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Old Jun 05, 2010, 12:13 PM
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Poor Man’s $5 Regenerative Discharger

If you’ve been using lipolys for any period of time you’ve become well-aware that lipolys develop higher internal resistance over time, and this condition is particular exacerbated by storing lipolys in a fully charged state. Also, if you’re a typical weekend flyer like myself, you invariably return from the field with fully charged lipolys that may stay fully charged for several weeks until you can make it to the field again. Problem.

To deal with this, many newer chargers on the market have a ‘storage discharge’ feature that will discharge to below 4.0 volts / cell. Other newer chargers have a ‘regenerative discharge’ feature that will discharge packs into a large source battery. John has also developed a wonderful little DIY circuit shown here that can be used to discharge into a load (lightbulbs, power resistors, etc.). Each of these work beautifully to bring lipolys to a voltage that will help preserve their longevity.

Another solution that I have been using for a while that has proven to be easy to use, very effective, and silly cheap is a simple power resistor that can be placed in between a fully-charged pack and a discharged pack (photos below). Very easy to make with a few connectors, wire, and power resistor. The resistor will limit current flow and bring the fully charged pack to a safe ‘storage voltage’ and has the additional benefit of bringing a discharged pack to a higher storage voltage that will allow for faster charging the next time the pack is used. Naturally, this only works for packs of the same cell count. Packs of different cell count should NEVER be connected together in parallel.

When returning from a day of flying, I simply place the power resistor between a full pack and discharged pack and leave it until there is no current flow and packs have equalized to the same voltage. I have made several of these ‘regenerative dischargers’ in various flavors (.1 ohm, .25 ohm, .5 ohm, 1 ohm, ….) and will use the appropriate resistor to keep current at a manageable level (current can be easily measured by placing a wattmeter or current shunt in series). A suitable wattage resistor should be used to limit heating. I’ve found that a good ‘one size fits all’ value for up to 6 cell lipolys is a 1 ohm, 10 watt (min) wirewound resistor (available from Radio Shack). Lower resistor values require higher wattage ratings due to higher current flow. My .1 ohm resistors are 50 watt, for example. Note that resistors may be placed in parallel to lower resistance and in series to increase resistance. Multiple resistors may also be used to increase power handling.

Packs need not be of the same capacity but must be the same cell count. I have used one of these dischargers to discharge a 5000mAh pack into a pair of 2100mAh packs. I also will hook several small packs in parallel and discharge them into another bank of small packs that have already been discharged.

Please chime in with your results if you use something similar. All comments welcome.

Mark
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Last edited by mrforsyth; Oct 19, 2013 at 12:15 AM. Reason: added photo
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Old Jun 05, 2010, 08:56 PM
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Excellent tip, thanks for sharing it.
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Old Jun 06, 2010, 04:59 AM
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Good thinking Batman

I wonder if the simple addition of a 10-20A polyfuse in series with the power resistor would be a worthwhile backup safety feature?
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Old Jun 06, 2010, 10:06 AM
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I plug mine together , but without resistors. Many times I just use one of those power distribution blocks so I can plug in any type of lead to the female banana jacks.
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Old Jun 06, 2010, 03:38 PM
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Good thinking Batman

I wonder if the simple addition of a 10-20A polyfuse in series with the power resistor would be a worthwhile backup safety feature?
Thanks Ken.

I did think about adding a polyfuse to protect against inadvertently connecting packs of different cell counts or the rare pack with a cell that has gone bad but didn't have any of appropriate value at the time. I've gotten myself into the habit of always metering my packs prior to connecting in parallel so it hasn't been an issue for me. Agree that it's worth considering though.

Using power resistors is painfully simple and even easier than setting up my discharger. Bringing my discharged packs up to storage voltage is an added bonus.

Mark
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Old Jun 06, 2010, 03:47 PM
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Originally Posted by gulio View Post
I plug mine together , but without resistors. Many times I just use one of those power distribution blocks so I can plug in any type of lead to the female banana jacks.
While possible, connecting a full pack in parallel with a completely depleted pack of the same cell count is generally discouraged as current flow can be quite high, depending on the packs being connected.

I once connected a fully charged 3S 2100mAh 25C pack in parallel with an identical pack that was depleted and got over 19 amps of current upon connection (as measured with my wattmeter that was in series). Doing the same with much larger packs or packs with lower internal resistance would result in even greater current. Simple resistors ensure that current is always at safe levels.

Mark
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Old Jun 06, 2010, 06:30 PM
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I checked most of my packs at thier worst possible difference and decided it was very insignificant. I'm not advising anyone to do it unless they are comfortable or can do what you did.

I'm curious to find out how quickly that 19 amps became a "reasonable/acceptable" current.
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Old Jun 07, 2010, 10:08 AM
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Originally Posted by mrforsyth View Post

Other newer chargers have a ‘regenerative discharge’ feature that will discharge packs into a large source battery.

Which one?
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Old Jun 07, 2010, 10:32 AM
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Which one?
The iChargers and the new FMA PL8. There may be more but these are the ones that I am aware of.

Mark
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Old Jun 07, 2010, 10:56 AM
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The iChargers and the new FMA PL8. There may be more but these are the ones that I am aware of.

Mark
As I have both of these I will add that they are capable of discharging up to 30 amps. or 500 watts into a 12V rechargeable source such as a 12V deep cyle and up to 1,000 watts into a similiar 24V source.

Real world figures that means nonimal 120 watts per cell for a LiPoly up to 4S (480 watts) into 12V or 960 watts into 24 volts.

Both are also capable of running cycles and now days my CBA IIIs and my CC-400s do not get used very much at all and neither does my CellLog8 controlled LiPoly discharger.

Charles
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Old Jun 07, 2010, 11:27 PM
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There are also some very expensive European chargers that have this feature, the Elprog Pulsar and some of the Schultze models IIRC. However the power levels are much higher with the Junsi iChargers and FMA PL8.
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Old Jun 08, 2010, 02:44 PM
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Gulio.
Here's a bit of an answer to your question. Not precise as I didn't time it with a watch. It's just a loose observation as I do this from time to time & I just noticed this thread, so I thought I'd post what I did last night.
Charged pack....Turnigy 5800mah, 4s, 25c, at 4.18v/cell
Discharged pack.... Turnigy 5800mah, 4s, 25c, at 3.78V/cell
Initial amps.... 24A
After 30 secs....18A
After 60 secs....12A
After 90 secs.... 7A
It then took about another 15 or 20 minutes to trickle down to zero.
Again, these are approximations as I wasn't paying close attention to it.
I'll do a proper analyses of a couple next time.
Oh. the final voltage of each pack was 3.98V/cell.
So the question is, does 24A cause a problem in the battery being charged?
As you can see it is practically a fully charged pack & pretty much a discharged pack. By that I mean too low to run another 5 lap race in one of my boats at 100A- 120A draw.
Cheers.
Paul.
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Old Jun 08, 2010, 05:42 PM
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Thanks for the info. The better the packs the worse the condition , but the more can be tolerated too so it may be a wash. I notice that my packs are all very old and not very good accounting for plenty of resistance. I'm not afraid to plug any of my packs directly into each other. I could live with yours as well , 25 amps at start wouldn't bother me a bit on those packs.
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Old Jun 08, 2010, 07:23 PM
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Originally Posted by 785boats View Post
...

Charged pack....Turnigy 5800mah, 4s, 25c, at 4.18v/cell
Discharged pack.... Turnigy 5800mah, 4s, 25c, at 3.78V/cell
...

Oh. the final voltage of each pack was 3.98V/cell.

...
It's interesting to note that the final stabilized voltage of the two packs is exactly the same as the average of their initial voltages. That seems very efficient.
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Old Jun 08, 2010, 07:42 PM
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Does the current go up as the cell count goes up?

Given all identical cells and all identical SOC Here's two extremes: 1S vs 10S packs.

1000 mah 4.2 plugged into a 1000 mah 3.7 = x current (5 amps at start for example)

1000 mah 42 volt plugged into a 1000 mah 37 volt = x current (5 volts differential instead of .5 volts)
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