# Battery Discharge Testing - lowest NiMH voltage



## odessit (Jan 29, 2009)

Just tipple-checking:
All my previous battery discharge tests were set not to go below 1V.
Is this the standard practice?
Should I set my battery tester's minimum voltage to 0.9V or even lower?
Thanks


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## Anders (Jan 29, 2009)

Hello odessit.

If you test one cell, 0,9 Volt is a good value.
It doesnt harm the cell.

You are talking about 1,2 Volt NiMh cell?

Anders


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## odessit (Jan 29, 2009)

Yes, NiMH 1.2v cells - Eneloop and Sanyo 2700 specifically.
The thing is - I would like to be consistent in my testing, but if 0.9v is a better real world example - this is what I will switch to.


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## Mr Happy (Jan 29, 2009)

odessit said:


> Yes, NiMH 1.2v cells - Eneloop and Sanyo 2700 specifically.
> The thing is - I would like to be consistent in my testing, but if 0.9v is a better real world example - this is what I will switch to.


It depends on the discharge rate. At light loads, there will be little difference in the capacity measured by a 0.9 V end point or a 1.0 V end point. However, at higher loads (e.g. 1C or greater, or even less than 0.5C with high resistance cells) the difference will be greater. A high resistance or highly loaded cell could deliver quite a lot of energy with a terminal voltage under load of around 1.0 V.

This means that for single cell discharge testing, 0.9 V is a good end point to pick.

[Edit to add: The IEC standard test is to discharge the cell at 0.2C down to 1.0 V at a temperature of 25 Celsius following a standard charge. If you are set up to follow that procedure, that would allow you to compare with previously published results.]


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## odessit (Jan 29, 2009)

Thanks Mr. Happy. I will stay with 1V. My previous discharge rates were 500mA = 0.25C for Eloop and 0.185C for S2700.
Close enough for my crude tests, although my analyzer allows precise 0.2C currents.


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## SilverFox (Jan 29, 2009)

Hello Odessit,

The IEC standard uses the following cut off voltages...

Discharging at 0.2C, discharge down to 1.0 volts
Discharging at 1C, discharge down to 0.9 volts
Discharging at 5C, discharge down to 0.8 volts, and
Discharging at 10C, discharge down to 0.7 volts.

As you can see, and as Mr Happy mentioned, the discharge voltage cut off is dependent on the discharge current.

I use 0.9 volts as a general reference for the cells that I test, as do many battery manufacturers, as listed on their data sheets. I don't think there is a lot of difference between using 1 volt or 0.9 volts until you get up to higher discharge rates.

I test both single cells and battery packs. The 0.9 volt cut off works well for both. You have to be careful with battery packs that you don't run into cell reversal, and many people use a 1 volt cut off for battery packs.

In the end, pick a cut off voltage and stick with it to end up with a consistent data base.

Tom


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## InventPeace (Jul 17, 2011)

*Re: Battery Discharge Testing - lowest NiMH voltage, 0%, 100% 80%, charge voltage*

TO SilverFox:
Just want to check with you : you said for 1.2 v cells .9 v lowest discharge is not bad for low discharge rates (1c or less discharge rate). thanks for posting that. 

The questiion I have is what voltages are 0% and 100% ., I read somewhere that toyota says that their prius NIMH batteries show no degradation between 40% and 80% usage. So we are thinking just charge them to 80% so they will last much longer(perhaps 1.14v ?) . THinking if .9 v is low (~0%) then 1.2 is high (100%). 

But we know we have to charge the batteris at more than the 1.2v.. so that would mean perhaps 20% higher charging voltage (1.44 ?) Please advise! thanks. 

===================== RE: 2003 post====================
Re: Battery Discharge Testing - lowest NiMH voltage 
Hello Odessit,

The IEC standard uses the following cut off voltages...

Discharging at 0.2C, discharge down to 1.0 volts
Discharging at 1C, discharge down to 0.9 volts
Discharging at 5C, discharge down to 0.8 volts, and
Discharging at 10C, discharge down to 0.7 volts.

As you can see, and as Mr Happy mentioned, the discharge voltage cut off is dependent on the discharge current.

I use 0.9 volts as a general reference for the cells that I test, as do many battery manufacturers, as listed on their data sheets. I don't think there is a lot of difference between using 1 volt or 0.9 volts until you get up to higher discharge rates.

I test both single cells and battery packs. The 0.9 volt cut off works well for both. You have to be careful with battery packs that you don't run into cell reversal, and many people use a 1 volt cut off for battery packs.

In the end, pick a cut off voltage and stick with it to end up with a consistent data base.

Tom​


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## Mr Happy (Jul 18, 2011)

*Re: Battery Discharge Testing - lowest NiMH voltage, 0%, 100% 80%, charge voltage*



InventPeace said:


> The questiion I have is what voltages are 0% and 100% ., I read somewhere that toyota says that their prius NIMH batteries show no degradation between 40% and 80% usage. So we are thinking just charge them to 80% so they will last much longer(perhaps 1.14v ?) . THinking if .9 v is low (~0%) then 1.2 is high (100%).
> 
> But we know we have to charge the batteris at more than the 1.2v.. so that would mean perhaps 20% higher charging voltage (1.44 ?) Please advise! thanks.


You misunderstand how batteries work. Batteries have two voltages: they have a resting voltage when subjected to no load, and they have a working voltage present when they are either under load or being charged. The loaded voltage is less than the resting voltage and the charging voltage is greater than the resting voltage.

That said, a typical "good" NiMH cell will have a resting voltage of about 1.2 V when at 0% state of charge and a voltage of _*about*_ 1.35 V when at 100% state of charge. I say _*about*_ because it is difficult to tell the state of charge of an NiMH cell from the voltage. It depends on the type or manufacture of the cell and the time since it was last charged.

When under load the voltage will be less than the resting voltage, and will drop rapidly to 0 V when the cell is empty. Therefore we tend to stop discharging when the voltage reaches 0.9 V just as a matter of convenience.

When being charged the voltage will be higher than the resting voltage. Often the voltage will be as high as 1.6 V just before the end of charging. However as soon as charging stops the voltage will decrease rapidly.


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## SilverFox (Jul 18, 2011)

*Re: Battery Discharge Testing - lowest NiMH voltage, 0%, 100% 80%, charge voltage*

Hello InventPeace,

Welcome to CPF.

As Mr Happy posted it is not a voltage that is related to 0 and 100%. Well, you could say that 0% is when the cell reaches 0.9V under load. 100% is difficult to relate to a single voltage.

The maximum voltage depends upon the charging rate.

If you want to use your cells between 40 and 80%, you would have to first do a complete charge followed by a complete discharge to determine the actual capacity of the cell. Next you would need a way to measure the amount of capacity you put into the cell and limit your charge to 80% of the maximum capacity you measured. While using the cell you would need to measure the capacity used from the cell and stop when you are down to 40%.

Voltage monitoring is not sophisticated enough to do this, so you would have to come up with a way to measure amp hours or watt hours on the fly, and use that information to both limit the charge and limit the discharge of the cell.

Tom


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## 45/70 (Jul 18, 2011)

*Re: Battery Discharge Testing - lowest NiMH voltage, 0%, 100% 80%, charge voltage*

What Mr H and SilverFox are saying, although in a more detailed way, is that there is really no practical means to determine the state of charge of NiMh cells, by simply measuring voltage.

A rough estimate of remaining cell capacity can be made when the voltage is measured under a known load, and some testers such as the ZTS work using this method. However, when simply measuring the open circuit voltage of a NiMh cell, the only voltage reading that is reasonably accurate, as Mr H mentioned, is when the cell reads ~1.20 Volt. This typically, is the voltage of a discharged NiMh cell.

When measuring the voltage of a NiMh cell under an unknown, or varying current load, estimating the remaining capacity of the cell becomes more complicated, and there is no easy way to estimate the remaining capacity of the cell.

Dave


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## BGater (Jan 9, 2014)

*Eneloop Discharging-minimum volts ?*

I am doing a discharge/charge cycle on my AA and AAA Eneloops with a hobby charger. What should the minimum discharge volts, or "cut-off" be set at ? I have it set to 0.9 volts. Also, what should the max charge volts be ? My LaCrosse BC-900U shuts off at 1.47 volts. Seems high for a 1.2v batt. Im sure its been covered somewhere here but couldnt find it. Probably not searching the correct words. Thanks in advance.


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## Norm (Jan 9, 2014)

*Re: Eneloop Discharging-minimum volts ?*

Always pays to search before opening a new thread, most questions have been asked previously. Thread Merged - Norm


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## BGater (Jan 9, 2014)

*Re: Eneloop Discharging-minimum volts ?*

Thanks Norm. I guess I was using wrong wording in my search as I couldnt find what i was looking for. This thread answered all my questions. Thanks again.


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