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Old 10-01-2013, 11:31 PM   #41
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Quote:
Originally Posted by Brian B-P View Post
I'm familiar with that. The battery is not just a resistor.


As the battery charges, the internal voltage rises, so less current flows for the same applied voltage.


Check out the basis on which multi-stage chargers determine the point to switch charging stages, and consider the consequences for the ones based on current measurement of the measured charge current (out of the charger) not being the actual charge current (into the battery)... because Kirchhoff's current law says that if current is coming from two different chargers, the current into the battery is the sum of them... not just the output of one charger.

To be fair, any charger that is measuring only its own output current is doomed to be incorrect (in the opposite direction to the multiple-source case) when other loads are taking some of the current instead of the battery (Kirchhoff again)... and that's the normal RV converter/charger scenario.

A charger which responds only to voltage won't have a problem with either case; although that's not going to be ideal charging, maybe it's good enough.

BASIC ELECTRONICS --- 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.
A battery charging is simply a DC supply.
If you really understand it things can be broken down to some pretty simple concepts. Those that don't try to impress you with their foot work.
That's what happens when everything that's read on internet is believed.

Therefore to look at multiple charging sources is to look at multiple power sources tied to same load (resister or battery). Then look at the smarts, or sensing. The smarts can look at is current, or shut off the output and look at the voltage. 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.

Granted that the charging profile might be a bit different with two charging source vs one, but it's going to be different between the two charging source if used individually along with a different profile when charged from the tow vehicle. Add to that many solar charges are PWM (Pulse Width Modulation).


If I connect a battery via jumper cables to my truck I can get a 74 amp hour battery from dead to fully charged (as high as will charge) in about 15 minutes. Connect that same dead battery to my smart 10 amp charger it'll take 10 to 15 hours. Connect it to my solar charger at 4+ amps it'll take about 8 hours with direct sun. All different. If I connected all three at once the truck would still have the battery charged in 15 minutes.

The fastest profile wins.
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Old 10-02-2013, 10:16 AM   #42
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Interesting thread. Although I'm new to this site and to deep cycle marine batteries, I've used Battery Tenders for years on my motorcycles. My experience has been that I get about 10 years' use from a wet cell battery on an electric-start Harley when it's plugged in to a Battery Tender through our longish Northern Wisconsin winters.
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Old 10-02-2013, 01:43 PM   #43
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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.

Quote:
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.

Quote:
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.

Quote:
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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