Trickle charging NiCads

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So, charge at the ten-hour rate from a current source. 800 mAh battery (kind of a low-end AA) will want 80 mA for ten hours, roughly, so connect a wall-wart with enough volts into a resistor in series with the battery, or an unregulated supply with an LM317 + resistor regulated current source. The resistor should be sized to R=3D 1.25V / Icharge, and suitable to dissipate 1.25 x Icharge watts.

Problem is, it will be permanently connected to the charging circuit and will be charged 6-10 hrs a day regardless of whether it is discharged or not in between charges. Under those conditions, I don't think a C10 charge will be very healthy for the battery.

I'm ok with designing the charging circuit to any degree of precision as far as current regulation is concerned. I just want to avoid a drastic reduction in useful battery life by constantly overcharging it, while keeping the circuit simple by omitting a complex full-charge detection.

Perhaps this is a good time to explain the intended application. I want to make an emergency light using white LEDs that will turn on automatically in the event of a power failure. Nothing new in that. But this unit will be connected in parallel with a normal house light that's usually kept on every night from about 5 or 6 pm to 1-3 am.

The emergency light will sense the state of the light switch even in the absence of mains power and turn on only if the switch is in the 'on' position - a fully automatic fit-and-forget operation. I've designed and tested the circuit but am not sure what level of trickle charging would be a good compromise.

"pimpom"

** Most NiCd makers suggest a C/50 rate for constant trickle charging.

But why use NiCds at all??

NiMH cells have lower self discharge rates and less issues like the formation of "dendrites" that plague NiCd cells left on trickle charge.

...... Phil

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I was going to suggest a different battery type also. Why NiCds? If you don't mind the weight there are also lead acid gel cells. I haven't done much (read anything) with NiMH. But I thought I read that they didn't want to be trickle charged. They wanted pulses... I guess you can make a trickle pulser.

George Herold

I was going to suggest a different battery type also. Why NiCds? If you don't mind the weight there are also lead acid gel cells. I haven't done much (read anything) with NiMH. But I thought I read that they didn't want to be trickle charged. They wanted pulses... I guess you can make a trickle pulser.

** Trickle pulsing is probably a very neat idea - funny I have never seen it done other than in the standby-by mode of a pulse type fast Ni-Cd charger.

The OP could set up a 555 timer, in astable mode, to produce a 1 second pulse each 30 seconds and have a CCS deliver say 500 mA to the battery during that pulse. Averages out as 16mA or C/50.

OTOH - emergency systems nearly always use Gell Cells and with good results. Constant voltage ( current limited) charging is the go with them and is simple to implement

.... Phil

One possibility is to set your charger to charge so many hours per day with one of the cheapy wall timers. The C/10 or 10 hour rate is often used as an indefinite charge rate with Nicads, you can leave the charge on all the time and still have pretty good battery life. You could try a C/10 charger with a wall timer and set the timer for perhaps 1.5X the usage. For example if you use 50mA for 10 hours per day you could charge at 50mA for 15 hours per day. A little error would have minimal effect on battery life.

RogerN

I understand that NiMH cells have fewer issues than NiCds. The main reason for using NiCds is that they're much cheaper and much more easily available than NiMHs where I live.

Thanks for your inpuits, everyone. NiMH, lead-acid gel, pulsed and timed charging are all technically sound ideas. But ATM, I'm committed to the idea of using one of those compact 3.6V NiCd packs that come with cheap Chinese import gadgets such as a US$2 rechargeable flashlight.

My circuit uses 1 low-power transistor, one-half of LM393, 3x1N4007, 1 zener diode, 8 resistors and 2 caps on a 1"x2" pcb, and I want to keep it no more complex than that.

A charging timer would be superfluous here as the mains line to which it will be attached is regularly turned on every night during waking hours and turned off for the rest of the day - a pretty regular cycle. The uncertain factor is the discharge frequency which is neither regular nor predictable. It may go for days without being called upon to deliver power. You cite C/10 and someone else cited C/50. That's quite a big difference.

C/50 is a common trickle charge to keep the batteries topped off after they are peak charged. Back in the day when nicads were the standard rechargeable battery for cordless phones and the like, most came with instructions that said to charge before use for around 12-15 hours. These had c/10 chargers. Nicads can take the overcharging but in my experience I think they last longer if you don't leave it on the chargers all the time.

RogerN

Hmmm. It looks as if there's no definitive answer. Would you say that something like C/20 would be a reasonable compromise?

It would probably work just fine, nicads have pretty good life even when continuously overcharged at c/10 but I do get better life out of them by not overcharging. One charger I have charges at a higher current up to a certain voltage and then uses peak detection to terminate charge and then reduces to trickle c/50. You could probably make a design to charge at C/10 at lower voltage and taper off the current as the battery voltage came up. Perhaps an adjustable voltage regulator and a resistor, that way it would charge faster after being discharged and charge at maybe c/20 at maybe 80% charge. For example if you charged a 4.8V pack with a 5.8V power supply and

10 ohm resistor, it would charge at 0.1A with the battery discharged to 4.8V and the current would approach zero as the pack charge approaches 5.8V. (assuming at 5.8V the pack was near full charge)

RogerN

"RogerN"

** That is bullshit.

** Such a silly scheme will result in very long charging times AND the cells will never reach full charge.

There is also the risk of charged cells heating and hence their voltage dropping and hence the charge rate increasing on and on to destruction.

...... Phil

It's obvious you don't know what you're talking about. Almost every cordless home device (phone, shaver, etc.) uses c/10 charging for nicads. You can leave the phone on the cradle indefinately, battery life is not as good as properly terminated charge but they still last a few years. I know this to be so, I've had those cordless devices and that is the actual results, provide better data if you have any.

The right way is peak detection but that doesn't seem to be an option for this application. If you're going to charge nicads by constant voltage it would work to limit charge to c/10 and have it taper c/50 at or near peak. At C/10 nicads won't heat to destruction. Current limited, voltage limited is what is used on lead acid batteries and they have acceptable charge times.

My experience is with 28 years of flying R/C model planes and helicopters where a battery failure means a crash. I've learned most of what I know about charging from a battery manufacturers former battery expert, manufacturer information, microcontroller battery chargers, battery charger chip data sheets and application notes.

RogerN

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Pimpom, Try googling "battery university". There is a bunch of stuff there, perhaps some answers to your questions.

George Herold

Simple trickle charger with taper:

Use an LM317 circuit to set the current to C/10. Add a TL431 circuit on the output, set to the full charge voltage, and designed to steal current such that the current available to the NiCds drops to ~C/50 or less when the pack reaches full charge voltage.

A partial schematic:

Green + ---[LM317]---[D1]---+---[LED]---[R]---+ | | +-------+------[R]--------+ | | | [R] | | | [R] | Red [LED] | | | | | +-------+ [NiCd] | | [TL431] | | | Gnd ----------+-------------------------+

When green is on and red off, current goes only to battery. When red comes on, current is "stolen" reducing the charge rate to ~ C/50 with properly chosen resistors. You don't need the LEDS, they're just nice indicators. I use 1.43 volts per cell as the full charge voltage. Compute the parallel R's for ~1.8 volt drop when (C/10 - Iled) flows.

Ed

Here's some info:

2.13) So, what's the right charge current? Depends. If using an unregulated charger -- one that doesn't do any detection of full charge, then one must restrict your charge current to the overcharge capacity of your cell. All NiCd cells I have seen can handle C/10 (approx. 50 mA for AA cell) indefinitely without venting. This is not to say that one won't get voltage depression, but rather that one won't destroy the cell(s).

RogerN

Roger, Phil is has mental problems and either can't afford, or won't take his medication most of the time. Just kill file him, or he'll be butting into very thread you post to.

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And more:

What about trickle charging? As mentioned in Part 1, trickling is not really necessary for NiCd's, since the C/10 rate is OK for virtually indefinite charging without fear of overcharging. However, some chargers on the market do offer an automatic switch-over to trickle charging after completing the fast or overnight charge. A trickle charge is technically the C/20 rate or lower, i.e., the current number obtained by halving the C/10 overnight rate.

NiCd and NiMH cells have the advantage that they can be charged and discharged rapidly. They are poor at slow rate discharges. NiCd cells became popular because they can be charged gently indefinitely without gassing. A typical charging arrangement is to charge the cells for 1.4 times their capacity. A 1 Ah cell would normally be charged at 100mA for 14 hours but could be charged for 20 hours or more at 100mA without damaging the cell.

Slow charge rates (between 0.05 C and 0.1 C) are the most-often recommended charge rate, since a battery can be recharged in less than a day, without significant probability of damaging or degrading the battery. Slow charge rates can be applied to a battery for an indefinite period of time, meaning that the battery can be connected to the charger for days or weeks with no need for special shut-off or current-limiting equipment on the charger.

constant current charging acceptable at 10% capacity NiCd batteries are charged with aconstant-current profile. NiCd batteries can be continu-ously charged at the standard C/10 trickle rate indefi-nitely without excessive temperature rise or damageNickel-Cadmium: NiCd batteries are charged with aconstant-current profile. NiCd batteries can be continu-ously charged at the standard C/10 trickle rate indefi-nitely without excessive temperature rise or damageconstant-current profile. NiCd batteries can be continu-ously charged at the standard C/10 trickle rate indefi-nitely without excessive temperature rise or damagebatteries can be continu-ously charged at the standard C/10 trickle rate indefi-nitely without excessive temperatureRogerN