Battery Load Capacity Question

I have a 9.6 V, 2100 mAh NiMH battery pack that I am wanting to use to power a robot. What I haven't been able to determine, however, is what
kind of loads the battery pack should be capable of supplying. On the net I have seen statistics like .5 C (1.05A), but as far as I can tell, those are only recommended or best performance/life time, not what the battery is actually capable of. I have also seen at least one place that states 2C. Can anyone tell me definitively if this battery pack will be capable of sourcing around 2 A? I have been having some problems with it, but I am unsure as to whether that is a bad connection or an inability to supply the load. Thanks.
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The short answer to your question is: Determine what cells are in the pack (type and manufacturer). Often the only identification printed on cells in heatshrink packs or custom packs will be a dot-matrix part number. Almost always, Googling for this part number, or a substring of it, will lead to web sites selling replacements, whence you can glean the manufacturer and catalogued part#.
Now look up the datasheet for the cell and go from there.
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Thanks for the reply. The batteries in the pack I am using do not appear to have any sort of identification markings on them, part number or otherwise. At least, not that I can see without removing the shrink wrap. the pack I am using is this one: http://www.onlybatteries.com/showitem . asp?ItemID530.57 A quick google search for Hitec/JR battery packs does not turn up any real information, but I haven't had a chance yet to do a more thorough search. Thanks again.
wrote:

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Well, normally a 2100mah batter back should be able to handle that load, but you may not get a lot of time off of it, depending on the circumstances. You need to look up the NMH cells as per the manufacturer and see what their ratings are. To me it sounds like maybe you have a old battery pack and or bad cell(s) causing some of your problems.
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Thanks for the reply. Unless I got ripped off, the pack should be new. Run time is not terribly important to me, as long as it is more than a few minutes :)

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I use 1800-2100 mAh NiMh AA cells all the time to supply these levels of current, and more, in driving 16 servos ..... I can't however tell you how long they will last before discharging at these rates. Electric R/C planes use them at lots higher current levels.
http://www.oricomtech.com/projects/svo-load.htm
- dan michaels =======================
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Hmm. Interesting. Let me give a bit more information, perhaps someone can see what I am doing wrong. I have a wall adaptor rated at 12 V/1.5A that I have been using for testing purposes. With this connected into the system (2 bipolar stepper motors W/drivers, 5V regulator to step the voltage down to logic level, and then a few logic-level components, drawing around 100 mA) the measured current level is 1.88 A. With this, everything works perfectly, never mind that I am running above rated current. Once I replace the wall adaptor with the battery pack (measured no load voltage level about 10 V) the system stops working. The power light on my microcontroller board is on, but it is apparently resetting so fast that it never communicates with my computer (it has a built-in program for interfacing with a computer terminal). The motors feel locked (although not as hard as I would expect) but emit a strange "hissing" sound - again putting me in mind of them resetting several times every second. I have tried powering the logic level portion of the circuitry from a separate battery pack, but that doesn't seem to make much of a difference, except for somewhat stabilizing the microcontroller (I definitely have some issues with the connector I am using). Any ideas? Did I just get gypped with my battery pack? Thanks.

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It's not clear why the system works with the wallwart, but not with the batteries, but it sounds like you may need a lot more filtering on the motor loads to quench inductive spikes, current spikes, etc. Maybe some reversed-diodes across the stepper windings, plus a large electrolytic cap and bypass cap at the battery terminals of the stepper motor driver - which you didn't mention anyways.
Unless the battery leads are very heavy and very short, they'll have enuf inductance that the switching currents will produce some rather large spikes right "at" the battery terminals of the controller. The electrolytic will help control this. See the following ... and look around the 4QD site ....
http://www.4qdtec.com/pwm-01.html#cap
In any case, you need lots of filtering on the motors and also the motor-controller, and also the logic controller. Best to run the logic controller off a separate supply, at least until you get the system working properly. Motor systems are always a challenge to get working right.
- dan michaels www.oricomtech.com =======================
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Humm, it may be the battery pack is getting pulled down momentarily by the motor current surge, causing the MCU to reset. Measured current may be 1.88a, but what is the startup motor surge current. On some motors I can see 20 amperes or more easily enough. In RC cars 100amps or more surge is typical. Thus NMH batteries may not be able to provide that kind of current momentarily. The RC car people typically use sub-C size Nicad's as they can provide huge amounts of current for getting that extra kick to the motors. NMH batteries can't do this. Try running the MCU circuit off a separate battery from the motors. Like Dan stated, filtering for RFI and EMI is important where motors are involved.
Oh, yeah I almost forgot, did you limit the max current going to the steppers? If the stepper is rated at 2 amps max , then you need to limit the current so it does not exceed 2 amps at the voltage you are running them at. Some stepper controller IC's have a current sense pin that can be used, but a simple expediant method is to use a 5,10,15,20+ watt wirewound ceramic resistor for this. Without current limiting the steppers can suck up a lot of current, thus starving the rest of the system for power.

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Thanks for the continued assistance. The stepper motors I am using are rated at 600 mA/phase, two phases per motor, for a total of 2.4 A. 12 V rating, 20 ?/phase. with the voltages I am using, there should be no danger of them exceeding rated current, thanks to V=IR. In theory, at least, I would think this would also limit any inrush current- the supply is seeing an equivalent resistance of about 5? (4*20? in parallel), so with the 10V battery pack, there should be a maximum possible current of around 2A. If there is any inrush current above this, it goes by too fast for my multimeter to catch.
Re: Filtering, the motor driver IC's I'm using (allegro micro 3977, http:// www.allegromicro.com/sf/3977/) have two power inputs each, and one logic level input. I have 47F capacitors from each of the power inputs to ground (for a total of 188F), and .1F capacitors between the logic inputs and ground. I could certainly add more, if necessary, this is just the configuration recommended by the manufacturer.
Just tried running the logic systems off a separate supply. This stabilized the microcontroller, as expected, however it did not help the motors to function properly. Not really surprising, as the entire logic system pulls a negligible amount of current compared to the motors.
One interesting point I just noticed: the 1.88A I stated was for the wall wart. When I hook up the battery, though, the current drops to about 200 mA, and the voltage at the battery terminals drops to about 5. 6V. This is even though the battery reads a full 10 V when measured by itself. which brings up a potential, if annoying, solution: Might my battery simply not be fully charged, even though it is reading the full voltage level? How can I tell? My charger gives no indication. IF this is the issue, I am SO going to kick myself. Which also brings up a related question: Using the 12V, 200 mA charger that I was sent (and told was appropriate for this battery pack), about how long should it take to fully charge this battery pack? And How can I tell if it is fully charged?
Thanks again for the assistance.

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It sounds like the internal resistance of the battery is a bit high. Is this a new battery? What is the date code on it? (These things have a shelf life).
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...snip...

..snip..
I am currently wrestling with similar issues, so I though the following might be useful:
1) When you test a battery by just placing a VOM across the terminals, you are NOT testing it under load. The current passing through your meter is probably < 1 mA (if not lower - depends on the meter). This does NOT measure in any significant way the capability of the battery to supply power to your circuitry... A battery should really be tested under some sort of load.
2) Each battery type also has a maximum discharge rate that it can operate at. One might think that you could take (for example) a 1AH battery, and get either X amps from it over 1 hour, or X/2 amps for 2 hours, or even 10X amps from it for 6 minutes --- but it doesn't work that way. A battery can only discharge at some maximum rate. This maximum discharge rate is the result of a number of things - including the specific chemistry used in the battery, and its design (such as the size of the internal electrodes, for example). So, if the required output current exceeds what the battery can supply, the voltage will drop (as you saw: due to, obviously, V=IR).
So, once you hooked up the battery to your circuits/motors, it was then under load, and the voltage drop to ~5.6V is the result of the current draw exceeding that particular batteries' discharge rate. It just couldn't provide enough power to maintain that 10V. That 200mA you measured was effectively the maximum discharge rate the battery was capable of supplying at that moment (as the battery discharges over time, the output mA also changes, as you might expect).
The wall wart didn't have this problem because (generally) they effectively have a "discharge" rate (if I can use that term here) of their full rated Amperage. Lots of current and lots of voltage for them to draw on in providing the rated output - so they are not limited in the same way a chemical battery is..
(Of course, if you upped your voltage/current draw high enough then the wall wart would start to behave in this respect just like a battery... But most systems don't operate anywhere near that regime... unless we're talking about space heaters, here... ;-)
The maximum rate that a battery can discharge depends on a whole host of things, such as: battery size (AA, C, D), chemistry (carbon-zinc, alkaline, NiCd, NiMH, Lithium), how old the battery is (they do "spoil" (discharge), you know, just by sitting on a shelf), what temperature they are stored at, etc... I have also seen differences between manufactures, and even within type (due to variations during the manufacturing process). For rechargeables, the specific charger and the charging time/schedule also make a difference. (And this is only the short list....)
Lots of variations: I have actually seen two identical batteries (same manufacturer, size, type and mAH ratings) exhibit quite different discharge rates...
3) The above also relates to another issue: many people confuse a batteries mAH rating with its' ability to provide power. If you do a little research, you'll find that a specified mAH rating is misleading in terms of determining how much current you can get out of it over some length of time. What the manufactures do to get the mAH value is to discharge the battery at a relatively low rate for 10 or 20 hours, then measure the remaining capacity and calculate the mAH value based on discharge rate, remaining power, and time on discharge. In other words: its' a derived value based on assumptions that are probably worthless to you if you want to know how big a battery you need for a specific application.
Although a 2000 mAH battery certainly holds more power than a 1500 mAH one (assuming the above variables are roughly the same), it tells you very little about how much current you can get out the battery, and for how long. (About the only thing the mAH rating is actually good for is to allow you to roughly compare batteries to each other (but only roughly)).
Over the years, batteries have become so ubiquitous that people nowadays hardly give them any thought... But there are some wrinkles you need to be aware of when you design them into a system...
So... given the particular circuits/motors that you are trying to power, how do you determine how large a battery you need? Unfortunately, as indicated above, you really can't go by the mAH rating. You need to know both the constant and peak current and voltage requirements of your system. Then you need to find a battery (or battery configuration) that can provide the required voltage and current.
You may just have to do some experimentation (as I am currently in the middle of... ).
Good luck! ;-) Chet
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My two-pennies worth !!!
The hissing noise you got from the motor is exactly as you suspected i.e. the power cycling 'several' times a second. You get exactly the same effect using PWM (at the wrong frequency) !
Battery charge.....make sure the batteries are flat before charging. Two reasons for this. If they are plain NiCad's then they can develop a memory which reduces capacity - but (and here's the clincher) even if they are newer NiMH or such, the fancy micrprocessor controlled chargers that are becoming standard nowadays, will switch to standby (trickle charge) before they are fully topped-up. Took me ages to figure this one out !!!!
Finally.... there are three rules for preventing motors etc interfering with your processor.
Rule 1. Use separate batteries for the processor. Rule 2. Use separate batteries for the processor. Rule 3. Use separate batteries for the processor.
Honestly, I used to try all sorts, to limited effect. Now I have a separate set of rechargeables just for the thinking bits, and a separate set for the motors.....tie the negatives together...and no more spurious resets !!
Toodleoo
Dave.
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I fully agree that the world is much simpler if you use a pair of batteries with a common negative.
However, there is no need to routinely discharge your batteries before charging them. You're just spending lifetime. If the battery is more than 5 or six cells, you risk permanent damage from cell reversal.
--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR
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Dave VanHorn wrote:

I'll second the discharge opinion. There is no reason to do this under normal circumstances (unless you're getting ready to store the cells for a long period). Instead, spend the money up front on a good charger -- it's cheaper in the long run. This advice applies to batteries based on any chemistry.
--
(Replies: cleanse my address of the Mark of the Beast!)

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Thanks for all the replies guys. I took another look at my charger, did some calculations, and realized that it would take about 10 hours for a full charge (since the battery pack was new, and, presumably, nearly completely discharged)-2100 mAh battery, 200 mA charger. I had previously let the battery charge for about two hours, obviously nowhere near enough. Plugged it back in, let it charge for the rest of the day, and viola, everything started working. At least until the motor controllers fried - both of them, simultaneously. Still trying to figure that one out, as the current through them was nowhere near limits, even at worst case (about 500 mA/phase, vs. ratings of 2.5 A/phase). As best as I can figure, a surge got through to the 5 V systems (I had not yet hooked them up to a separate supply, that is now done) and the motor controllers were simply more sensitive than my other components. I do wish I knew for sure though. Thanks again for the info and assistance!
Kap'n Salty wrote:

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