Member Since: 06 May 2005
Location: Y...... because I can
Posts: 10438
Lots of batteries a Q
As some of you know I am off to Morocco this Wednesday. I have been (and still am) prepping the Red Dwarf up for the occasion. I have a 1800w inverter that has its own custom built battery pack. I also have a big fridge that has its own yellow top. In addition to this the vehicle has a twin battery set up. The inverter and fridge battery's are connected to the vehicle via Anderson connectors. So 2 on the vehicle 3 in the inverter and 1 on the fridge totals 6 battery's
I new that the inverter battery's and fridge battery's were flat and I have been working on the vehicle for the last few days which would have drained the vehicle battery's. Last night I thought I had better stick a charger on to top them up. Sure enough the charger display indicated a very flat battery. I then had to move the car pushing a 110 is not a pleasant option so I thought lets see if it will start. The thing is it did easily even though the 'new' charger indicated a flat battery.
Anyway the question
Would the charger take a look at all the battery's, add the amps then give a reading of flat if the total charge was not available. For instance lets say that in total I have 500 amps available and there is only 75 amps left would this give a flat reading even though the car will easily start on 75 amps.
Sorry for the long winded explanation. And I don't know if 'amps' is the correct term.In the Beginning there was nothing, which exploded.
13th Oct 2008 8:20 am
catweasel
Member Since: 05 May 2006
Location: Bundaleer
Posts: 4805
a batteries state of charge is relative to its voltage level not current. how are they all connected?
13th Oct 2008 8:44 am
robsmith
Member Since: 02 Sep 2007
Location: Staffordshire
Posts: 2392
Its a matter as to how you've got the batteries connected to each other and the charging circuit, and which set the Anderson is connected to.
If all batteries are in parallel then the volts measured are essentially those for them all.
But, if you've got the secondary batteries in parallel, and the "car" battery on its own, separated from them by a split charger then if you measure at the Anderson (which I guess is connected to the secondary batteries) then its the volts on the secondary batteries, and the "car" battery "has a life of its own", and even if you flatten the secondary batteries there should be enough in the car battery to get you going.
Sounds to me that however you've got them connected is going to be good for starting
Have a good trip to Morocco Rob Smith
Silver rools OK
13th Oct 2008 8:47 am
wiggs
Member Since: 03 Sep 2006
Location: Manchester
Posts: 14368
Andrew ...if you dont think you have enough batteries ...i have a few you can borrow
Click image to enlarge
G4 Gone ...but not forgotten
13th Oct 2008 9:21 am
10forcash
Member Since: 09 Jun 2005
Location: Ubique
Posts: 16534
Andrew, from memory the main battery is isolated from all the others, so the aux battery, inverter pack and fridge pack can be flat but the vehicle battery will not be discharged by them or connected aux. loads when the vehicle is standing, which battery did you connect the charger to?
If you have all (or several) of your batteries connected in parallel you will find that even a relatively low voltage will start the vehicle.
A low voltage would normally indicate a lack of available current (Amps) with which to start the vehicle - so a single battery reading 11 Volts would possibly not supply enough current to crank the engine. This is because when under the kind of loads experieinced when the starter motor cranks the engine the voltage will drop significantly. This is noticeable in a car when the lights dim upon starting - in older vehicles the radio may switch off etc - in modern vehciles this tends to be because they are isolated when the starter motor runs to free up as much power as possible and so not to cause a voltage "brown-out" or "spike" that can be potentially harmful to modern electronics.
What happens if you add another identically flat(ish) battery in parallel is you double the available current.
Dispite double the current being available a battery meter will still read "flat battery" as it simply measures voltage. Doubling the number of batteries reading 11 Volts will still give you 11 volts (in parallel).
There are some intelligent battery testers that measure current while a high current is drawn to simulate engine cranking - this measures the drop in voltage relative to the current drawn and gives a better or more true reading of battery condition. Quite often a battery when getting old, will accept a full 2.2Volts per cell (lead acid) and in 12 Volt applications this means a voltage of 13.2V but when any demand is placed on the battery the cells can not supply the current required and the Voltage drops significantly - this is the only way of really knowing if a battery is beyond redemption or just needing charged.
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pushing a 110 is not a pleasant option so I thought lets see if it will start. The thing is it did 
easily even though the 'new' charger indicated a flat battery.
even though the car will easily start on 75 amps.
