Power problems

Discussion in 'Tech/Mech General' started by mlp, Mar 25, 2009.

  1. mlp

    mlp Deleted User

    Hi all,
    I'm running Eric an old Bedford CF and I'm having power problems!

    I have upgraded his Alternator to a 70 amp version charging a 70AH main battery.

    I have set up a 2nd 110AH battery with a common - earth and a relay switched + connection between the two batteries.

    My problem is the 2nd battery doesn't seem to reach full charge, even after a 3 hour run my inventor will only run my laptop for an hour or so (about 40w) I haven't tested the draw on the battery from the inverter but it's spec mentions a .5A running draw, my basic electric understanding makes this under a 4 amp draw.

    The Alternator was tested when i uprated it and was providing a good charge, the only thing I can think of is that it's something to do with the battery mismatch.

    Any help would be greatly appreciated as some power would be nice.

    Cheers Mark
  2. Pete45


    Jan 29, 2009
    Milton Keynes Bucks
    Hi,if you have a 70ah main battery and the 2nd is a 110ah,that is a mismatch,2 70ah or 2 110ah.that work ok for me.hope that help in some way.. Pete:thumb:
  3. Tony Santara

    Tony Santara

    Jul 26, 2007
    Cheshire (ex Mancunian)
    It sounds like your running an old battery with a new one, as well as being mis matched
    an old battery will tire out a new one quicker , also a bigger battery (110) will drain quicker because its supporting a smaller one( 75)
    The draw for a laptop is quite big
  4. hilldweller

    hilldweller Funster Life Member

    Dec 5, 2008
    Is the 70Ah battery the starter battery ?

    Anyway, when you connect two batteries together, one fullish the other emptyish the current flow could be BIG, maybe big enough to burn the relay contacts. Check that the relay is in fact connecting the two with no voltage across the contacts.
  5. oldun


    Mar 23, 2008
    St Neots
    Don't forget that the alternator charging current is controlled by the voltage of the vehicle battery.

    When the vehicle battery voltage is low the alternator will pump out 70A. As soon as the vehicle battery is topped up the alternator output will be drastically reduced so the charging of the leisure battery will be at a very low rate.

    Posters on other self build forums have suggested mods to the alternator control circuit to overcome this problem but I think that would be (a) a bad idea with a new vehicle under guarantee and (b) beyond the capabilities of most of us.
  6. scotjimland

    scotjimland Funster Life Member

    Jul 25, 2007
    A Solution

    Odd as it may seem a vehicle alternator isn't designed to charge batteries .. it's primary function is to provide 12v for the vehicle electrics.. it's secondary function is to replace what was used for vehicle starting, it cannot fully charge any battery, it's not designed to, let alone a second leisure battery .. also, it should never be used to try and recharge a fully discharged starting battery .

    There is a solution, well, two solutions, one was mentioned by djchapple,.. dismantling the alternator and modifying the engine wiring harness which is either not desired or possible due to technical or warranty reasons ... the other is to fit an Alternator to Battery charger ..
    This is not a cheap solution but I'm afraid there is no other if you want to fully charge the leisure battery.

    Sterling Alternator to Battery Chargers

    If you are a serious wild camper this is a worth while investment..
    Last edited: Mar 25, 2009
  7. mlp

    mlp Deleted User

    Thank you all for your advice, the jist i get, is my problem is my missmatch!
    I can see that the ideal fix would be to get another 110 amp battery for the main system(this is seperate with the ignition off)...
    But pennies won't run to this at present, if I swap arround the 110A to the main and the 70A as the leasure! would this help?

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