Battery maintenance - Page 4 - Fiberglass RV

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Old 10-02-2013, 01:43 PM   #43
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Name: Brian
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Originally Posted by Byron Kinnaman View Post
A battery is a considered a load to a DC supply. There are only 3 types of loads, resistive, capacitive, and inductive. In DC there's only resistive.
If a battery were just a resistive load, the relationship between current and voltage would not change during the charging process... but it obviously does.

A battery is not just a resistor, but it's also not just a capacitor or just an inductor... or just any combination of them. There is more in the electrical world than just resistance, capacitance, and inductance. A battery also not just a load - it is an electrochemical power supply.

By the way, capacitance certainly does exist in DC circuits, although I agree inductance in the battery can be ignored. In a steady-state DC circuit the capacitance of a component won't matter, but charging a battery is inherently not steady-state.

So how to describe a battery? Resistors don't store energy so that's not the principal component. Inductors don't store DC electrical energy, so that's not it either. A capacitor does store energy and is sort of close to a battery. A battery could be modeled as a huge capacitor, in series with a low resistance, and paralleled by a high resistance. The capacitor stores energy and the resistor impedes current flow in or out, while the high resistance around the capacitor leaks charge away over time. If this were a complete description of a battery, chargers would only need two stages (bulk and maintenance); even then they would only approach full charge very slowly if the charger was fixed at the fully-charged voltage (due to the series resistance). The ideal charger would run at a higher voltage for the bulk charge stage, and would need to measure or determine the battery's internal voltage, without the voltage drop due to current passing through the series resistance.

Real batteries are buckets of electrochemical soup, and cannot be accurately described by such a simple capacitor/resistor model. Too bad - a capacitor can be a nice store of energy if you can make it small and cheap enough. Some large hybrid vehicles are using very large capacitors instead of batteries... but they're far from cheap or small.

Originally Posted by Byron Kinnaman View Post
A battery charg[er] is simply a DC supply.
I agree, although to be most effective it is a variable DC supply, responding the charge condition of the battery. The battery has a charge condition not strictly dependent on the current applied voltage, because it is not just a resistive load.

Originally Posted by Byron Kinnaman View Post
When the battery gets close to being fully charged one of the smarts will shut off that source and leave the other until the battery gets a bit charge.
I agree that the last charger running determines the end state of charge, but that still might not be ideal, especially if the last one running is not very well managed. It certainly might be close enough to ideal.

Originally Posted by Byron Kinnaman View Post
If I connected all three at once the truck would still have the battery charged in 15 minutes.
Yes, with chargers of very different current capacity the big one will dominate. In this combination the battery would be mostly charged (regardless of other little connected chargers), and still would not be fully charged... but close enough for automotive purposes, at least.

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