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Model Forum / Radio Controlled / Air Models / May 2004Basic battery maintenance questions
OK, I've done some searches about charging batteries and found various info on charging Nicad and NiMh packs. Having read all that theory I'd like to hear some practical information. So now for the questions: 1. Is there a simple (and safe) method for determining the max charge rate? 2. What's the minimum permissible voltage per cell? 3. What's the max permissible voltage per cell? 4. Is there a realistic method for determining safe discharge rates? 5. How do you detect the difference from a "warm" battery to a "hot", is it significant? I'm asking specific questions for a reason but I won't bias my questions by spelling it out. Regards  SignatureThe Raven http://www.80scartoons.co.uk/batfinkquote.mp3 ** President of the ozemail.* and uunet.* NG's ** since August 15th 2000. On 5/14/2004 6:41 AM Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge: > OK, I've done some searches about charging batteries and found various info > on charging Nicad and NiMh packs. Having read all that theory I'd like to > hear some practical information. So now for the questions: > > 1. Is there a simple (and safe) method for determining the max charge rate? A SAFE charge rate is 1C ie you have a 1200 mah battery, the safe charge rate is 1.2 amps or less. > 2. What's the minimum permissible voltage per cell? NiCAD and NiMH is .9 (point 9) volts PER CELL. LiPo is USUALLY 2.8 volts PER CELL > 3. What's the max permissible voltage per cell? NiCAD and NiMH is USUALLY 1.2 - 1.3 volts PER CELL about 1 hour after being removed from the charger. LiPo will USUALLY read about 4.2 PER CELL fresh off the charger and then drop to about 3.7 - 3.9 about 1 hour after being taken off charge. > 4. Is there a realistic method for determining safe discharge rates? DO NOT EXCEED 1C That is a SAFE charge rate. SOME manufacturers say you can safely charge their batteries at a higher rate. IMHO, perhaps you can, however, you are most likely shortening their life. > 5. How do you detect the difference from a "warm" battery to a "hot", is it > significant? It is significant. A warm battery you can touch COMFORTABLY. A hot battery is VERY UNCOMFORTABLE to touch. A hot battery, if it is not damaged, is either being discharged too fast, or being charged at too high a rate. Either can damage a battery and/or shorten its life. In extreme cases, you also have a fire hazard. If you go to http://www.rcbatteryclinic.com/ Red can answer your questions much better than I can. Red is considered by many as "THE BATTERY GURU". > I'm asking specific questions for a reason but I won't bias my questions by > spelling it out. > > Regards Ted, You HAVE to exceed 1C discharge rate. Otherwise, you would only get 3 amps out of that 3000mAh pack and that would hardly get a speed 400 plane off the ground. Also, if 1C was the max, we would be flying a lot of planes for an hour on NiCds! -- Paul McIntosh http://www.rc-bearings.com > On 5/14/2004 6:41 AM Ted shuffled out of his cave and grunted these > great (and sometimes not so great) words of knowledge: [quoted text clipped - 43 lines] > > > > Regards thats 1C **Charge** rate. Not discharge... and i agree. I get almost 10c discharge from my KAN 1050's and they barely get warm. Good cells, IMO. > Ted, > [quoted text clipped - 58 lines] > > > > > > Regards If your dumb enough ( like I was )to put a Ni-cad glow driver in your pant pocket and have it short out on a key you will udertstand the term hot. It felt like someone had poured boiling water in my pocket. It burned a place the size of a quarter on my right leg. If you have not seen the safety alert from AMA on Li-poly batteries and use them, go to there web-site and check it out. Donald | On 5/14/2004 6:41 AM Ted shuffled out of his cave and grunted these | great (and sometimes not so great) words of knowledge: [quoted text clipped - 7 lines] | A SAFE charge rate is 1C ie you have a 1200 mah battery, the safe | charge rate is 1.2 amps or less. Most NiCds will tolerate up to 2 C, and some (like sub-C cells for cars) will do 3 C. But as mentioned, 1 C is a good rate for most NiCd cells. Faster rates WILL wear them out faster. For NiMH, don't go over 1 C, *ever*, unless it's a sub-C cell and explicitly rated for it. 1/2 C is better. | > 2. What's the minimum permissible voltage per cell? | | NiCAD and NiMH is .9 (point 9) volts PER CELL. LiPo is USUALLY 2.8 | volts PER CELL NiCd and NiMH cells don't mind being discharged to zero. The danger is that if you have a battery (made of many cells) and you discharge the entire battery down to zero, that some cells will have gone down to zero before the others, causing the others to `reverse charge' them as they discharge, which is very bad. So for a pack, 0.9 volts/cell is a good guideline. For a single cell (like for a glow driver), go ahead and go down to zero if you want. For LiPo, don't go much below 3 volts, ever. (But you didn't ask about LiPo.) | > 3. What's the max permissible voltage per cell? | | NiCAD and NiMH is USUALLY 1.2 - 1.3 volts PER CELL about 1 hour after | being removed from the charger. LiPo will USUALLY read about 4.2 PER | CELL fresh off the charger and then drop to about 3.7 - 3.9 about 1 hour | after being taken off charge. There is no set max voltage, but do keep the battery cool and don't exceed 2 C for NiCd or 1 C for NiMH. I doubt that any sort of NiCd/NiMH cell (unless it's really old and worn out) would ever require over 1.8 volts/cell to charge at a safe rate (and 1.8 volts/cell is a high estimate -- 1.6 volts/cell seems more reasonable.) Fresh off the charger, NiCd and NiMH cells read around 1.4 volts for a little while. | > 4. Is there a realistic method for determining safe discharge rates? | | DO NOT EXCEED 1C That is a SAFE charge rate. SOME manufacturers say | you can safely charge their batteries at a higher rate. IMHO, perhaps | you can, however, you are most likely shortening their life. He said discharge, not charge. It depends on the battery. Some won't handle over 2 C well, some can handle 30 C. For AA or AAA cells, it's probably on the low end, and for sub-C cells, it's much higher. The best method is to ask the manufacturer, but I imagine you could measure the voltage drop as you draw more and more current and pick a rate where the drop (and therefore the heating of the battery) isn't too high. Easier to just ask, though. Again, the faster you discharge a battery, the faster it'll wear out. | > I'm asking specific questions for a reason but I won't bias my questions by | > spelling it out. You'd probably be better spelling it out ... SignatureDoug McLaren, dougmc@frenzy.com Time flies like an arrow; fruit flies like a banana. >> 5. How do you detect the difference from a "warm" battery to a "hot", is it >> significant? [quoted text clipped - 4 lines] >high a rate. Either can damage a battery and/or shorten its life. In >extreme cases, you also have a fire hazard. 'Hot' battery is also confounded by chemistry. NiCd chemistry is most tolerant of excess heating, NiMh much less so. You can kill a NiMh pack with excess heat. IIRC the lithium chemistries really don't like excess heat _at all_, and you can literally blow those packs up and set your building shed (car, house, whatever) ablaze. The new GP Triton charger has an optional thermal probe, and the book recommends using the probe on NiMh and all lithium types. The manual is available online, and you can read the various limits and recommended settings to get a 'baseline' on the various chemistries : http://www.electrifly.com/gpmm3150.html Cheers, Fred McClellan The House Of Balsa Dust home.mindspring.com/~the-plumber Thanks to everyone who reponded to my questions, the information was very good and helped clear up some issues. I have read most of the higher profile websites on battery care etc and have decided that my safe rate, until I gather more experience, will be to charge my batteries at approx C/2. While 1C seems acceptable to many I'd rather spend a little more time charging and get more life from the batteries. Once I get a spare set or two then it may be different. Minimum discharge voltage seemed to be 0.85-0.9V per cell with max being normally determined by delta peak (which means it could be signifcantly more than rated levels). Discharge rates probably follow the basic website info that indicated that C/5 was the most common factory method of rating. Warm and Hot seems to be the difference between feeling warmth from the battery and the battery feeling too hot to hold. Either way, I have no desire to push things. The main reasons for the questions was my recent acquisition of a Supernova 3000. The 3000 lets you specify every setting yourself with no simple "Tell me what it is and I'll charge it" setting. It does have an automatic setting but how much faith can you put in a fast charging guesstimate? One charge was done on automatic and it seemed a tad aggressive. It might be fine for the serious racers etc but that's not the way I want to treat my batteries. This left me to consider manual settings, hence my questions. Based on your responses, plus the help of other fliers, I've figured out what should be a safe set of parameters. Tested yesterday in less than ideal weather the battery pack (for electric flight) did really well. Unfortunately, on the second flight near disaster occurred when the wind picked up excessively. Handing controls back to my instructor gave him some fun landing experience, particularly when a gust of wind opened the lower hatch (which I'll have to modfiy) and then let the battery plummet out of the aircraft. Luckily, altitude was less than 5ft and the aircraft did an acceptable landing (for one with no controls). SignatureThe Raven http://www.80scartoons.co.uk/batfinkquote.mp3 ** President of the ozemail.* and uunet.* NG's ** since August 15th 2000. |
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