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Old 02-03-2017, 02:14 PM   #1
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Default Home made LIPO discharger w/ low voltage cutoff questions

So I'm getting back into the hobby and am extremely bored so I decided to build my own discharger using four 100W 1Ohm heat sinked resistors which will be cooled by 2 or 4 fans depending on how warm everything gets (Can't find a datasheet on these resistors). I plan to design a circuit to cut off voltage at ~6.40Vdc. I may add two sets of 7 segment displays to display voltage as well (One for each cell)

I have a few questions.

1. Does anyone discharge each cell of a 2S individually or just worry about balance in the charging process?
2. Is anyone using a low voltage cutout circuit w/o having to use a high current relay?

Thanks much

Dave
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Old 02-03-2017, 02:59 PM   #2
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What is the purpose of discharging to such a low voltage? It's too low for storage. Have you also considered that you may need to slow the discharge rate as you get close to the end voltage?
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Old 02-03-2017, 03:05 PM   #3
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I'm just cycling the cells. Some of the top guys at my track were saying that they were able to lower the IR of cells by doing this and quite honestly it's just a project for me. It may or may not make any difference but it's something to do. The discharge current is going to be reduces as voltage is reduced. (33.6A@8.4V0dc and 25.6A@6.40Vdc). I'm not an EE just someone who wants to play around some to be honest.

Dave
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Old 02-03-2017, 03:16 PM   #4
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Originally Posted by TT_Vert View Post
I'm just cycling the cells. Some of the top guys at my track were saying that they were able to lower the IR of cells by doing this and quite honestly it's just a project for me. It may or may not make any difference but it's something to do. The discharge current is going to be reduces as voltage is reduced. (33.6A@8.4V0dc and 25.6A@6.40Vdc). I'm not an EE just someone who wants to play around some to be honest.

Dave
25A is still way too high when you're close to the target voltage.You'll want to taper down to around 1A or even less when you're very close. The voltage drops fast when you're around that voltage level. Also, at that high current the battery will have a different load voltage to rest voltage, so when you cut the discharge, the voltage may jump back up to 7V or so. Unfortunately you'll need a much more complicated circuit than just a resistor with a cutoff to taper the current.

Cycling at high amperage can lower IR temporarily, but you'll wear out your batteries pretty quickly doing that. If you're still keen to try that, save the cycling for the more important races.
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Old 02-03-2017, 03:28 PM   #5
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I played around with this and killed some of my batteries. I don't care about it anymore. I'm cured I found that discharging them that hard, they don't discharge evenly and one cell can dip too low and damage it. If you want to do that, which probably only the high level processionals and battery wizzards are going. I would get their setup and learn their tricks. I have an Icharger 308 Duo and I can cycle batteries, and I do see a drop in internal resistance. I'm also a very good driver, but I don't see or feel much of a difference on the track. Not enough of a difference that I feel I need to work my packs over before I run them.
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Old 02-03-2017, 03:30 PM   #6
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Also. The discharger I made got over 500 degrees and I was grilling baby carrots on it.
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Old 02-03-2017, 03:31 PM   #7
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Quote:
Originally Posted by TT_Vert View Post
So I'm getting back into the hobby and am extremely bored so I decided to build my own discharger using four 100W 1Ohm heat sinked resistors which will be cooled by 2 or 4 fans depending on how warm everything gets (Can't find a datasheet on these resistors). I plan to design a circuit to cut off voltage at ~6.40Vdc. I may add two sets of 7 segment displays to display voltage as well (One for each cell)

I have a few questions.

1. Does anyone discharge each cell of a 2S individually or just worry about balance in the charging process?
2. Is anyone using a low voltage cutout circuit w/o having to use a high current relay?

Thanks much

Dave
1) If the cells are wired together in a brick pack, then the connection to the node between the two cells is made with wire far too light to carry a high discharge current; it's only for balancing at, say, 1A or less.

2) A MOSFET will be much better than a relay.

If you are interested, I can provide you with details on the discharger I recently designed.
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Old 02-03-2017, 03:47 PM   #8
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Thanks for all the input guys. TO prevent oscillation I'd definitely have to work some hysteresis into this. Are there MOSFETS that can handle 40A continuous current? I may need to go w/ some type of IC also to do this but it's just something I wanted to toy with. I never will be good enough to probably notice a minor IR reduction but it's just something fun to play with and expand my knowledge I suppose. I'd certainly be interested in your schematics Howard

Thanks
Dave

Last edited by TT_Vert; 02-03-2017 at 04:13 PM.
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Old 02-03-2017, 07:17 PM   #9
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You'll want plenty of hysteresis, and it's also a good idea to include a short time delay between the comparator switching and when the MOSFET gate is driven to prevent oscillations or other erratic operation due to transients from switching a high current.

There are many MOSFETs capable of handling 40A. For further info on this and other items, I just created a separate thread here:
Discharger Design for 1s and 2s LiPo
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Old 02-03-2017, 07:28 PM   #10
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Thanks much Howard I'll mosey on over there.
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