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Do SLA chargers fight if paralleled?

Discussion in 'Power Electronics' started by flippineck, Feb 21, 2018.

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  1. flippineck


    Sep 8, 2013
    I have a set of 3 'leisure' SLA batteries, all broadly similar in terms of age and rated capacity, connected in parallel, powering a handful of very low power equipment - a small LED nightlight type device and a small alarm system. Very very occasionally an inverter which supplies 240V AC to around 4 - 8 7W LED lamps is connected for at most, an hour or two every couple of weeks.

    I have 3 solar panels, I think they are 80W each, connected to an Ebay solar charge controller device, the output of which is sent to the battery bank described above.

    All works okay, except for the fact that during the UK winter months, the amount of sun drops low enough for the bank to discharge so low that the charge controller no longer functions. At this point the controller simply drops out and won't recognise when the sun appears again - so I come back to site after a few weeks absence to find completely flat batteries.

    I had an idea (uh oh). I figured that since the times of low sun in the UK more often than not coincide with times of high wind, why not supplement the system with a wind generator.

    So, I sourced an old car alternator from a scrapyard and fitted it with some homebrew turbine blades.

    It's an old Ford Focus Mk1 1.8 TDi alternator. More than that I haven't Googled yet.

    My worry now is that if I simply take the output from the alternator, and connect it straight across the battery bank whilst leaving the solar charge controller connected, this might cause all sorts of conflicts.

    What might be the nature of these conflicts and how could I aim to avoid them? The simplest Idea I have had is to go for complete manual control and have some kind of physical 2-position changeover switch to avoid any conflict completely, but I'd like to be able to have the system rigged up so that it will look after itself without any manual intervention.

    Not sure whether to start thinking about voltage sensing and relays, some kind of comparator setup, not sure..
  2. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    I wouldn't expect a conflict because the alternator should have a diode to prevent current flowing backwards and a voltage regulator that will keep the output low if the other charger has the voltage range up.
    Only problem I see is the charging current could go too high if both chargers are on at the same time. For that reason you probably want to interlock them.
  3. duke37


    Jan 9, 2011
    Have you tried the alternator on its own, they are speeded up a lot from the engine so I would expect that 5000rpm would be needed.
  4. flippineck


    Sep 8, 2013
    No live testing at all as yet. Definitely in the shed stage at the moment.

    The alternator pulley on the focus seems to be about half the diameter of the crankshaft pulley so i guess that makes for a ratio of 50%.

    In my experience charge warning lamps seem to go out round the 700,800 rpm mark so I was hoping to be able to start getting at least some current with a low battery at 1400, 1600 rpm or so. Which, I'm hoping might be achievable in a fair wind. I do get plenty of wind.

    If it does need to get up to the 5000 mark, I'd for sure need a belt drive or somesuch & that would maybe neccessitate a very large fan..


    May 20, 2017
    The whole point of using alternators on cars instead of the old dynamo's is that they will give a useful charge from relatively low RPM.
    If using a car alternator, you will need enough energy in the battery you are charging to power the armature (that's how they work). The current into the armature is varied by the control circuit so that the output volts are the correct value and is load dependant. So you will need to acquire a controller from an old car or, build one.
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