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Old 04-16-2021, 01:21 PM   #1
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Converter/Charging Question

Hello, I have what is probably a stupid question, just to get that out of the way right up front. We have ordered a large fifth wheel, which will be coming with an inverter with a subpanel, and a separate/traditional converter/charger- I'm assuming / hoping it's a multistage unit. I would much prefer a larger inverter/charger but alas I may not have a choice, at least right out of the gate, though I'll probably add one later. Our first RV had an old school AC to DC converter (2008), and our second RV had a Magnum inverter/charger, so I was able to run the gen and charge the battery bank relatively quickly, and I could also specify the shore power, let's say I'm moochdocking and hooked up to a 15A or 20A circuit, I could make sure the charger wouldn't try to pull more than that and pop the breaker.

My question is how does that work with the newer multistage chargers (old term converter)? How do they know about the amperage limitation so as not to go hog wild and dump a ton of DC current (by elevating voltage too high) unless the generator is running or there is real (50A) shore power available?

Thanks in advance!
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Old 04-16-2021, 03:11 PM   #2
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Most are only 55 to 75 amps, so the draw isn't that high on the A/C side.

I have a PowerMax 55 amp converter/ charger. It has 4 stage regulated charging like the big boys, just not the 125 amp output.

If Your looking for more fast charging on generator, just add another, switched, converter/charger near the battery bank.

If the batteries are low, both will charge away.
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Old 04-16-2021, 03:15 PM   #3
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Originally Posted by twinboat View Post
Most are only 55 to 75 amps, so the draw isn't that high on the A/C side.

I have a PowerMax 55 amp converter/ charger. It has 4 stage regulated charging like the big boys, just not the 125 amp output.

If Your looking for more fast charging on generator, just add another, switched, converter/charger near the battery bank.

If the batteries are low, both will charge away.
Cool, good advice, thanks!
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Old 04-17-2021, 07:36 AM   #4
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twinboat makes a great point. our 5th wheel has a 70 amp converter. that is dc amps out from the converter which equates to 7 ac amps into it, plus a bit more for inefficiency of the converter.
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Old 04-23-2021, 10:09 PM   #5
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Originally Posted by dbottomley View Post
My question is how does that work with the newer multistage chargers (old term converter)? How do they know about the amperage limitation so as not to go hog wild and dump a ton of DC current (by elevating voltage too high) unless the generator is running or there is real (50A) shore power available?
Amperage is pushed by voltage against the load. Standard lead acid batteries have a pretty high internal resistance so you can't dump current into them. Multistage chargers measure the voltage in the battery and drop their output voltage as the battery rises to meet it. Internal timers and such keep them from going too far even at low voltages. In short, your electrical logic is backwards.
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Old 04-24-2021, 07:35 AM   #6
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Amperage is pushed by voltage against the load. Standard lead acid batteries have a pretty high internal resistance so you can't dump current into them. Multistage chargers measure the voltage in the battery and drop their output voltage as the battery rises to meet it. Internal timers and such keep them from going too far even at low voltages. In short, your electrical logic is backwards.
Current is determined by both voltage and the load. Since voltage is constant on these chargers, the load determines the current, not the other way around.

Chargers never measure voltage, they measure current. The set output voltage of the charger is constant until the battery is fully charged. Early in the process when the charger canít supply enough current for the load, the voltage is low simply because thatís all the charger can produce. As the battery charges, the charger voltage rises because the load is reduced. Only when the charger detects significantly reduced current (1A is a common threshold) will it reduce its output voltage because it uses that reduced current to determine that the battery is charged.

The idea of multi-stage chargers is much overblown. The charger doesnít do anything different in the absorption and bulk stages. Only when the battery is fully charged or very near fully charged do chargers change their set output voltage.

Typical lead-acid batteries have internal resistance around 10 Ohms, making it fairly easy for them to draw large currents.
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Old 04-24-2021, 10:26 AM   #7
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Since voltage is constant on these chargers, the load determines the current, not the other way around.

I have to disagree. Most modern charges are multi-stage and voltage varies by stage and sometimes even within a stage. Some chargers maintain constant current in the bulk stage by varying voltage, but in any case the voltage is higher than the Absorption stage. Absorption stage is always constant voltage with amperage decreasing as the load increases, and Float stage is a still lower constant voltage.
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Old 04-24-2021, 01:23 PM   #8
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I have to disagree. Most modern charges are multi-stage and voltage varies by stage and sometimes even within a stage. Some chargers maintain constant current in the bulk stage by varying voltage, but in any case the voltage is higher than the Absorption stage. Absorption stage is always constant voltage with amperage decreasing as the load increases, and Float stage is a still lower constant voltage.
Since I have designed chargers operating on military equipment and spent nearly 11 years testing batteries and chargers, Iíll respectfully tell you are wrong. Float stage is the only time chargers adjust anything, and that is only voltage. There is no absorption or bulk stage on a charger. That is a description of how the battery state affects voltage and current, but the charger does nothing different. A charger is set at a constant voltage and current throughout the charging process (neglecting temperature changes). The battery determines the actual voltage and current at all times. When deeply discharged, the battery draws maximum current from the charger and the charger puts out as much voltage as it can up to itís set point.

The point when the charger is finally able to reach its set voltage is when the current begins to decline, but the charger does nothing at all at that point. It never adjusts voltage or current. It continues to function as a simple dumb fixed voltage DC power supply until it senses that the current has dropped below a specific fraction of C. Since commercial charger manufacturers canít know what battery is hooked to the charger, they often choose 1A as the point where float mode begins.

The idea that chargers are smart or that they have multiple stages is nothing but marketing. A smart charger is a simple fixed voltage DC supply that switches to float voltage when it detects that the battery is drawing 1A or less. A few specialized AGM chargers have a timed finishing stage which runs a slightly elevated float voltage for a few hours at the beginning of the float, and converters tend to have a mode where the charger operates at an intermediate voltage when small loads are applied to the system after full charge is reached.
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Old 04-24-2021, 02:19 PM   #9
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Assuming we are talking about lead-acid batteries, batteryuniversity.com is very concise:
Charging lead-acid batteries
Lead-acid battery C-rate
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