Glasshopper Posted July 5, 2009 Share Posted July 5, 2009 Can any one suggest a circuit diagram to discharge li-po batteries safely at up to ,say, 3amps and a minimum safe voltage of 3v. per.cell. I use batteries from 500ma 2cell to 3300ma 3 & 4 cell. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 6, 2009 Share Posted July 6, 2009 Buy a decent charger/cycler....???? Quote Link to comment Share on other sites More sharing options...
Bert Posted July 6, 2009 Share Posted July 6, 2009 Glasshopper Why do you want to discharge your Lipos? If it's for storage I don't bother, it doesn't seem to matter in warmer weather. If you want to cycle them - don't. Lipos don't need cycling - it only shortens their useful life. Bert Quote Link to comment Share on other sites More sharing options...
Glasshopper Posted July 6, 2009 Author Share Posted July 6, 2009 Thanks steve what would you recommend? Quote Link to comment Share on other sites More sharing options...
Glasshopper Posted July 6, 2009 Author Share Posted July 6, 2009 Bert ..........why do lipo chargers have discharge functions if its not necessary? Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 6, 2009 Share Posted July 6, 2009 Too many chargers around to recommend a specific one....it really depends on what you intend to use it for. All I would say is to build in some future proofing...you may only need 3S 3300mah cells at the moment but larger cells might be just around the corner!!! Quote Link to comment Share on other sites More sharing options...
Glasshopper Posted July 6, 2009 Author Share Posted July 6, 2009 Steve, tks: obviously one of the uses would be to discharge my lipo's.What do you use and how long does it take you to discharge a 3300 ma. li-po ? Bert thinks that discharging batteries is a waste of time. Is it? Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted July 6, 2009 Share Posted July 6, 2009 I have never bothered to discharge mine even though I have a discharge function on my charger. Waste of time? It is as far as I am concerned. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 6, 2009 Share Posted July 6, 2009 Agreed. Best way to discharge them is to fit and fly! Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted July 6, 2009 Share Posted July 6, 2009 Incidentally, even if the charger has discharge function, this is usually severely limited...often to just 20 Watts or so. Therefore discharging a 4 cell pack would be limited to around 1A maximum at start so your 3.3A packs could take 3 hours. Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted July 6, 2009 Share Posted July 6, 2009 hello glasshopper-bet you wished you had said nowt.......... ken anderson............. Quote Link to comment Share on other sites More sharing options...
Glasshopper Posted July 7, 2009 Author Share Posted July 7, 2009 Ken..... no........ always ready to sit at the feet of master and learn Timbo....... you have hit the nail on the head ! That's why I am looking for a decent circuit. I have an I max with a discharge function which is inadequate. The "E" in RCM &E used to mean something not any more! . Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 7, 2009 Share Posted July 7, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Bert Posted July 7, 2009 Share Posted July 7, 2009 Glasshopper I still can't see why you want to do it but if you do, there are some circuits HERE that will do the job. Bert Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted July 7, 2009 Share Posted July 7, 2009 glasshopper-watch you do'nt go discharging them to much or below the cut off point or it will be ........ ken anderson........ Quote Link to comment Share on other sites More sharing options...
John Cole Posted July 7, 2009 Share Posted July 7, 2009 A number of sources recommend partial discharging for long-term storage, and cool storage. Most recommend 3.75 volts, which is about 40% charged / 60% discharged. That's the voltage all my new ones come charges at, and the supplier confirms this is for maximum life. LiPos degrade slowly if full charged and at room temperature. Here's something I got off the web, from US Army tests. Quote Link to comment Share on other sites More sharing options...
Glasshopper Posted July 7, 2009 Author Share Posted July 7, 2009 Phil wood thanks I would need to ensure that the voltage in each cell does not fall below 3v. I have been wondering if an esc or a voltage regulator could be adapted. Some of the newer esc's from HK have individual cell protection as well as the overall limiter Bert....... thanks...... very intellesing...... I am thinking about it Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 7, 2009 Share Posted July 7, 2009 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
John Cole Posted July 7, 2009 Share Posted July 7, 2009 Glasshopper: you still haven't said WHY you are trying to discharge your LiPos. If it's for storage you need only go down to about 3.7 volts / cell, and there's no advantage (and much danger) in taking them down to 3.0 v. If you're trying to do that and exact balance is not needed (because you'll balance them when you recharge) then you need only discharge them through the main output lead. For this all you need is a kitchen timer, a resistive load such as a 12 v bulb (or several in parallel) on your usual power connector, and a digital voltmeter (preferably with a connector that will plug single-cell into your balance lead). Then measure the voltage per cell for each cell, select the lowest, select how much you want to come down, calculate what mAH discharge that means using a volts / capacity chart, set the timer accordingly and plug in. Then disconnect when the timer rings. That's all there is to it. For a connector to a JST-XH free socket (as is found on many LiPos now) take what's called a JST (2-pin) connector - strictly a JST-EHY connector - as used for the battery power connection on very small models. Cut a few mm off the plug shroud and this will give you a 2-pin connector with exposed pins at 2.5 mm spacing. This will fit into contact pairs in an XH socket (and some others) as well as Futaba Rx battery connectors. For a LiPo volts / capacity chart, look at my pictures; you'll see one there with the spreadsheet table used to graph it. This is based on third-party data and I use it all the time for checking rsidual capacity / state of charge. Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted July 7, 2009 Share Posted July 7, 2009 The only useful reason I can think of for discharging lipos is for storage, and even that only needs to take 15-20% out of the cells to get them down to below 3.9v. The most effective way to do this is simply to plug them into the model they're normally used in, wander out into the back gardern and run the motor for a minutes or two. You could even do this indoors, but the revolving prop indoors is a tad risky and the draught can blow things around... PDR Quote Link to comment Share on other sites More sharing options...
John Cole Posted July 8, 2009 Share Posted July 8, 2009 Pete: the sources I found suggested 40% charge / 60% discharge for optimal long-term cool storage - so at around 3.75 V per cell, see my earlier posting. And as I said: that's how they come from the manufacturers, and they should know. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 8, 2009 Share Posted July 8, 2009 Glasshopper I think its all been said above.....I'm no expert but I don't see the need for discharging /cycling your Lipos either. Cycling became popular with Nicd/NiMh cells because of the so called "memory effect" (which does or does not exist according to who you listen to!!!). This caused cells which only used, say, a third or a half of their normal capacity when in use to retain the memory of that discharge curve & when asked to deliver more capacity couldn't supply& efectively became "flat" The occasional cycle from fully charged to fully discharged & back to fully charged got rid of the memory effect & also allowed you to see what sort of capacity the cells retained. Lipos have no such issues hence the need for cycling/discharging of the cells simply disappears!! Quote Link to comment Share on other sites More sharing options...
Glasshopper Posted July 10, 2009 Author Share Posted July 10, 2009 Thanks for all your contributions ! I John Cole gave one of the reasons for discharging ; to bring a fully charged battery back to a fit state for storage among others. It was interesting to see the reaction and interest of subscribers to the other web forum, linked by Bert, where the same basic question was posed . But then the solution was elegant. Quote Link to comment Share on other sites More sharing options...
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