Internal resistance weirdness?
 

I have a Revoletrix Cellpro PowerLab 8 (V2) charger. It has a cool function that measures the internal resistance of every cell in the battery.

I read (somewhere) that it is good for the individual cells of a battery to have an internal resistance that is matched to the other cells, but I may also be remembering the old days when I ran matched and cycled NiCd cells. I also remember reading that as cells age or are left charged, their internal resistance will go up. I don't think anyone will say that high internal resistance is a good thing.

So, I got bored today and tested a few of my batteries. All were charged at 1C. These batteries have between 50 to 100 cycles and have not been treated as well as they should have been.

• $135, 6500mAh Orion, 90C, Cell1 = 6.9mΩ & Cell2 = 4.6mΩ. Hmm…
• $99, 5500mAh Reedy, 60C, Cell1 = 12.9mΩ & Cell2 = 7.0mΩ. Hmm…
• $35, 3000mAh Reedy, 25C, Cell1 = 4.0mΩ & Cell2 = 4.0mΩ. Wow!

I just purchased a new shorty pack and I am a bit worried about the numbers. All of the above batteries have been abused a bit so I wouldn't expect perfection, but the new battery is, like, new.

• $115, 4000mAh Revtech, 65C, Cell1 = 3.3mΩ & Cell2 = 8.0mΩ. Hmm…

Should I be concerned? $115 is a lot of money.

FWIW, all of the above batteries work fine, although I have not tested the Revtech except for a few 10A discharge cycles. The cheap Reedy Wolfpack is my Stock 2WD Short Course battery of choice. I just wanted a shorty battery to test out moving weight forwards and backwards in the chassis.

Thoughts?

A battery is only as good as its weakest cell. This is true for all battery types.

Also numbers dont lie. Past 3 years a group of my fellow racers have been experimenting with discharging/charging of lipos on race days. Its true with lipos that they DO get flat and DO need a discharge from time to time, NOT every time.

I would discharge a orion 3800 30C battery with a integy 30 amp discharger with a astro flight watt meter in between. I watch the numbers very carefully. On a 3800mah pack, fully charged, I can pull out around 3000mah at 30 amps with no issues. I usually like to leave around 1000mah as a buffer.

Let that battery rest for a little bit, then charge it up at 2c rate. The difference in performance is incredible. The internal resistance drops way down, and it runs like a brand new pack. Lipos do get a small bit of memory if charged the same way each time. Breaking that memory is using a discharger. Even a 20 amp discharger performs well with lipos.

In your experiment, I would discharge at a higher rate and then see what the numbers are. Im even thinking higher numbers for discharge in the future 40 amps or more. As long as I dont exceed the amp discharge rate of battery, all things should work well and even better numbers may come out, or I will reach a limit where the discharge will give no increase in performance after X amount of amps.

There are plenty of people saying you dont need to do this, its all false..
3 years is a long time of doing this process and I still use the batteries.. Never had a blown, swollen, or damaged pack...

Im not saying everybody should do this, you need the right equipment to make it work and to monitor the packs. Numbers dont lie....

Here is what the numbers dont tell you. If you are a racer, here is some info. When you race, you know how your vehicle runs. Throttle is more top end, bottom end or middle...

When I first started playing with this, some batteries had way more bottom power than top, others had way more mid range power. Other had more top end. So I would use packs to the type of track lay out. If it was a tight track, I would use the packs that had more punch, wide open track, more top end...etc. The numbers dont show this. You as a racer know by how the truck/car handles.

Sorry for the book... :)

Solution: http://www.rcgroups.com/forums/showt...0#post21729823

:)