Charging voltage for NiMH battery

[BeBuLamar]

Using a CV/CC power supply I tried to charge some of my old NiMH AA batteries which I think are bad. I found that to get to C/10 which is very slow charging the voltage has to be about 5V. I would expect to be able to get 1C rate with the voltage set to 1.8V or so. Is that because my batteries are bad?

[Jwillis]

NiMh batteries are best fast charged then trickeled for the remaining 30 Minutes . Long slow trickle charges  cause "memory" . 1.4V to 1.6V per cell at 1C is the recommended charge rate with temperature monitoring . Then trickle at 0.05C .
I have never used a power supply to fully charge batteries but have kick started LiPo's that won't charge on the charger. Then finished with the charger.

[IanB]

Using a CV/CC power supply I tried to charge some of my old NiMH AA batteries which I think are bad. I found that to get to C/10 which is very slow charging the voltage has to be about 5V. I would expect to be able to get 1C rate with the voltage set to 1.8V or so. Is that because my batteries are bad?

Charging at C/10 for 16 hours or so is a good way to rejuvenate tired NiMH batteries. However, if they have such a high internal resistance that you can't get even 200 mA through them at extreme voltages then they are presumably dead and useless. Best just to recycle them.

[IanB]

NiMh batteries are best fast charged then trickeled for the remaining 30 Minutes . Long slow trickle charges  cause "memory" . 1.4V to 1.6V per cell at 1C is the recommended charge rate with temperature monitoring . Then trickle at 0.05C .

Really not. Charging at 1 C is extreme and is likely to cause overheating and venting, regardless of what the datasheets might say. Don't believe everything you read.

There is a happy medium, somewhere around 0.3 to 0.5 C, that will give maximum life to the cells as long as proper charge termination is applied.

[magic]

For kicks, I would charge them at 0.01C and see if they will manage to drive something like a wall clock for more than a week, but needing 5V per cell to reach 0.1C is a very bad condition indeed. Some old cells I have which have been abused with "super fast" chargers still take 0.1C in a normal charger.

[beanflying]

Using a CV/CC power supply I tried to charge some of my old NiMH AA batteries which I think are bad. I found that to get to C/10 which is very slow charging the voltage has to be about 5V. I would expect to be able to get 1C rate with the voltage set to 1.8V or so. Is that because my batteries are bad?

What are the NiMh batteries you are trying to charge (brand/construction)? The impedance of this technology swings wildly from Brand to internal construction before you get to age and former abuse/storage. Even within AA's there is a lot of variation. So suggesting a charge rate on unknown cells is not of much use. As a bit of a general rule 'reasonable' AA cells in ok condition will handle a 1-3 hour charge but some of the stinkers will need slow charging at 10-15 hours and really really dislike attempts at faster charging. All that is before you get into the issues of memory and long term discharged storage will have done to the chemistry.

Going back into the past of R/C NiMH abuse days (post NiCad) we used to fast charge at 2-4+C and pull 100A+ back out but they were limited to 1-2 specific cells/manufacturers. Lifespan even at this abuse level was hundreds of cycles while maintaining near peak performance.

Generalist advice this is a good source https://batteryuniversity.com/learn/article/nickel_based_batteries and there is a lot to recommend adding a reasonable R/C charger to your collection that will consider cell impedance and charge most common battery types (Lithium/Nickel and Pb), the sometimes patchy Imax/clones are the bottom end of that or spend a few $ more and enjoy.

[BeBuLamar]

The batteries are Energizer brand NiMH rated at 2500mAH. I had them for almost 20 years and not using them for a good 10 years now. The question is really not what rate to charge but why do I need 5V just to push 200mA into the battery?

[beanflying]

Because more than likely they are chemically damaged internally due to prolonged uncharged state. Also the older 'high capacity' cells (over 2000mAh) had the highest internal impedance for a start and were absolutely a slow charge technology. The other issue with the higher capacity of that era was greatly increased self discharge compared to the then new Enerloop (2000mAh) technology.

I have more than a little usage of AA cells for Receiver packs (4 cell) of that era and before and we in most cases avoided AA sized cells because of impedance and we went to A or SubC sizes as even for this application we needed more punch for Digital servos and planes meeting the ground  due to poor packs were common.

[atmfjstc]

20 years? Regardless of whether they've been used or unused, after 20 years the batteries are toast, finished, kaput. I'd get new ones if I were you.

[IanB]

The batteries are Energizer brand NiMH rated at 2500mAH. I had them for almost 20 years and not using them for a good 10 years now. The question is really not what rate to charge but why do I need 5V just to push 200mA into the battery?

It was widely believed that the Energizer brand NiMH of 20 years past were some of the worst cells on the market. So you are not starting from a good beginning.

Why do you need 5 V just to push 200 mA into the battery? Because they are expired cells, dead and useless. They are paperweights. Do not waste your time on them.