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Old 05-29-2019, 12:33 PM   #1
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Battery upgrade to lithium - Wiring size vs circuit protection

Which is the better approach - a) size the wiring to maximum possible amperage that could be applied with a dead short or - b) size the wiring for the needed/expected load and fuse the circuit to protect the wiring size choice?

My assumption is that it is better to size the wire for the expected load and protect the circuit from overload. Breakers will be a better solution than fuses. Since the converter only provides 55 amps, then that is the expected bank loading.

Project scope - upgrade from a dual group 24 wet cell bank on the tongue to a triple lithium Battleborn cell bank under the bed.

Objective - reduce tongue weight and increase amp hour capacity to allow two days of overnight furnace use. Agenda - leave the generator at home except for a very limited set of off grid circumstances that require a stay of three to five days duration.

Background - the coach is a FC23FB. The converter is a PD4655 and will be upgraded to the version with a lithium charge profile. A battery monitor will also be added. We normally use full hookup rv parks, but on one or two nights each trip we find ourselves off grid. Initially it was a rest stop or parking lot, but with better planning it will be a dry campground. Because of our travel and storage profile, we have no desire to install a solar system. We disconnect the umbilical when parked for extended periods. We do not use inverter power, but have the OEM 1000watt unit installed.

The searching of upgrade threads seems to focus on solar or inverter support requirements. The Battleborn installs are either imbedded in other system installs or assumed to be direct replacement solutions. Consequently, the start of this thread. My apology if my ignorance missed the concepts in prior threads.

Your review and input is appreciated. Pat
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Old 05-29-2019, 09:26 PM   #2
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Which is the better approach - a) size the wiring to maximum possible amperage that could be applied with a dead short or - b) size the wiring for the needed/expected load and fuse the circuit to protect the wiring size choice?

My assumption is that it is better to size the wire for the expected load and protect the circuit from overload. Breakers will be a better solution than fuses. Since the converter only provides 55 amps, then that is the expected bank loading.
That's correct, if you never run a "big" inverter. Each one of the new "BattleBorn" LiFePO4 batteries is capable of 100A. The wire size which would be needed to support the 3-battery string with maximum loading doesn't even have a name (it's much bigger than AWG 4/0). Instead: size for the Inverter or the highest anticipated simultaneous total loading (appliances, fridge, and lights), whichever is greater.

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Background - the coach is a FC23FB. The converter is a PD4655 and will be upgraded to the version with a lithium charge profile.
Don't do that. The "Lithium" switch on newer PD converters simply changes the multi-stage logic into two-stage CC/CV, with maximum Voltage 14.6V. It depends on BMS to actually stop the charging. Most BMS boards are also set to stop at 14.6V, and I personally feel that's a little too high. I own a PD 4655L "MBA WildKat" 55A Converter, but I'm not going to use the Lithium switch. Instead, I'll be using the pendant to invoke boost mode @ 14.4V (as if it were an AGM battery). When the battery voltage begins to increase to about 13.2V, the battery is almost fully charged. When I see that happen, I will stop repeating "bulk charging" invocations from the pendant, and let the voltage settle back to "normal" mode at 13.6V. (I have spoken with PD engineeers, they agree that this way is better for the batteries.)

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A battery monitor will also be added.
As you know, LiFePO4 batteries maintain nearly constant voltage (12.8V) through most of the SOC range. When they're falling from 15% SOC down to 5% SOC, you'll begin to see a drop - and it happens pretty fast. When charging, nearly all of the Voltage change occurs during the last 5%. You will need a "coulomb counter" battery monitor, rather than a mere volt meter with temperature compensation. They show voltage pretty accurately, but their main job is to measure total energy input versus removed.

I very recently got mine from China. Here's a picture of my various parts, on the living room floor before installation. The big green box is my inverter; the PD 4655L "MBA WildKat" as at lower left. (A solar controller is shown above the Converter.) The 150ah battery is on the right, and the not-yet-connected "coulomb counter" is sitting on top of the battery, with its shunt. A 150A Circuit Breaker is not shown, and I will be using larger 'primary wires' with the battery.
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Old 05-29-2019, 10:33 PM   #3
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The Battleborn web site has information under the BMS FAQs that states each battery will deliver 100 amps continually and a 200 amp surge for 30 seconds. In the event of a higher draw the BMS will deliver maximum battery capacity for 1/2 second. Still investigating what maximum battery capacity will be.

Rick, I understand that the PD 4655 can be operated the way you suggest, but it was my understanding that Battleborn recommends the higher voltage charging. Sounds like a topic that I need to broach with them.

Thanks for the perspective. Pat
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Old 05-30-2019, 09:00 AM   #4
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Size your wires based on the amperage the line will handle (don't forget to calculate the losses for length). Then size you your fuses to protect the wire. If you're running multiple items off that wire then size your fuses for what is connected to it. That way your wire is protected and your loads are protected. Think of it like your house. You have a heavy gauge coming in (100 or 200 amps) and then your loads at 15, 20, 30 or 40 amp breakers that protect the load and it's circuit. Most equipment like in your home has additional fusing built into it (ie. microwave, toaster oven, TV, etc). The same concept holds true for 12 volt items. Your inverter has built in protection, but you still want to add protection to it. Same for anything else. You never know when something goes wrong. And in a moving bouncing trailer anything is possible.
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Old 05-30-2019, 09:05 AM   #5
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find the max continuous load, then go one size smaller

you want to do this once
you want to be safe.
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Old 05-30-2019, 09:16 AM   #6
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Smaller wiresize numbers mean larger wire diameter.

Find and check an “Ampacity table”. It will give you design data.
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Old 05-30-2019, 09:39 AM   #7
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Originally Posted by waninae39 View Post
-- snip -- one size smaller -- snip -- safe.
What is the logic for going one size smaller vs one size bigger? Is it that more is a waste, because with electric current no safety factor is needed?

See RMs post - Mechanical brain translates size to diameter as opposed to sizing designation. Which concept is correct. Smaller or larger wire cross section area?

Different point - should the max load be what we expect to use or worst case for all loads that could be applied? I think this is a moot point as a 55 amp converter can/will only deliver 55 amps and therefore the DC loading is not much, unless an inverter is used. The inverter is 1000 watts, so that is about 85 amps. Would expect that to be worst case for the inverter circuit and the 55 amps to be worst case for the converter circuit. Is that a poor or valid set of assumptions.

Yes, we do not want to create a problem that ruins our AS experience.

Appreciate the input. Pat
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Old 05-30-2019, 10:26 AM   #8
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Battery upgrade to lithium - Wiring size vs circuit protection

Example: 14 gauge wire is good for 15 amp load and breaker/fuse. 12 gauge wire is good for 29 amp load and breaker/ fuse. As the gauge number goes down, the wire diameter goes up.

I guess it’s traditional measurement method that’s not going to change anytime soon. Adding to the fun, the wire actual diameter and current rating also depends on the temperature rating of the insulation.

An “Ampacity Table” covers all that as well as lengths of the wires, that affects voltage drop, which is another consideration. Longer wire runs need larger diameter wire to avoid excessive voltage drop at a given current.

Spent a lot of time in engineering school figuring this stuff out mathematically. The tables make it a lot easier.
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Old 05-30-2019, 10:35 AM   #9
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Except, of course, until you get to 1/0, 2/0/3/0 ..... up to size 1000. Thanks for the explanation. Pat
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Old 05-30-2019, 10:59 AM   #10
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Yup. Absolutely correct. It’s not real consistent overall. It drives me nuts keeping track sometimes.
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Old 05-30-2019, 01:12 PM   #11
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the same goes with nails "4 penny nail" and as they get bigger, they go into inches (;
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Old 05-30-2019, 10:10 PM   #12
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BB recommends a 300 amp fuse for a triple cell bank. Pat
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Old 05-30-2019, 10:16 PM   #13
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Use "power distribution blocks", with different output wire sizes.

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Different point - should the max load be what we expect to use or worst case for all loads that could be applied? I think this is a moot point as a 55 amp converter can/will only deliver 55 amps and therefore the DC loading is not much, unless an inverter is used. The inverter is 1000 watts, so that is about 85 amps. Would expect that to be worst case for the inverter circuit and the 55 amps to be worst case for the converter circuit. Is that a poor or valid set of assumptions.

Yes, we do not want to create a problem that ruins our AS experience.

Appreciate the input. Pat
The connection to the battery string must be sized for the inverter. 1000 watts (AC) will be close to 90A on the "12V" DC, because the Inverter isn't 100% efficient. You should probably use #4 wire (bigger than #6). and definitely nothing smaller than #6. Length is important - if it's less than 2 feet of distance, I "feel" that #6 is probably OK (because generated heat can be distributed by the terminal ends, as well as the wire jacket insulation). That's what I'm using with my LiFePO4 battery to my adjacent inverter. (But LFP batteries run at slightly higher voltage, with less current needed to provide a given amount of power.) 4-AWG is definitely OK, and probably a better choice for your installation.

The wire going back to your 12V Converter/Power Distribution Panel doesn't need to handle all that. 6-AWG is more than enough, and even 8-AWG might be adequate. And so, you should use a pair of "power distribution blocks" (one for negative, one for positive) next to the battery, providing multiple connections. The BIG circuit breaker goes within the +12V short #4 wire into the battery. Then, on the "downstream" wires, you use smaller fuses or breakers which correspond to those wire sizes. (The Converter Wire is smaller, with a fuse within the Converter. The Inverter wire is bigger, and it can be without a fuse because the Converter already has a DC fuse or breaker on the inside.) Here is a suitable device, from Amazon: https://www.amazon.com/Elecer-Distri...dp/B07FCG3FWG/
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Old 05-30-2019, 10:29 PM   #14
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WHOOPS! corrected some of my numbers.

corrections within the quote.

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When the battery voltage begins to increase to about 13.5V, the battery is almost fully charged. When I see that happen, I will stop repeating "bulk charging" invocations from the pendant, and let the voltage settle back to "float" mode at 13.6V. (I have spoken with PD engineers, they agree that this way is better for the batteries.)


As you know, LiFePO4 batteries maintain nearly constant voltage (12.9-13.3V) through most of the SOC range. When they're falling from 15% SOC down to 5% SOC, you'll begin to see a more rapid drop - and it happens pretty fast. When charging, nearly all of the Voltage Increase occurs during the last 5% of SOC. You will need a "coulomb counter" battery monitor, rather than a mere volt meter with temperature compensation. They show voltage pretty accurately, but their main job is to measure total energy input versus removed.
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