2 leisure batteries

[irish.david]

OK, i'll give you my take on the whole battery charging thing.

First important thing is called the batteries open circuit voltage. This is the voltage across the battery terminals with no load across them (ie. measured with the earth strap disconnected). For a fully charged battery this should be between 12.5v-13v. If you apply voltage to the battery terminals that's above this open circuit voltage you create a voltage difference, current flows and the battery charges.

The most common cause of duff battery syndrome is that the battery won't hold it's charge. This can be due to a number of reasons (coating on electrode plates, electrode plates corroded, etc) and can be casused by a number of things (long time fully discharged, too high charging voltage on the terminals, etc). This means that the capacity of the battery (amp/hr's) is reduced but the open circuit voltage is unchanged. Batteries are made up from a number of cells that each supply a portion of the total voltage. If one of these cells shorts out then you will get a reduction in the open circuit voltage, but this only happens rarely.

Your alternator supplys a current with a voltage of 14v. This means that when you connect it to the battery, current will flow to try and make the battery terminals 14v as well. If the battery is flat then the battery terminals will start at maybe 12.5v and gradually increase until they're at 14v. If the battery is fully charged it'll reach 14v much quicker. If the battery is duff in the "won't hold charge" way then it would appear flat, but will get to 14v very quickly when connected to the alternator.

This means if you've got a duff "won't hold charge" in a 3 battery system it's battery terminal voltage will rise quickly and the other 2 good batteries will take the majority of the charge and it will appear as if you've got 2 and a bit batteries in your system. If your duff battery has a shorted cell then it will try and drag the other batteries down to it's lower open circuit voltage.

I don't think that a diode is a good idea in the split charge systems in our vans. I can understand why some people use them, as it's the best way to stop very flat leisure batteries from sucking some juice out of a starter battery when connected to a split charge system. I know that some boat systems use them but i assume that they're wired differently than the vans with the leisure batteries and starter battery on different feeds from the alternator.

With the forward voltage drop of a diode you're limiting the maximum charging voltage on you're leisure batteries to 13.3v so even with perfect cabling with no resistance the batteries will never get fully charged. As the split charge system on the vans is wired so the feed from the alternator goes firstly to the starter battery, then to the leisure batteries you'll get a natural voltage drop across the cable from the starter to leisure battery (due to it's resistance). This means the starter battery will alway get more charge than the leisure ones when they're all flat. Also, if your main cable from the alternator to the battery (starting cable) is in good condition you'll reduce the amount that flat leisure batteries suck out of a charged starting battery to a neglible amount as the alternator is able to hold the voltage at the starter battery terminals over its open circuit voltage.

Phew.....

Dave