battery charging

Discussion in 'Tech/Mech General' started by vandad, May 6, 2011.

  1. vandad

    vandad Read Only Funster

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    Hi,
    I'm currently using a Smartcom voltage sensitive relay to give me a leisure battery charging facility when driving. The Smartcom is fed directly from the vehicle battery but I read a thread a few months ago that suggested it would be better to have a higher capacity normal relay and use the voltage sensitive Smartcom to open the circuit. Am I correct in thinking that the Smartcom relay regulates the charge and that a normal relay doesn't and if I'm drawing the charging current direct, via a normal relay, from the starter battery and not from the alternator (does an alternator give a regulated charge?) am I going to have a problem. I sure could do with some expertise - every time I think I've got a grip of 12v auto electrics something crops up to worry me.
     
  2. Terry

    Terry Funster

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    Hi and welcome Vandad :thumb: Don't know any detail on how your smartcom works but my mate who did all the wiring on my self build just kept things simple (for me :Rofl1:)He simply wired in a twin relay to charge the l/b and run the fridge keeping the starter battery separate.The twin relay was under a tenner took him a few mins to connect and has proved faultless for 4 yrs :thumb:
    terry
    if nothing else it will bump you to the top where someone may know about your Smartcom :Rofl1:
     
  3. DESCO

    DESCO Read Only Funster

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    Hi and Welcome to MH Fun:welcomefunster:

    Sorry can't help but will put you back at top again.
     
  4. hilldweller

    hilldweller Funster Life Member

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    As far as I can make out a Smartcom relay switches when it detects something like 13.6 V on the starter battery, in other words it's being charged, so it connects the leisure battery to the starter battery and they both get charged.

    A "normal" relay does not have an accurate switching voltage.

    The only reason I can think for switching a normal relay by the smartcom is if you choose a more robust relay.

    Voltage regulation is all done by the alternator. Relays just switch on/off.
     
  5. vandad

    vandad Read Only Funster

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    The reason I want to use a standard relay is that the Smartcom is rated at only 30amp and if I use a higher rated, say 70amp, relay the l/b should in theory charge quicker - the wiring would of course have to be upgraded. What I can't understand is that any 12v piece of equipment, battery included, pulls a certain amount of current (amps) and for example a 48watt television will pull 4amp (watt/voltage) but what does a discharged battery pull in amps - a mains operated battery charger is usually rated in a certain amp output so what is the output of a charging system using a relay fed from the starter battery.
     
  6. hilldweller

    hilldweller Funster Life Member

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    That is an extremely good question and I can't answer it. If you connected 12V to 13.6V ( which is what your relay does ) it would be BANG but clearly that does not happen. A battery is a complex load, it's all about chemical reactions taking place in a fluid so I can just guess that this regulates the inrush current otherwise every MH on the planet would long ago have gone BANG.

    The ultimate current is whatever your alternator can supply ( ?? 50 - 90A ?? ).

    You can be sure you'll not get 50/2 into each battery. But eventually they will even out at the alternator's float voltage of about 13.6V. A good battery charger will run up to 14.6V ( some over 15V ) for a while to stir up the acid and losen any deposits on the plates.

    So if you want the maximum into your leisure battery you'll need thick wires and a bigger relay switched by you smartcom. Most commercial MHs have this bigger relay switched by the alternator D+ line, no smartcom needed.

    Don't forget your fuses. One next to each battery.
     
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