Would this work? switch to put leisure battery in parallel?

Discussion in 'Mech Tech' started by stuming, Nov 1, 2011.

  1. Hi all,

    Don't think this has been asked before, so here goes:

    I installed a leisure battery and a relay to charge it when the alternator light went out. You know, by clipping the switch wire onto the blue wire from the voltage reg I think.

    It worked well, leisure battery always charged but only started charging once the engine was running.

    The only issue I had was a "slightly" difficult to turn started motor: if the main battery wasn't fully charged, or maybe the starter motor was in its "sticky" place, it wouldn't turn over. I bought one of those battery boosters that you can charge at home with the built in jump leads, and it works well, but is heavy and a pain in the @rse as you have to take it everywhere.

    Now, I recently got my refurbed engine back and fitted, which is good, and when I checked I noticed that he'd connected the the switch wire from the relay to the coil, which is something I know this particular guy always does. The upshot of this is that I never have any trouble starting the bus even if it's been standing for 2 weeks like it was when I came back from holiday on Sunday. The starter never sticks and spins faster it seems.

    I'm guessing this is because both the batteries are acting in parallel to power the starter motor. The downside will be if I use the fridge and drain the leisure battery sufficiently that when I go to start, the power from the parallel batteries wont be enough to start.

    So, to my idea of fitting a selector switch so I can choose to route the switch wire to charge the leisure battery when the alternator light goes out, or to have the switch wire routed to the coil to boost the power to the starter if the main battery needs help.

    Would this work? Any idea where I'd get the components? I bought the JK heavy duty relay kit so I'm not sure of the type of switch, or gauge of wire to use. Any problems with a setup like this?

    Sorry for the long post!

    Cheers

    Stuart
     
  2. matty

    matty Supporter

    Taking the signal from the coil is dangerous and could lead to a fire.

    Using the coil to signal the relay means the starter is taking power from both battery's to start, you will get away with it if the main battery is ok as it electric with take the path of least resistance

    If the main battery is low the starter will taking the power from the leisure battery overloading the wiring look at the size of cable difference coming from the batterys. Also the leisure Battery is not designed for the high loads of starting and may explode spilling hot acid.

    The main battery is already charging with the engine running all the spit charge relay does is join the battery's together so the alternator is charging both at the same time when the engines running.

    It sounds as through the main battery is on its way out or has drain on it when parked up if its not holding its charge.
     
  3. I've got a voltage sensing solid state relay, and I wanted the ability to top up the main battery from the LB for emergency use. I have a switch which can select between main and LB for stereo and cig lighter so the terminals are close together at the front of the bus. I think I used 30A cable so I wouldn't turn the engine over like that though. If you really want this you should probably connect you relay back to the voltage regulator and install a relay bypass switch across the relay terminals

    I would say that solving the original problem by parallel connecting the batteries is like masking the original problem, which may be something like a failing battery or a bad connection somewhere - wouldn't you be better off fixing that instead?

    Is there a real risk of the LB exploding? I thought that Leisure batts just couldn't deliver the cranking amps required, that is unless they are already over specc'd for the job anyway - 120Ah should turn over the engine ok, smaller ones may not?
     
  4. matty

    matty Supporter

    Leisure batteries are designed for providing a smallish steady current over a long period, and are capable of being pretty well discharged before rechargng again.

    starting batteries are intended to provide a high current (several hundred amps) over a short period. They also prefer not to become too discharged before recharging.

    Although the Lead/Acid chemistry is pretty much the same, the way in which the lead plates are arranged inside the battery vary between the two types to give them the characteristics needed for their designed use.

    By using a leisure battery on a car to start it will probably buckle the plates inside witch could course the casing to split relising acid and explosive gas
     
  5. what they all say ^^^

    leisure batterys are designed for small currents over long periods
    the wiring for split charger is only rated to around 50amp not 250amp and will burn out due to the extra load

    your linked batterys are only a band aid to your initial starter issue, i would say look at your starter asap before that finally fails and lets you down
     

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