Question On DIY Battery Charger

The tendency of NiCd battery packs to die over a short time annoys me. Some cells die sooner than others creating a second problem. IMO they cost more than they're worth and they should carry a "USE BY" label. I'm going to find a way around it. I have even thought about hacking something together using a 12V SLA and a buck boost converter.
My question is 'can you parallel charge a set of cells at their rated voltage and amperage without overcharging them?'


  • Might be worth giving this site a try.
  • There is some good info in this document.

    Like you I don't seem to get the value out of rechargeables like I would hope.
  • MacTuxlin and MikeDYur, Wow. Great info. Thank you both.
  • ercoerco Posts: 19,736
    Roomba NiMH batteries were expensive and notorious for going bad due to a single cell failure. With series 12 cells, you're sure to find the weak link. Frustrating! Ebay is rife with replacement kits for DIYers: I bought several of those, they work for a while until another cell fails.

    Fortunately they have switched to Li-Ion cells in the new models.
  • Update: After studying the links and watching several YouTube videos I replaced all the cells from my dead battery pack with cells from a battery pack that was unused. (I bought a cheap cordless drill some years ago for the motor.)
    I cut the spot welded tabs down the middle on the bottom side of the new pack which gave me seven pairs and a single cell. Then I matched the tab and polarity pattern of the bottom side of my dead pack and placed those pairs 'tab side up' into the enclosure to keep the proper shape. Next I dumped them out onto my work surface and held them together with a rubber band. I soldered hookup wire to the tabs on the other side to match the tab and polarity pattern on the opposite side of my old pack.
    It worked.
    I also found out they are called SC cells or "sub C" and I can buy them with tabs. Tabs will keep me from overheating the cells with solder. Thanks again!
  • A few years ago I figured out sub-C cells were easily available on the internet, and replaced the batteries in an old cordless drill, and a Dustbuster. These things are built like tanks, no point in tossing them when all they need is fresh batteries!
  • lardomlardom Posts: 1,583
    edited 2017-09-06 - 17:53:16
    Jeff, you're absolutely right. In addition to an 18v cordless hammer drill that I paid a lot of money for I also have a 15.6v cordless vac which I'm not about to throw away. :smile:
  • Just be careful when soldering those tabs! You don't want to overheat the battery. That's why they're spot-welded at the factory, since spot-welding is too fast an operation for much heat to spread.

  • Phil, I'll be careful. I'd love to have a spot welder but I can't justify the expense. I think balance charging should be standard practice. My NiCd packs are series discharged and series charged. I now see that as a flaw that accelerates problems between cells.
    I go through a lot of batteries as a hobbyist. There are some projects I'd like to scale up so I want to look into building custom Li Po packs
  • Buck RogersBuck Rogers Posts: 1,815
    edited 2017-09-10 - 06:01:30
    Oh wow good news. In fact all of my projects are powered by the famous 18650 batteries. (Yes the ones from the RS issues.)

    Since that out-of-business firm forgot about battery holders I bought mine from Tinkersphere here, who also sells the same size.

    However................ erco were you aware that one of yours has been visiting several developers?
  • Larry, I had some more documents that may help:

  • MikeDYur, I copied the docs to a folder on my desktop. Thanks
  • lardom wrote: »
    MikeDYur, I copied the docs to a folder on my desktop. Thanks

    Sure no problem, may the source be with you.
  • MikeDYur wrote: »
    Sure no problem, may the source be with you.
    Actually, the source is probably with erco. Most of us on this forum know that erco is good at robotics and finding good 'sources'! :smile:
  • Hi

    My Experiience-
    In the distant past I used NiCads all the time for RC gliders, and over many years don't remember ever having a failure. The batteries, AA size 500mah were all soldered as a block using either thick multi-strand wire or little slips of tin plate cut from food tins.
    They were left on continuos trickle charge at 50ma and only taken off charge when used to fly. To counteract the memory effect they were famous for I discharged the pack - slowly to 1 volt per cell- I made a little 2 transistor circuit that stopped discharging when at 4 volts and then switches to trickle charge.
    I could fly all day (gliders -slope soarers- don't come down until you make them- (usually)) on one set of 500mah batteries (two in series in the transmitter), so obviously the discharge rate/current was quite low and may be why they lasted so long.
    With model racing sailing yachts some people had superpowerful sail winches which could discharge the cells much quicker, so a fast recharger was built based on the negative delta effect to halt charging- it required a processor to monitor the small voltage change from peak voltage. Well before the prop I'm afraid- I used the Z8. As far as I know the NiCads took this treatment without complaint.
    For electric cars and electric flight when the batteries were discharged in very much shorter times involving huge discharge currents the situation may be quite different.

    What are the discharge rates you are using?
    And- Nicads? I thought they were phased out in favour of the NiMhd and Lipo technologies.

  • tritonium, I didn't know much about batteries at all until the problems I was having with my cordless drill and cordless vac bothered me enough to ask a question on this forum. I disassembled my cordless vac today and I just plugged it into the charger. I will check the voltage of each cell first. I think the next thing I could do is discharge the good cells through a resistor until they are at the same level. Then I'll recharge them a second time. Does that make sense?
  • Hi

    Sounds good- discharge slowly- you need to know the amp/hour capacity and then discharge at say a rate that would take 5 hours if fully charged. NiCads will allow very fast discharge rates normally but for now go easy on them.
    Do not over-discharge.
    You could even charge them at a 10 hour rate (ie a/h divided by ten) for twenty hours. NiCads will take overcharging as long as the rate is low, and then you will know they are fully charged.
    Then you can do a test discharge and find the actual capacity.
    Repeating this might bring the capacity up.
    Ideally you would do this individually per cell and if they are reasonably matched then make a multi-cell pack of them.


  • tritonium, thanks for sharing your experience. I plan to leave my 15.6v vac disassembled for at least a week so I can do some 'learning' experiments. (Using cut strips of food cans sounds pretty clever!)
  • Hi

    Have fun :-D
    There's loads of info on the web...

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