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Low cost DIY Lithium camper batteries.

Lithium LiFePO4 LFP DIY Batteries

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#1 rando

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Posted 14 October 2017 - 11:13 PM

I have been convinced for some time the lithium batteries were the future for marine and RV type applications and that lead acid's days were numbered.   Lithium batteries have many major advantages over lead acid, including 4 times the energy density, much flatter voltage curve and less picky charging requirements.   What has stopped me from installing lithium batteries in my Fleet flatbed was the huge 'early adopter' premium of ~$10/Ah as opposed to $2-3/Ah for high quality AGM or $1-2/Ah for flooded lead acid.   Recently browsing on ebay I found a guy selling new old stock LiFePO4 cells for ~ $1.50/Ah (at 12.8V) and decided to take the plunge.    The only draw back being that these batteries are 3.2V, 5.5Ah cells, so it would take some work to make an appropriate 12.8V, 150Ah battery pack.    I figured I would describe the process and results in case anyone else was interested in a winter arts and crafts project that results in a large LiFePO4 battery pack at about 1/5th the cost of a commercial lithium pack and 1/4 the weight and 60% the cost of an equivalent lead acid battery bank.

 

1. Batteries.   There are a lot of surplus Full River/Tenergy LiFePO4 cells available on ebay and amazon at the moment for very reasonable prices.  These are new old stock (manufacturing dates from 2013 and 2014) from a reputable battery manufacturer in China.  I bought 120 of the following 3.2V, 5.5Ah batteries:

http://www.ebay.com/...=item51efabf285

From seller 'dougdeals' for $180.   If you want to make a different capacity battery pack there are plenty other options for different quantities and sizes of batteries.  These are amazingly cheap - this is the equivalent of a 150Ah at 12V, for $180!   A similar sized battery would be about $1500 from manufacturers such as Battleborn.

 

2. Pack Assembly.  This is where the arts and crafts comes in.   In order to get the right voltage (12.8V nominal) you need to put 4 cells in series.    So I divided my 120 cells into four groups of 30 cells, and went about making 4 larger 3.2V batteries each comprised of 30 cells in parallel.  There are two ways of assembling individual cells into packs - spot welding and soldering.   I don't have a spot welder, so I went with soldering with a high wattage (140W) soldering gun and plenty of flux.   Before soldering up the 30 battery packs I needed a way to physically assemble them.   For this I used my newly acquired 3D printer to print out 5x6 battery plastic frames to hold the cells in a nice grid, but you could also glue the cells together or build a wooden box to hold them.   I then soldered all the cells together using 12AWG copper wire:

IMG_0523.jpg

The first one took me several hours, but once I had my technique down, it was probably only 90 minutes or so per pack.   It is very much craft work and is not hard to do

 

Once the packs were all soldered up, I attached about 20cm of 12AWG wire to the positive and negative of each pack, covered the exposed contacts with some 1/8" ABS I had lying around and then shrink wrapped each pack into some massive heat shrink tube (also from ebay).    All in all this was a weeks worth of evenings to do, and I ended up with four 3.2v 150Ah battery packs each about 7" wide, 8" long and 2.5" tall. 

 

3. Putting it all together.  To make the final product you need to wire the 4 packs in series to make the 12.8V battery.   This is where a couple of caveats about lithium batteries come in.  First, it is very bad to discharge lithium batteries below about 2 - 2.5V per cell.   While not a fire hazard or anything like that, it has the potential to ruin the cells.   Secondly,  lithium batteries don't 'self balance' between cells like lead acid batteries do.    That is to say that over many cycles the voltages of the individual cells can drift apart, so that some can end up over charged (not a big deal) and some undercharged (big deal).    Both these issues can easily be solved with a Battery Management System (BMS) which monitors the individual cells and shuts things down if they get out of wack and also applies corrective action to balance the individual cells.   Luckily BMS are now very cheap and effective.  I happen to have this one from a prior project and used it in this project:

http://www.batterysp...-30A-limit.aspx

But there are now much cheaper options on ebay and amazon. 

 

I wired all the individual cells to a blue sea terminal block and then used jumpers to put the cells in series:

IMG_0535.jpg

 

Continued...


Edited by rando, 15 October 2017 - 02:34 AM.

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#2 rando

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Posted 14 October 2017 - 11:16 PM

The terminal block made it super easy to wire up the BMS, and I also added a JST balance connector so I can watch and adjust the individual cell voltages (more later):

IMG_0534.jpg

 

I added another sheet of ABS and some standoffs to cover the BMS board and terminals, and added a couple of stainless M6 screws for the battery posts:

IMG_0531.jpg

 

Finally I added a battery balancer/monitor to display the battery voltage and to manually balance the cells, although my BMS balances automatically so it shouldn't be needed.    It plugs into the JST balance connector.  This connector also allows you to use an RC hobby type battery charge to apply a balance charge to the pack if thing did get out of whack at some point.

 

Continued...


Edited by rando, 14 October 2017 - 11:40 PM.

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#3 rando

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Posted 14 October 2017 - 11:33 PM

The whole thing is strapped to a 20" x 8" plywood panel with battery straps with some webbing handles for easy handling.   This fits easily in the battery compartment in my Fleet, but you could arrange the cells in other configurations (and pack them much tighter) to fit other spaces.   

 

4. Results The final product weighs 40lb (10lb lighter than the 75Ah lead acid battery that came with the camper with 3 times the usable capacity) and is 20" x 8" x 6":

IMG_0532.jpg

 

I charged it up using a standard 15A battery charger and ran it against a battery analyzer to measure the capacity - 150.346 Ah with an 8A load:

IMG_0536.jpg

 

I only charged the battery to 14.4V (3.6V/cell) and could probably get an extra 10% by charging it up to 14.8V, but 150 Ah is more than enough for me.   This is drop in compatible with my current camper electrical system, I just need to change the settings on my Victron MPPT charge controller for 14.8V bulk charge and 13.6V float, and the settings on the BMV-700 to represent the new much larger battery pack. 

 

5.Conclusion  For a few (well maybe more like 12-16) hours work and a total cost of ~$300 you can have a kick arse 150 Ah (or smaller, or larger) cutting edge lithium battery system at 1/4 the weight of lead acid that should outlive your camper! 


Edited by rando, 15 October 2017 - 04:27 PM.

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#4 rando

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Posted 14 October 2017 - 11:34 PM

PS If you are interested in trying this, I would recommend acting soon.   Once the word is out on these cheap surplus batteries, they likely will run out pretty quickly. 


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#5 Vic Harder

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Posted 15 October 2017 - 12:42 AM

very cool project rando!


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2012 ATC Puma, BattleBorn LiFePo4, Victron MPPT and BMV, AIMS 3000W inverter, Induction cooktop... loving it... 


#6 K6ON

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Posted 15 October 2017 - 02:14 AM

Thanks for the tip Rando, I just ordered two cases.


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#7 Happyjax

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Posted 15 October 2017 - 03:43 PM

Impressive build Rando! Do the balancers have to be connected to each cel? I didn't see as many wires as I would have expected.... Lots to learn about lithiums ::)


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#8 craig333

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Posted 15 October 2017 - 04:08 PM

Tempting.


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#9 rando

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Posted 15 October 2017 - 04:22 PM

Impressive build Rando! Do the balancers have to be connected to each cel? I didn't see as many wires as I would have expected.... Lots to learn about lithiums : :)

 

The balancers are only needed where the 3.2V packs are in series.   Parallel cells are all at the same voltage as their terminals are tied together so they don't need to be individually balanced.   For my pack there are two 'high current' terminals (black and red 12 awg wires) which are the main power lines on the -ve of pack 1 and +ve of pack 4.  There are also 3 'balance' terminals (yellow 16 awg wires) that are at the internal junctions of cell 1&2, 2&3 and 3&4.  Here is the wiring:

Lipo-cells-connection-anode-cathode-term


Edited by rando, 15 October 2017 - 04:23 PM.

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#10 rando

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Posted 17 October 2017 - 10:44 PM

I have a couple of cycles on the battery by leaving my fridge running in the driveway.  Seems to be working well - ~90% SOC first thing in the morning then back up to 100% before noon.   It recharges much faster than the AGM it replaced.   

One minor issue though - my Blue Sea ACR is always connected as the lithium battery has a resting voltage around 13.5V.   Not a big deal, but kind of waste to have an ACR that is always on.   I am looking for a new battery isolator that has an adjustable set point. 


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