Ni - MH battery charger - what current do I have to use?

[Aldo]

Hi guys,

I have been using Ni Cd batteries for more than 20 years in several devices that I have. But now, with the Ni MH batteries at the same prices that Ni Cd, but with much more power in the same size, I got some of these batteries to use in cameras and other devices.

The problem I am struggling now, is about how to charge these batteries in the right way.
I have seen AA batteries, rated at 1500 mA, that have to be charged at 205 mA. I thought that the right procedure was to charge it with 10 % of the total amount of current stored in the battery.
I am interested in the Eveready AA Ni MH rated at 2100 mA, but there is nothing on the battery that specifies the current for charging it.
Should I use 210 mA like a conventional Ni Cd?? :?:
Is it 14 hours the right timeframe for charging Ni MH?

If anyone knows more about this matter, I would appreciate any help.
I got a pretty simple circuit with a Mosfet that develops the current and protects the cells, but I m not sure about on how many miliamps I have to set the current to avoid any life shortage on the cells.

Thank you very much in advance
Have a very good day

Al

 

[audioguru]

Hi Aldo,
Your NiMH battery cells are rated at 2100mA/hr, not 2100mA. The charging process is not 100% efficient (some charging is lost as heat), so you must charge for about 40% more than calculated (14 hours instead of 10).
So 140% of 2100 is 2940, and you can charge at 210mA (0.1C) for 14 hours, or any lower current for a calculated longer time.
The charging time is based on a discharged cell (0.9V) but if the cell isn't fully discharged then overcharging will result. Overcharging causes heat build-up that may cause the cell to "vent" flammable gases and the resulting loss of chemicals reduces its future capacity.

Haven't you seen Energizer's website? It has all the details. Click on "Technical Info" in the top right corner, then on the next page from the Datasheets Product Group select "NiMH Rechargeables", then the next page has lotsa good stuff like datasheets, FAQs and a 35 pages "NiMH Applications Manual". https://www.energizer.com/

 

[k7elp60]

I have build many battery chargers for Ni-Cad and Nickle Metal Hydrid batteries. The easiest way is to take the capacity of the cell (Ma/hr) and multiply by .095. Set up a LM317T to current limit charge at the calculated rate.
Make sure that the output voltage under the charge current will be equal or greater than 1.5 times the number of cells in series. Doing it this way you can leave the batteries on charge indefinately and they will not overcharge. Yes they will get warm to the touch, but they still will not over charge.
To figure the value of the resistor for the LM317T, divide 1.25 by the calculated charge current. This value of resistor sometimes is not a standard value, so go up to the next standard value. The charge current will be a little lower than calculated, but thats okey.

 

[audioguru]

Hi K7,
In their Applications Manual, the manufacturer keeps talking about "maximizing the cell's life" and recommend a trickle charge current that is 1/4 of what you use, beyond the battery being fully charged. They say that overcharging with your method will reduce the life of a cell. See PDF pages 18 to 20 showing a timer that switches the overcharge from 0.1C to 0.025C.

 

[Aldo]

Hi guys, and thanks for all the information that you are providing me.
I was checking the Energizer web site, but I couldn't find the pdf files that you audioguru mentioned above, regarding cell life.
I am gonna try again,
Sorry about my misunderstanding regarding mA/hr. I got it !!.

I have seen some circuits with timers, but the one that I have doesn't have timer....I just remind myself to disconnect the cells after 14 hours every time that I use it.....I will consider an easy to build timer...

Thanks again guys

Al

 

[audioguru]

Haven't you seen Energizer's website? It has all the details. Click on "Technical Info" in the top right corner, then on the next page from the Datasheets Product Group select "NiMH Rechargeables", then the next page has lotsa good stuff like datasheets, FAQs and a 35 pages "NiMH Applications Manual". https://www.energizer.com/

Hi Aldo,
The link to their Applications Manual is in the middle of the page. Can't miss it.

A CD4541 makes a pretty good overnight timer.
What are you going to do about timer-charging batteries that are not fully discharged? They will get overcharged.