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Charge vs Voltage of NiMH batteries

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techie

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Can someone please point me to a graph indictating the charge vs voltage of a NiMH battery. I want to make a device that has to have a smart charger and an indictor showing battery charge left.
 

Monitoring NiMH charging is a bit tricky issue. The usual way seems to monitor the battery temperature rise, and the battery voltage: When the battery is full, the temperature starts to rise, and the voltage has a tiny dip. In practice the stop criteria is that dV/dt turns from positive to zero (and actually a bit negative for a moment). A temperature sensor is a good security measure, NiMH cells can not take too much overcharge without damage.

To monitor discharging is even worse: You can not in practice estimate the remaining charge by any simple measurement. I have worked with a cordles phone solution where the processor simply "gestimated" the remaining charge by monitoring (and integrating) the power consumption; it seemed to be the only way.

You can find some basic info about NiMH charging in:
**broken link removed**
The article has also some diagrams illustrating the issue of charging. Other charger chip producers have similar papers.

In practice it is easiest to use a ready-made charger controller. The next easiest is to use a microcontroller (specially if you happen to have one present anyway) and use it to monitor the voltage and temperature.

Good luck,
Ted
 

You mentioned that NiMH batteries are easily damaged by over-charging due to heat. What happens if it is "trickle-charged" by say 20mA on a continuous basis. Does this small but continuous over-charging damage the batteries or reduce its life.
I have seen many simple charges like on cordless phones that keep charging the batteries without any monitroing. Is it OK to do so.
 

acccelerated failure

I would expect that the heat generated by the power not put into charge would accelerate the degrading of the capacity. For what you pay for the batteries, it is a small extra charge to have a good charger.
 

Flatulent, if the current is very small like 10-20mA, the heat generated will be negligible. What I am asking is that whether this small heat which will not raise the temperature of the battery by even 2 degC, will damage the batteries.
 

no fires

I have a similar system working for a portable 2m radio that is only used indoors. The batteries have not shorted out or caused a fire. I have no idea how long they will hold a charge now. The current is set for the radio draw plus a few mA. It runs all of the time.
 

There seems to be differing opinions about the long term effects of trickle charging of MiMH batteries. I have no personal experience about it, but you can read for example an article:
**broken link removed**

The consensus seems to be that battery manufactures do not recommend real long term (months) trickle charging. It might degrade the capacity, or it might not. But IMHO propability of explosion is likely very low due to virtually non-existent heating effect of very small curent.

I would use "intermittent trickle charging" technique for maintaining of charge just to be on safe side regarding battery ageing/loss of capacity.
 

HI


I know that the optimum charging current is 10% of the amp / hour capacity of the
battery .
 

HI

Once long time ago I worte a sofisticated algorithm to calaculate battery guage but to day fortunately there are electronic components who do that you can find them from company like :

Microchip

Ti

Linear-tech


All the best

Bobi
 

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