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Old 05-30-2023, 10:11 AM   #1
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To Shunt or not to Shunt,

That is the question.....

I am getting ready to get new lithium house batteries in my 2007 classic FB. In my rv I have an older "BMS"(its the old LED on the galley wall) that provides level of charge for lead acid batteries. So, I assume that it detects voltage from 12.8 down to 12.5,, or so, that gives a reading of 100% to empty (about 50% left on the lead acid).
I have looked at discharge curves on Lithium and from what I can find the discharge curves are fairly flat from full charge down to about 20% left. then the voltage drops from about 12.8 down to 12.5, where at 12.5 there is about 5% left. My thought is that the monitor that is currently in the airstream would detect and start to register the drop in voltage in this latter range. Correct??
Also, if my understanding is correct of the remaining capacity, assuming I have two 100AH batteries, at the 20% capacity left in the battery I would still have around 40AH. Still quite a lot and compared to lead acid, at this amount of capacity left, the lead acid would be at 60% state of discharge.
I don't need to monitor my daily consumption, just need to know when I might need to turn the generator on or plug in to charge the batteries.
Would the monitor in the trailer be adequate?
Thanks for your thoughts
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Old 05-30-2023, 10:14 AM   #2
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Not in my opinion. Any gauge, lights etc which base their reading on voltage is inadequate and wildly inaccurate for lithium. Don't feel all alone, my son's brand new trailer with the "lithium upgrade " has a digital monitor based on voltage. I fixed that.

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Old 05-30-2023, 11:41 AM   #3
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Shunt.

I switched to lithium hoping to get by on voltage readings — found they were hopelessly inaccurate and went back and installed a shunt (Victron 712). While you may not “need” to monitor daily consumption, the ability to know what you are using at any time and learn the consumption of each device is very useful and entertaining.
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Old 05-30-2023, 01:40 PM   #4
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To Shunt or not to Shunt,

Just FYI - I canít answer your specific technical question but wanted to point out an option. I picked up some Renogy brand Lifepo4 batteries, they have one 100ah option with Bluetooth monitoring built in and another option with a Bluetooth CAT5 dongle. I went with the latter option (fits better in the battery box) and so far itís been great, on-demand monitoring of SOC and real-time amperage status, without needing to install a shunt. Very simple turnkey solution, if you can live with not having BB or Dakotas. (I got the Bluetooth dongle for free with a Motherís Day promotion as well as a dedicated monitor that I sold on Ebay.)
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Old 05-30-2023, 02:27 PM   #5
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The Victron shunts are worth having. If nothing else, to avoid an anxiety about the State of Charge (SOC). Voltage is not very helpful.
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Old 05-30-2023, 03:33 PM   #6
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Part of my reasonig, other than trying to conver on the cheep, is that my converter/charger has a limiting voltage output of 14.26 measured at the battery terminal during absorption phase. It is a three stage charger with float at 13.5. Most of the battery suppliers “require” 14.4 volts. BB is an exception at 14.2-14.6. I cannot for the life of me understand why BB can be charged so low and others “require” 14.4 to14.6. Just wonder if it’s that way just to sell more chargers.
Renology sales gave me the 14.4 requirement and also indicated that lithium chargers have a different charging “profile”.
I have 8 year old cabela group 30 agm batteries that are finally failing. Seems as if my charger works great for them
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Old 05-30-2023, 03:47 PM   #7
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get a Victron shunt. it is very accurate and is easy to install
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Old 05-30-2023, 03:47 PM   #8
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The idiot who installed my lithium system (not me, a professional idiot) neglected to move the jumper on the new Progressive Dynamics converter into the lithium mode. So, it treated the battery as if it was lead acid. The result was that it maintained the charge at 13.2 volts, which is only about 60% of a lithium battery SOC. Negated the entire point of having lithium batteries.

I agree you might make do without a shunt, but you must have a lithium compatible converter.
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Old 05-30-2023, 06:08 PM   #9
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Quote:
Originally Posted by starlite29 View Post
Just FYI - I canít answer your specific technical question but wanted to point out an option. I picked up some Renogy brand Lifepo4 batteries, they have one 100ah option with Bluetooth monitoring built in and another option with a Bluetooth CAT5 dongle. I went with the latter option (fits better in the battery box) and so far itís been great, on-demand monitoring of SOC and real-time amperage status, without needing to install a shunt. Very simple turnkey solution, if you can live with not having BB or Dakotas. (I got the Bluetooth dongle for free with a Motherís Day promotion as well as a dedicated monitor that I sold on Ebay.)
We also have the Renogy Lithium batteries with the cabled battery monitor. It is very convenient. I do not know how they calculate the SOC, but I doubt it is based on voltage. In my experience it is very accurate. I also have a "dumb" shunt installed that I like because it is always displaying the current draw (or charge) for the batteries.
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Old 05-30-2023, 09:19 PM   #10
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My impression is that most believe in monitoring through a shunt or bms. Thanks
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Old 05-31-2023, 03:14 AM   #11
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Try thinking about it this way. Youíre going out on a date to a fancy restaurant, do you want to guess if you have enough money in your wallet?
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Old 05-31-2023, 09:25 AM   #12
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In a nut shell voltage reading on lithium batteries remains constant right up to the end unlike lead acid so voltage reading really does not indicate charge level of lithium batteries - you need the "shunt" + monitor that connects to it regardless of manufacturer (there are others besides Victron) also when converting you require a charger/convertor designed for lithium which uses 14.5v charging cycle. also, if you have an older existing solar setup the charge controller is probably a PWM and requires an upgrade to MPPT. When we converted our system in our 2004 International 25 CCD we basically had to review and change everything - you can check out our conversion experience here if you want https://www.marriedwithairstream.com...-introduction/
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Old 05-31-2023, 10:21 AM   #13
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Check out Renogy batteries or something similar. Built in Bluetooth to monitor your battery on a phone app. Very accurate and easier install.
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Old 05-31-2023, 10:38 AM   #14
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Shunt!
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Old 05-31-2023, 11:00 AM   #15
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Even with lead based batteries ---- not having a way to measure capacity [ capacity is measured by current with a shunt in series { not parallel - voltage } is using the lack of SMART engineering .
If you can not measure ----- you do not know !
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Old 05-31-2023, 01:06 PM   #16
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Problem is that lithium batteries start with a higher nominal voltage (12.8v vs 12v for lead) and maintain it throughout most of the discharge cycle. With voltage based monitors which are designed for lead based batteries, you do not know you are about fully discharged until you are about 90% out. Basically, you do just fine until suddenly, the lights go out. With a 400ah bank, you do not know until you have only 40ah left... usually this is about 1am. About your solar charger... the older PWM are about 70% efficient. To start, you can expect about 80% solar panel efficiency. So with a 100w panel - expect 80w. With a PWM, lop off another 20-30% and the panel will provide maybe 60w. With a MPPT charge controller, efficiency is more like 99%. So you will get more like 80w out of the 100w panel. More if you have perfect sun angle, a clear sky and perfect panel temp (75F).
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Old 06-04-2023, 09:20 AM   #17
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Quote:
Originally Posted by Sport 16 STL View Post
Check out Renogy batteries or something similar. Built in Bluetooth to monitor your battery on a phone app. Very accurate and easier install.
The app on my iPhone has allowed me to easily monitor my Renogys with no problems. Considered a shunt, but did not see that there was any advantage.
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Old 06-06-2023, 07:37 AM   #18
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Shunt! We built a good solar system in our previous trailer and boondocked a lot. We plan to continue having off grid days mixed with utilities days. It opens up lots of locations to camp.
A shunt shows you your rate of energy use/gain so you can know your status and modify your use.
A good info resource on solar and battery monitoring is "HandyBobSolar.worldpress.com" He's a little eccentric but definitely a science head with lots of experience living off grid, in Montana. Not all of his data applies exactly to RV based systems, but he has lots of info.
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