What is good-NiCd or NiMH

[Ravi]

Friends,

I have a VHF handheld Uniden receiver which is designed to operate from 04 AA size alkaline or NiCd batteries.

Due to difficulites in getting NiCd's in my area,I thought of switching to NiMH types.Now I am wondering whether:-

1.Is its OK to use NiMH on the radio since it is designed for NICd?
2.Is the EMF of a NiMH is as same as NiCd?If not will there be any problems in the receiver?
3.Can I use my NiCd charger to charge NiMH's the charging rate of which is 160mA for 14 hours?

Appreciate your expert views

Thanks
Ravi
Colombo
Sri Lanka

 

[williB]

Friends,

I have a VHF handheld Uniden receiver which is designed to operate from 04 AA size alkaline or NiCd batteries.

Due to difficulites in getting NiCd's in my area,I thought of switching to NiMH types.Now I am wondering whether:-

1.Is its OK to use NiMH on the radio since it is designed for NICd?
2.Is the EMF of a NiMH is as same as NiCd?If not will there be any problems in the receiver?
3.Can I use my NiCd charger to charge NiMH's the charging rate of which is 160mA for 14 hours?

Appreciate your expert views

Thanks
Ravi
Colombo
Sri Lanka

NiMH are better than NiCds ..
as far as i know they can directly replace NiCd cells
how many mA hours are your NiMH cells ..?

 

[Oznog]

NiCd/NiMH work the same on a trickle charger, but fast chargers- not the 10+ hr ones like the one you mention- may or may not turn off at the appropriate time since the charge properties are slightly different.

Otherwise, the cells are basically the same. They usually have greater capacity than NiCd. Voltage is the same, the ability to deliver extremely high surge current is typically less than NiCd but this only comes in things like race cars which drain the battery in a matter of minutes. That's changing too, NiMH keep getting stronger and can now compete even in that area.

 

[Ravi]

Many thanks for the information. The capacity of my NiCd is 1600 mAH and it is being charged @ the rate of 10% of it's capacity for 14 hours.I also charge them @ high rate of 20% for 5 hours. My charger (designed by me) is capable of delivering currents up to about 400mA.Charger use the principle of monitoring barrery's EMF and when it reaches to say 85%, the comparator output goes high and switches off the transistor.It also has a timer capable of giving timing ranges of 5,14 & 20 hours.

My NiMH's capacity is 2300mAH so that I think I will have to alter the charging current to say at least about to 230mA.Is that right? I was made to understand that fast charging of a NiMH is not a problem and it does not suffer from "memory effect".But it discharge more quickly than nicad and has a shorter recharging-cycle lifespan, while nicads consistently offer some times more than 1,000 cycles in the real world.

I hope my understanding of the position is correct

Thanks again
Ravi

 

[audioguru]

Ravi,
Go to www.energizer.com and click on Technical Info. For battery type, select Ni-MH Rechargable. The datasheets for all their cells are listed and there is a big tutorial about charging them with cautions and a comparison to Ni-Cads. They do charge differently. They don't make Ni-Cads anymore but the datasheets and tutorial about them are still on their website.

About the only things the same between a Ni-Cad and a Ni-MH are voltage, weight, flat voltage discharge curve and reduced life if over-charged. My Ni-MH AA cells reach 1.45V when fully charged at a 10% rate.

Look at their non-rechargable AA lithium cells that have a very flat voltage discharge curve and high capacity. :lol:

 

[Ravi]

Thanks audioguru.The site is very informative.
Have a nice day!

Ravi

 

[Oznog]

Problem is knowing when to terminate the charge. The current and voltage don't really change much when they reach and exceed a full charge.

NiCd have this dv/dt drop the charger needs to detect. With a constant current, once the cells near full charge, the voltage actually drops. It's small and temporary, eventually it will rise again. So it's hard to catch. The same thing happens with NiMH but it's substantially smaller, and a charger designed soley for NiCd will probably not catch it. Then only a timer or a temp sensor will stop the charge.

 

[bryan1]

On my smart battery charger I made it has a provision for using a thermistor to measure the battery temperature this feature is used for nicad's, nimh and lithim ion batteries. Once the battery hit a certain temperature the charger cuts of the supply current and just trickle charges to keep the battery topped up. If you want I can look thru the specs and find the temperatures for you if you like. I must say the battery charger project is the most complicated kit I've built to date but it's sure worth all the time I spent.

Hope this helps

Cheers Bryan

 

[samcheetah]

if you want to design a charger then a really easy to use NiMH/NiCd charger IC is the MAX713.

 

[Jay.slovak]

It's not a good Idea to replace NiCd's with NiMH's in devices that have high current consumption! NiMH can't supply such high current and they will most likely get damaged or the device won't work correctly.

 

[audioguru]

Hi Jay,
How much current do you call "high current consumption" from a tiny AA battery cell? :?:

Energizer's datasheet for their AA Ni-MH cell shows its 30 milliohms internal resistance when fully charged and 40 milliohms when half-discharged. It also shows how well it supplies 5A. :lol:

The datasheet for their obsolete AA Ni-Cad cell shows that it was worse! 8)

 

[Jay.slovak]

Hi Jay,
How much current do you call "high current consumption" from a tiny AA battery cell? :?:

Energizer's datasheet for their AA Ni-MH cell shows its 30 milliohms internal resistance when fully charged and 40 milliohms when half-discharged. It also shows how well it supplies 5A. :lol:

The datasheet for their obsolete AA Ni-Cad cell shows that it was worse! 8)

Well you can take battery of 10 AA NiCd's, and you can start your car with it, and the NiCd's will "survive" this without problems... You can't do this with NiMHs.

 

[Jay.slovak]

And also some External PhotoFlashes that run on NiCds, you can't use NiMHs there!!!

 

[audioguru]

The spec's for Energizer's obsolete AA Ni-Cad shows it performing poorly with only a 3A load. Start a car? I don't think so! :roll:

I wonder if Sanyo still make their excellent Ni-Cads? :?: :?:

 

[Nigel Goodwin]

The spec's for Energizer's obsolete AA Ni-Cad shows it performing poorly with only a 3A load. Start a car? I don't think so! :roll:

Energizer NiCd's must have been pretty crap then! - I can't see 10 NiCd's starting a car either - BUT they should provide GREATLY more than 3A.

You have to be extremely careful with NiCd's, and with NiMh as well, as both have very low internal resistances, and both will provide enough power to destroy themselves.