery will die prematurely.The best way to prolong battery life is to take the device off the dock when it is fully charged (for example, put it on charge in the evening and take it off charge next morning). Then leave the device off the dock and use it until it shows signs of running low. When that happens, recharge it and repeat.(Ah, I see what a Razor Ouroboros is. The reason it doesn't charge any more is because the NiMH battery is shot. The mouse can use an alkaline battery or an NiMH battery. It has a detection circuit so it doesn't try to charge an alkaline battery. To this circuit, a bad NiMH battery looks like an alkaline battery, so it won't try to charge it. Replace the bad NiMH battery with a good one. And don't allow the mouse to charge it, use an external charger.)

author: XOIIO
author_rank: Super Contributor
timestamp: December 14, 2014, 09:56:25 pm
content: Well, it's definitely not a simple low cost device, the mouse itself monitors the battery charge level when it's not charging, I figure it would stop charging the battery, but maybe not.

author: IanB
author_rank: Super Contributor
timestamp: December 14, 2014, 10:00:49 pm
content: Quote from: XOIIO on December 14, 2014, 09:56:25 pmIt may not be a simple,low priceddevice, but it may still have simple, low cost internals. Without detailed knowledge of its battery charging circuit, I wouldn't trust it.

author: Someone
author_rank: Super Contributor
timestamp: December 16, 2014, 02:35:55 am
content: Quote from: XOIIO on December 14, 2014, 07:18:57 pmAs mentioned above there are some different ways to terminate charging for NiMh chemistry, see more in this document:http://na.industrial.panasonic.com/sites/default/pidsa/files/panasonic_nimh_chargemethods.pdfWhen I charge my NiMh batteries manually I leave each cell connected to a 0.5C rate with a voltage limit of 1.5V and wait for the delta T (T for temperature) to indicate they are done, easy to do when you have a bench supply next to you for long periods so you can monitor it.