Does battery voltage correlate to its remaining life?

[johnny13oi]

Hey guys, I was wondering if a batteries voltage correlates to the batteries life in any way. Like would one be able to guess the remaining life of the battery given the voltage. Thanks in advanced.

 

[LuxLuthor]

Hi Johnny, I don't think you can tell just based on a random voltage reading, unless it is so low it indicates the cell is dead.

It depends on which kind of battery you mean (Rechargeable-NiCd, NiMH, Li-Ion, non-rechargeable versions, etc.).

With some rechargeables like NiMH, it also depends on how the battery responds when you discharge it, then how much it recharges...often cycling it several times properly, or doing a "forming" 0.1C x 16 Hrs charge can dramatically rescue them.

(The 0.1C means the capacity of the battery..i.e if you have a 2000 mAh NiMH AA battery, then 1C means you are charging it at 2000 mA rate, and 0.1C is charging at 200 mA rate....sorry if you already knew that.)

 

[johnny13oi]

Hi Johnny, I don't think you can tell just based on a random voltage reading, unless it is so low it indicates the cell is dead.

It depends on which kind of battery you mean (Rechargeable-NiCd, NiMH, Li-Ion, non-rechargeable versions, etc.).

With some rechargeables like NiMH, it also depends on how the battery responds when you discharge it, then how much it recharges...often cycling it several times properly, or doing a "forming" 0.1C x 16 Hrs charge can dramatically rescue them.

(The 0.1C means the capacity of the battery..i.e if you have a 2000 mAh NiMH AA battery, then 1C means you are charging it at 2000 mA rate, and 0.1C is charging at 200 mA rate....sorry if you already knew that.)

No need to be sorry for explaning ... I didn't know that until recently when I joined these forums and its a commonly asked question. Yeah I was referring to the rechargeable NiMh 1.2V batteries I was just seeing if there was a way to tell how much battery life is remaining so that I can charge it accordingly. Thank you for the answer.

 

[Curious_character]

For NiMH, the answer is a definite no. You can't judge the remaining capacity by the open-circuit voltage. Even a test of battery voltage under load gives only a very rough idea. The remaining capacity of alkaline cells can be estimated so-so by looking at the voltage under load, but not the open-circuit voltage. You can make a fairly good estimation of the remaining capacity of lithium primary, Li-Ion, and lead-acid batteries from their open circuit voltages, but they have to have been disconnected from a load for some time.

c_c

 

[johnny13oi]

Is .9V considered the point where NiMh batteries should be recharged or is there a higher point for longer life of the cell?

 

[Curious_character]

You don't extend the life of NiMH cells by discharging only slightly -- they're actually happiest if fully discharged each time. By the time they hit 1.0 volt per cell, they're fully discharged, that is, there's no signficant amount of energy left. (You might possibly need to get down to 0.9 volt under load if you're drawing a very large amount of current.) It doesn't hurt to discharge them lower. The only thing you need to avoid is reverse charging them. While some claim to be able to withstand a limited amount of reverse charge without damage, there's a point where they'll be permanently damaged.

c_c

 

[LuxLuthor]

It doesn't hurt to discharge them lower.

I just learned something there. I thought it must hurt them since all the disharge profiles stop at that 0.9 to 1.0V for NiMH of chargers that I have. thanks

 

[Curious_character]

Not long ago I ran across a Sanyo app note where they described methods of recovering NiMH cells which had been stored for a long period. They found that most would recover after discharging to about 0.9 volt, then recharging at 0.1C for 16 hours. But some cells required deeper discharging, to around 0.4 volt, in order to recover.

The curves stop at 1.0 or 0.9 volt simply because there's no real energy left beyond that point, and the discharge curves become essentially vertical. Anyone designing a device intended to run from NiMH cells needs to make it work with a cell voltage down to that level, but there's no need to make it function at lower cell voltages.

c_c

 

[SilverFox]

Hello Curious character,

Be very careful when discharging NiMh cells below 0.9 volts. If you are using a heavy load (1C or higher), it should not be a problem, however under very light loads, it is possible to damage the cell and cause it to vent through deep discharging.

There are actually two phases that occur during the deep discharge. The first involves the depletion of the active material of the positive electrode. At the end of this phase, hydrogen gas begins to form. The negative electrode is larger and begins to absorb the hydrogen gas. The second phase involves the complete depletion of the negative electrode. Once both electrodes are completely depleted, the negative electrode tends to absorb oxygen and the capacity of the cell is reduced. The generation of hydrogen and oxygen gas builds up the pressure within the cell, and it is possible for the cell to vent. Venting electrolyte leads to a further loss of capacity.

The exact voltages where these transitions accrue is very close to 0.4 volts, but varies depending on the condition of the cell.

Time is another variable. A short excursion to a lower voltage is not as damaging as shorting a NiMh cell out and leaving it a 0 volts for an extended period of time.

We have found that lower capacity NiMh cells are more tolerant of discharge abuse than the higher capacity cells. You can completely ruin a high capacity cell by allowing it to sit overnight with a resting voltage below 0.9 volts. It seems that the pressures cause separator damage and the cell develops soft shorts that lead to high self discharge rates.

When you are trying to break up large crystal formations by slow discharging to 0.4 volts, make sure that the cell doesn't spend any time at this discharge level. As soon as you hit 0.4 volts, immediately start a 0.1C charge to bring the cell back up to 1.0 volts. From there you can charge at your normal rate.

Most manufacturers consider a cell fully discharged at 1.0 volts. We run test data to 0.9 volts to get the extra little bit out of the cell.

Tom

 

[johnny13oi]

I dont know if this is relevant or not but I have an LED light that dies at exactly .900V.

 

[LuxLuthor]

Thanks for that additional info, Tom. So when should we generally do a D/C lower than 0.9....maybe down to 0.6V as a setting on my Hyperion with about a 0.5C discharge rate?

I didn't know that sometimes going lower (i.e. down to 0.4 or 0.5V) can accomplish something beyond 0.9V, but is there something to look for that makes you think of doing the deeper D/C?

 

[Mike abcd]

Hello Curious character,

Be very careful when discharging NiMh cells below 0.9 volts. If you are using a heavy load (1C or higher), it should not be a problem, however under very light loads, it is possible to damage the cell and cause it to vent through deep discharging.

There are actually two phases that occur during the deep discharge. The first involves the depletion of the active material of the positive electrode. At the end of this phase, hydrogen gas begins to form. The negative electrode is larger and begins to absorb the hydrogen gas. The second phase involves the complete depletion of the negative electrode. Once both electrodes are completely depleted, the negative electrode tends to absorb oxygen and the capacity of the cell is reduced. The generation of hydrogen and oxygen gas builds up the pressure within the cell, and it is possible for the cell to vent. Venting electrolyte leads to a further loss of capacity.
...
Tom

Hello Tom,

You appear to be describing an NiMH cell being driven into reverse voltage when using multiple cells in series and not just a cell being deeply discharged.

According to Sanyo, both the initial hydorgen gas generation and later oxygen generation happen only at negative voltages. That can't happen with a single cell regardless of the depth of discharge.
Section 2-2-4 "Polarity Reversal"
http://www.sanyo.com/batteries/pdfs/twicellT_E.pdf

I'm sure c_c is aware of this as he previously pointed out to me that simply overdischarging an NiMH cell didn't damage it as I thought.

Mike

 

[Curious_character]

Hello Curious character,

Be very careful when discharging NiMh cells below 0.9 volts. If you are using a heavy load (1C or higher), it should not be a problem, however under very light loads, it is possible to damage the cell and cause it to vent through deep discharging.

There are actually two phases that occur during the deep discharge. The first involves the depletion of the active material of the positive electrode. At the end of this phase, hydrogen gas begins to form. The negative electrode is larger and begins to absorb the hydrogen gas. The second phase involves the complete depletion of the negative electrode. Once both electrodes are completely depleted, the negative electrode tends to absorb oxygen and the capacity of the cell is reduced. The generation of hydrogen and oxygen gas builds up the pressure within the cell, and it is possible for the cell to vent. Venting electrolyte leads to a further loss of capacity.

The exact voltages where these transitions accrue is very close to 0.4 volts, but varies depending on the condition of the cell.

Time is another variable. A short excursion to a lower voltage is not as damaging as shorting a NiMh cell out and leaving it a 0 volts for an extended period of time.

We have found that lower capacity NiMh cells are more tolerant of discharge abuse than the higher capacity cells. You can completely ruin a high capacity cell by allowing it to sit overnight with a resting voltage below 0.9 volts. It seems that the pressures cause separator damage and the cell develops soft shorts that lead to high self discharge rates.

When you are trying to break up large crystal formations by slow discharging to 0.4 volts, make sure that the cell doesn't spend any time at this discharge level. As soon as you hit 0.4 volts, immediately start a 0.1C charge to bring the cell back up to 1.0 volts. From there you can charge at your normal rate.

Most manufacturers consider a cell fully discharged at 1.0 volts. We run test data to 0.9 volts to get the extra little bit out of the cell.

Tom

Thanks very much for the information. I'm glad to have the opportunity to learn more about this. Not being professionally involved in battery technology, my knowledge comes only from reading the engineering manuals provided by the manufacturers, now on line. I've found the Energizer manual particularly helpful, although Sanyo and Panasonic also publish good information. But all three indicate that the discharge of the positive electrode happens at a voltage of about -0.2 to -0.4 volt, that is, when the cell is reverse charged, not at a positive voltage like you've said. While none recommend intentionally reverse charging to this level, they state or imply that permanent damage occurs only when the negative electrode also discharges at an even more negative voltage. Positive end points for discharge voltage seem to be based solely on the need to prevent substantial reverse charge in series cell configurations.

Since what you've said seems to contradict the information published by these manufacturers, could you point me to literature -- reliable web sites or texts -- which explain the apparent contradiction?

I do note that the manufacturers caution against leaving cells connected to a low current load while stored, but it looks like the damage this causes has a different cause than what you're talking about. Please correct me if I'm wrong about this.

Thanks!

c_c

 

[Mike abcd]

...I've found the Energizer manual particularly helpful, although Sanyo and Panasonic also publish good information.
,,,

c_c

c_c,

Do you have a link for the Energizer manual? The only Energizer doc I found was fairly limited compared to the Duracell "OEM" and Sanyo tech docs.

The Duracell doc I found is here.
http://www.duracell.com/oem/rechargeable/Nickel/nickel_metal_tech.asp

Mike

 

[SilverFox]

Hello Mike and Curious Character,

As you have pointed out, polarity reversal with in a cell involves negative voltages and is associated with battery packs of several cells. However, similar damage occurs when a single NiMh cell is over discharged.

If you notice the last paragraph in the Sanyo document…

In addition, if the battery is left connected to a load for a prolonged period of time, the battery voltage will become 0V, and the likelihood of leakage will increase…

This means that in addition to cell reversal, the same damage can occur if you over discharge to 0 volts.

We have two cases here which result in similar damage. One is when a series of cells is discharged and one cell of lesser capacity is driven into reverse polarity. The other is when the cells are deeply discharged.

In the 0.0 – 0.4 volt range, the NiMh battery chemistry is unstable. This instability is what causes the cell damage. The amount of damage is dependent on a number of variables including the state of health of the cell, temperatures, time spent at the discharged condition, and so on.

I would love to be able to explain what is going on with the chemistry, but I find it hard to understand myself, let alone explain it to someone else.

I got this information from the "Handbook of Batteries" third edition.

If you review the Polarity Reversal graph in the Sanyo document and transpose the battery voltage curve a little, you have a graphic of what this looks like.

Here is a test showing this. This is a 0.003C discharge on a high capacity cell. The damaged started at around 0.26 volts.

What I find very interesting about this is that during the last voltage plateau, the battery temperature rose 2 F.

Tom

 

[Curious_character]

c_c,

Do you have a link for the Energizer manual? The only Energizer doc I found was fairly limited compared to the Duracell "OEM" and Sanyo tech docs.

The Duracell doc I found is here.
http://www.duracell.com/oem/rechargeable/Nickel/nickel_metal_tech.asp

Mike

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf. Figure 14 is apropos to the current discussion.

c_c