How to charge NiCd at 28.8V - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st July 2005, 12:49 AM   #1
diyAudio Member
 
Sunsun22's Avatar
 
Join Date: Jul 2004
Location: Hong Kong
Default 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?

Thanks
  Reply With Quote
Old 1st July 2005, 06:44 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
charge each pack alone.
Use relays to connect them to their own charger and then reconnect them in series for pre duty.
Check that each battery in the pack has a diode to bypass when discharged. This is to prevent back charging which kills Nicads. You can open up the packs and do this yourself.
__________________
regards Andrew T.
  Reply With Quote
Old 1st July 2005, 09:03 PM   #3
AuroraB is offline AuroraB  Norway
diyAudio Member
 
AuroraB's Avatar
 
Join Date: May 2001
Location: Norway, -north of the moral circle..
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.
  Reply With Quote
Old 2nd July 2005, 11:39 AM   #4
diyAudio Member
 
Sunsun22's Avatar
 
Join Date: Jul 2004
Location: Hong Kong
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)?

Sunny
  Reply With Quote
Old 2nd July 2005, 01:08 PM   #5
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
MEMORY EFFECT IS AN URBAN LEGEND.

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.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 2nd July 2005, 01:33 PM   #6
AuroraB is offline AuroraB  Norway
diyAudio Member
 
AuroraB's Avatar
 
Join Date: May 2001
Location: Norway, -north of the moral circle..
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!"
  Reply With Quote
Old 3rd July 2005, 01:53 AM   #7
diyAudio Member
 
Sunsun22's Avatar
 
Join Date: Jul 2004
Location: Hong Kong
I will not consider charging while using the pre-amp. Yet, what voltage transofrmer and current should I select to build a simple charger without jepordizing the NiMH?
  Reply With Quote
Old 3rd July 2005, 09:02 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
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.
__________________
regards Andrew T.
  Reply With Quote
Old 3rd July 2005, 11:17 AM   #9
diyAudio Member
 
Join Date: Jun 2005
Location: Pennsylvania
Send a message via ICQ to sndscietist Send a message via AIM to sndscietist Send a message via MSN to sndscietist Send a message via Yahoo to sndscietist
Quote:
Originally posted by AndrewT
Nimh ??
Where are Nimh available for the same price as Nicad?
Here you can get NiMh relativly cheaply http://www.goldmine-elec-products.co....asp?dept=1240 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.
  Reply With Quote
Old 3rd July 2005, 05:08 PM   #10
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
charge pump jamesrnz Solid State 1 13th August 2008 03:35 PM
charge pump jamesrnz Power Supplies 0 8th July 2008 12:23 AM
trickle charge psu? nafunga Tubes / Valves 6 19th December 2005 02:00 AM
Nicd recharger unit NickC Parts 1 4th June 2003 12:37 AM


New To Site? Need Help?

All times are GMT. The time now is 12:44 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2