Charging a car battery "in-situ" - can it be done?

Discussion in 'Car and Van Talk' started by diymostthings, Nov 1, 2012.

  1. diymostthings

    diymostthings Well-Known Member

    Normally I would remove it (last car was a Montego) but with these new fangled jobbies (Mondeo) I've heard that disconnecting the battery can play havoc with the computers/radio etc. What I'd like to do is just connect up my charger (16V-18V dirty DC open circuit) to the battery while it's still connected to everything. Does anyone know if this will do any harm?

    Thanks for any advice/comments.

  2. The modern up to date chargers are ok to connect to the battery while still connected.
  3. Lokkars Daisy

    Lokkars Daisy New Member

    I'm almost sure it wont be a problem, the existing battery will regulate the dirty dc
  4. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Dunno about that LD. The terminals of the charger will go straight to the car electronics, possibly 18v.

    Mr. HandyAndy - Really
  5. There you go, then - clear enough?

    DIY, you would almost certainly 'get away' with it - a car's chasrging system regularly goes over 14-odd volts when it's running anyways, so I wouldn't worry about your charger possibly pushing out a few more V over that - even tho' it's unlikely to reach anything like 18V in practice (that's 'open' voltage as you say - add any load and it'll drop instantly.)

    The 'dirty' nature of the supply is another matter - I wonder how many volts are going t'opposite way in that ripple? Even then, tho', I'd be amazed if the car's electronics wouldn't take it in its stride - the sensitive computer stuff is gonna have it's own regulated and smoothed input to cope with the car's slightly dirty output as well as fluctuating voltage.

    But, depending on how dirty your charger is, I guess there has to be a risk.

    I suspect the main reason manufacturers don't recommend it - apart from simply enjoying putting together a safety/don't do list as long as your arm - is that you could end up connecting the charger the wrong way around. That would almost certainly be instant alternator - and many other bits - fry-up.

    My neighb charged up his completely flat battery overnight, connecting it up in the dark and leaving the battery in situ as it was cold... Yep, you guessed. Next day, his battery was fully charged. With reverse polarity.

    Turned the key, and whallloooopppp. He fried the ECU, the central-locking/alarm processor, the indicators, the... you name it. He wrote-off the car he had owned for around a week. Yes, it was only a 2nd hand Rover 45, but he paid over £3.5k for it...

    I'd recommend at least buying a new charger - they are much smaller these days, and have controlled outputs. No doubt more smooth too...
  6. diymostthings

    diymostthings Well-Known Member

    Thanks for that re-assurance DA. Well its a homemade charger - transformer secondary is 18V 8 Amp feeding a 45 Amp bridge rectifier with a 50 mF smoothing cap. Waveform looks a bit grotty on the 'scope (as you feared,  slight negative "spikes" now and again which dissapear when charging) but I'll take the risk. Thanks for the tip about reversed polarity!

    And thanks to everyone else who replied.

  7. seneca

    seneca Screwfix Select

    You dn't want a "smooth" DC for charging battery's DIY, that's why chargers don't normally have  smoothing or reservoir capacitors in them. If you have that 50mfd capacitor straight after the rectifier it's actually a reservoir capacitor, which means that your 18 volt ac from the ****** is going to be more like 24 volts DC!
  8. teabreak

    teabreak Screwfix Select

    Not sure about all the electricimical fings, but worth remembering that a battery in an enclosed box may get quite hot, also I used to find that a battery with the caps loose for charging would sometimes "spit" drops of acid during the process, not good for paintwork:(

  9. diymostthings

    diymostthings Well-Known Member

    Thanks for that Seneca - well I didn't want to bore you with the entire circuit! I did of course appreciate the rise in voltage from a reservior capacitor. I actually made up a switchable 10 amp potentiometer with some lovely 10AMG nichrome wire I had, with copper brushed contacts and a spring loaded wiper. Works well and gives me a variable 0 to 8 amps on a good car battery. Thanks for the warning anyway.

  10. seneca

    seneca Screwfix Select

    That's ok DIY but it wasn't just the voltage I was talking about, more to do with the ripple, apparently battery's don't charge properly with smooth DC,
  11. Lokkars Daisy

    Lokkars Daisy New Member

    Nah Andy, recall the days of CB radio, I remember many a good buddy running their cb radios off a 12v car battery which was at the same time being charged by a common battery charger. The voltage across the batt terminals remained constant and smooth.
    I charge my Transit that way, no harm done so far
  12. A 50uF or 50mF smoothing cap?!
  13. Lokkars Daisy

    Lokkars Daisy New Member

    what do you want explained da
  14. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    A 16v 1000uf 670 ripple, now you're talking.


    Mr. HandyAndy - Really
  15. Lokkars Daisy

    Lokkars Daisy New Member

    It's a battery charger handy not a psu for electronics
  16. Hi Lokks. Only that 50uF would be far too small to smoooooth, but a 50mF quite impressive!
  17. seneca

    seneca Screwfix Select

    mfd and uF are the same DA.   ie, microfarad and u for micro
  18. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Aye, no, it's a PSU.

    Mr. HandyAndy - Really
  19. Lokkars Daisy

    Lokkars Daisy New Member

    50 Farad would be quite impresive
    Mr Penfold recommends 1mF per mA  
    However it's a battery charger so no smoothing required
  20. Lokkars Daisy

    Lokkars Daisy New Member

    why do my posts keep vanishing

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