On Sun, 26 Oct 2014 10:27:50 -0700, Kevin McMurtrie <mcmurtrie@pixelmemory.us> wrote:>In article <af4q4ahnkem0ojcv7ot8o3k5pm0ecstg1f@4ax.com>, > Jeff Liebermann <jeffl@cruzio.com> wrote: > >> On Sun, 26 Oct 2014 11:08:32 +0100, Robert Roland <fake@ddress.no> >> wrote: >> >> >On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann <jeffl@cruzio.com> >> >wrote: >> > >> >>One accidental overcharge and your >> >>battery is dead. >> > >> >Not true. >> >> I beg to differ. >> >> >Early NiMH batteries were quite intolerant of overcharging, but modern >> >batteries are much more robust. Not quite as robust as NiCd, though. >> > >> >In fact, some amount of overcharge is necessary in order to keep the >> >cells in a battery balanced. >> >> Let's see if I understand what you're saying. In a series string of >> NiMH cells, one must overcharge the good cells in order to bring the >> not-so-good cells up to the good cell voltage? Is that what you're >> saying? If so, this sounds like a variation on "battery conditioning" >> which works well at killing batteries.>Series cells (and capacitors) naturally unbalance due to slight >variations in self-discharge. There's eventually a risk of some cells >being reverse-charged while others still have enough voltage to make the >pack seem usable. That's immediately the end of the battery pack's >life, maybe in a violent way. > >Balancing overcharge is very mild; just enough to compensate for varying >self-discharge. NiMh, NiCd, and lead-acid have no problem at all with >this. It ages lithium-ion batteries more rapidly so some packs have a >shunt voltage regulator on each cell.For lead acid batteries, it's called an "equalizing" charge: <http://batteryuniversity.com/learn/article/equalizing_charge> For NiMH, the common method is to use a "discharge tray" which discharges all the cells to a specific setpoint, so that series charging will bring them back to the same level. This covers the methodology: <http://www.rctech.net/forum/rookie-zone/313578-balance-nimh-equalize.html> There are NiMH "equalizers" but if you read the instructions, they do the same thing as a "discharge tray" and do NOT overcharge the battery. I've not heard of equalizing NiCd batteries, although they should work and survive better than NiMH if overcharged. One of the responsibilities I've assumes is keeping this collection of lead-acid batteries alive since about 1993: <http://members.cruzio.com/~jeffl/k6bj/K6BJ%20Repeater/slides/Batteries.html> The trick is to threaten to soundly thrash anyone that even suggests punching the "equalize" button on the charger: <http://members.cruzio.com/~jeffl/k6bj/K6BJ%20Repeater/slides/40A%20battery%20charger.html> I use a method recommended by a former employee of C&T Tech, manufacturers of the batteries. Instead of overcharging cells that are lagging, I discharge the other cells down to the level of the lowest cell, and then charge them back to 100% normally. It's a somewhat tedious procedure, but it works well. I've only needed to do it twice in 20 years. I also have the EoC set to between 90 and 95% of full charge, in order to reduce outgassing and sulfation. We add water about once every 5 years. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
NiMh charging formula
Started by ●October 25, 2014
Reply by ●October 28, 20142014-10-28
Reply by ●October 29, 20142014-10-29
"Jeff Liebermann" <jeffl@cruzio.com> wrote in message news:6ph05ah1q36d48106ciccsimpu1ti0hnj0@4ax.com...> On Sun, 26 Oct 2014 10:27:50 -0700, Kevin McMurtrie > <mcmurtrie@pixelmemory.us> wrote: > >>In article <af4q4ahnkem0ojcv7ot8o3k5pm0ecstg1f@4ax.com>, >> Jeff Liebermann <jeffl@cruzio.com> wrote: >> >>> On Sun, 26 Oct 2014 11:08:32 +0100, Robert Roland <fake@ddress.no> >>> wrote: >>> >>> >On Sat, 25 Oct 2014 20:45:38 -0700, Jeff Liebermann <jeffl@cruzio.com> >>> >wrote: >>> > >>> >>One accidental overcharge and your >>> >>battery is dead. >>> > >>> >Not true. >>> >>> I beg to differ. >>> >>> >Early NiMH batteries were quite intolerant of overcharging, but modern >>> >batteries are much more robust. Not quite as robust as NiCd, though. >>> > >>> >In fact, some amount of overcharge is necessary in order to keep the >>> >cells in a battery balanced. >>> >>> Let's see if I understand what you're saying. In a series string of >>> NiMH cells, one must overcharge the good cells in order to bring the >>> not-so-good cells up to the good cell voltage? Is that what you're >>> saying? If so, this sounds like a variation on "battery conditioning" >>> which works well at killing batteries. > >>Series cells (and capacitors) naturally unbalance due to slight >>variations in self-discharge. There's eventually a risk of some cells >>being reverse-charged while others still have enough voltage to make the >>pack seem usable. That's immediately the end of the battery pack's >>life, maybe in a violent way. >> >>Balancing overcharge is very mild; just enough to compensate for varying >>self-discharge. NiMh, NiCd, and lead-acid have no problem at all with >>this. It ages lithium-ion batteries more rapidly so some packs have a >>shunt voltage regulator on each cell. > > For lead acid batteries, it's called an "equalizing" charge: > <http://batteryuniversity.com/learn/article/equalizing_charge> > > For NiMH, the common method is to use a "discharge tray" which > discharges all the cells to a specific setpoint, so that series > charging will bring them back to the same level. This covers the > methodology: > <http://www.rctech.net/forum/rookie-zone/313578-balance-nimh-equalize.html> > There are NiMH "equalizers" but if you read the instructions, they do > the same thing as a "discharge tray" and do NOT overcharge the > battery. > > I've not heard of equalizing NiCd batteries, although they should work > and survive better than NiMH if overcharged. > > One of the responsibilities I've assumes is keeping this collection of > lead-acid batteries alive since about 1993: > <http://members.cruzio.com/~jeffl/k6bj/K6BJ%20Repeater/slides/Batteries.html> > The trick is to threaten to soundly thrash anyone that even suggests > punching the "equalize" button on the charger: > <http://members.cruzio.com/~jeffl/k6bj/K6BJ%20Repeater/slides/40A%20battery%20charger.html> > I use a method recommended by a former employee of C&T Tech, > manufacturers of the batteries. Instead of overcharging cells that > are lagging, I discharge the other cells down to the level of the > lowest cell, and then charge them back to 100% normally. It's a > somewhat tedious procedure, but it works well. I've only needed to do > it twice in 20 years. I also have the EoC set to between 90 and 95% > of full charge, in order to reduce outgassing and sulfation. We add > water about once every 5 years. > > > > > -- > Jeff Liebermann jeffl@cruzio.com > 150 Felker St #D http://www.LearnByDestroying.com > Santa Cruz CA 95060 http://802.11junk.com > Skype: JeffLiebermann AE6KS 831-336-2558One of the main reasons for doing an equalize charge is to stir up the electrolyte in those cells. The overcharge produces bubbles of H2 and O2 that cause stirring of the acid. The specific gravity will stratify over time. You run the risk of sulfation if you do not do this. Now where you are located, the earthquakes just may take care of it. I ran a set of 4 volt, 550 AH batteries in a 28 volt plant that is over 35 years old and still going strong. We did an equalize charge every 6 months and topped off the cells with distilled water.
Reply by ●October 30, 20142014-10-30
On Tue, 28 Oct 2014 23:12:41 -0400, "Tom Miller" <tmiller11147@verizon.net> wrote:>One of the main reasons for doing an equalize charge is to stir up the >electrolyte in those cells. The overcharge produces bubbles of H2 and O2 >that cause stirring of the acid. The specific gravity will stratify over >time. You run the risk of sulfation if you do not do this.Good point. It was my understanding that stratification was only a problem if the battery was perpetually undercharged. I don't know for sure if that's true. <http://batteryuniversity.com/learn/article/water_loss_acid_stratification_and_surface_charge> We were warned about the problem but decided that anything that would produce bubbles was good enough to stir the electrolyte. At least once per year, we run a test to see how long the repeater will run on battery power. Typical is 2 to 7 days depending on usage. The failures are never what was expected with all manner of devices not working quite well at low battery voltages. I usually stop the test at about 70% of full charge to minimize damage to the batteries. When recharged at 40 amps, there are plenty of bubbles to stir the mix. <http://members.cruzio.com/~jeffl/k6bj/K6BJ%20Repeater/slides/Batteries.html> Note the white caps on the corners of each cell. Those are electrolyte sampling tubes that are used to get a hydrometer sample from the bottom of the cell. This mess was before someone else cleaned it up and replaced some of the corroded hardware: <http://802.11junk.com/jeffl/crud/k6bj-batteries-02.jpg> C&D KCT-720 cell: <http://www.cdtechno.com/product/vla/kct_kt.html> The manual: <http://www.cdtechno.com/pdf/ref/rs_1476_0610.pdf> on Pg 17 recommends an 8-24 hr equalizing charge every 3 months. No clue where they got that idea, but nobody does it that often.>Now where you are located, the earthquakes just may take care of it.Last quake was 1989, so that's not a reliable method. The building is built on a cold war era gun platform, so it's not going anywhere in a quake.>I ran a set of 4 volt, 550 AH batteries in a 28 volt plant that is over 35 >years old and still going strong. We did an equalize charge every 6 months >and topped off the cells with distilled water.Nice. The batteries in the photo came out of a local telephone central office and were retired after about 15 years of use making the batteries now about 40 years old. On paper, they were dead on arrival 25 years ago. I've kept them alive mostly by preventing others from tinkering and arranging maintenance. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
