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Old 03-14-2016, 10:13 PM   #181
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Batteries that are in the 50% to 80% charge state can be safely recharged at a rate equal to about 20% to 25% of their capacity.

If you have a battery bank 232 AH capacity (my two 6V batteries) that has been depleted below 80% charge, they can be recharged at about 46-50 amps without overheating and excessive gassing of the batteries. So a converter bigger than 55 amps (to allow for other DC loads) is a waste of money.

To use the capacity of a 70 amp converter would take a battery bank of around 300AH or more.

At the other end, if you only have a single Type 24 12V battery (80AH), any more than a 30 amp converter is a waste.

Recharging from 80% charge to 100% charge is a slow process, that slows even more as you go above 90% and approach 100%.

So often heavy battery users with periodic recharging will run their batteries in the 50% to 80% range to allow fastest possible recharging while keeping battery life reasonable.

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Old 03-15-2016, 09:39 AM   #182
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Fred is correct when considering the amperage spread evenly in a battery bank (wiring of the batteries must be balanced electrically) and no single battery is exposed to that kind of amperage.

Some additional thoughts:

1) A converter will power the DC needs of the camper first and provide charging amps to the battery second. The higher the DC needs of the camper, the less amps available for Stage 1 charging.

2) The chemical reaction that takes sulfates deposited on the battery plates during discharge and forces that back into solution is an exothermic reaction; it generates heat. That heat can cook off the water in the electrolyte. Lower charge rates; less heat generated.

3) The high charge rates of Stage 1 (called "Bulk" among other names) requires a battery discharge state below 50% in order for the battery plates to have enough "holes" on the surface to force charger electrons into. Using this Stage will cause a lot of heat internal to the battery and can cause a permanent loss of capacity and reduce the total charge/discharge cycles the battery is capable. See attached graph of the total cycles possible based on the depth of discharge before recharging. Stage 1 is a pretty violent way of charging a battery and is to be avoided if possible. FYI - Cycles on the Vertical and Depth of Discharge on the Horizontal.

4) Once those "easy holes" are filled (at about 50% capacity), the charge rate must be cut back to under 10amps to avoid "boiling" the electrolyte and permanently damaging the battery by plate warping or cracking. Called Stage 2 (or Absorption), a converter, no matter how many amps rated, will only charge about 8 - 10 amps once the battery reaches 50% charge.

5) At 90% of capacity, nearly all of the remaining useful "holes" have an electron in it. Finding a vacant one is more difficult so the charge rate again must be reduced to a rate that matches the time the electron takes to find a "home". This is charge stage 3 (or Float). In float, the charge rate is reduced below 2 amps (again regardless of available capacity to deliver). It will stay in Float until the battery will no longer take any more electrons. This process can take several days.

6) Since high charge rates permanently reduce capacity and can damage the battery, it is recommended that you never discharge a deep discharge battery below 50%. This number is in all the literature and now you know why.

7) Using an inexpensive dedicated 10 amp (or so) battery charger-maintainer to maintain your 50% or less discharged battery when boondocking, puts less wear and tear on your expensive converter and will require the exact same time on your generator.

8) Using that 10 amp dedicated charger will take longer to charge a more severely discharged battery but it will cause less long term damage than Stage 1 (28 amps or so is all you are going to get anyway before a current limiter opens at 30 amps - most towable campers).

As an aside, the standard 4 light battery condition indicator system used in all campers will indicate the battery has fallen below 50% when the 2nd light from the top goes out. As a refresher, the top light is "charging" (not full), the second is "good" and will stay lit until the battery needs to be recharged. The 3rd light is "Fair" and will stay lit until the battery is actually dead. The fourth light will always stay on until that battery (or batteries) needs to be replaced due to failure.
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Old 03-15-2016, 02:48 PM   #183
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7) Using an inexpensive dedicated 10 amp (or so) battery charger-maintainer to maintain your 50% or less discharged battery when boondocking, puts less wear and tear on your expensive converter and will require the exact same time on your generator.
So what you are saying is a 30 amp charge and a 10 amp charge are both going to take the same amount of time to charge a battery.

That just doesn't sound right.
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Old 03-15-2016, 03:43 PM   #184
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So what you are saying is a 30 amp charge and a 10 amp charge are both going to take the same amount of time to charge a battery.

That just doesn't sound right.
Actually, what he is saying is that at some SOC level, the 30 amp converter will only be putting 10 amps into the bank as it comes up to charge. At that point, both the 30 amp (or 40, or 50 or 70) will only be putting 10 amps into the batteries and it would then be equivalent to a 10 amp dedicated battery charger. However, that is why I have the pendant on my PD9270, so that I can force it into boost mode when running the generator to maximize the value of generator run time. Before we get all crazy about overheating the batteries at 14 or 14.5 volts in boost, remember that the alternator generally runs at 14 volts or little more anyway.

However, I do not subscribe to the"wear and tear" argument. In addition, most 3 stage converters also have a 4th stage "equalization" pulse mode when in storage mode that keeps the electrolyte mixed every so many hours. a conventional charger won't do that.

To go beyond just theory and conjecture, I am installing small automatic data logger that will connect with and log my Trimetric in real time to a micro SD card. Once I have that, all of these questions will be easily answered with real data that can show SOC, voltage and current from loads, converter, alternator and solar in real time at relatively high resolution. In addition, it should also allow me to compare battery SOC bank vs resting voltage as my batteries age going forward. Analyzing the Trimetric log data will pretty much tell you how well everything is working in an RV electrical system.The meter display is great to tell you what is happening at a given instant, but knowing what happened for the past days and weeks as the RV is used will tell all!
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Old 03-15-2016, 03:46 PM   #185
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What is this "pendant" you speak of. I'd love to be able to force my WFCO into bulk mode while running the generator.
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Old 03-15-2016, 03:59 PM   #186
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What is this "pendant" you speak of. I'd love to be able to force my WFCO into bulk mode while running the generator.
Unfortunately, you can't add one to a WFCO converter, only to Progressive Dynamics converters. Time will tell what it is worth for me once I can see what is actually going on. My converter is a 70 amp unit and I can force it into boost when I run the generator, but the 22 feet of #6 wire between the converter and the batteries costs me some voltage and therefore charge current. I am perplexed that FR used both a positive and a negative cable to go to the batteries when they could have saved wire (and half of the voltage drop) by simply grounding the negative from the converter to the chassis in the rear power bay. That is an easy fix however, but I won't mess with it until i can see what is really happening over time.

There is also some data that indicates that some RV power converters don't do so well when running on the strange AC waveform that comes from Onan generators vs connecting to shore power.

We'll see.
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Old 03-15-2016, 04:08 PM   #187
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Actually, what he is saying is that at some SOC level, the 30 amp converter will only be putting 10 amps into the bank as it comes up to charge. At that point, both the 30 amp (or 40, or 50 or 70) will only be putting 10 amps into the batteries and it would then be equivalent to a 10 amp dedicated battery charger.
Until it gets to 90% SOC the higher amp rated charger should bring it up quicker to that point thus reducing generator run time. Right?
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Old 03-15-2016, 04:10 PM   #188
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stupid question, what happens if my charger was unplugged from DEC until now. How will this effect my 2 6 volt batteries
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Old 03-15-2016, 04:16 PM   #189
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stupid question, what happens if my charger was unplugged from DEC until now. How will this effect my 2 6 volt batteries
If you have no battery disconnect switch, then they might be dead.
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Old 03-15-2016, 04:17 PM   #190
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stupid question, what happens if my charger was unplugged from DEC until now. How will this effect my 2 6 volt batteries
If everything was turned off and your batteries were charged you should be down a bit, but get it plugged back in and charging. lead acid batteries only lose 2 to 5 percent charge per month when sitting, less if its cold.
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