Battery Runtime Tests

[oldtamiyaphile]

OK, maybe a little snide, then

Had I known your basis for the 2.2amp thing, I could have pointed to the nessesary info earlier. Frankly, shorting out the charge terminals would have never occured to me, it's for voltage measurement only.

I have thought about 'real' run time tests (gentle peak charge and discharged in a car), but we know mathematically that a 100mah car should run twice as long as a 50mah car. I've killed a motor on one charge HEHE.

[Namuna]

...but we know mathematically that a 100mah car should run twice as long as a 50mah car....

True, IF we're talking about batteries that were charged to their prospective capacities...Unfortunately (I'm sure you'll correct me if I'm wrong here ) I believe some of the rated capacities are based on C/10 or even C/16 charge rates. Even if you found someone that had equipment that could put out such miniscule juice, I have to wonder if anyone would SERIOUSLY consider waiting that kind of time to play with their little cars.

I think perhaps the 'best' test is to see what the highest Cx can be used to still achieve the rated capacity.

[SuperFly]

Thanks for the tests, Namuna.

I'll have to measure what my custom charging pad is putting out using the methods described. I use a 1000mah multi-voltage adapter at 3v running into a MotorWorks charging station, and I usually charge between 2-4 button pushes. My results are fairly consistent with Namuna's info.

One thing that has been alluded to, but to me is one of the most important issues is having fun. I like my charging to be fast, and my run times around 10-12 minutes. By then, I'm ready to switch cars or go get a beer. To be honest, I could care less about battery life. I have yet to wear one out, and I have a jar full of extras when I do.

[oldtamiyaphile]

As I pointed out earlier, the 50 and 150mah cells displayed a linear correlation between rated capacity and discharge time. The 100 cell faired worse only because it's of inferior quality. IE, despite the high charge current the 150 still ran about three times as long as the 50.

WRT the whole charge time/run time theory. Since so much charge current is being wased as heat, you might well find that a 4 min charge at say 500ma would actually yield a longer runtime, as opposed to 1000ma for the same lenght of time.

At this point I must assume your charger only goes down to 1amp, which might have 'decided' your charge current?

Ni-Cads are rated in the following way. I'm sure there are some asian copy cat rogues that don't use this methodology though, but it is the electronics industry standard.

Charge rate is C/10 for about 16 hours (delta peak monitored cut off). They are then left to sit on a shelf for two weeks to self discharge, after two weeks capacity and voltage are measured. The above tests should be conducted at 25 degrees C.

What does this mean?

The Mah/Voltage rating is a guaranteed minimum. Typically a correctly, freshly charged cells will have 1.4 volts and between 20-30% higher capacity than their rating.

So, it's entirely possible to charge at say 300ma and get a cell full to 100% of it's rated capacity.

Perhaps a second round of testing is in order. Use only one type of cell and report on the affects of charge current on final capacity.

Superfly, you're not charging at 1000mah. You could connect the charger to the mains (provided you stepped down voltage to ~3v DC) and the car would have 100's of amps at it's disposal, but would still only draw the 300ma it normally does. As I've said, Ni-Cads in a controller can put out 100amps (or close) but they certainly don't deliver all their current to the car. Matter of fact, you're giving the charger less current than alkalines could deliver.

[boho]

What effect would I have if I used higher voltages to charge? Why should I use 125~150%?

[Namuna]

Okay, if I read you right...NO MATTER how high the Charge rate you use, the batteries will accept ONLY a certain level (based on internal resistances) and the rest is translated into heat.

So to find the MAX, SAFE Cx I could probably set my Peak Charger (it's capable of 100mAH up to 5A, in 100mAH increments) to say 500mAH and while the batt is charging, I'll have my DMM on there and monitor how much the batteries will accept (assuming it'll be lower than 500mAH)...

For the sake of argument; Let's say I have a 100mAH rated, NIMH battery connected and I found that as I was charging at the 500mAH rate, my DMM was only reading 250mAH. That means for this particular battery, the 'threshold' is 250mAH.

With that, I could charge at a relatively 'safe' rate of 200mAH or I could push it further at 300mAH (to hit its threshold) and deal with the heat of the wasted 50mAH.

This is actually GREAT to know, I've got plenty of AA and AAA cells. Some are reputible name brands, and others are no-names. Doing these tests would let me know just how hard (but still safe) I could charge them at.

[SuperFly]

Superfly, you're not charging at 1000mah. You could connect the charger to the mains (provided you stepped down voltage to ~3v DC) and the car would have 100's of amps at it's disposal, but would still only draw the 300ma it normally does. As I've said, Ni-Cads in a controller can put out 100amps (or close) but they certainly don't deliver all their current to the car. Matter of fact, you're giving the charger less current than alkalines could deliver.

This might be one of those areas where theory and practice diverge. The quality of the charge I get from the 1000ma DC adapter is noticably different than what I get from batteries, even brand new alkalines. It is also different with adapters rated at different mah output. I'll have to re-read the posts to see what I'm missing, but my experience has been that the charge the battery receives is different depending on what I use to charge it.

[Lord_Nick_D]

I have put rechage ni cads in my controler and it does make a huge difference as you said
you also get better range I think
it well worth trying atleast

[SuperFly]

WRT the whole charge time/run time theory. Since so much charge current is being wased as heat, you might well find that a 4 min charge at say 500ma would actually yield a longer runtime, as opposed to 1000ma for the same lenght of time.

Could you explain this in more detail?

[oldtamiyaphile]

boho, we use up to 1.5 times voltage because a cell reachs up to about 1.6v during charging.

Namara, you can't use a DMM to measure how much a cell can 'accept'. What you described would only measure charge current. Charge current is regulated by the charger alone and not the cell, so your reading would be what you set the charger to.

All you can do is charge a different currents, discharge the cells and then determine at what point the cells begin to struggle.

I measured the current a stock Bit charger puts out.

Nicad: 300ma
Alkaline:200ma

I also did your short circuit test

Nicad 500ma
Alkaline 300ma

As you can see, it far from puts out the max current of the AA's.

Superfly, yes, you will get different results depending on what batteries or else you use to power the controller, as you can see, Nicads charge faster than Alky's so you'll get a more complete charge (even if one charge is still only about 5-10% full). Given that the most current the charger draws is around 500ma, adding a bigger supply won't have an effect though, might be something to do with voltage (3v vs 2.4?).

If you have a MM, try measuring current/voltage of different sources.

Could you explain this in more detail?

If you charge at 500ma instead of 1000ma, a lot less current will be wasted as heat. If a cell generates enough heat, it will begin to vent gasses (to prevent expolsion), the point where this occurs is surprisingly low. Also hot cells self discharge much quicker than cool cells. If you leave a cell at 60 degrees C, it will go flat much quicker than a cell left at 30 deg C. Heat is the enemy.

What this boils down to is that you might be able to charge for shorter than a peak and still get the same runtime. It would take a lot to fully explain it. I hope this makes sence.

[Namuna]

Namara?

I also did your short circuit test

Nicad 500ma
Alkaline 300ma

As you can see, it far from puts out the max current of the AA's.

Okay, let me give an EXACT detail of how I got my 2A+ readings and let's see what I'm doing wrong...

This is my DMM (from the shack):
http://tinyrc.com/albums/motortests/aab.jpg

When testing:
1. I take a standard Bit Tx/charger and turn it on (this is with Alkalines)
2. With the DMM set to test in the Amps range (have to switch the red lead to the 10A max plug) I put the leads onto the prongs of the charger
3. I get readings in the 2.2A to 2.5A range

With the red lead in its' usual plug, it has a max range of 400mA...I've already blown a fuse trying to test in this range and learned my lesson there.

What's being done wrong?