If you are going to fit a split charge relay (better a VSR) to your van, may I suggest you contact Vitron to get a wiring diagram of how to connect the wires up to make 100% sure that no damage is done to your MPPT controller. I would imagine that the terminals from the scr are connected to the leisure battery and when the engine is running the controller will see the battery as fully charged and switch the controller off, but I would check just to make sure.
The state of charge of a battery is only determined by voltage for a sealed or maintenance free battery. (If its a none maintenance battery with inspection and filler plugs then a hydrometer is also another method.)
So if your voltmeter shows 12.8/12.9v then you have to assume that your battery is fully charged. If you have a 105 amph battery then again you have to assume that you have 105 amp of available power. If you had a 110 or 115amp leisure battery then you would use those figures.
This is for lead acid batteries. I haven't studied new calcium batteries but their composition will be the same - just using different chemicals.
As the battery ages it looses it capacity due to sulphation of the plates. It will still show up as fully charged on your volt meter. At one time the only way you could test the capacity of the battery was to fully charge it and then put a load test on it for a duration of time and see the results. So if the battery went flat when the load had taken 50 amps it would mean that the battery's capacity had dropped from the original 105 amp to 50. You will still be able to use your pump and would have no problem for a while if you used less than 25 amps a day. (Remember the 50% charge 'rule'. ) But sooner or later the battery will deteriorate until it didn't manage to get you through the day.
Some of the suppliers have come up with fancy equipment that supposedly can tell you the remaining battery capacity electronically with a short test. But I'm old and cynical.
Voltage and amps aren't the same thing although they are very dependant on each other. The voltage is the force or pressure (energy) applied to the current where amps is the amount of current being pushed at a certain point.
Now you know that energy in a battery is the result of a chemical reaction thats happening in the battery's guts. So when my volt/amp meter tells me that its my alternator is charging my battery at 14.5 volts and 3 amps, its basically telling me that the amps are what its taking to complete the chemical reaction inside the battery.
BUT the fuller the battery gets the slower it accepts charge to complete this chemical reaction. So if I take my pump and run it for an hour and take 5 amps from my battery, it will take longer for the charger to replace it due to the deminishing charge acceptance of the battery. During the recharging state you could see an intial spike of the charger at 5 or 6 amps, but will quickly drop to 3 and then 2 and then 1.
You posted this earlier assuming that your charger pushed 3.5 amps into your battery for 2 hours. I'm not sure this is correct. In all probabilty the battery was fully charged (as near as damit) to start with and its my belief that it took 2 hours to replace the 3 amps shortfall.
Well i fitted the Numax battery today and what a difference , i was not hoping the sun would come out to give me a power boost , I worked from 10 to 3.30 , the solar took 10 amps in and the pump and power reel took 13 amps out , so on paper i used 3 amps all day , i decided to put the battery on the smart charger on the sealed setting to see what it would put back in , it took 2 hours to get to float mode , sealed setting provides 3.5 amps an hour , so 7 amps went in to make up the 3 amps taken out , not exact maths but close enough for me , after doing some sums i worked out that i got home with the battery at 90% charged .
Every day its back up to 12.9 or higher by the time i have finished pottering about , is there a connection between amps and volts or could it still display 12.9 v and be short of say 20 amps ?
If your battery shows 12.9 volts after its stood idle for 4 hours then its fully charged. If your battery was 20 amps short it would mean that the voltage of the battery would reflect that as it would be lower.
Lets go back to our 105 amp example. If your battery is short of 20 amps, it means that you have 85 amp left. That is just over 80% charged. The battery voltage when left to stabilise will probably be in the region of 12.5 to 12.6v on your voltmeter.
PS. As far as I'm aware the only battery that will charge fast and at the same rate until fully charged is a Lithium ion battery. But there is no way I'm going to spend £950 for a 110 amp battery for that feature.
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