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May 21, 2012, 10:39 PM
jayman
Discussion

NiCd-NiMH fully discharged?


Somewhere I think I recall hearing that some batteries should be fully discharged before they are recharged. It has something to do with battery memory, whatever that is. I think it has to do with either NiCd or NiMH batteries or, perhaps, both.
Any suggestions?
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May 21, 2012, 11:15 PM
Registered User
Drifter1soc's Avatar
If I remember correctly Nicd batteries will get what some call memory if they are not fully discharged. I could be wrong, it has been a long time since I heard that.I do remember they eventually would have a shorter lifespan when not fully discharged enough times in a row. I use pretty much nothing but lipos anymore, totally new set of rules with them.
May 21, 2012, 11:15 PM
Grumpa Tom
Kmot's Avatar
Memory only applies to NiCd's. They need to be fully cycled to prevent it.
May 21, 2012, 11:27 PM
Big Boats Rule!
boater_dave's Avatar
Nicads have a memory effect and like it better when they are fully discharged and fully recharged. The process is known as cycling and must be done once in a while to get the full capacity out of a pack. Especially after long storage.
Nimh's aren't supposed to have a memory, but I still cycle my nimh packs. Nimhs have a bit more self discharge, and may loose a significant ammount of their charge every day.

Dave
May 22, 2012, 02:56 AM
Registered User
yeah having to almost always cycle is a Nicad thing... as said above Nimh aren't supposed to develope a memory .. but putting them through a cycle after a few uses (or extended periods of no use) will keep them up to optimal performance

ofcourse the better your battery (generally the more $$ ) .. the less issues you'll run into
May 22, 2012, 04:52 AM
Registered User
Don't forget, a fully discharged NiCad cell still has a reading of 1.1volts per cell.

Regards Ian.
May 22, 2012, 05:36 AM
Registered User
An unattended NiCad will self discharge over a period of time, and during self discharge will alter its chemistry a bit, needing to be cycled to re-form itself.
In any pack, one cell will be less good than the others, and will wind up with a little less charge than its companions. This makes it discharge earlier, and less able to take a full charge.
These are the causes of the memory myth.
May 23, 2012, 11:14 PM
1/2 a bubble off
Apismelifera's Avatar
When I ran my RC10 car a lot, I used exclusively NiCd batteries. Out of the first six I bought all lasted over five years. Still having about a seven minute run time. Half of them lasted a year more. Ran them every weekend four or more times each.

I used an el-cheapo timer type charger when I was out at the shopping center to recharge from the car battery, one at a time. And a home made constant voltage charger with a built in discharge at home. Plug in all six batteries at the same time, press the discharge button and down they went, till the discharge cycle was completed. Then the charger would bring them back up to full charge.

They don't need to be fully cycled every time. I usually fully cycled mine every fourth time I used them.

During the same period my son replaced his every other year. He used a charger that only charged and did not have a discharge function.

Now our batteries did not fail outright, as in they all of a sudden stopped working. They just gave progressively less and less run time. Mine only lost about 30% capacity over a five year period. My sons batteries lost about 50% capacity over a two year period. I credit the difference in lifetime to me fully cycling mine once in a while, as opposed to just charging them when the car started to slow down.
May 26, 2012, 05:41 PM
jayman
I have a Venom 1800 7.2 NiCd pack.
What voltage should I have when it is fully discharged?
What voltage shouls I have when it is fully charged?
Thanks, Jay
May 26, 2012, 07:29 PM
Registered User
When nicads are fully charged any additional power fed into them by the charger can only be turned into heat. It is this overcharging that does most of the damage to a nicad as it eventually dries out the cell and causes a drop in capacity. The drop is not sudden. It is gradual over time and sort of sneaks up on you. Nicads have a small vent in the casing to prevent pressure build up and a potential explosion.

Due to the voltage curve relative to charge state it is virtually impossible to figure out how much charge is remaining in a cell. The best way is to discharge the pack to the equivilant of 1.1V/cell. A 4 cell RX pack would then be 4.4V. 1.1V is starting down the steep part of the voltage curve and the pack can be considered flat. Discharging to less than 1.1V/cell can result in the failure of a cell in a milti-cell pack. That's another story.

Recharging at the 10 hour rate for 16 hours should see the pack fully charged with the minimum amount of overheating and the minimum amount of cell damage as well. The extra 6 hours is due to inefficiencies in the conversion process within the battery which produces some heat. That is why batteries are warm while being charged. If the charging is kept at the 10 hour rate the heat generated will be able to be lost through the casing of the battery and the temperature will not get high enough to cause venting to occur.

Charging packs with an unknown starting level of charge with fast chargers and relying only on a timer will result in a shortened life as heating will be very high after the cells reach full charge.
May 27, 2012, 06:11 PM
jayman
Not long ago I was my LHS and saw a battery display that reminded me I needed 8 AA batteries because I forgot to turn off my transmitter. I bought the top of the line Energizers and paid through the nose. I put them in the transmitter and used it for about half an hour. Two days later I looked at the transmitter and noticed it was still on and the brand new batteries were fried. All that money down the drain. Never again.

I went to Harbor Freight and bought a charger and 8 NiMH AA batteries for less than I paid for the disposables. They work just fine. From now on, I plan to buy only rechargable batteries.

Now, this started me thinking. Why not use NiMH batteries in the model in place of the LiPO? That would solve problems with LiPOs like charging, storing and using. I think the last straw for me was when learned that you also had to be concerned with the rate of discharge. Too much for me.

I looked for a NiMH battery pack that would provide about 12 volts but could not find one. If I can't find a 9 to 10 AA battery holder, I will make one out of styrene sheets. A 9 battery configuration will yield about 12 volts. I will then go to Harbor Freight and buy a dozen NiHM batteries for $18 to $24.

If a battery won't take a charge, just get rid of it. This is better than getting rid of a whole pack because of bad cell. This is a low cost versatile system and the weight is about the same as a NiCd 7.2 pack (9 oz) and only 3 oz more than my Lipo pack.

But if this idea was so good, everyone would be using it. So, what is wrong with this? What are the drawbacks?
May 28, 2012, 05:18 AM
Registered User
Before LiPos, NiCad and NiMH were the norm. Building your own pack or going to a specialist battery supplier to have a custom pack made to get the voltage / capacity / size / shape / connector wanted was and still is almost as simple as buying separate bits over the counter.
May 28, 2012, 07:41 AM
Registered User
Jay, you don't say what you are going to use the batteries for.

It depends on what discharge rate you are looking at. Ni-MH batteries are I believe, limited to about 1.5 to 2 amps and will be damaged beyond that. Ni–Cd batteries are good for about 15 to 20 amps and Li-PO batteries can be much higher than that. Some are rated at 45C now so a 2200 mA/H 45C Li-PO can supply about 100A.

I use Ni-MH batteries in my transmitters. They are great things for that. But I use a Ni–Cd pack in my yacht because the current required by the sail winch and rudder servo together would be pushing Ni-MH cells towards their limit.

I prefer Li-POs in a boat with a motor but that is because I have got used to them flying planes but in a boat I use a on board battery alarm to avoid doing them any damage.

Ken
May 28, 2012, 12:47 PM
jayman
Thanks guys for your comments.

I am using this in the Lindberg 54' Chris Craft Constellation. It is 30 inches long and weighs about 3 pounds.

It is powered by 2 M.A.C.K. 1885 motors geared to 2.5:1. The data plate states 6 to 12 volts and 0.8 Amps. It has Graupner nylon props, 35mm in diameter. Pitch is unknown. At 2400 shaft RPM there is a 25% reduction in RPM when placed under load. The servo is a small Futaba and the ESCs are Mtroniks tio 15 Marine units.

Somehow, I just knew it was not really that simple. So, would you recommend NiCd instead of NiMH? The NiCds are even cheaper: $3.98 for a 4 pack. BTW, would anyone care to guess at the current draw for this motor set-up?

Thanks, Jay


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