DIY NiMH battery packs (R/C makers?)

Discussion in 'After Hours Lounge (Off Topic)' started by Jay H, Feb 12, 2004.

  1. Jay H

    Jay H Producer

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    Has anybody wired up a NiMH battery pack before? I am looking at making a 6V NiMH battery pack around 8AmpHr, I know that wiring batteries in series, will add the voltages, such that 5 1.2V AA NiMH batteries will total a 6V output. I've also learned that running a bunch of AA battery "packs" in parallel will also increase the amphr, however, I think there are more electronics involved though for reliability sakes than simply wiring a bunch of batteries together in parallel. Something about added diodes to ensure the proper flow of power, etc. etc. Just wondering how hard it would be to do, would I be able to get the diodes and what to look for at an electronics store.

    What I propose is to make 4 "packs" of 2100mAH 1.2v AA NiMH batteries wired in series. Then each "pack" will be wired in parallel and then a fuse attached to the end before the output. In this way I should have a 6V complete battery with a ampHr rating of about 8.4ampHrs. I'm wondering how to wire up the diodes that I am recommended to use or whatever else electronics I need.

    I've also been told that overcharging is baaaddd for NiCADs and NiMHs, my battery charger that I already have appears to be a simple walwart thing that takes 20hrs to charge, but it doesn't look like it has any kind of voltage or thermister sensor built in to prevent overcharging, I'm wondering if anybody has ever built one or bought a cheap one. I would need to make the output custom as the battery pack will have a small 1/8" like connector on it so I would have to buy the charger and then wire up my connector to it.

    Jay
     
  2. Brett DiMichele

    Brett DiMichele Producer

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  3. Seth_L

    Seth_L Screenwriter

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    You should only use 4 AA batteries in series to make a 6V pack. Alkalines start out at 1.5V, but as soon as you put a moderate load on them they will sag to about 1.25V. Ni-Cad and Ni-MH start out at ~1.25V and stay there even under a heavy load.

    That said, you can take 16 AAs and wire them into 4 parallel strings of 4 AAs in series and get your desired goal. I wouldn't put any Diodes or anything else in them.

    What do you plan on using this for?
     
  4. Brett DiMichele

    Brett DiMichele Producer

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    Seth is spot on.. Everyone whines about NiCd and NiMh being
    1.2V vs. Alkaline being 1.5V but they neglect to take into
    account the Alkaline sag to 1.2V almost immidiately under
    load where as the NiMh's will retain 1.2V through 89% or
    greater of thier capacity.

    My biggest complaint with NiMh technology is that they DO
    suffer from Memory (contrary to popular misconception) if
    they are charged too fast you get a buildup of crystaline
    on the Annode which will alter how much charge the cell will
    take on and the only way to reverse it is to use a good
    intelligent charger with a pulsed flex negative delta
    charging scheme which can break down the crystaline scale
    and restore proper capacity.

    Also Nikel Metal and Nikel Cadnium technologies both have
    fast bleed off rates at room temperature. It may not seem
    like a problem when you only have a few cells but when you
    have a whole bunch it becomes a royal pain in the ___.

    I bought 24 Powerex 2300Mah Cells for my Digital SLR Camera
    and I use 8 at a time in the battery grip + 4 in the flash
    and I have a complete other set I keep on standby. The
    problem is that unless I freeze the batteries they bleed off
    the charge before I get around to using them. When I use
    them they power the camera great as I can get about 1500
    shots out of a full charge. But if I let the camera sit for
    2 weeks without storing the batteries in the freezer they
    are dead! Storing at cold temperatures slows down the bleed
    by slowing down the chemical reaction inside the cell.

    I am going to purchase a Sealed Lead Acid battery because
    I will only have one battery (with exception to the 4 the
    flash takes) when you charge an SLA it will hold for a very
    long time. SLA's don't vent Hydrogen or Oxygen into the air
    they recycle the fumes thus they are safe and approved for
    airline transport and shipping is easy. They can be used in
    any position as there is no liquid acid, rather they capture
    the acid in Absorbent Glass Matting (AGM) there is nothing
    to spill or leak and the batteries are in tough polycarbonate
    cases that can stand up to a fair amount of abuse. And best
    of all.. They are cheap! Like $9.00 to $20.00 for one 6v
    Battery ranging from 4 to 12 AmpHours.
     
  5. Jay H

    Jay H Producer

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    Thanks folks, I am using this to power a set of Marwi dual beam bike lights that I use every day for commuting (So I don't have to worry about daily loss rates via NiMH). I like the NiMH because it's way lighter than SLA and a little more enviromentally friendly, (no Lead , however there is mercury in the NiMH, I believe).

    I found this smart Delta-V charger:

    http://www.batteryspace.com/product....08&1=337&3=353

    And these battery packs on sale:

    http://www.batteryspace.com/product....17&1=227&3=169

    Seems the perfect fit, I can wire them up in parallel to get 6V and an 8AmpHr charge and then when recharging them separate them and charge them up not in parallel. I've read that charging NiMHs in parallel is a bad idea but running them/discharging them is good, so to be able to disconnect the parallel packs is a going to be ideal.

    My other alternative was to buy a bunch of 1800mAH tabbed AA batteries and make my own holder, but those packs seem like it would be easy. With the 8AmpHr 6V 12W bikelight that I use, I should get around 4 hours of light time which is enough for 5 days or 1 week of bike commuting.

    A brand new Marwi 8AmpHr battery is $99 on sale so these would be cheaper for me to make and more flexible (and a better charger to boot, unlike the standard walwart charger that I got from marwi).

    Jay
     
  6. Brett DiMichele

    Brett DiMichele Producer

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    I still say SLA is the way to go.. High Capacity NiMh cells
    are heavy trust me.. 8 Powerex cells weigh more than 12
    regular AA's and an SLA would wind up weighing in about
    the same and lead batteries are easier to dispose of than
    NiCad.. Nikel Metal can be pitched anywhere (no mercury)
    but NiCad contains Cadnium which is a very toxic heav metal
    and far worse than lead.

    But whatever route you go I am sure it will work just fine.
     
  7. ChuckSolo

    ChuckSolo Screenwriter

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    Two great place to get info on building your own battery packs are:

    www.rccaraction.com
    www.airsoftplayers.com

    Both these sites are for hobbyists that use battery packs for their equipment, i.e., RC cars/trucks and Automatic Electic Airsoft guns.
     
  8. Jay H

    Jay H Producer

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    So, I took my old Marwi 8aH 6v battery apart and I can now say with authority that the cells in there are toast. They're spilling their guts out and are definitely toast. It appears to by 2 packs of 5 1.2v cells in parallel for a total of 10 batteries. The cells themselves are green with no writing on them, however what I found interesting is that there is a small silver thingajig wired in series, one on each pack which I can't tell what it is, a fuse or a diode? It's a really small silver rectangular box that has the line "PEPI N" on one flat side and on one of the edges says "J020+120" whatever that means... I would think that J is usually a schematic for some kind of jumper and the 020 would be an identifier but don't think that's right. Anybody have a clue what it is?

    What do you also know about charging NiMH batteries connected in parallel, I'm told it's a bad idea, however the charger provided can only charge them in parallel. Which is why I really want to build my own, something that I can build that can be charged as an individual pack and then connected in parallel for use. But what are those little silver things? What do they do and I'm wondering whether I should recreate that... Anybody know?

    Jay
     

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