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  #1  
Old 11-16-2009, 06:26 AM
dawg dawg is offline
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Default Lipo battery discharge project

Hi everyone.

This is my first post, I hope you don’t mind me making some mistakes along the way or ask stupid questions

I am trying to make a battery discharger for r/c lipos.

It’s purpose is to cycle new batteries and discharge any unused batteries for storage..

Lipos are the biggest thing since sliced bread for radio control flying things.

Anyways, there are times when I need to discharge the batts, and since they are high capacity no battery charger with discharge capabilities can load up more than a few amps, if that.

So, I am making a high capacity discharger for batts from 2s (2 cells) all the way up to 8s (8 cells)

The lipos have a min discharged voltage of 3.0 v/per/cell up to 4.2 volts/p/c. ie: min = 2s x 3 = 6v, max = 2s X 4.2 = 8.4v

When using lipos, I discharge the lipos up to 80% capacity.

Therefore 80% of say 5000mAh is 4000mA. This is what manufacturers say to discharge them to. More than 80% and its no good for the batt.

The discharge load will consist of 3 x 55w halogens in series (36v) and 3 x 12v fans also in series but across 7.4v minimum and 36v max..

Here is the control circuit




Now comes the bit I need help with. My math is bad so please keep it simple math wise.

Each lipo has different low voltage so I need to be able to select battery cell count.

These I like :
LM339 or similar
TL431 or similar

2s = 7.54 v ~ 8.4 v
3s = 11.31 v ~ 12.6 v
4s = 15.08 v ~ 16.8 v
5s = 18.85 v ~ 21.0 v
6s = 22.62 v ~ 25.2 v
8s = 30.16 v ~ 33.6 v

I am thinking Fig 2 or Fig 3 is a cell count circuit which can display an led when the voltage is within its voltage range to show the number of cells
Both circuits are from the makers Datasheet

Fig 2
Using 2 x TL431



OR

Fig 3
Using 2 x TL431


Eg:
3s lipo min discharge voltage of 11.31v up to 12.6v fully charged.

The next lipo say 4s has minimum discharge volts set to 15.08v.

This allows a gap of around 3v so it shouldn’t be hard to have a circuit show the number of cells in the battery.

Simply put, the cell count circuit shows a labeled led array the correct cells for a battery.

This concept is fine but I would need 6 of these circuits calibrated to the specific lipo cells. 2s to 8s..

Which ever lipo is connected will only switch the relevant led automatically while the other circuits remain OFF.

******************* Well this is where I’m stuck ******************L

Is there a better way to identify 6 different voltage ranges (using led display to show the cell count) ?

==========================

Stage 2

Voltage cutoff

When I plug the battery in, an LED lights up to show the cell count, 2s ~ 8s

So when I press a button for 2s ~ 8s, the circuit will stop discharging at :

Button 1 2s = 7.54v
Button 2 3s = 11.31 v
Button 3 4s = 15.08 v
Button 4 5s = 18.85 v
Button 5 6s = 22.62 v
Button 6 8s = 30.16 v

To prevent the battery from over discharge, some sort of voltage alarm or cutoff is needed

[When the lipo voltage reaches its minimum voltage (see above) it enables a relay and opens a n/c contact (RL3,C1) and stops discharging.]

I have seen many voltage cutoff circuits.

Is there also a more efficient way to monitor a number of voltages, instead of using 6 x voltage cutoff circuits ?

=============================

Other thoughts

1
A transistor circuit might have a too wide switch zone where it might cycle the alarm cutoff on and off.
This is because when the battery load is removed the battery will increase in voltage. (not much though [0.2v –ish]

2
Picaxe also comes to mind ????

3
A common relay for the voltage cut off circuits ie: (RL3,C1) normally closed

4
A common buzzer or siren for the cutoff circuit

5
Op amps are ok I guess but I would rather stay with comparators or zener shunts if relevant


Any thoughts greatly appreciated. J
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  #2  
Old 11-16-2009, 03:35 PM
Audioguru Audioguru is offline
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A Lipo battery does not need to be discharged for "conditioning". It has a limited number of charge-discharge cycles and wasting cycles just makes it go bad sooner.
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  #3  
Old 11-17-2009, 03:17 AM
dawg dawg is offline
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Default discharger

Hi,
that's fine, it's that I would like to have them discharged before storage..

Sometimes I don't fully use up the lipo and would like it to be in storage charge to make it easy on the battery..

Well I guess I'll have to stop cycling new batts slowly.
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  #4  
Old 11-18-2009, 12:30 AM
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Bernard Bernard is offline
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Are all of the batteries 5 Ahrs??
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Old 11-18-2009, 04:22 AM
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Read: http://www.batteryuniversity.com ,Part one, esp. Discharging & Storage

Last edited by Bernard; 11-20-2009 at 03:33 PM. Reason: missing info.
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Old 11-18-2009, 05:18 AM
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SgtWookie SgtWookie is offline
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I wouldn't discharge batteries connected in series, even if they're all the same part number made by the same manufacturer.

As batteries age, they will have different charge capacities. If you try to discharge them in series to a specific voltage level, you will wind up discharging some of the batteries more than others.

I don't recommend connecting batteries in parallel, unless they are charged to the exact same voltage, or connected by a resistive load. If you connect two batteries together in parallel that are at a different voltage level, one battery will try to charge the other battery at a high current level. This will cause both batteries to heat up, and possibly rupture.
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  #7  
Old 11-18-2009, 02:03 PM
dawg dawg is offline
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Default lipo discharger

Hi,
thanks for the replies,

the batts cells are exactly the same volts when charged, and this happens always when charged up.

I monitor the cells during discharge

I check the imbalances every time they are used.

On new packs the cells are < 1 ~ 2 mΩ

Used cells after many hours are within 2 ~ 3 mΩ of each other after sitting around for a while.

And that is what I call a 'bad' pack..

Lipos not used for a bit require a few 'cycles' brings All the cells into perfect harmony. ie: reduce differential resistance per cell..

that's why I need to 'cycle' them.

I do not want to simply charge them and use them straight away as the internal cell resistance are 'fighting' the pack..especially when things are sucking over 30 amps continuous



The lipos are from 2s to 8s and vary from 2000mAh up to 5 amps hr

I charge my batts at no more than 0.5c

I baby thems lipos and have been using some packs as old as 3 years, and show no sign of stopping..

Those 'old' packs are used around 180 ~ 200 times per annum.

Most lipos I have are all series.

regards,
Adrian
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  #8  
Old 11-18-2009, 07:03 PM
bountyhunter bountyhunter is offline
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Quote:
Originally Posted by dawg View Post
Hi,
that's fine, it's that I would like to have them discharged before storage..

Sometimes I don't fully use up the lipo and would like it to be in storage charge to make it easy on the battery..
I believe that is backwards. Storing a rechargeable battery in a discharged state usually screws them up over time.

Ditto on the fact that it is dangerous to deeply discharge cells stacked in series: the lowest cell will go dead and reverse polarity in the string and be destroyed. You have to monitor EACH cell continuously when doing this and stop discharging when the lowest cell reaches it's end of life voltage.

EDIT TO ADD:

I didn't realize LiPo was "lithium polymer". I worked with Valence Technology here in San Jose some years back (mid 90's) which was the first commercial company who made LiPoly batteries. Those batteries were VERY sensitive to discharge, and in fact theirs would get hot enough to catch fire if ischarged too deeply because they would grow internal shorts and the cell would dump quickly and burn itself up. Their first major contract was batteries for the Army's night vision goggles and the batteries were burning up. AFAIK, they never found a solution to that problem.

Be VERY careful with them as they contain enough energy to generate a ton of heat instantly if there is any kind of a problem with the cell or the load.

Last edited by bountyhunter; 11-18-2009 at 07:19 PM.
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  #9  
Old 11-18-2009, 08:42 PM
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Might be dooable-but messey[ lots of parts ]. Use a LM 3914N as LED display volt meter, in dot mode; outputs feed LED's and inverters, into D latches. Selected latch closes switch,4066, connecting battery via V divider's to a comparator. Comp. output disables constant current discharge ckt. when battery falls to 40% charge V. The circuit cannot determine A hr rating of battery, so set to lowest capacity to serve all batteries, or hand select discharge rate. Each LM.. step represents one cell. so that #8 step=8 cells, adjusted to represent a 50% charge V. A deeply discharged battery might give an eronious cell count. I think it would be easier to look at the battery and turn selector switch to proper cell count, set discharge current, take a long walk.
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  #10  
Old 11-19-2009, 12:15 AM
dawg dawg is offline
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quote
I think it would be easier to look at the battery and turn selector switch to proper cell count, set discharge current, take a long walk.

Yeah, good idea, thanks Bernard.

[I will make your circuit anyways, for learning something new..] : )

quote
Be VERY careful with them as they contain enough energy to generate a ton of heat instantly if there is any kind of a problem with the cell or the load.

I accidentally shorted a screwdriver with the terminals once across 25v, no more screwdriver. These lipos can produce over 200 Amps burst for 30 sec.

The current is spooky.

Maybe I'm over designing this ???
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