NiCd and NiMh batteries

Roberto

Roberto

Well-Known Member
Joined
20 Jul 2001
Messages
5,849
Location
Lorient/Paris
sybrancaleone.blogspot.com
I would like to use these rechargeable batteries instead of the normal alcaline ones in small equipment like portable gps and vhf, at 4/6 elements at a time the cost would be easily amortised and esp there would not be the need of looking for batteries disposal places which hide away every time they see me with a Duracell in my hand -.<)

But, my portable vhf and gps instructions specifically state "do not use nicd or nimh rechargeable batteries"

What is the reason ? is it only a disclaimer for some remote danger, a bit like "do not use for navigation" for gps, or is there some valid technical reason ?

thanks
 
There are several main differences between the types of cell.
NiCd/NiMH provide a slightly lower voltage, 1.2V, which shouldn't matter as alkalines have an end point of 0.9V anyway.
They have a smaller capacity, a bit more than half an alkaline but again, that's your problem and the machine will be ok.
They also have a lower internal resistance, which enables them to deliver more current into the same resistance. This shouldn't be a problem for a properly designed power supply, but they have been known to cause problems e.g. with flash guns.

IMO the danger is small but real. They are probably just covering their backsides, but then again its not my gps. I use alkalines 'cos they last longer.
 
rechargeable batteries generally produce a slightly lower voltage even when fully charged ie about 1.2 - 1.3 volts instaed of 1.5 - 1.6 in the case of AA or AAA size. Therefore 4.8 - 5.2 as opposed to 6.0 - 6.4 for duracell type. Some equipment will see a bank of rechargeables as partially discharged because of the voltage and shutdown or fail at a certain voltage (about 5 volts for a Garmin GPS for example) so you therefore get much less time between recharges!
 
I usually ignore the statement and use NiMH in ALL my portable stuff - never had problem.

Biggest 2 differences between rechargeables and Dry-cells ....

a) They actually are lower voltage than Dry's .... 1.2 instead of 1.5V. BUT they are more stable on voltage under load ... where a dry-cell will go lower ...

b) They can discharge at fantastic high rates causing fire if shorted badly ... but Dry's just die !! Ive burnt my leg carrying a couple of recharge AA's in my pocket !!
 
Are you all sure that the time time between charges being less than normal dry cells. I have 2000mh nimh AA batterys in my camera and they last about 5 times as long as normal dry cells.???
 
Doing a quick check-up on the Duracell website and the RS catalogue, the typical alkaline manganese AA cell has a nominal capacity of 1.1Ah (1100mAh) and the typical NiCd has about 600mAh, or about half. NiMH is actually listed at 1.2Ah, so I stand corrected: they are the same capacity as an alkaline cell.
Not sure where you get your 2Ah AA cells from, but it looks like you are comparing them with zinc carbon cells, which are only any good for powering cymbal-playing bunnies.
 
If you do decide to ignore the manufacturer's concerns and change, go for NiMh rather than NiCd cells. The reason is that NiCd cells have a "memory" of the point where they were recharged. If your GPS or whatever switches off when they are half discharged, and you then recharge them, their charge capacity is reduced slightly each time. If you do use them you need a good quality recharger which inludes a discharger (resistance) to fully discharge them before recharge.
 
[ QUOTE ]
Not sure where you get your 2Ah AA cells from.....

[/ QUOTE ]

There have been huge advances in rechargable NiMH batteries in the last couple of years. Have a look here for 2.6Ah AAs.
 
Apart from the voltage difference which has been well aired on the forums there is one major disadvantage of NiCd and NiMh rechargable batteries over 'normal (non rechargable) batteries' such as Duracell.

This is that the rechargables, when fully charged, do not hold their charge 'on the shelf', for anything like as long as Duracell's.

I would never for example put them in a grab bag to drive any equipment in an emergency.

I am very much in favour of using rechargable batteries and use them on many items around the house.

Iain
 
[ QUOTE ]
Not sure where you get your 2Ah AA cells from, but it looks like you are comparing them with zinc carbon cells, which are only any good for powering cymbal-playing bunnies.

[/ QUOTE ]

Yesterday I bought 12 1.8 Ah NiMH at Mapling for 9.99, cheaper then some Alkalines.

Like others, I have used NiMH in all sorts of equipment at not had a problem.

I am about to try rechargable alkalines in gps. They have 1.5 volts and do not suffer self discharge (the handheld is only a back-up). However, they do need correct charger.

Ian
 
I use re chargeable NiMh all the time in all my portable gear: GPS, cameras, VHF, PDA etc etc. They are cheap to buy, quick to recharge (a bit over an hour), last many times longer than the average Alkaline AA, and although initial voltage is slightly lower, they maintain it until they are nearly flat.

That is the main problem: internal battery checkers do not register them becoming exhausted until - in the case of my camera - they are literally about to give up. I have around 30 seconds once the 'battery low' indicator shows up, before the whole thing goes dead. Some gear - particualrly the PDA - does not cope well with power outage caused by the batteries suddenly failing like this, and both data loss, and software probs can arise if I allow it to happen. I guess this is why manufacturers dislike rechargeables.

Also in the event of an internal short, rechargeables can pack a huge discharge rate - enough to totally fry the PCBs.

But my camera lasts around 70 frames using alkalines, and will give around 800 - 1000 frames per charge using 2ah rechargeables. No choice!
 
Rechargeable batteries operate at 1.2 volts instead of the 1.5 volts of conventional batteries - in most instances no problem but lower voltage means higher amps which could result in a fried resistor or two.

I use rechargeable (NiMH) in LEDs, torches, motorised gadgets but never in wirelesses.
 
Since I never read / saw any such warning, I have used nothing but rechargeable NiMnH batteries in my Garmin 76 without problem. Battery life seems excellent. They're in my camera, in my keyboard and mouse, various torches etc. No probs so far.

I am told that the internal resistance of MiMnH batteries is so low that a dead short can give rise to a short burst of quite high current. Maybe thats what they are getting at. But I take such warnings as arse covering these days - after all, Garmin say you musnt use your GPS for navigation. Presumably they think we buy them to hang round our necks as a sort of nautical medallion. Pillocks.

Made the mistake of buying some Hitachi NiCad AA batteries recently and discovered that the capacity was about a third of the NiMnH cells.
 
I've tried rechargeable alkalines - beware these are the only batteries that I've used in the last 20 years that leak!

I do use the batteries as recommended by the manufacturer and I also use their own charger.

The shelf life is longer than Nicad and NiMH but you need to keep checking them for leakage. I wouldn't use them again in expensive equipment, they're false economy.
 
Ni-MH batteries have a much larger capacity, but they self discharge faster. Ni-cad batteries are much safer for memory back up purposes.

Philip
 
[ QUOTE ]


Garmin say you musnt use your GPS for navigation. Presumably they think we buy them to hang round our necks as a sort of nautical medallion. Pillocks.
.

[/ QUOTE ]

Could not agree more! I recently bought a marine hand bearing compass. The instructions warned it was not to be used for navigation.... what the !!!! IS it for then?
 
Made the mistake of buying some Hitachi NiCad AA batteries recently and discovered that the capacity was about a third of the NiMnH cells

Total life capacity of NiCds is generally better than NiMH though as you should get many more charge/discharge cycles out of them.

As many are saying, I also use NiCd's and NiMH's in everything, regardless of "warnings" - the lower voltage is the main matter to be considered, but they hold that voltage right throughout their discharge cycle which is an advantage (especially in handheld radios cos the transmitted power is normally directly related to the battery voltage).

An advantage of NiCds is their low internal resistance compared to even NiMh's so can deliver higher currents without voltage drop (voltage drop lowers tx power). This is of advantage for VHF handhelds during transmit, so I (for my own use) buy handhelds with battery pack holders which I can load with NiCds and not often use the NiMH pack that comes with the radio. For the same mass the NiCds have lower capacity than the NiMH's though so have to be charged more frequently (but as said, their total life capacity should be greater).

To the comment one has made about memory effect, is an old wives tale. The professionals working in critical power supply situations agree, and I have never experienced it despite the fact I just charge batteries whenever I think about topping them up. Like any battery you can lower their capacity by abusing them (so can be confused as a "memory effect") but you are much more likely to do that by the practice of discharging them completely than by just charging them when convenient, regardless of state of charge, but always before fully discharged.

John
 
I also use NiMH batteries but instead of having a memory effect like NiCd they seem to fail by increasing their resistance while keeping their capacity. In other words the voltage dips under load more and more until the equipment shuts down immediately you turn it on once the batteries are slightly discharged. Then you check them and they still seem charged. ... I havent had many fail yet compared with NiCd.
 
You must log in or register to reply here.

Share this page

Top