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?
Basically NiCad are heavier
Basically NiCad are heavier and have a lower power capacity but deliver higher current. Also cadmium is bad for the environment.
NiMH are lighter and hold more energy but can`t deliver as much maximum current as the older NiCad cells.
Most robot builders choose NiMH over NiCad because of its better weight/power ratio and that is what I would suggest doing too.
Ah…
Thanks for the advice and sorry for the late reply. I actually forgot about this thread.
However I was planning to get NiMH batteries, but then I read that their voltage supply is much more unstable than NiCad batteries once you start pulling a few amps. In fact I read that it was recommended to use NiCad if you’re using more than 8 servos. Since I’m planning to use 12 (for starters) I thought perhaps I would get a lot of decoupling problems and whatnot using NiMH. But NiCad batteries do seem like a bit of a hassle so I’d rather avoid them if possible…
Adding NiMH in parallel =
Adding NiMH in parallel = less current draw per cell = less voltage sag under heavy load.
Hmmm…
Yes but I think then I wouldn’t be able to use 4 x 1.2V (AA) batteries as I would like…I would need 8 batteries…correct?
I mean if I connect batteries in parallel I increase the current capablility and in series I increase the voltage right?
Nevermind that…I asked mr. google and he confirmed my belief…
The thing is 8 AA batteries is to big and heavy…but perhaps you had something else in mind?
Nope, 8 AA is what I had in
Nope, 8 AA is what I had in mind. Are you sure 8 will be too heavy on a project that uses 12 servos?
NiCd AA’s don’t store much power compared to NiMH so you may find the lifespan of a fully charged set quite low when drawing high currents. If you go for an RC-style NiCad pack then you’re already be adding quite a lot of weight.
Yes
8 AA batteries is definately too much. It’s a small lightweight quadroped and I’m be using 12 x DAGU 8gr microservos. The thing is though these cheap small servos have surprisingly high (peak) current draws. I know that the somewhat equivalent HTX900 and TowerPro SG90 peak around 750mA, so I presume mine are about the same. I’ll do some testing ASAP though.
I guess I’m mainly worried because an important part of this bot is that I’ll be measuring the current draw of ALL the servos in realtime in an atempt to create a sorta robotic nervious system. I’m currently working on a shield for my Arduino with 12 x 0.51 Ohm shunt resistors, 1 x shift register and 2 x multiplexers to be able to do this.
Anyway too much voltage sag would severely mess with my readings I’m afraid. Perhaps I should find a way to measure the actual voltage supplied by the battery pack (also in realtime) and use this as a reference to get more precise current draw measurements?! 
If the shunt resistors are
If the shunt resistors are between the servo and ground then you don’t need to calibrate for varying supply voltage =)
Classic Ohm’s Law will give you [servo current] = [shunt voltage]/[shunt resistance], where [shunt voltage] = [shunt high-side voltage] - [shunt low-side voltage].
In this case however [shunt low-side voltage] = 0V, because it’s connected to ground, so [shunt voltage] = [shunt high-side voltage] - 0V = [shunt high-side voltage]
Then your equation becomes [servo current] = [shunt high-side voltage]/0.51Ω, which is nice and easy, with no need to worry about the supply voltage.
Are you planning on getting analog readings for the servo current draws?Edit: Heh, totally forgot about that big thread you started back in September!
I think the main point of
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.
Don’t forget the memory…
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.
NiMH seems to be popular
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
Yes!
I also read that. NiCads have to be FULLY discharged before you recharge them. Another good reason to go for NiMH!!
Well
I didn’t forget about this idea that started back then (my 1st thread on LMR). I’m just slow due lack of time, lack of electronics/mecanics skills, limited access to parts and the fact that I started another project in the meantime: a wheeled mapping robot. I was originally planning to use the walker as a mapper as well but decided to do two seperate projects instead. The plan is to merge them at some point though.
And yes the shunt resistor IS between the servo and the ground. Basically I ended up with my original idea just with a 0.51 Ohm resistor instead of a 6 Ohm resistor. Like this:
I did quite a bit of testing and it works fine.
I'm glad to hear that the stability of the voltage supply wont affect my readings. Allthough I don't fully understand it, it does make sense. I originally calculated that I needed a 6 Ohm resistor to get a full resolution (Arduino ADC = 0-5V) like this:
Rsense = Vsense / Iservo => 5V / 0.75A = 6.666.. Ohm (given a peak current draw of 750mA)
And I notice now that the supply voltage ISN'T part of that equation. So I presume you're right :)
However if the batteries simply can't supply the current that a servo is TRYING to draw at a given moment it would still give me a wrong idea about the stress/load of the servo?! Or did I misunderstand something again?! :/
Thanks guys!
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
Considering it came from a university I’m shocked at that article… a ‘coulomb’ © 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
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
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 = 101.8A = 18A average current, you’ll get a pitiful 6mins run-time… but if you’re only going to pull 1C = 11.8A = 1.8A average current, you’ll get an estimated 1/1 = 1hour from full charge.
Perhaps
But here is a quote from this article:
"We highly recommend NiMH batteries in applications that call for long duration but not a high amp load. If you have an aircraft with very large servos that pull a lot of amps or more than 8 standard servos we recommend using NiCAD batteries for the best results."
So I dunno?!
OK
Thanks…
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
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.”
