question about modifying SLA charger

Discussion in 'The Projects Forum' started by Thuktun, May 21, 2015.

  1. Thuktun

    Thread Starter New Member

    Apr 2, 2015
    15
    0
    Hello everyone and thanks in advance. Forgive my ignorance on anything I may say, I'm new to this. I've built a few guitar pedals and simple projects but I'm not educated in anyway about electronics except for the little bits I soak up online.

    I put together an ebike and made the battery pack from 3 12v 10Ah SLAs. I have them wired in series for riding but to charge them I disconnect everything and reconnect them in parallel and charge them from a 12v 2A charger. This is a pain in the butt, I'd much rather charge them in series. I can get a 36v charger on ebay for under $20, less then I could build one for, but they all seem to be 2A output which would be too high to charge my 10Ah batteries.

    My question is, is lowering the output to 1A as simple as putting a 36 ohm resistor between the charger and the 36v pack? Am I right in thinking this resistor needs to be rated for 36w? I was thinking 3 100 ohm 15w ceramic resistors in parallel would get me close enough and more then cover the 36w. Am I right in expecting a voltage drop of 1.2v? Would this be too much? Do you have any better ideas?
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,036
    1,662
    Why do you think 2 amps too much for 10 Ah batteries that obviously handle driving an electric motor that likely put loads of 30 - 40 amps on them? o_O

    I typically charge 50 - 100 Ah LA batteries with a 200 amp continuous capacity charger and quite often use it on my lawn mower batteries that are maybe 15 Ah at best.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    SLAs have the maximum allowed charging current specified on their data sheet. Look it up on the web. I'm guessing that 10Ah SLA will allow a maximum charging current of at least 4A. The charger should limit the charging current during the initial part of the charging cycle to no more than the batteries allow.

    The more important thing is what does the charger do to control voltage during charging.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    2A should be fine. What you really need is an automatic mechanism to drop the voltage to a float charge once a certain voltage is reached, followed by a complete shut off at a certain stage.

    This is what SLABs looks like when they are overcharged:

    [​IMG]
     
  5. Thuktun

    Thread Starter New Member

    Apr 2, 2015
    15
    0
    The 12v charger I have now has all the auto shutdown features I need. What I've decided to do is get 2 male and 1 female 6 plug trailer connectors. Wire the 6 points on the female to each of the battery contacts and 1 male on the bike wired so it's series and the other on the charger wired so it's parallel. Seems the cheapest and easiest solution and avoids cheap eBay chargers. Thanks for everyone's input.
     
  6. Thuktun

    Thread Starter New Member

    Apr 2, 2015
    15
    0
    I had another idea. If I do the 6 prong plug thing I'm going to get arcing on the contacts every time I plug in the bike, there is a big rush of current charging the capacitors which I avoid now by clipping a resistor across the contacts for a few seconds before plugging it in.

    30A switches aren't that easy to come by so how about this. 2 15A DPDT, when both are "up" it's 36v 10Ah and the current is shared across both sides so can handle up to 30A and when both are "down" it's a 12v 30Ah battery. Will this work? I don't think there's a short when I am switching them, if one up and one down but could someone double check that for me? Should be 1 24v and 1 12v when switches are in opposite positions I think.

    1432383077920-412576755s.jpg
     
  7. Thuktun

    Thread Starter New Member

    Apr 2, 2015
    15
    0
    No, that's a mess. This is what I will do. Back to 2 male 6 prong connectors, one mounted to the bike with switches to avoid the arcing.

    1432384931968-412576755s.jpg
     
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