How to charge NiCd at 28.8V

I wish to build a battery supply for my pre-amp which has + - 15V rails after regulation. I will be using NiCd battery packs at 7.2V for remote toy cars. Four of them in serial will provide 28.8V that is around + - 15V for my preamp. How can I charge them?

1. Charge the battery pack by my toy's charger at 7.2V separately, then connect them in serial when to be used in the preamp.
2. Connect them together and build a charger that can charge 28.8V.
3. Any other recommendations?

You can of course charge each pack separately, - but this seems rather inconvenient, though...

NiCd's normally requires constant current charging, - this means that you need a voltage supply of appx 45-50 volts and a current regulator. Charging current is normally 10% of the battery capacity, although many modern NiCd's can be fast charged at up to 150% of rated capacity, but this will reduce lifetime somewhat, and is not recommended as normal practice.
This also means you cannot charge while the battery while in use.
NiCd's also have a bad habit of "voltage memory", if partly used and recharged.
NiMH batteries don't suffer from this habit....and is otherwise used and treated as NiCd's.
Thank you for the information. I'll try to look for NiMH batteries and see if there is any available here.

Meantime, I saw some circuits that use IC to limit the current and sense on voltage of batteries. If the battery has low voltage, it will charge at higher current while up to certain voltage, the circuit will change to float charge. My question is: -

I can only find IC to perform the above operations for 6~12V. Is there any IC that can control 28.8V (or better still 31.2V)?


Mmmm'k. Just wanted to get that out. You're welcome to search on why.

You can of course charge while in use, hmm let's say your project draws 20 to 100mA from lowest to highest, while on, and batteries are rated at say 1Ah. Running a CCS set at 500mA will fully charge in 2 to 2 1/2 hours, whether on or off.

If your circuit draws closer to rated current (with likewise much shorter run time), you'll get a wider difference in charge time (for a given charge current), but I don't really see how you can build something which drains batteries in 10 hours, charges them in 2 and not expect the charger to power the device simultaneously with an only slight increase in charge time. Matter of fact, once the device starts draining at a rate equal to charging, they nullify and no juice goes to the battery. If it drains in 2 hours and trickle-charges in 10, then yes you'll have to turn it off first.

The NiCd memory effect is indeed no joke,- provided that a continous process of partial discharge and recharge is used. The remedy for this disease is a repeated full decharge and recharge, which *can* restore the cell. All users of rechargeable instruments know this effect.
In later times, the use of dedicated, "intelligent" chargers have somewhat remedied the problem.
OTOH- as cadmium is a major environmental problem, - why use NiCd's at all??? NiMH is appx. the same price, at least in my corner of the world....

As for the "charge while in use" question,- if your battery pack is followed by regulators, - no problem,- except for the noise introduced by the charger, - which I guess was the noise you wanted to avoid in the first place....??
Without regulators , your circuit must sustain the potential overvoltager introduced by the charger, if you want an efficient charging process.

I regularly make 28V battery packs for aerospace purposes, but we don't use intelligent chargers, - it's " charge, test,recharge, then fly - and forget!"
charge current C/10.
max temp 40degC
max volt for Nicad about 1.4v per cell. Nimh ??
Use a combination of temp and voltage to turn off the charge rate or you may reduce to about C/100, but test this value first to ensure the cell temperature drops. If you can push more charge in at a lower temp then you will get more back.
No response on the diodes? Did you understand?

Where are Nimh available for the same price as Nicad? In UK Nimh are about 2 to 3 times more expensive.
AndrewT said:
Nimh ??
Where are Nimh available for the same price as Nicad?

Here you can get NiMh relativly cheaply in fact Li-ion aren't that expensive anymore provided you know where to look. as for charging NIcad and Nimh they are extremly simple provided that you have a good filtered power supply to do so.

for my single/double cell chargers i typically use series resistors and LED's
i made this nice double cell charger with a 15V wall-wart a 1/2 watt 1K and a high brightness red LED. it charged at 12 Ma which means it took almost 3 days from dead to full.

i'll use this as an example you have a 700 Mah ni-cad AA on your hands and you want to trickle charge it at 10 ma and lets say you have a 35 volt supply you should put a 3.5K 1 watt resistor in series with the battery. that should charge it in roughly 12 hours.
keep in mind that it doesn't stop when the batteries are charged.
NiMH and Li-Ion batteries do not contain cadmium, but their electrodes (by design) are irreversibly corroded by the electrolite, dramatically increasing the output resistance over time.

This means that these batteries have a known limited lifespan and a constant performance degradation with time that starts just after manufacturing, no matter how they are used. After a certain period of time they become useless, even if they were not used at all (between 1 and 2 years). Most people throw these batteries to the ordinary thrash can, so they represent no ecologic improvement at all over NiCd.

They only have a single advantage over NiCd: the absence of memmory effect. But they have lots of disadvantages: Higher prices, very limited life, charge current limited to 1C (6C for NiCd), much higher voltage drop for a given current as they discharge, etc...

On the other hand, NiCd batteries are free of such internal degradation phenomena. Theoretically, with correct use they show almost unlimited lifespan. Furthermore, the capacicy loss due to the so called 'memmory effect' is fully recoverable by repeated full charge-discharge cycles.

I would never use NiMh or Li-Ion batteries. They were introduced in the market as an exotic thing just because manufacturers wanted to sell more and more batteries but NiCd units lasted too long and it was posible to DIY recycle them.

As an alternative I recommend lead acid batteries, these also show almost unlimited lifespan when used correctly. Furthermore, their sulfation process is reversible by electronic means, so they may be recycled again and again. You may even get some old and sulfated SLA batteries for free, recycle them and use them.

I'm just recycling two 6V 10Ah SLAs now. They were very sulfated and discarded from a boat. Despite they are 8 years old, they are progressively recovering their capacity.

Remember that no recycling process exists for NiMh nor for Li-Ion batteries,. After one or two years of use or storage these batteries become just unusable toxic chemical waste. Allways ask for manufacturing date before buying them.


2005-01-09 9:51 am
Really? Is the lifespan of NiMH so short? I have some ones kept & used for several years and they are still useable.

How to DIY recycle NiCd ones? I once collected many unuseable ones, try to recycle them electrically, but failed. Some article suggest drill them and inject water into them, or steam them in high pressure pot ... too radical.

About lead-acid battery: They Don't have almost unlimited lifespan. Frequently & heavily loaded ones used in electric bikes just die in a year. Their positive panel is corrupted and can't be recycled anymore.
thanks a lot for the very interesting information here and in the other theread about SLA. May I join Kenshin's request for further information about NiCd recycling? I guess it's a matter of using the appropriate resistor and let the current bleed until the voltaage reaches some value, but what resistor and voltage?

I would make my wife happy if I could be able to revive her portable Electrolux vacuum cleaner without throwing three NiCd batteries in the (appropriate) wastebasket... :hug:

Muchas gracias!
I have not experimented yet with apparently dead NiCd cells, altough I have a bunch of them. I know just what I have read from several sources, for example :

I also recommend to search for info on Yury Podrazhansky and his charging algorithms patented by Advanced Charger Technology (his company).

The recovering process is based on charging and discharging pulses with dead times inbetween. Timing appears to be critical.

About lead-acid battery: They Don't have almost unlimited lifespan. Frequently & heavily loaded ones used in electric bikes just die in a year. Their positive panel is corrupted and can't be recycled anymore.

This happens because standard non-deep-cycle lead-acid bateries are not intended to be discharged by more than 30% and must be recharged as soon as possible. They may last for dozens of years when used properly. Also, this positive plate 'corruption' is mostly due to sulfation, so its recoverable.

There is a lot of lead-acid battery premature failure and wasting due to improper usage and improper charging.

Same or worse happens with NiCd.


2005-06-05 12:52 am
The amount of misinformation is this thread is simply incredible. I don't even know where to start but I'll try;

- Memory effect is typically not a problem with NiCd. It's true that it "can" happen, but for it to happen the cells must be discharged to VERY near the same voltage every time. This is quite uncommon for DIY projects, the typical scenario where it occurs is when a device has a low-battery shutoff detection circuit, thus battery drain is always stopped at same point. Mainly the choice between NiCd and NiMH is shelf charge life (longer for NiCd) or power density (higher for NiMH).

- NiCds do NOT require constant current. That is ridiculous and there are very few NiCd chargers that use a constant current charge. It's certainly possible to use CC but not at all necessary. The primary issue no matter the charge profile is that the pack does not continue to receive higher current (perhaps higher than C/8) after fully charged as too great a % is turned to heat. While it is a great idea to have a thermal cutoff on a battery pack or charger, the typical implementation is not a routine way of shutting off charging, it is an emergency cutoff. Thermal switches in these uses are cheap and failure-prone in themselves.

- Neither NiCd or NiMH are worn out in 1-2 years whether used during that period or not. Of course if someone constantly drained them and immediately recharged, they could end up cycling them so many times that it did wear them out, but the shelf life on either type is at least 3 years, and even then it is only a "life" meaning the battery still meets certain capacity criteria- it will still work at a lower capacity fine for a few more years unless subject to extreme treatment like lots of high current charger & discharge cycles which keep getting it too hot.

- Trying to trickle at 10mA is pretty much pointless. Even a constant 24/7 trickle should be higher, around C/30 - C/50. The more ideal "trickle" scheme is not to constantly leave them charging but charge at a more moderate C/5-C/15 rate with off/on cycling to periodically refresh the charge. A common scheme based randomly on human habits is to suggest an on cycle or two every one day, that current being calculated as discharge and recovery time necessitates.

- Charging a battery while in use is no problem, but on some devices like a preamp you'd probably want a better-than-otherwise-necessary charging circuit to keep noise low. It doesn't necessarily need to be post-regulated either, as the current drawn by the battery pack makes the voltage collapse and you can indeed measure the resultant voltage before even attaching anything else if you suspected any risk. Further, the typical regulated charger isn't going to have a significantly higher voltage than battery pack peak. There's margin built into just about any device, if your DIY gear is so close to it's upper limit that a couple dozen percent voltage spike is going to kill something, you're designing a piece of gear with too little margin.

The original design targets don't make sense. A +-15V supply off of 28.8V? Why? The voltage is at least 10% too low to split then well regulate, but higher than necessary if you're going to step it up with a switcher. Before we speculate much more on the battery and charger it would be good to better understand EXACTLY what you're doing here and why.
An excellent example of the inherently short lifespan of NiMH and Li-Ion are mobile phone batteries. When you buy a brand new handset of the latest model, the battery may even last for a week or more at idle. However, if you get a phone that has been stored or used for several months you may surprisingly find out that the battery only lasts for three or four days.

After one or two years you end up charging the battery daily because it isn't even capable of powering the pohone for two days, output impedance has become too high. Most people will have thrown their old handset and got a new one by that time since a spare battery costs far more than a new handset. That reason together with small size is why NiMH and Li-Ion have so quickly become the standard for mobile phones. These batteries feature small size and a conveniently short lifespan, and manufacturers just love this fact (altough some young people are so wasteful that they throw away their fully working handsets every three months or so).


2005-06-05 12:52 am
Mobile phone batteries are a horrible example.

1) Higher current charge and discharge.
2) Constant drain unless phone completely OFF, very high # of cycles.
3) Optimized for smallest size reasonable/affordable.

It's not the amount of time that wears out mobile phone batteries, it's the specifc, atypical use.


2005-03-26 2:31 pm
Kenshin said:
Really? Is the lifespan of NiMH so short? I have some ones kept & used for several years and they are still useable.

How to DIY recycle NiCd ones? I once collected many unuseable ones, try to recycle them electrically, but failed. Some article suggest drill them and inject water into them, or steam them in high pressure pot ... too radical.

About lead-acid battery: They Don't have almost unlimited lifespan. Frequently & heavily loaded ones used in electric bikes just die in a year. Their positive panel is corrupted and can't be recycled anymore.

I have revived NiCd batteries before by charging a capacitor to a voltage about 70V (using my power amp PS) and discharging it into the battery. After doing this a few times, if the battery voltage comes up, then it's good again. I got the idea from a magazine and tried to built a circuit to do the job, but for some reason the momentary swtich kept fusing together due to the arc.