Hi All

I have build a micro controlled 'smart charger' for my Dust Buster - reason NiCad batteries were not charging properly/memory issue. Works by charging with constant current and terminated with negative delta voltage.

The battery pack capacity is 1500mA and the charge current is constant at 300mA.

As I am using a relay to switch the charge circuit my question is about top up/maintenance charging: how long should charge and what delay between charging?

Many thanks
Al

 

The general statement for NiCd batteries is that you can charge continuously at 0.1C. To allow for less than perfect or older batteries, I would limit continuous charge to 0.05C.

 

my question is about top up/maintenance charging: how long should charge and what delay between charging?

Topping up isn't recommended for NiCd due to the infamous memory effect.

If you want long life, the batteries need to be discharged before charging again. Keeping them "topped off" for convenience will reduce battery life. If the batteries were never properly "initialized", you won't see the designed capacity or service life.

 

the infamous memory effect.

The memory effect was observed in NiCd batteries in their early days but not now. I did an investigation into this effect in NiCd batteries for the company I worked for which made battery powered equipment. I could find no such effect. This was in the late eighties.
http://batteryuniversity.com/learn/article/how_to_restore_nickel_based_batteries
"The modern nickel-cadmium battery no longer has cyclic memory, but it suffers from crystalline formation. The active cadmium material is applied on the negative plate and with time, a crystalline formation develops that reduces the surface area and lowers the battery performance. In advanced stages, the sharp edges of the forming crystals can penetrate the separator, causing high self-discharge that can lead to an electrical short."

 

Thanks for the responses guys. I was getting such bad performance from the batteries that i am not so worried about reduced battery life. I think memory effect was part of the issue and dud wall pack.

AlbertHall: The Dust Buster is used regularly with 2 kids around and 0.1C is too slow. With a micro controller and ADC I can charge much faster with NDV termination.

I read something somewhere about pulsed charging helping to break up the crystal formation - while not precisely set up to charge this way I thought I could achieve something from pulsing after charged... but if topping off is not recommended then i'll discontinue this.

dl324: Okay I'll disable the top off. I have a button available and will program that to start a maintenance cycle to discharge then recharge. I can run this when the memory effect is causing issue.

The next step is lithium ion batteries but I'll have to internally balance the cells...

Thanks
Al

 

Hello there,

When i had problems with my portable drill i looked into this also. The battery packs would die too soon and also i wanted the drill to be ready any time i needed it without overcharging the pack.

What i found was that after a normal charge all i had to do was replace the amount of charge lost by self discharge, but it could not be a constant charge because a constant 24/7 charge would require too low a current which would not reach the level where the charge acceptance of the cell would kick in. That means the cell would not charge with too low of a current, and would over charge with a current that was enough to actually charge it enough to replace the charge due to self discharge.

The solution to the above is to charge at the normal charge rate (say 100ma, 200ma, or whatever is needed for a normal charge) but decrease the charge time significantly, only charging once per day (or maybe once every two days, etc.). I went with charging two times per day for 15 minutes each time. I made the time adjustable though in case i needed to increase it in the future.

The total daily time is based on the charge lost per day by self discharge. You can calculate that, and then using a charge acceptance factor of maybe 1.2 you can figure out the time needed to replenish the lost charge each day and charge for the time needed JUST for that lost charge each day.

The result was that the pack lasted 5 years instead of just 1 year with constant over charging. Constant overcharging is what happens if the pack is left plugged in 24/7 and although they say that NiCd's can take an overcharge better than other chemistries, they still dont survive as well as when done right with the daily replenishing cycle instead of constant charge.

If i remember right, my charge current level was always 150ma but may have been as low as 120ma. I long since discarded NiCd altogether and went to using Lead Acid for the drill operation. With a 7Ahr battery i get almost four times the run time of the drill because the NiCd sub C size only go up to about 2Ahr.

If you go to NiMH you have to get good quality cells that are made for very high discharge. I have not tried that myself and probably will never do that.

Funny you should mention the dust buster. That was one of the first problem child's i ran into that got me thinking about what is wrong with the way NiCd's are being charged in simple appliances.

Just to note, with negative delta voltage termination the charge current level has to be high enough to make sure that can be accurately detected. If the current is too low it gets harder to detect as the cell ages. The higher the current, the more pronounced the change is and also the better the charge acceptance is. 300ma sounds somewhat reasonable, but 500ma would be even better.

 

MrAl Thanks for your great reply and discussion.

Your solution is kind of what I have implemented however I have estimated the self discharge through listening to the dust buster when on - i.e higher voltage = spins faster = noisier. As these batteries are old and I don't want to spend the time with capacity testing my empirical evidence through listening will have to do.

Interestingly I've been keeping a running total on the MCU of charge stats: count of start and termination types - the negative voltage detection works reliably at 100% of termination with the timeout not being triggered.

Cheers
Al