How do I know NiMH's are charged?

[Biggoggs]

Hi all, got a question

I have a simple AA/AAA/C/D/9V charger designed to charge NiCd's & NiMH batteries, but I'm unsure how long I should leave them charging. On the back of the charger it says it charges at 4x(1.4V =... 165mA 0.23VA), and a table of charge times. In particular;
"AA - 500mAh - 5 hours
AA - 600mAh - 6 hours
AA - 700mAh - 7 hours
AA - 1200mAh - 12 hours"

Now the problem is, that's as far as it goes- there's no `2000mAh NiMH' time.
Extrapolation of existing charge times would say 20 hours, an electrical contractor told me 4 or 5, maths says 18- what is the actual charge time required, and why?

Also, I've noticed the Eneloops hold their voltage until they're dead, is there any way to tell they're at maximum capacity? Will the voltage rise to a particular level?

 

[mdocod]

most chargers are supposed to charge till full then shut off or go into a slow trickle with an indicator of sorts (led change color) to let the user know they are done, if you have to just guess, then you will always be over or under charging your cells. Sounds like a bad option to me.

 

[lctorana]

I agree with your maths link - about 19 hours at that charge rate.

Rule of thumb, as I've always understood it, is 1.4x for NiCd, 1.5x for NiMH.

 

[MrAl]

Hi all, got a question

I have a simple AA/AAA/C/D/9V charger designed to charge NiCd's & NiMH batteries, but I'm unsure how long I should leave them charging. On the back of the charger it says it charges at 4x(1.4V =... 165mA 0.23VA), and a table of charge times. In particular;
"AA - 500mAh - 5 hours
AA - 600mAh - 6 hours
AA - 700mAh - 7 hours
AA - 1200mAh - 12 hours"

Now the problem is, that's as far as it goes- there's no `2000mAh NiMH' time.
Extrapolation of existing charge times would say 20 hours, an electrical contractor told me 4 or 5, maths says 18- what is the actual charge time required, and why?

Also, I've noticed the Eneloops hold their voltage until they're dead, is there any way to tell they're at maximum capacity? Will the voltage rise to a particular level?

Hi there,

According to your chart there they are dividing the milliampere hour rating
by 100 to get the charge time. Using this technique, a 2000 mAh cell
would require 20 hours charge time, a 2200mAh cell 22 hours, and a
2500mAh cell 25 hours, etc. See how the hours follows the mAh rating?
Take the 2000 and chop off the last two zeros and you get 20, the hours.

The 1.5 factor shown in the link might be right for the low charge rate
of 165ma but for higher rates it gets lower.

 

[BlackDecker]

If you really want good info, get a new charger that has a LCD display. I use a LaCrosse BC-900 which has a separate LCD readout for each of the 4 battery slots.

 

[Biggoggs]

I figure a more expensive charger would make things simpler, but armed with a simple charger, a stopwatch/alarm, and a multimeter, I should be able to charge my Eneloops properly, just with a little more effort involved. Sounds like the general consensus seems to be somewhere between 18--20hrs.

If the battery gets warm, does that mean it's charged, or over-charged? eg, could I take it off the charger when it starts to get warm- would slow-charging give me a bigger window of error?

Thanks for the replies so far guys

 

[lctorana]

According to the graph in this months' Silicon Chip, when the batteriesa get hot they are (just) overcharged.

 

[TPA]

While your goal is admirable, you're better off getting a higher quality charger. There's a good chance that cheap charger has a timer in it and won't charge after xx hours, thus leaving your batteries not fully charged. There are plenty of good chargers out there which aren't terribly expensive such as Maha's 204W.

 

[NiOOH]

Hi Biggoggs.

Your charger is a slow unsofisticated charger. If it can charge C and D cells most probably, it doesn't have a timer, or if it does it is set to a very long times. For 2000 mAh cell your charging current is below one tenth of the rated capacity (165 mA/2000mAh=0.0825). Although charging at such low rate is not very parctical, you can still do it. Calculate your charging time as follows:

time (hours)=(capacity of the charged cells/165 mA)x1.5.

In you case charging 2000 mAh AA Eneloop cells this translates too
(2000/165)x1.5=18.18 hours.
The factor 1.5 comes from the fact that in low charging currents the charging efficiency is low, i.e. not 100% of the electrical input from the charger is passed to the cell .
Usually a factor of 1.4 is recommended at charging current of one tenth of the rated capacity. In your case I'd recommend to charge for a little longer using a factor of 1.5 to ensure full charge.
If you don't want to purchase a new charger, I'd recommend to, at least, connect a timer to the charger, in order to keep with the times.