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I have two 10 cell NimH stacks I bought about three years ago that I've left laying around on the bench since first charging them. Just charged them again and one has reached 14V and the other only 12.5V. Is the lower voltage one shot?
You could try cycling them both. NiMhs do lose charge, as Havelock, over time but a few discharge / charge cycles can sometimes restore them. I usually charge and discharge cells I have not used for some time before recharging prior to a sailing session.
Hi Alex Yes sounds like one cell has gone. You can probably feel the heat from the faulty cell after charging and discharging.
If you are competent you could split the pack and identify the failed cell. A meter across each cell whilst running a small motor will show up any low voltage cells. Chances are the rest will be ok and will take and deliver a full charge. If so and you can use a soldering iron and can purchase replacement cells of the same make and capacity you could make the pack OK.
I'd even consider rescuing all the good cells and making a lower voltage battery, 5 amp batteries being the price they are.
It would be great to salvage something from it so I would be happy to get the soldering iron out. The pack is the stick type with all the cells end to end, do you know how these would be connected together?
Hi Alex They will be connected in series with the positive connected to the negative. I suggest you carefully cut off the heat shrink covering, taking care not to cut into the cells. You can then check each cells voltage. The nominal is 1.2v rising to 1.4v when charged. Any around 1v or less will never hold a charge so mark them with a marker pen. Hopefully it will be one of the end cells that has failed. On your type of pack the bottom of the battery (negative) will be in a metal case attached to the top (positive) of the next cell. I use a flat blade screwdriver to separate the cells (they are spot welded) and sometimes you can get the case off the bottom of the dud cell, leaving it attached to the positive of the next cell. I suggest you then charge the remaining good cells to see if they all take a full charge. If they are OK you can get a new cell or just make a lower voltage pack. I use a piece of stranded wire to repair the joint. You will need a 40+watt iron and some solder paste, and may need to scratch the battery case and pin for the solder to take. If you were careful with removing the heat shrink you can use it to cover the pack with a bit of electrical tape to make good.
Please ask if you need any guidance with the process. I have been doing this for many years and may not have explained in enough detail if it's new to you.
Do remember if the battery is charged it can short in its unwrapped state so do make sure you bench is uncluttered and kept clear of any metal objects.