16 replies to this topic

[enelson]

We've recently purchased a K2 model for my Tundra.  I wanted to set up a modern battery system using solar and the truck power.  After a bit of research I chose lithium iron phosphate (LiFePo4) batteries.  They are a safer and cheaper alternative to the lithium batteries in laptop PCs and cars like the Nissan Leaf.

 

LiFePo batteries are around half the weight of lead acid and can also be discharged much deeper than lead acid.  I don't have the numbers off the top my head but my 160AHr batteries are similar to a 250AHr lead acid battery in terms of the amps that can be pulled out per discharge.  In addition the LiFePo batteries have a life time around four times longer.  This is important since the 160AHr worth of batteries set me back around $800 by the time they were delivered.

 

The batteries are cheapest when ordered as cells and then assembled into the 12V battery.  The cells are 3.3V and four cells in series comprises the battery.  It is important that the cells be "balanced" so that the voltage is very close across all cells.  This balancing is accomplished through a set of cell balancing electronics.  I my case, I chose and integrated charge controller, cell balancer, and battery management system (BMS).  For lithium batteries there are very few integrated BMS that are designed for solar recharging.  An engineer from Siemens has a hobby product that he offered through a Kickstarter campaign.  The system is the Electrodacus SBMS4080.  The best intro to the system is in this video.    The system can be used with 4 cells instead of the 8 cells normally used in a 24V off the grid home.  If anyone happens to get there hands on a SBMS4080 I can walk them through setting it up for 4 cells.

 

For solar I chose the Global Solar 90W peel and stick panels.  They are a good deal from the eBay seller, 

http://www.ebay.com/...l-/261358136079  These are produced in Germany so I was more assured in the manufacturing quality control than some other panels.

 

I made a frame for the batteries out of 1" 8020 and mounted the SBMS4080 to the frame. The BMS needs to be close to the batteries.  While I don't have a remote display the access hole that Bill had built for me is enough for me to check the system whenever I want.  The 160AHr of LiFePo batteries is a huge amount of power for our needs.  We could easily go 3 nights without recharging and run the frig and furnace no probem.  A few pics below will give a better idea of what it all looks like.

 

 

Edited by enelson, 05 January 2015 - 02:22 AM.

[enelson]

More pics

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[photohc]

Thanks for the post! I'm interested in this bms after seeing MJLorton's review of the Electrodacus SBMS4080 a few months ago. I missed his kickstarter but understand that he will be starting another Kickstarter later this year for version 2. 

I'm not clear on how you manage the power coming from the truck alternator, is there a input for another source other than solar?

Also, the center photo on your second post, is that the battery bank? Where and who did you get these cells from? What is the weight of the entire battery bank? Thanks. 

[enelson]

I ordered these http://www.evassembl...hdjo5ammek28g66  I guess it was more like $1000 by the time shipping was added.  I'd say for us the 160AHr are overkill.  She had to have a microwave when I was planning the system but changed her mind and uses the stove.  I would probably go 100AHr if I were to do it again.

 

When using 4 cells with the SBMS4080 you nee to tie together the unused voltage sense lines and then shunt them to the negative terminal of cell 7.  Get the board and I will explain in more detail later.  The new unit will take up to 100A input and the 120A output.  I will probably buy one of these in case she decides we need a big inverter.

 

One thing we noticed in AZ over the holidays is that with the panels mounted horizontally with the low winter sun angles we were lucky to get 6A out of the two panels.  I'm going to mount two more 90W panels on the roof.  There wasn't enough roof line to get the extra two panels to fit.  I'm making so mounting boards so I can mount the additional panels east/west.

[enelson]

The weight is on the spec sheet.  I didn't weight the whole assembly but I'd say around 50lbs.

[enelson]

The truck power feeds into the same input as the solar panels.  The power comes from the trailer plug and is the power that is intended to charge a backup battery for electric trailer brakes.  This line can source around 6A of current.  I'm satisfied with the small charge from the truck since these LiFePo batteries can be damaged by charging too quickly in temperatures below 25F.  In cold weather the batteries should only be charged at a current of capacity / 30.  I'm my case this is 5.33A.

[photohc]

Sounds like a super cool setup. Do you know if his current BMS is available for purchase? I looked around his kickstarter info and didn't find an option to secure a board. (I guess I could email him.) 

Yes, with the angle of the panels on the roof, the production from them diminishes quickly if the sun is not directly overhead. I solved that by having a light weight flexible 100w panel mounted on a plastic corrugated panel that I moved around during the day. With 140 w panel on the roof and the 100w flexible panel, I can usually get the batteries back to 100% by 1pm. I have two AGM 70ah at a total of 120#'s. I would love to get rid of that weight.

I have a little experience with LiFePo batteries having a set I use with my ham radio. It's amazing how much power they have for the size and weight.

Thanks again for the information. Hard

[enelson]

I'm hoping to hinge an east/west mounted panel so I can tilt it up 60 degrees for winter camping.  The portable panel is probably the way to go, but I'll have 360W on the roof soon and I will be able to handle the reduction in charging current from low sun angle.

 

A 100AH LiFePo bank would give you more useable capacity and would have a long lifetime if properly charged.  I think I ended getting in on the BMS purchase after the Kickstarter but before he shipped out the units.  I set a donation to his PayPal account via a link I can't find anymore.  I'd try emailing him.

[photohc]

I had an email chat with Dacian last night. He is making more of the SBMS4080 this month but also offered more info on his next generation version. I'll wait to see what that will look like in March. 

I'm thinking about the 100AH version of the LiFePo4. How did you bind the 4 cells together?

[enelson]

Used some materials I had around work to strap them.  The stuff is called Smartband and is used in subsea installations.  The band material doesn't really like going around 90 degree corners and isn't the best solution.  A set of ratchet straps would work.  If you have access to a press brake you could form some aluminum straps.  Also, there are LiFePo4 cells out there that all already banded together.