Tuesday, Aug 30, 2011 at 10:55
Hi Nigel, When did you last find a person with adequate formal electrical qualifications running a battery
shop? They have all sorts of commercial and marketing knowledge about batteries but little understanding of electrical science.
The charge (current) delivered to a battery, any battery, is foremost a function of the voltage difference between the source (alternator or charger) and the battery. The impedances of both the source and the battery also play a secondary part in the current. So when two or more batteries are connected in parallel to a charging source and the batteries begin to charge they each draw current in accordance with their internal voltage and impedance. Should the smaller battery tend to become more (or fully) charged, its internal voltage rises to almost the same as the source voltage and current flow to it diminishes or ceases (it is now 'floating') whilst the larger battery continues to draw current until its internal voltage also reaches the same as the source whereupon its charging current tapers off. Both batteries are now at a fully charged state. In practice, both batteries charge at their proportional rate (due to their rising internal voltages) and arrive at the fully charged state together.
A similar scenario takes place on discharge of batteries in parallel. Current initially flows from them in proportion to their relative impedances and as their voltage falls the battery which can better sustain its voltage (the larger one) will deliver more current. This continues with the voltages balancing the discharges and the batteries each maintain the same State of Charge.
Your dissertation of "the 120 will not be able to charge above 80a/h" makes no attempt to explain why this would be so and is incorrect. My expressions are not just 'theory'. They are established and demonstrated electrical engineering principles. All electrical engineers understand and work with these principles every day.
"Water Analogy" does not always accurately apply but it does in this instance and for those who appreciate it here is the example:
Two water tanks, say 1000 and 500 litres sit side by side and are connected together with a pipe which also connects to a source pump. As the pump feeds, both tanks receive a flow (amps) and if one tank tends to rise above the other then its increasing internal head (voltage) opposes the flow and the two tanks rise in level (state of charge) until pump delivery ceases. Each tank has taken a different quantity (Ah) but is at the same level.(proportional SoC). Exactly the same occurs on discharge from the tanks. Each delivers in proportion to its head so that the level (SoC) of each tank is identically maintained.
I have no desire to "spend an afternoon gathering all the facts". I have already spent a lifetime gathering electrical knowledge, some in formal education, some in practical experience, and have established my proficiency. Be cautious of Google Nigel. It provides as much inaccuracy as information..... just as does shops and the bloke at the pub!
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