NiMH .....after charge up, what voltage do you regulate the cell to?

[treez]

NiMH .....after charge up, what voltage do you regulate the cell to?

Does it depend on ambient temperature?

Or do you simply give it trickle charge at a certain constant rate.

I looked on batteryuniversity.com and many other sites but this is not explained.

 

[FoxyRick]

You don't, usually.

Charge termination from full charge current (or change over to time-limited trickle charge) is by detecting ΔV of about -5mV to -10mV per cell, or by dT/dt of about 1°C to 2°C per minute.

Voltage detection/limit is only really for preventing damage in case something goes wrong. Likewise the timer.

NiMH do not like to be trickle charged for long periods. They cannot handle it the way NiCd can and will be damaged.

 

[tpetar]

Negative delta V (NDV) Cut-off charge system graph:



Batteries are charged at constant current of between 0.5 and 1.0 C rate. The battery voltage rises as charging progresses to a peak when fully charged then subsequently falls. This voltage drop, -delta V, is due to polarisation or oxygen build up inside the cell which starts to occur once the cell is fully charged. At this point the cell enters the overcharge danger zone and the temperature begins to rise rapidly since the chemical changes are complete and the excess electrical energy is converted into heat. The voltage drop occurs regardless of the discharge level or ambient temperature and it can therefore be detected and used to identify the peak and hence to cut off the charger when the battery has reached its full charge or switch to trickle charge.

dT/dt Charge system NiMH batteries do not demonstrate such a pronounced NDV voltage drop when they reach the end of the charging cycle as can be seen in the graph above and so the NDV cut off method is not reliable for ending the NiMH charge. Instead the charger senses the rate of increase of the cell temperature per unit time. When a predetermined rate is reached the rapid charge is stopped and the charge method is switched to trickle charge. This method is more expensive but avoids overcharge and gives longer life. Because extended trickle charging can damage a NiMH battery, the use of a timer to regulate the total charging time is recommended.

https://www.mpoweruk.com/chargers.htm

 

[BradtheRad]

Just to state my experience for comparison...

When I take NIMH's off the charger, and let them settle for several hours, they usually read 1.38 V.

When fresh off the charger, they usually are at 1.44 V.

Similar readings for Nicads (give or take a percent).

Seems to me that a safe volt level to hook them up to on a permanent basis, is whatever does not make current flow through them continually.

I'd be inclined to say about 1.35 V, but after seeing the above replies, I think it would require testing in order to discover precisely what level will yield both (a) longest lifetime, as well as (b) longest emergency backup power.

I have nicads and nimh's that are 13 years old. Most of them are still good, although I can't be sure what percent of their capacity they've lost.

 

[treez]

I'd be inclined to say about 1.35 V,

do you think this voltage depends on the ambient temperature?....for emergency lighting use, how do i ensure our nimh's are kept topped up all the time?

 

[BradtheRad]

do you think this voltage depends on the ambient temperature?....for emergency lighting use, how do i ensure our nimh's are kept topped up all the time?

A battery has a maximum voltage that it will hold, based on its internal chemistry.
It means that if you send current through it at a tiny rate, maybe C/50, it will not rise above that inherent volt level. (I think of this as 1.38V, but that is my DMM.)

Now if you 'push' the battery by applying a high charge rate, it heats up. It elevates the volt level artificially. Readings are invalid for several hours, until the battery settles.

My observations of charging, measuring, etc., are all at room temperature.
You ought to do tests on a battery pack at various temperatures. Charge it at C/20, say, until the volt level stops rising. Then reduce applied voltage until no current flows in. Wait 24 hours, then see if there is any change.

NIMH's have a higher self-discharge rate than nicads. It can be measured as milliamp-hours per month. Your aim is to apply a volt level, so as to replace amp-hours lost through self-discharge.

What volt levels are we talking about?
Suppose you maintain a nimh at 1.25 V. Doubtless that is a 'safe' level and will not overcharge it, nor reduce its useful life. However it will power your light for a disappointingly short time.

Suppose you maintain it at 1.30 V. In my mind that is still safe, it will not overcharge the nimh, and will provide a bit longer emergency operation.

Or suppose you maintain it at 1.35 V. Emergency operation time should be maximum. This level should be okay at room temperature, but only real tests will tell you if it does or doesn't overcharge at higher or lower temperatures.

It's my guess that maintaining it at 1.40 V will result in overmuch trickle charge in all temperatures, reducing its useful life. However I have not done definitive tests. So my guess may be useless when it comes to your batteries.

 

[tpetar]

Graphs :

**broken link removed**

**broken link removed**


Read this material:
Panasonic NiMh Handbook
http://industrial.panasonic.com/eu/i/21291/Handbook2011/Handbook2011.pdf

 

[FoxyRick]

treez, NiCd cells are probably more appropriate for your intended use, rather than NiMH. NiCd can be trickle charged very simply, whereas with NiMH to ensure long service life you will likely need a periodic charge cycle between which the cells are not charged at all. If you are intending this as a commercial venture than the slight increase in complexity might increase the production cost too much.

One of the problems I have experienced has been in electrical goods with built-in rechargeable cells, where these have been replaced by NiMH due to easier availability. Unfortunately the manufacturers often have not changed the charging method and so the NiMH cells barely last a year. NiCd cells would have lasted several years.