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NiCad vs NiMH batteries

I wanna buy a charger and some batteries so I've been doing a bit of research on the different types. However the information I found is somewhat confusing and outright contradictory. And in any case I'd like to hear about the experiences you guys have had using NiHM and/or NiCad batteries...

Which would be the better choice? Pros and cons?

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I really needed some practical advice on this one. I have decided to go for NiMH for sure. And I'll presume that 4 AA NiMH batteries can supply enough current for 12 microservos.

So now I have to make sure that the ones I get are capable of supplying relatively high currents. I understand from articles like this that I need to look out for the CAPABILITY and NOT the CAPACITY alone. Here is a quote:

"The power capability of a battery is measured in coulombs and denoted by a C. You will NOT find this measurement advertised on the battery or its packaging but you can often find a rough estimate online and a more accurate measurements through the battery manufacturer's datasheet, if they are so kind.

The power capability speaks to the amount of amperage a battery is comfortable with supplying to a circuit. A coulomb is measured in amps and is proportional to the capacity of the battery. For instance, if a battery has a capacity of 1Ah and its power capability is 0.1C then this battery is comfortable supplying 100mA or less steadily."

So I'm still open for advice ;)

EDIT: These certainly seem to do the trick: 1800mAh * 10C => 18A !!! That's what I'm talking about :)

PS: I'm also interested in tips regarding chargers. Obviously I want it to charge as fast as possible, so any particular things I should be looking out for (I know you can't allways trust the specs)?

Considering it came from a university I'm shocked at that article... a 'coulomb' (C) is the unit of electric charge (Q). 1coulomb is equal to 1amp.second, or 1farad.volt; if something is measured amps the only thing that can be is current, not charge.

C is also used to mean the nominal discharge rate, in amps, which is the current you'd expect from a cell that was totally discharged in one hour under constant conditions. This is no doubt what they were referring to, but putting coulombs in there is just wrong =/

Note that at a discharge rate of 10C, the batteries will run dry in 1/10th of an hour, i.e. 60min/10 = 6 minutes!

I also noticed that it 1st says: "The power capability of a battery is measured in coulombs" and then later "A coulomb is measured in amps". That doesnt make ANY sense.

And this article seemed like one of the more serious and thorough I could find?! :/

Anyway when you say: "at a discharge rate of 10C, the batteries will run dry in 1/10th of an hour", do you mean it will ALWAYS discharge that fast even if the current being drawn is litle or nothing? I mean I need the batteries to be CAPABLE of supplying higher amps for peaks, but generally the current draw will be relatively low. And obviously 6 minutes of runtime is completely useless...

It's another one of those confusing naming things - 'C' can be the maximum discharge current that the battery will supply, but it's also used to mean the discharge current that you're drawing from the battery.

For the example of those high-discharge NiMH AA cells, they've got a maximum rated discharge of 10C, so you can't expect to be able to pull more current than that at any one instant.
However the estimated run-time of the battery will be C/(nC) = 1/n in hours, where nC is the average discharging current you're expecting to pull from the battery. If you want to pull nC = 10C = 10*1.8A = 18A average current, you'll get a pitiful 6mins run-time... but if you're only going to pull 1C = 1*1.8A = 1.8A average current, you'll get an estimated 1/1 = 1hour from full charge.


And off course I don't really need 18A. However I should get some capable of providing atleast say 6A at peaks?! I guess I'll just have to look around a bit and see what I can find. But it does seem like most NiMH batteries, having a capability of only 0.1-0.2C, wont suffice?!

Although most NiMH are rated at 0.2C discharge, they don't really suffer any for discharging at 1C, which I believe Wikipedia will back me up on...

 Edit: Yep, Wiki says "Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of around 1×C (full discharge in one hour), it does not differ significantly from the nominal capacity."

NiMH seems to be popular which is the conclusion I came too. I assumed NiCad would be the thing to get but thats because my rebound 4x4 many years ago used a NiCad pack. But NiCad is indeed worse for the environment, heavier, and of course the annoying depression memory charging. So assuming NiMH has been chosen as the battery of choice heres a few questions:

It's said that 1500 is roughly the NiMH magic number for capacity. This is because NiMH is prone to discharge and higher capacity will drain when not in use faster. Also NiMH tends to have voltage drop under load which obviously is a large problem. On top of this, a slow charger is often good unless you pay a lot for a good fast charger because trickle charging is good for the batteries.

Now to me I would instantly think buy the highest capacity and the fastest charger for the cheapest I can for convinience. Anyone have opinions/experience on this? I'm going from advice found online with no real evidence backing them up other than its the said thing. Also LiFePO4 and A123 are said to be good so I'll throw them names into the melting pot of ideas and comparisons

Nicads have a memory. Use them 1/2 way and then charge them they will eventually only give you half a charge worth of juice. NiMh's you can charge any time you want.
I also read that. NiCads have to be FULLY discharged before you recharge them. Another good reason to go for NiMH!!
I think the main point of NiCad or LiPo maximum current draws are for brushless motors which really pull the amps. You should have no problem with a dozen servos on NiMh.