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NiMH rechargeable battery question ?

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tron87

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When designing circuits that are powered by these batteries is a low voltage cutoff circuit essential to protect the cell from being discharged too much?

How low can the voltage on a 1.2v aaa NiMH rechargeable cell go before damage is done to the cell ?

How low can the voltage on a 8.4v pp3 NiMH rechargeable battery go before damage is done to the battery ?

How do you tell when a NIMH cell is above the threshold voltage where damage is done due to over discharge but below useful operations in supply power and ready for recharge ?
 
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You should stop at 1.0 volts per cell. If you have more than 1 cell in series, the stronger cells can damage the weaker cells by forcing them all the way to a reverse polarity.
 
How do i calculate how long to charge a NIMH cell for if its an aaa 700mah 1.2v i discharge it until it drops to 1v the lowest it can go before over discharge ?

My NIMH charger does not cut out when the cells are recharged so calculating charge time is very important, that charger puts out 50ma for charging aaa NIMH batteries if it helps.
 
Hi,

If you discharge it all the way then you can divide the 700 (the capacity) by 50 (the charge current) and then multiply by about 1.2, which is:
t=1.2*700/50=16.8 hours approximately.
It is possible that the charge acceptance is not that good, so you may have to multiply by 1.3 or even 1.4 instead of 1.2 or so. A test would help determine the best time for your cells at that charge rate.
 
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When you say discharged all the way do you mean that under load the cell provides just 1v or when the cell is not under load the cell outputs 1v or the cell is driven truly flat <1v ?

How can you tell when a 1.2v NIMH cell has been overcharged ?
 
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For their Ni-MH cells, Energizer recommends using a smart battery charger IC that detects when the cells are fully charged and refuses to over-charge cells that are already charged. They say to shut off the charger when the cells are fully charged or to limit the trickle charge current to 1/40th C.

Your charger has a continuous output of only 50mA so it is probably made to charge and over-charge old Ni-Cad cells. Ni-Cad cells were trickle-charged at 1/10th C. So a 500mAh cell was trickle-charged at 50mA.
 
This is what I call, "notes to myself". I gathered this from Duracell and Energizer web sites, for my own use. You are allowed to check their web sites, too.

Caution: Most measurements cited by the manufacturers assume a charge rate of 1C.
1C is unhealthy for NiMh batteries. It causes too much heat. This makes designing a good charger rather difficult because the specifications are only valid for a poor quality charger. Maybe a way around this is to use a C/2 or C/10 rate until the battery is mostly full (SC is approaching 1.45V) and then use a 1C charge rate for a few minutes while measuring dt/dT (d temperature / d Time).

The usual way to detect full charge is dt of 1º- 2º C / minute (1.8º-3.6º F) @ rate = 1C
There is a peculiar characteristic during charging, in that a voltage drop of -.02 V per cell happens at about 94%. It is possible to use this as a method to detect 94% full. The last 6% is delivered at C/10 (3.6 minutes).
A trickle charge is defined as .05C to .03C.
The “nominal” rating of NiMH batteries is 1.2 volts. They really deliver 1.25 volts at ½ amp from 90% to 20% full. A 2.5Ω, 2 watt resistor is an important tool for measurements on single cells, like AA or AAA batteries.
Zbatt =.05 Ω This is why NiMH is better than alkaline for current surges, like recharging a photo flash.
If a charging event achieves 1.78 volts per cell, the cell is overcharged!
If a charging event achieves 140ºF (60ºC), the cell is overheated!
More likely numbers for a full charge are 1.41V to 1.45 V, NL.
If a memory effect happens, exercising the battery down to 1.00 volts per cell and then recharging will correct it in 3 or 4 cycles.
Voltage reversal can happen, but it is very bad. Do not allow loaded voltage to go below .8V
Self discharge might be as high as 5% to 10% in the first 24 hours. After that, ½% to 1% per day.
The recharge cost in amp-hours is 50%. (A 1 amp-hour cell will require 1.5 amp-hours to recharge.)
The chemical result of overcharging is the creation of free oxygen gas. The cells can not survive undamaged if O2 is generated at greater rate than C/10. They might even explode.
 
Basically is a nimh cell fully discharged (within safe limits) when it has 1 volt across it and is not connected to anything or when it drops to 1v under load ?
 
You would be best off disconnecting the battery when it is 1V under load, this way once the load is removed then the cell voltage will increase slightly and no damage will be done to the battery. Depending on what the voltage is, there are several different ICs that can disconnect. A good one will have hysterises so it does not oscilate. The one that I used to protect the batteries in my radio, I used an ICL7665 it has an internal reference voltage so it can be programed for any voltage above 1.3V. As for charging you can use the 7665's sister part, the MAX712 which is able to stop charging once it detects a full charge and will not damage the battery with overcharging.
 
With an internal resistance of .05 ohms for a AA battery, even a whole amp of load current will only throw the measurement off by .05 volts and a whole amp out of a AA battery is a lot of current for such a small package.
 
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