Simple and safe NiMH charger?

[antomas]

Hi everybody!
I'd like to add a battery charger for a 4xAAA NiMH pack to a portable headphone amplifier. Charging speed doesn't matter to me. After a bit of search and some thoughts (possibly wrong: I'm not a EE!) I had the idea of using a trickle charger at 0.05C with a voltage limiter. In my mind, even if voltage limit is not considered a proper charge termination strategy, my charger should:

- not overcharge the cells, if mistakenly left ON forever (yes, I'm most of the time absent-minded);
- not discharge the cells, if left OFF forever (except for the few uA leakage of the 1N4001).

Would my schematics work the way I expect?
Thanks very very much in the anticipation for every comment about it.
Massimo

 

[Conrad Hoffman]

A good place to read up is here. The dilemma with NiMH is they reportedly don't like to be trickle charged at low levels indefinitely. There are various battery charging chips that will sense the voltage change and terminate properly, probably for very little money.

 

[theAnonymous1]

Maxim makes nice battery charger IC's. Maybe one like this......

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666

Not very expensive and they can be "sampled".

 

[antomas]

Hi Conrad,
Thanks for your link.
In fact mine is not a real trickle charger. If I'm right, when the battery reaches 5.5V, the trickle current should flow through the zener and do not bother the battery pack. Possibly...
Massimo

 

[antomas]

Anonymous,
thanks a lot. My hope was to put together a safe charger with just 4 components.
Ciao, Massimo

 

[Conrad Hoffman]

If you limit the voltage to a safe level, you'll never get the cells topped off and lose a lot of the energy capacity- energy capacity that's already pretty bad with AAA cells. It *will* work however, though I'd think seriously about going with AA cells and using the chip!

 

[theAnonymous1]

Anonymous,
thanks a lot. My hope was to put together a safe charger with just 4 components.
Ciao, Massimo

Four....... fourteen; what's the difference.

At least you would know the batteries are being properly taken care of.

 

[TheMG]

When it comes to batteries, charging can be summed up into two categories:

-standby charging, where the charger is left on and the voltage from the charger is pretty much equal to the battery's fully charged voltage, as the name suggests this is for standby use, such as battery backups, where the very long charging time and slightly reduced capacity does not matter much

-cyclic charging, usually a much higher voltage is applied to charge the battery, resulting in higher charge current, faster charge rates, and the battery gets properly "topped off". This method necessitates proper end-of-charge-cycle termination. This is the most common charging method used, especially for portable devices.

Actual charging voltages, currents, and termination method will depend on the type and size of battery being charged.

 

[EWorkshop1708]

What about using a comparator to measure the battery's voltage, and shut off charging when a certain voltage is reached?

 

[antomas]

TheMG,
Maybe I had been too concise. I'd like to use this circuit onboard on a portable headphone amplifier, so many generic or recycled wall-warts can be used as a power source. Therefore usage is cyclic and termination is voltage based. I'm aware that, for some unknown (to me) reasons, this is not the recommended charge termination method for NiMH cells.
So the question is: is current limiting at 0.05C + voltage-based termination harmful for NiMH cells?
Or more precisely: could it reduce lifespan of the NiMH cells? How much?

EWorkshop1708,
Indeed a comparator can be used at the price of additional components. The only advantage I see is the possibility of fine-tuning the voltage level used to terminate the charging process. As alternative, one could fiddle with different zener and "valve" diodes. For instance, replacing the 1N4001 plus a 1N5817 Schottky diode should give around 5.8V, A 6.8V Zener plus 1N4001 around 6.1V, etc. Another solution uses a L200 regulator. Page 9 of datasheet shows a schematics for a battery charger like mine (five components) that should be altered a bit to prevent small battery packs from being discharged through the R1-R2 resistors.

Conrad, theAnonymous1,
I understand that the Maxim chip is the perfect solution, but... sorry, the circuit doesn't seem to fit the limited space on my board.
With my circuit I just hoped to find a simple solution for charging the 4xAAA pack (4xAA don't fit my enclosure) without reducing their lifespan and using easy-to-find components.
If you have any advice about that as well as about the proper voltage termination level, please let me know. In the many battery datasheet and papers I checked I just found that charging should be absolutely stopped ad 1.8V per cell, but this value seems to me a bit too much to be used. Do you think 1.55V per cell (i.e. 6.1V per pack, using a 6.8V Zener) would be ok to "fill the bag" reasonably?

Gentlemen,
thanks for your help and patience and best regards.
Massimo

 

[Dan2]

Sorry i don't have any usefull info but i also need to build a battery charger - only thing is i don't know what type of battery it is - so i want to know if there are any differences between the chargers. in other words can i use a nicd charger for nimh batteries??

actually the battery i have looks like a backup battery for a sliding gate motor - and its pretty heavy so my guess it is a sealed lead acid battery.

 

[antomas]

Dan,
don't worry about your question. Diyaudio.com is a great place because you may make any (most) questions and great people shall provide you with competent answers (of course I'm absolutely not talking about myself).

Your battery looks like a SLA (sealed-lead-acid) type.
Some information here:
http://individual.utoronto.ca/janzen/electronics/tutorials/battery/sla_working_with.html
Check your battery type, manufacturer and capacity in Ampere/hour. There should be something written on the top or on the side of it.
According to an italian SLA battery manufacturer, the best (don't ask me why) way to charge a SLA battery is voltage and current limiter (I tend to remember at 0.1xC - one tenth of capacity).
The simplest circuit I found around for a SLA charger is on page 9 of the L200 chip or here:
here

Hope it helps,
Ciao!
Massimo

 

[Dan2]

FANTASTIC!! thats what i am looking for.

just one problem - there is no writing on the battery, its just a black plastic enclosure with a red dot by the positive and a black dot (on a black casing, i know) by the negative terminal.

my plan is to use the battery to hold the memory on a car radio - so its not i high power application. maybe i will build a 0.5A charger 'cos i doubt the battery will be less than a 5Ah.

 

[gfiandy]

Hi, Just had a quick look at your circuit. I assume that the intention was to creat a 50mA curent source. If this is the case then you need to draw the current through the 25R resistor to the load then take the end of the 25R to the ref input. At the moment the current doesnt flow though this part so it won't work.

Can't really comment on battery charging schemes as I don't know enough about them.

Regards,
Andrew

 

[antomas]

Aaaaaaaaaaaaaaaaaahhhhhhhhhh!

Andrew, you're my saver!
I was completely drunk when I wrote the schem.
Even worse, I was still drunk when a designed a pcb for it (not yet etched).
Heartfelt thanks, (drink moderately!)
Massimo

 

[antomas]

Without shame of threadjacking my own thread

Dan,
before maing any further experiments with your battery, you can do two things:
1. check that your battery is not discharged below 10V (i tend to remember this is the "non-return" level; do also a google search for SLA and "deep discharge"); if so, throw it away (in the appropriate bin;
2. in lack of further info about your battery, compare the size of your battery against the size of batteries of some reputable manufacturer (Yuasa, Varta, Fiamm, etc.) to guess the "right" capacity (C).
Ciao!
Massimo

 

[antomas]

Ready to test

Here is the final schem I shall build and test. Changes are:
1. corrected the current source (thanks very much, gfiandy!)
2. a bit less current, suitable for AAA batteries rated at 750 mAh or more, possibly not appropriate for the few batteries rated at 550mAh;
3. a bit more voltage; 1.5V per cell seems safe; if you like to squeze more from you battery pack, use a 7.5V Zener AFTER checking the specs of the battery manufacturer.

Hope this may be of help for someone. If you want to charge AA cells, increase current by decreasing the value of Ri, using the formula:
Ri = 1.25 / (C / 10)
where C is the capacity of your battery pack. Maybe the Zener could get hot when the charge is over. Who knows...
That's all for the moment.
Good aftenoon and happy charging.
Massimo

 

[theAnonymous1]

A lead-acid battery it technically at zero capacity when the voltage is <11.6v. Draining the battery any further will take big chunks out of it's life expectancy. Sulfation will occur if the battery is let sit for any length of time with a voltage <11.6v.

 

[antomas]

I forgot to mention that the 6.8V/5W Zener is 1N 5342B.

In old Europe:
RS part no. 485-3965
Distrelec part no. 603608

Ciao a tutti!
Massimo

 

[antomas]

Anonymous,

A lead-acid battery it technically at zero capacity when the voltage is <11.6v

Thanks for pointing it out. To help the absent-minded people like me not to forget, this is also stated in the page mentioned a few posts above.
Ciao, Massimo