Hi,

I am reconditioning a battery from a HCH-1 but my question applies to all battery work. How do you measure capacity of the individual sticks accurately when they rebound from the low (discharging) or high (charging) measurement of voltage significantly? For example, I discharge a battery to 6V but when I go to charge it, it is already at 7.1V. This happens even if I discharge it up to 3 times at gradually decreasing amperage. If I just charge it immediately after discharging it the rebound interferes with the charger's ability to detect what is going on and it will not give me an accurate measurement of charge anyway. Plus it is likely to shut off early. I need to have a relatively accurate measure of charge and discharge so that I can tell which batteries need replacing. Thanks in advance.

Jen

 

I just received my new Imax B6 V2 chargers. These are the genuine ones as well as a version upgrade. The ones I had been using were not. These have a "re peak" function that I set for 3 times with a delta max of 5mv. The re peak, at face value, doesn't seem to apply to discharging. I will see if these new chargers solve my problem. I am currently (no pun intended) charging NiMHs at 0.7amps and discharging at 0.8amps.

Jen

 

Jen,

I'm very sorry you've gone down this path. There is so much evidence of the benefits of grid charging/discharging that render stick work unnecessary except in the cases where sticks must be replaced. That requires high current discharge testing on the order of 36X (180W vs. 5W) the power the Imax can deliver.

I have a dozen of the chinese Imax knock-offs. They are terrible at deltaV detection and frequently cut out well before the stick is close to full. It has nothing to do with voltage bounce of the sticks, it has to do with them being crappy pieces of crap. You can sometimes get some use out of them by setting the deltaV detection to 20mV and keep your charges under 3A. You may have to go as low as 0.7A.

While the SkyRC is a quality product, it is terrible for this application. You should see an improvement in correct charge termination. The best move would be for you to return it/them and obtain something with a useful rate of discharge.

Voltage bounce is normal and desirable in NiMH cells. In your case, when you start a discharge after fully charging the sticks, the capacity is the number the Imax reads when the discharge terminates at 6V and ~0.8A, that's the capacity of the stick... at 0.8A.

The issue with the B6 is the discharge rate is way too low to tell you anything useful about the health of the sticks. They can only identify the most crappy of sticks. The car subjects the sticks to a 100A load for short duration and 20-50A routinely. Sticks perform at 50A+ much differently than they do at 0.8A.

I recommend the following:

1. reassemble pack
2. build DIY grid charger and discharger in my sig (about $85-90)
3. grid charge to peak voltage for 8 hours but do not exceed 24 hours. Blow a box fan through the pack for cooling.
4. discharge to 135V with 2X 200W or 250W bulbs
5. switch to 60W bulbs and discharge to 60V or 12V as time permits.
6. Grid charge to peak voltage for 8 hours but do not exceed 24 hours
7. Install in car

The above process will accomplish what it will take 2-3 weeks to do with your IMAX B6 (assuming you only have one that works). 3 of them would get you down to one week, etc., but that would require that you time your cycles perfectly and don't lose any time or have any false charge terminations.

The grid charger takes about 30 minutes to put together.

If it doesn't work, you have a sick stick or two or more, and you'll likely never find it with the Imax.

Good luck,

Steve

 

Hi Steve,

You know a lot more about this topic than I do. However, I don't want to give up on the Imax B6 Version 2 yet. So I want to ask a couple more questions. These chargers seem to be working. They aren't shutting off prematurely when charging. That said, how many volts should a 7.2 nominal voltage (HCH1) be charged to? At a delta peak of 5 the chargers are not shutting down at 8.5V and I got chicken and shut them down manually. They were on the Auto setting.

I am getting excellent changes in IR, like from a reading of 75 reduced to 62 mOhms per stick. The new chargers measure this. What is considered a good level of IR and is that upon being charged or discharged?

I have ten 2A car light bulbs that are connected in parallel. They also have a low voltage shutoff inline with them. Can I use this gadget to test the capacity of the sticks and if so, how long should a good stick last?

Thanks,
Jen

 

Did you buy these:
http://www.skyrc.com/index.php?route...product_id=201

For $10 more, you could get a charger that can charge at 20A AND discharge at 20W internally (like 4 IMAX chargers) or 300W regeneratively (discharge into another battery like a 12V car battery). You will need a 12V power supply, and that's about $20 on amazon, or you can use a 12V car battery with a car battery charger attached to keep the 12V topped up. I use a $90 122Ah capacity deep cycle marine battery from Walmart.

Reaktor 300W

It has 6X the charge capability of the IMAX and 4X the discharge capability internally or 60X the discharge capability of the IMAX.

You see why I say you should return them? Overpriced and under-capable.

NiMH should be charged to peak voltage. That peak is a little different for every stick, but it's typically in the vicinity of 9.2V at 1C charge. The 6A charge of the V2 is close enough. Voltage rise accompanied by HEAT is the best external indication. When your voltage is 9+, and your sticks are about 100°F, you're pretty close to 100%. The chargers should terminate before the sticks get to 120°F. Note that there can be significant variation in individual cells within a stick, so it's a good idea to find the hot one and use it as your guideline. An IR thermometer makes this quick work.

Keep in mind that these things charge at 50A in the car.

IR changes with state of charge. The charger IR measurement is useless in a real-world sense, but it is useful as a comparative tool. The Reaktor does this as well. You should charge all sticks to 100%, wait 1 hour and then check IR. make this a standard measurement for comparison purposes.

Note that IR measurements are EXTREMELY sensitive to the quality of the contact. If you're just attaching your alligator clips to the ends of the sticks, your connection is likely interfering with the results. Thread the bolts into the ends of the stick partially and then clip onto the bolt. Sandwich the clip between the bolt and the stick and so you get a lot of contact area. You don't need to torque it as you'll crush the clip. Get it finger-tight using a 10mm socket for a little extra finger leverage.

Your discharge load is decent. If I remember my circuit basics, your total resistance is lower, so your load at bulb voltage should be 20A. Assuming those are 12V bulbs, you'll be putting out > 12A at single stick nominal voltage. Bulb resistance changes with filament temp, so their resistance decreases at lower voltage, so they don't fall off as fast as a straight resistor. While this isn't a great load, it's way better than what the Imax can do.

6.5Ah/12Ah = 32 minutes. Say 80% due to age and capacity loss, so 26 minutes minimum, so between 26 and 32 minutes if they're in really good shape, BUT this is for low current discharge. Says nothing about what they'll do at 100A. Do you have a automotive 12V battery load tester?

Once you have balanced the sticks, if you have one, you can do a 5.4V/30 second test. You apply the tester for no more than 30 seconds. If it hits 5.4V before 30 seconds, terminate record time to 5.4V. If it makes it to 30 seconds, record voltage. You should then be able to sort them first by descending voltage (the ones that don't hit 5.4V are the best), then by descending time (the ones that go the longest before hitting 5.4V are the second best).

Good luck!

Steve

 

I meant to ask you, is the low voltage shutoff a feature of the bubs you're using, or a different device. I'd like to know the details.

Thanks,

Steve

 

Hi Steve, I posted another message with a link in it that directs to the type of Imax charger I have but it got sent for moderation. I will see how this one does.

The discharger is this on eBay:

http://www.ebay.com/itm/270303431090

I wanted to ask also, it is my understanding that the super low amp charges and discharging is for the sake of balancing the capacities among the 6 cells that make up the stick? Any thoughts?

 

Here is the link to my chargers:

http://www.hobbyking.com/hobbyking/s...ischarger.html

Thanks again,
Jen

 

Thank you for the links. That's a curious little discharger. It doesn't suck.

Having seen the intent, don't take my calculations to heart. They are very approximate.

Again, concerning the IMAX B6AC v2, it's a quality unit, and I'm glad you bought it from Hobbyking, but it's very limited. For the price of 2 of those, you can get 1 Reaktor 300W that has the equivalent charging capacity of 6 B6AC and internal discharge capability of 4 B6AC. With regenerative discharge mode, it's equivalent to 60 of them. Of course you're only working with about 180W max with a single stick, so it's more like 36 of them.

They also have a 1000W/30A AC powered version that has an internal discharge capacity of 80W, which is equivalent of 16 B6AC for the cost of 4 of them. You'll be able to discharge at 10A, which is frequently good enough to sort good from bad.

Low current charging (C/10 or lower) is to "top balance" all cells to 100% SoC. The low current charge tops off the weak cells and is converted to heat in the stronger cells at a safe rate.

The low current discharge is to drive cells to below 0.78V to extract capacity at the voltage depressed state while minimizing the potential for cell reversal at high current. What usually happens is that 1-2 cells go low voltage and discharge in the 0.78V or lower range while the others stay strong. Once these completely deplete, their voltage falls off and the strong cells can't support the 6V total, so the discharge terminates.

The next charge "resets" the chemistry of the weak cells to eliminate the voltage depression enabling them to discharge their full capacity at >1V.

As you can see from the above concepts, the whole-pack grid charge and deep discharge accomplishes this on all 120 cells at once. It does it all at low current to prevent damage from excessive charge and prevent damage from polarity reversal at low voltages.

Steve

 

Thanks Steve, there's a ton of info in these posts and I am going to have to digest them thoroughly. I got the load tester and infrared thermometer. - Jen