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NiMh charging formula

For lead acid batteries, it's called an "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: 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: The trick is to threaten to soundly thrash anyone that even suggests punching the "equalize" button on the charger: 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.

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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
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Jeff Liebermann
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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.

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 to
Tom Miller

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. 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.

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:

C&D KCT-720 cell:

The manual: 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.

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.

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
Reply to
Jeff Liebermann

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