Scarfing Power from 5V Pin

@inq My LiFePO4 12V 100AH battery is configured for Charged Voltage of 14.2V. Allowed is anywhere between 14.2 and 14.6 so I keep mine at the low end but I am thinking of bumping it to 14.4 to make sure I am filling them up since we get so little sun here. Actually I have no idea what the per cell V is as it's a sealed case, but somewhere along the line the number 6 came into my head so 14.2 / 6 is 2.4, your 3.65 could be legit if there are 4 logical cells which actually makes sense since that is 14.4 that means I am at 98.6% BUT since the mfg recommends 14.4 as charged and allows 14.6 then 14.2/14.6 means I am at 97% when fully charged.

All I know for sure is these batteries are way better than lead for a lot of reasons and will last me the rest of my life or until I sell the RV. 

Posted by: @zander

@inq My LiFePO4 12V 100AH battery is configured for Charged Voltage of 14.2V. Allowed is anywhere between 14.2 and 14.6 so I keep mine at the low end but I am thinking of bumping it to 14.4 to make sure I am filling them up since we get so little sun here. Actually I have no idea what the per cell V is as it's a sealed case, but somewhere along the line the number 6 came into my head so 14.2 / 6 is 2.4, your 3.65 could be legit if there are 4 logical cells which actually makes sense since that is 14.4 that means I am at 98.6% BUT since the mfg recommends 14.4 as charged and allows 14.6 then 14.2/14.6 means I am at 97% when fully charged.

All I know for sure is these batteries are way better than lead for a lot of reasons and will last me the rest of my life or until I sell the RV. 

 

That's very interesting.  So... you know it is LiFePO4?  Does it have only the two normal terminals or does it have separate input (solar cells) / output (usage)?  Got a Internet link handy?

Why I ask... not being a hardware guy, I don't know how it'd be hooked up with 6 cells.  2.4 V is well below recommended cell voltage for LiFePO4 and exactly four cells are required to make the "12V car battery replacement".  

I know there are two basic types of BMS's... being the main goal after safety is to keep the cells balanced.

• Passive are by far the most common and simply burn off charge of cells that are higher than the other cells.  They use very small gauge wires and will only waste mA in attempt to keep the cell balanced.
• Active are uncommon, but move current from "too high" charged cells to "too low" cells.

Maybe your battery is using some sophisticated way of only "hooking up" four cells at any one time, while they charge "off-line" cells individually.  I don't know if or how'd they go about doing that being mainly a hardware problem.  The software wouldn't be that difficult.  

@inq The battery is totally sealed so I don't know how it's constructed. I do know they weld a bunch of small cells into the battery. likewise the BMS. These are very popular amongst the RV community.

https://battlebornbatteries.com

I suspect we are talking about two very different things.

@inq HI, was just reading your post to @zander about the LiFePo4 batteries, and found this article about them.. https://batteryfinds.com/how-do-i-calculate-the-lifepo4-battery-capacity-i-need

It appears that the LiFePo4 batteries have a cell voltage of 3.2 V, so for a  ~12 v system, that would be 4 cells = 12.8 V.

https://lifelinebatteries.com/battery-basics-lifepo4-cells/

As far as the cell balancing goes, the Passive and Active approaches are what are commonly used, with the Passive one being predominate. Although the Passive approach waste power, it's usually in mA..  and it more expensive to do Active by taking current from higher voltage cells, and putting it in lower voltage cells, it would depend on your requirement of how much DoD ( depth of discharge) you can live with.

I hope this information is useful to you both, because it certainly helped me with my understanding of how to pick battery systems for home/RV/boat etc...

kind regards,

LouisR

@inst-tech I don't understand the entire discussion TBH, it's chemistry. The volume of chemicals determines the Voltage and to the best of my knowledge there is no 'standard' volume for a cell. I have seen YT vids of various LiFePO4 batteries being disassembled and generally there are several cells inside the size of a thick book. They may be made of smaller bits, I have no idea. The batteries I use however are constructed from cells that look like 18650's. Most of those cells are made in Japan, but as demand has grown some are now made in other countries but so far as I know not in China for some reason. When I sized my installation it was to allow for 5 days off grid with minimal solar. Due to covid and the US political situation we will never travel in the US and in fact will leave the RV fall 2023 after one last trip across Canada so we will never get a chance to see if the batteries will in fact get us through 5 days or not. If @inq really wants to know more about BB just call them, they are very friendly and given his interests and abilities they might well be interested in sponsoring him, in any case I am certain he will enjoy talking to the VP, he is a gearhead for sure, the Pres is more low key but still super knowledgeable.

@zander - I've heard good things about Battle Bot.  For your situation where you want a trouble-free battery, they're perfect.

I wanted to learn about the inner workings, tinker with them and save money.  BB was about 5.5X the price of my battery at that time.  And yes... I got most of my information/knowledge from Will's forum that you mentioned before.

@inq Will is an amazing young man, I also got my initial education on LI batteries from him.

@inst-tech 

As you pointed out, four cells in series make the "nominal" battery.  3.2V is pretty close to being fully discharged.  Nominal is in the flat spot where most of the capacity is being utilized... 3.3V => 13.2V.  

At one time, I gave some thought... mainly lots of hand waving while drinking beer/wine/favorite poison of the month that I wanted to build a battery with a bunch of cells.  It would have +/- terminals for output as normal, but have a separate pair +/- for hooking up to solar panels.  I'd use an ESP8266 that would monitor each cell's Voltage/SOC and independently hook up groups of four cells in series to the output bus and independently hook up ones or combinations of cells in series depending on the solar panel's current output in voltage and current.  I optimistically (delusion-ally) thought it would be superior to a passive or active BMS and it would also allow eliminating the solar charge controller.  I have a good handle on the software side, but my limited hardware knowledge finally augured-in trying to come up with relays or Mosfets that could handle hundreds of amps. 

VBR,

Inq

@inq Do you come up with these innovative ideas before or after imbibing?

@inq while it's my limited understanding that because one of the benefits of LiFePo4 batteries is the weight to power ratio that some claim to be 4 to 1 over SLA type, that's because of the chemistry, which is also re reason why the DoD is so different in other type of Li -ion batteries.. Since the cell balancing is a crucial part of the life cycle of the batteries, I really don't think that you can eliminate the charger..lol But hey, what do I know. As far as I'm concerned, anything is possible these days with all the information out there in cyber-space, and people willing to try to build a better mouse trap!.. That's what free enterprise is, and thank God for that! I'm always amazed at what others think up, and eventually produce.. being mostly a hardware guy, I do however, appreciate those with the gift of the software side, and have learned much from their ideas.  You just just keep thinking, ( and I appreciate the wine,beer, and spirits of the month) as I too get inspiration them as well,... lol

regards,

LouisR

@inst-tech Indeed the single LiFePO4 is 33 lbs and 100AH, the Trojan T-105's they replaced were 67 lbs each and 6V so 134 lbs for 12V and a usable 112.5 (better at 75 actually) so basically a 4:1 power to wt ratio as you said. So many other benefits as well for LiFePO4 makes it a no-brainer regardless of cost. Some benefits are not easy to quantify like speed of solar charging with LiFePO4 vs FLA, the FLA are sooooooooo slow compared to the LiFePO4. What I have never seen in a review of these batteries is that when they are 'done' means they are ONLY 80AH now instead of 100. Even at the most conservative 3,000 cycles, a once a day full cycle is 8 years. Waaaaaay more than FLA at 500-1,000 cycles IF you take good care of them. BB thinks 5,000 cycles is doable without doing anything too special other than keeping the DOD to between 20%-80% most of the time. 

Hi @inq,

    I am sure you realise this, but I thought others might like to note that the graph you show above is open circuit voltage ... the only thing connected is the voltmeter. You can use this as a guide to estimating the state of charge,  but in terms of providing useful power you need to know the voltage when it is delivering a realistic current.

At modest current discharge levels, whilst state of charge is reasonably high, lithium-ion batteries are usually good at maintaining their voltage level, but the low state of charge end can be disappointing.

Most electronic loads have power supplies which aim for constant power demand ... so as the state of charge gets low, if the voltage drops, the power supply demands more current, so the battery discharges at a faster rate.

Best wishes, Dave

Posted by: @davee

Hi @inq,

    I am sure you realise this, but I thought others might like to note that the graph you show above is open circuit voltage ... the only thing connected is the voltmeter. You can use this as a guide to estimating the state of charge,  but in terms of providing useful power you need to know the voltage when it is delivering a realistic current.

At modest current discharge levels, whilst state of charge is reasonably high, lithium-ion batteries are usually good at maintaining their voltage level, but the low state of charge end can be disappointing.

Most electronic loads have power supplies which aim for constant power demand ... so as the state of charge gets low, if the voltage drops, the power supply demands more current, so the battery discharges at a faster rate.

Best wishes, Dave

 😊 I'm not sure this thread will go down in the annals of history as a reference for anybody, but it is important to note in case someone bounces here.  I think @zander and I discussed that exact subject... where my discharge test curve didn't fit the expected, main stream, Internet curves... as I watched it discharge and the rate increase.  https://forum.dronebotworkshop.com/postid/28690/ and the next two or three posts after that.  But that was a mere 18650.

I can't say I have a lot of experience with battery banks like @zander and the rest of you might have, but I was surprised at how resilient and powerful these Lithium based batteries are.

A couple of cases...

First, the real sad/disappointing lead-acid battery.  It's an 80 Amp-Hour deep-cycle type.

Powering 3 car type 12V incandescent bulbs... 18W all night (regulations) and use the interior lights (same type) for a couple hours per day and it barely could make it through a long weekend.  I put a 300 Watt AC Inverter on it and did some tests... I was able to get a 280 Watt load on it pulling ~25 amps.  I ran it till the Inverter minimum voltage kicked it off.  This was well below the 50% SOC recommendation of the lead-acid batteries... but after it "rested" (poor thing)  It bounced back to above 12.5V.  It crapped out at about 20 Amp hours usable at that load.

The other extreme - the 272 Amp-Hr LiFePO4 was a nearly bottomless reservoir.  I did a 0.2C test by nailing a 54.4 Amp load.  It lasted 5:04 minutes for a pretty exacting capacity test.  In real-world use, I went a whole week in Florida summer heat.  It ran all lighting (now LED).  It ran a chest type refrigerator continuously.  It ran music and charged phones.  Occasionally pulling the full load the BMS can handle (120 Amps) I can run a 1500W Inverter Microwave.  At the end of a week (with no charging) it was at 75% capacity.  It's amazing.  

@inq That's impressive. The other thing about them that is important to me is the charging performance. When I had FLA it took forever to charge that last 10% or 20% but LiFePO4 is basically a straight line. Not only is that great for solar in my solar challenged location but in the winter when the power goes out sometimes for a day or more the generator only has to run for a few hours because we are dumping 120A into the bank all with a small 2,000 W inverter generator. In order to beat the small possibility I couldn't charge in the winter (limit is 24F/-4C) I placed them in the interior basement instead of the exterior basement. Where I am located it seldom gets that cold and now that they are in heated space it's no problem. As it turns out enough heat leaks through to the forward bay that it would not have been a problem there either. Now they have heated batteries so camping in even colder weather is possible if your RV can handle it.