Rebuilding NiCd Battery Packs, and Memory



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This is being written to shed some light on two very common misconceptions about NiCd battery packs.

The first is the misconception of "memory." I donít know how many times Iíve read or been told that NiCds can develop a memory if Iím not careful to recharge them at different points in their discharge cycle. Let me point out that the manufacturers tried for a long time to duplicate the problem seen on one or two satellites when they were told of the problem, as observed by NASA. When they managed to do it, they found that it took computer accuracy in timing the discharge/recharge cycles and even then it was difficult to duplicate. Bottom line? Donít worry about it. Use the radio and recharge it when you will.

The second misconception is that you can rebuild a NiCd battery pack with off-the-shelf cells and get as good results as buying a pack from a distributor. Sounds good, saves money, is fun to do, and think of the operating fun youíll have telling everyone you rebuilt the pack "from scratch!" Why is this a misconception? Itís because those battery packs you buy from an insert provider are pre-selected so that the discharge and recharge characteristics of each cell are within a given tolerance.

I know of three places where NiCd battery backs have been used and are still being used. You may also know that there are now farad capacitors and that these are making headway in replacing NiCds in some places. They are light weight but have a "short" shelf life, not good for stand-by radios, but electric hobby cars/planes might be a good use for them. The three areas I will use for my discussion are these: radio (ham, business, police, etc), hobby radio-controlled electric cars and planes, and satellites. A typical pack for an HT costs about $40; for a hobby car/plane, about $100+; for a satellite, in the $1000s. If they are the same beast, why the difference in cost? If I can build the same thing with off-the-shelf cells again, why the difference in cost?

The reason is PRE-SELECTION. What is pre-selection and why does it cost so much? Pre-selection is when the battery company pre-selects cells of similar charge/discharge characteristics for inclusion in a battery pack. For amateur radios, the tolerances are for packs that will last for a few hundred discharge/recharge cycles, for the hobby car/plane people this would not be worth it since they commonly charge their units 10 to 20 times a day. Having to buy a new pack every 10 to 20 days of use would quickly kill the industry! A satellite discharges and recharges once every 24 hours every day itís in orbit. If it only lasted 200 to 400 cycles, it would be dead in about a year. Doesnít sound cost effective to me! Satellites need about five to ten years of minimum life from the battery packs, or from 2000 to 4000 cycles. If a radio-controlled electric car/plane hobbyist goes out 25 weekends a year and wants at least two years of enjoyment, weíre talking about 1000 to 2000 discharge/recharge cycles.

Bottom line, we pay for what we get. If we pay $40, we get about 200 recharge cycles for our radios. If we pay $24 or less for the cells off the shelf, we get no guaranty of cell characteristics matching, and in all probability weíll get far fewer recharge cycles out of the pack than we had hoped.

So what is it that kills a battery pack? One problem is something called reverse charge. This is when a weaker cell has depleted its charge and the stronger cells still have some charge left. As you continue to use the device, the stronger cells reverse charge the weaker cell(s). After awhile, the weaker cell can never be properly recharged and youíll notice that the pack is down from 7.2 volts to about 6 volts, if only one cell goes bad.

Another method of death for a battery pack is called over-charge. Letís say that the battery pack has a built-in shutdown when the voltage drops too low. This is designed to protect the weaker cell(s). If this continues to happen, the time to shut down will get shorter and shorter as the weaker cell(s) never gets a full charge, or the strong cells get over-charged and they start venting. Venting is when the cell gets too hot and pressure builds up to the point that it vents the hot gasses, thus losing some electrolyte and capacity. Last, if the pack has under-voltage and over-charge temperature protection, we get to the point that the weak cell(s) is never getting fully charged no matter how long we try. It is protecting the weak cell by shutting down before it gets reverse-charged, and the temperature protection protects us from over-charging the stronger cells. Thus the weaker cell(s) will never get fully charged and we will think we have a MEMORY condition! (So hereís one place where people are sure their battery pack has a memory. Another is when the cells vent and lose capacity. This also looks like the memory problem because the cells say theyíre fully charged but they donít last as long as they should.)

What to do? I have a suggestion that has worked for me on some battery packs I've been using since 1992. This method will work great for when the weaker cell is not being fully charged. I have two battery packs I can open. Every year I do so to recharge each cell individually once or twice. So far this has kept the weaker cells from being reversed charged by the stronger cells and kept the pack alive now since 1992 and counting.

The manufacturers seem to be going more and more to the sealed battery packs. I have two packs for a newer Yaesu HT. These are of the newer sealed design. They cannot be opened and the smaller pack died after only three years. If I could have gotten into the pack, I could have kept it alive much longer. Another example: My friend Joe/WA3OHI has an older Kenwood 2-meter HT. It has one of the best battery pack designs Iíve ever seen. Push the two voltmeter probes down two holes and the battery pack pops open. One Sunday in July of 2000 I did this to see what was going on. There are seven cells in this pack, PB-26 @ 8.4V, and they are tied together in three sets of two, plus the leftover cell. One set of two was measuring 0.00V, another set was at 1.11V, the third set was at 1.25V, and the last lone cell was at 1.22V. I charged these cells in their sets and brought the pack back to its nominal voltage, but after 24 hours, the pack was down by over 1.25 volts. It turned out that one cell in the 1.11 volt pair was down to 0.00 volts. I removed enough of the shrink-wrap so I could recharge the 0.00 volt cell. After 8 hours that cell was still over 1.30 volts. It looks like we have saved this pack for a few more years but weíll have to check it from time to time just to be sure. This pack is only about seven years old and we let it get a bit too far this time, but so far it seems that the pack is fine. We were lucky the weakest cells were not at negative voltages, which would mean weíd definitely have to buy a new battery insert.

Bottom line, if you can get a pack you can open, or one of the empty cell packs for alkaline batteries, get them! With an empty pack you can buy off the shelf cells, put them together in a pack, recharge them individually, and get far more recharge cycles out of them than from a standard sealed battery pack. We can estimate that if satellite NiCd packs can last for 2000 to 4000 cycles, we should be able to get even more than that if we charge each cell individually.

The downside of using off-the-shelf cells is that youíll probably need to charge them every time, individually, for best results. I strongly recommend buying a battery pack insert, but if you are determined to try making one from individual cells then I recommend checking a rebuilt pack after every recharge cycle to see how the weaker cells are doing. See how long it takes for the weakest to be only minimally charged before the charger cuts out. There will be weaker cells just as there are weaker links in a chain. The object here is to catch them at the point of being not fully charged, and charge them individually before they get reverse-charged during normal operation. In this way you will be able to extend the life of the battery pack.

Good luck, and I hope Iíve dispelled some of the myths surrounding NiCd battery packs and that you understand how to keep that rebuilt or new battery pack alive longer than you would otherwise.

73 de KE3FL
Philip Karras
Assistant Emergency Coordinator
Carroll County MD
ORS, OES, and VE
ARRL Life Member,
FISTS, ARCI, NWQRP, ARS
ke3fl@arrl.net


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