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Low cost DIY Lithium camper batteries.

Lithium LiFePO4 LFP DIY Batteries

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#31 Vic Harder

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Posted 08 November 2017 - 07:50 PM

Vic, I am getting closer to 13.8-14 volts at the camper battery.

 

cwd

You will see that for sure, once the current flow drops.  The more/faster current flow, the lower the voltage.


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#32 cwdtmmrs

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Posted 12 November 2017 - 07:01 PM

Rando, using the measurements you posted for the 30 pack cell of 7"x8"x2.5", standing the packs on edge would yield a battery about 7"Wx8"Hx13'L, which is almost the same as a group 31 battery. Does that sound right?

 

Thanks,

cd 


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#33 rando

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Posted 12 November 2017 - 08:17 PM

Your dimensions sound about right - you do need a little space for the wiring and the protection circuit - but not much and you would have enough extra length in the 13" dimension to accommodate this.

 

One other thing to note - if I were to do this again I wouldn't bother with the 3D printed frames to hold the batteries.  They were time consuming to print, and didn't add much to the finished product except making the 4 individual packs very rectangular and wasting space between each cell.   Doing it again, I would glue the cells together, and then strap each pack together with strapping tape.   I would assembly the whole battery in a plastic case (pelican case or the like as we discussed before) with closed cell foam padding to fill in the gaps.  The overall battery would be smaller and easier to handle and more rugged.


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#34 cwdtmmrs

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Posted 12 November 2017 - 08:51 PM

Thanks! Good info on the gluing together because I my next question was going to be about the program for the 3D printer!

 

With my small requirement of AH/day, this battery would last me almost a week.

 

I remember reading about limited or no charging on the LiFePo batteries in below freezing temps though. Any thoughts?

 

cwd


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#35 rando

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Posted 12 November 2017 - 09:32 PM

The low temperature charging can be an issue, depending on your climate and configuration. 

 

I was originally quite concerned about it, living in CO and using the camper year around, but it has turned out not to be an issue for us.  Charging below freezing is OK, as long as the charge rate is low compared to the battery capacity - Victron suggests a maximum of 0.05C.    We primarily charge with solar, and in the winter my 240W flat mounted panels only produce a peak of about 7-8A anyway, so we are around 0.05C of our 150Ah battery.  To make sure, I set my MPPT charge controller to limit the charge current to 9A when the camper is just sitting in the driveway (easy to do in the app).   When we are using the camper, we keep the inside > 0C to keep our water from freezing, so I remove the current limit. 

 

In other cases this could be more of an issue.  Some BMS will prevent charging (but not discharge) below 0C.  The one I used doesn't have this limit, but I did come up with a simple modification to add it, which I never ended up implementing.    You could also add a limit to your battery isolator - if you have the blue seas SI-ACR, place a 0C thermostat  between the isolate terminal and 12v and it will prevent the batteries charging below 0C. 


Edited by rando, 12 November 2017 - 09:39 PM.

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#36 rruff

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Posted 12 November 2017 - 09:32 PM

I believe that limited charging at below freezing temperatures is the case.

 

"Li ion can be fast charged from 5°C to 45°C (41 to 113°F). Below 5°C, the charge current should be reduced, and no charging is permitted at freezing temperatures because of the reduced diffusion rates on the anode. During charge, the internal cell resistance causes a slight temperature rise that compensates for some of the cold. The internal resistance of all batteries rises when cold, prolonging charge times noticeably.

Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a sub-freezing charge. This is permanent and cannot be removed with cycling. Batteries with lithium plating are more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers (Cadex) prevent charging Li-ion below freezing.

Advancements are being made to charge Li-ion below freezing temperatures. Charging is indeed possible with most lithium-ion cells but only at very low currents. According to research papers, the allowable charge rate at –30°C (–22°F) is 0.02C. At this low current, the charge time would stretch to over 50 hours, a time that is deemed impractical. There are, however, specialty Li-ions that can charge down to –10°C (14°F) at a reduced rate."

 

http://batteryuniver...ow_temperatures


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#37 rruff

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Posted 13 November 2017 - 05:07 PM

This is probably a dumb question, but would it be ok to remove the paper wrapper before gluing the cells together? It looks like there is a plastic insulation ring on the + end. And the rest of the casing is -, correct?
 


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#38 rando

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Posted 13 November 2017 - 06:57 PM

As long as you can still isolate the +ve terminal, I think this would be fine.  I don't remember if the plastic insulating ring covers the whole top of the battery or not.   You just need to be careful handling them as you are right that the entire case is -ve.    Another option would be to switch out the paper wrapper for heat shrink tubing which would make them easier to handle and glue. 

 

If you do want to use battery frames, they are now available on ebay:

https://www.ebay.com...9IAAOSwzQFZ8wGB


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#39 rruff

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Posted 20 November 2017 - 05:18 PM

I ran across some discussion of low charge rates not being good for LiFEPO4.

https://www.eevblog....anging-current/

 

 

"Lithium based rechargeable batteries can be damaged (over time) by slow charging currents.  This has to do with dendrite growth on the edges of the plates.  You do NOT want to trickle-charge these batteries.  The only exception I know of is Lithium Titanium Oxide [LTO] chemistry, which is very tolerant to slow charging (but not trickle charging).  LTO batteries are still very new and they do not have the nice flat discharge curve that LiFeP04 batteries have.  LTO batteries are currently used in very high reliability applications (like medical implants).

If you have a source of power that is unpredictable (such as solar panels, or some other energy harvesting device), it is best to store up energy in a capacitor, and then dump this charge into the battery at a C/2 rate, then let the cycle repeat.  The repetition cycle can be as rapid as your source will allow.  This type of charge profile will minimize dendrite growth and maximize the lifetime of your batteries."

 

I was thinking of a 150ah battery so I'd have plenty of room for expansion later. Initially I'd just have a 160W panel. At 10a charge I'd have a .07C charge rate and it wouldn't get much higher than that. A lot of the time it will be lower, with the sun at an angle and cloudy days.

 

Thoughts?


Edited by rruff, 20 November 2017 - 05:22 PM.

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#40 enelson

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Posted 23 November 2017 - 05:39 PM

I've been charging our 160AH LiFePo batteries at a low C current for around 4 years now and I can't tell any loss in performance of the batteries.  I wouldn't worry about charging from solar cells or a solar/truck combination.  Charging at C/2 is a lot of current on large batteries.  I probably won't get 20 years out of the batteries and will be happy any use after 10 years.


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