I have found a box with some old batteries for an Hilti cordless drill. These batteries have not been charged for a long time, as well as having sat through cold and warm surroundings in the attic. The batteries are of type 3.0 Ah NiMH.
When plugged in to the charger, they start charging for a bit, but after about 15-30 minutes they stop, and the charger is indicating that they are fully charged. Then I try them with the cordless drill, and after 2 minutes, they are depleted to a point where the drill can't run with even the slightest resistance.
I have no known working batteries to test the charger with, so I don't know whether or not it is the charger being broken. I have no idea how one would test this.
Battery model: SFB 155 - 3.0 Ah NiMH
Charger model: C7/24
Is it possible, or even worth to try and fix?
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  • Enough youtube videos exist...
    – Solar Mike
    Commented Mar 16 at 8:30
  • a short high voltage spike could burn off the whiskers that have formed and making a short circuit, use safety precaution when doing it, battery can explode
    – Traveler
    Commented Mar 16 at 8:35
  • 1
    @Traveler that's only for NiCd cells which are shorted. These aren't as they accept and offer charge, just at low capacity Commented Mar 16 at 9:54
  • I performed the "burning of whiskers" with a car battery about 15 years ago. It worked great but only lasted for a little while. It was not worth it.
    – Evil Elf
    Commented Mar 16 at 11:43
  • Fwiw, hilti makes great tools but you’ll find greater breadth of tool types from other vendors (milwaukee, dewalt, makita) at a negligible step down in durability and a massive step down in price. Plus, new battery and motor tech is light years ahead of old stuff. Commented Mar 16 at 14:24

4 Answers 4


NiCds &/or NiMHs abandoned in the attic for years (or anywhere else for years) are recycling fodder, nothing else.

Price new batteries. Then either buy them, or decide that buying a whole new tool with better battery technology is worth it (likely.) If you really, really want to limp the thing along there are places that will rebuild battery packs for obsolete tools you can't get new battery packs for, but they are also expensive, and you're still left with the terrible limitations of NiCd/NiMH battery technology.

Tl:Dr - To "restore" NiMH battery capacity, trade in the old for recycling, buy new with capacity.

If the nominal voltage happens to be 12V, sometimes you can get use from the tool itself by gutting a battery pack and turning it into a cord you can attach to a 12V battery in a car, boat, tractor etc.

  • 2
    Upvoted, and doubling down in comment: Buy new tools. There was a time you bought NiCd/NiMh tools. Then there was a time you rubbed your tummy and patted your head to give their batteries a second life. Then there was a time Lithium tools were super expensive. But these eras have past. Today? You're dealing with what, 30 year old tools and you're going to do crazy stunts to convert them from a dead old POS to a weak unreliable old POS and meanwhile for like $150 you can buy a nice set of 2 or 3 new tools with a couple of new lithium batteries. It's time.
    – jay613
    Commented Mar 16 at 14:05
  • 1
    Yeah, you are both right. I just needed to be sure there weren't a somewhat easy way to restore the batteries. Thanks for the inputs.
    – 786yt
    Commented Mar 18 at 6:50

Cordless tool battery packs, regardless of brand or chemistry, are not consumer serviceable. They are designed to be recycled at end of life. I'm sure you can find YouTubers doing all sorts of weird things to battery packs, but it's not DIY territory. You're supposed to replace or have a professional repair it.

The only advice I can offer: Sometimes batteries that haven't been used for years require multiple charge cycles to get them up and running again. This will either work or it won't depending on battery condition.

  • I guess, like other said, that it might just be time for some new tools. Thanks for the input.
    – 786yt
    Commented Mar 18 at 6:51

If you look up the charging chemistry for NiNH batteries, it's somewhat scary. There are two reactions going on, one which releases oxygen and one which binds oxygen. They run at different rates, so quick charging actually involves building up internal pressure until the second reaction can pull it down again. You can get yourself in much more trouble with a bad charger for lithium cells -- explosion and metal fire -- but I still wouldn't put NiMH batteries on any charger except one designed for them.

You can certainly try fully discharging (through a suitable load) and fully recharging them and see if a few cycles of that helps.

If you're willing to void your warranty, you could open it up, discharge all the cells individually (again through a suitable load)), and see if that resynchronization improves the performance of the battery as a whole after it is reassembled.

Again, if you're willing to void the warranty, it may be possible to replace the cells... IF you can reliably identify them and order those replacements.

But I'm skeptical about claims that other procedures will "reform" the cells. The usual failure mode is metal dendrites growing into the electrolyte, I believe, and I haven't heard of any way to re-dissolve them. Admittedly I may be badly out of date.

Tool batteries are marketed like razor blades. Manufacturers discount the tools because they know they can make a good profit selling the replacement batteries. Figure that into your purchase decisions when choosing a wireless tool over a wired or air-powered one; you will pay for the convenience in maintenance cost and usually in reduced power. That may be a good trade-off or not; decide per tool and per task.

  • I don't feel like going down the road of doing that kind of repair. It is probably just a better idea to hand them in at a recycling center. Thanks for the input.
    – 786yt
    Commented Mar 18 at 6:53

If you suspect the charger, you can trickle charge cells with an external power supply to get them up to a full state of charge and then see how they perform.

Another possibility with packs is one of them cells is bad and gets empty fast, and the BMS shuts down the whole pack. Measuring cell voltages at that point will tell you the bad cell which you can replace.

For individual cells, a smart charger can do a capacity check by charging and discharging cells and counting how many Wh the cell can store. This is how you tell good cells from bad.

Once a cell is declared bad though there's not much you can do: the capacity is much reduced from spec and that's not fixable. Maybe you can use it in some application that is happy with the low capacity, but don't expect it to be good for a lot of cycles. At the end of the day those cells are really only good for recycling.

  • Downvoted because this answer would void warranty and contradict manufacturer safety instructions. Commented Mar 16 at 11:38
  • @RobertChapin the OP 'found a box with some old batteries'. The batteries are old so it's unlikely they can claim on the warranty. NiMH batteries are hard to catch fire, and none of my suggestions would put them in a position to doing that. Commented Mar 16 at 14:15
  • That kind of anti safety self justification is off topic here. Please consider discussing this in an electrical engineering forum instead of the DIY stack. Commented Mar 16 at 14:37
  • Upvoting to counter the down-votes. They are tatty old batteries that are almost certainly out of warranty, and you would have to try pretty hard to burn the house down with old NiMH batteries that won't even hold a charge properly.
    – Simon B
    Commented Mar 16 at 22:28
  • 1
    I haven't really done anything like replacing battery cells before. It is some old batteries, and I think it might just be time to hand them in at a recycling center, for them to take propper care of them. Thanks.
    – 786yt
    Commented Mar 18 at 6:55

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