Split charging system

Discussion in 'Mech Tech' started by gary gizamo, Feb 25, 2019.

  1. But you have in line fuses to protect against this.

    As a battery flattens the internal resistance increases so the amount of current it draws reduces. The alternator doesn't deliver 70 amps just because that's what it's rated at. It's the resistance of the downstream circuit that decides how much current it will allow through.

    I think you're more likely to blow a relay if it opens when you're trying to crank the engine and the leisure battery discharges through it. It shouldn't open however until it gets a signal from the regulator that the alternator is spinning and the engine has started. Then of course the big heavy current drain from the starter has been turned off.

    I'm just making this up from first principles so I'm keen to know if this is correct.
     
    gary gizamo likes this.
  2. mikedjames

    mikedjames Supporter

    As above.. one thing a relay off the alternator light cant cope with is the instant that you start an engine quickly with a healthy starter battery and a healthy but very flat LB. The relay closes and a lot of current flows from the starter to the LB. If your fuses in the split charger is the right size it can blow with the potentially over 100 amps current. No fuse no relay contact left. A bigger alternator will make it worse but even just the batteries by themselves can deliver scary current.. internal resistance of a 500 CCA battery might be 0.005 ohms. Its the thin wire and the dodgy connections on the average LB that are limiting the current, not the battery or any alternator.
     
    theBusmonkey likes this.
  3. theBusmonkey

    theBusmonkey Sponsor

    ^^ as I said previously;)
    And for that reason that's why I prefer heavier duty kit than the standard 30 or 40 amp relays most peeps fit.
    Nothing wrong with them, it's just very occasionally we push the limit with our style of travelling...:thumbsup:
     
  4. Your sort of on the right track. I havent't got time a the moment to write something up but it will do something, battery charging is complex and and people quoting irrelevant figures is misleading.
     
    Lasty and matty like this.
  5. But when you start the engine, the contacts on the split charge relay should be open, preventing a connection between the starter and the leisure battery. I suppose the alternator might kick in before the starter motor was turned off which would close the relay contacts. You'd see the red charging light on the dash go out pretty much instantly. But if fact there's a fair bit of a delay which takes care of this.

    One thing I don't know though is how much current a battery draws when it's charging. Could this exceed the rating of the relay?
     
  6. I'll try keep this reasonably simple.

    Lead acid batteries all work pretty much the same way, be it standard flooded, Gel, AGM, VRLA,. All have a fully charged cell voltage of 2.2V and an at rest voltage of 2.1V hence we have a 12V battery nominally at 12.6V fully charged, better technology such as AGM have a lower internal resistance and can have a higher voltage at the terminals around 12.8V, VRLA batteries can be even higher but the cell voltage is still the same.

    You can charge a battery two ways, constant current/constant voltage or just constant voltage.

    Constant current has the advantage by keeping the voltage just above battery voltage to start charging and keep it charging but not high enough to unduly raise the cell temperature the battery can be fast charged with current at around the amp hour rating of the battery till it is around 80-90% charged then it has to switch to constant voltage charging. In other words if we have a flat 12v 45ah battery reading 12v we can apply say13V and gradually increasing the voltage (the actual voltage will be dependant on battery type and temperature) we can bulk charge the battery at 45A till it is nearly fully charged. Myth, fast charging doesn't buckle the plates, deep discharging the does. Any buckled plates was the result of discharging the battery before it was charged. Constant current charging is usually used to recharge traction batteries such as milk floats, fork lifts etc.

    Constant voltage charging is what we normally use on vehicles normally set at 1V or 1.2V above the battery fully charged voltage but below the gassing level of the battery, hence we have voltage regulators traditionally set at 13.8V modern batteries can tolerate higher voltages before gassing so we see some regulators now set at 14.2-14.4V.

    Constant voltage charging is self regulating, in that the difference between battery and charging voltage may initially be high and high current will flow to charge the battery as the battery charges the voltage rises and as voltage difference diminishes less current is able to be pushed into the battery until they are nearly equal and only a very small current will flow which will take many many hours to fully recharge the battery.

    Because it takes so long to fully charge the battery, smart charging systems mains powered or some modern vehicle charging systems especially vehicles with stop start systems raise the charging voltage above gassing level to quicken the final charge and to equalise the cell voltages (not all the cells in a battery charge at the same rate) which can lead to problems with dissimilar battery types,condition capacity, state of charge smart charging has to assume some things usually the cells have discharged to the same level which can lead to overcharging a dissimilar battery while is trying to fully charge a another one. Batteries don't gas at the same voltage, different battery types gas at different voltages and more importantly the higher the battery temperature the lower the gassing voltage, modern car charging systems use the intake air temperature sensor to assume the battery temperature or are fitted with a battery temperature sensor we are only talking points of a volt here it is that critical.

    As Andyv mentioned discharged batteries have a higher resistance due to sulfation, the battery you might say has to be woken up before it will accept a high current charge, no mega inrush of current, with constant voltage charging the current will initially be low ramp up and then drop down it can be seconds to minutes depending on how much sulfation and the temperature of the battery.

    Even though our charging systems use a set constant voltage charging system, the higher the current the system has to deliver the system voltage decreases. As we can see with constant current charging you only need a voltage slightly above battery voltage and it will still charge the battery at high current, in fact get it right and it may charge quicker. Over speccing the split charging system to avoid any voltage drop is just wasting money.

    Some notes.

    Don't charge dissimilar batteries from the same source if it can go over the gassing level of the battery.
    Don't leave an engine idling to charge a flat battery it will damage the generator especially alternators, high generator outputs are assumed to be when a vehicle is being driven and the generator is spinning fast enough to pull an adequate cooling airflow through it, idling will cause it to overheat and can damage the diodes in the rectifier pack. Alternators don't charge better at low speed, dynamos and alternators charge about the same, just that alternators can have a higher RPM they are geared higher so can charge at a lower engine speed but with a Type1 engine the engine cooling fan speed is the limiting factor they are geared the same. Genrator output is measured at their normal running temprature a cold genrator can excede its rated output for a short time till warms up and a hot generator will produce less of it's output.

    Don't leave a battery discharged/partially discharged the soft sulfate will turn hard and damage the battery. recharge imediatly and regularly top the battery charge up if in storage

    Leaving batteries on a stone floor will discharge them is a myth, but batteries will self discharge quicker the hotter they are. Store in a cool place.

    Batteries don't like heat the cooler they are the longer the life, batteries mounted out of the engine compartment usually have a longer life, not so bad in a bay window van where when they are being charged there is a cooling airflow over them when the engine is running.
     
    Valveandy, Lasty, snotty and 3 others like this.
  7. Very useful info. Thanks for going to the trouble. It answers my query that a 30 amp relay may not be man enough for the job under all circumstances. I'll know though if this happens because the fuse to the LB will blow. So far so good.
     

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