DIY Lithium 18650 Portable Camp Battery Box

#21
What I have ended up with is a very light (around 3kgs) 21Ah useable box with 2x 5v / 2.4 Amp USB, 1x Engel, 1x Cig, 1x 240 Outlet using a 24v 600w Pure Sine Wave inverter. The battery I built using 18650 cells and is 24v so I have a BMS on it, a DC / Solar Charge controller feeding it that can step up and step down the power as required for the battery, a 24v to 12v buck converter and a shunt to measure whats coming out of the battery.
Hey - just stumbled across your LiPo battery and i am really keen to understand a little bit more. Do you have a parts list that you used? Potentially some links? I am sick of lugging around lead 100AH, especially when chasing the sun in wooded areas.

I will be attaching this to my solar panels and fridge, so i was wondering how you go about controlling the discharge and charge at the same time? I assume that all outputs go through your buck converter, and any input to the battery goes through a charge controller? So all power going out pretty much goes through the batteries first? Everythign else i am pretty good with, it is the simultaneous charge/discharge i am yet to get my head around.

Great looking project!

cheers,
 
#24
The charge side is handled via a DC / MPPT Solar Controller that can step up or down the voltage to suit the 24v BMS on the battery pack.

Example Charge Controller

I got mine from AliExpress at a better price but that link has a better diagram.

Another good thing about this unit is it is powered from the DC side and not from the battery so it does not add to the load when you are only discharging.

From the 24v battery / BMS we goto the 24v shunt and the busbar which has the 24v 600w pure sine wave inverter and buck converter on it.

Example Shunt
Example Buck

The buck then goes to all the different 12v outlets.

With this setup you can charge the battery from an unregulated solar input, a wall wart style charger (I use an old xbox one power supply) or the alternator in a car etc. If you are charging and discharging at the same time the shunt will tell you the delta, ie if more power is being generated than consumed or vice versa.
 
#25
Ah!

So you have the MPPT inside your box and you basically use that as a regulator and charge controller? Then you have raw solar feeding your box. you rely on the BMS to protect each cell.

The part im not sure of is the location of the Shunt in all of this. To pick up the delta, you'd need the battery charge circuit on one side and the solar input on the other. Though this seems like it would greatly confuse the MPPT and charge circuit.

I have attempted to draw my understanding of what you have said

Thanks! this is a fun little problem solve.

EDIT: The diagram below is wrong....
Code:
         +------------------------------------BOX-------+
         |  [BMS]----[CELL1]                            |
         |    |         |        +--[DISPLAY]           |
         |    |      [CELLn]     |                      |
         |    |         |        |                      |
         |    +---------+-----[SHUNT]----+-[24V/12V]----|--[12OUT]
         |                               |              |
         |                               +--[INVERTER]--|--[ACOUT]
         |                               |              |
[SOLAR]--|--[MPPT]-----------------------+              |
         |                                              |
         +----------------------------------------------+
 
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#26
I will draw you a diagram of how it hangs together, the MPPT controller tells me what is going into the battery and the shunt tells me what is coming out of the battery. Yes it's all fully self contained in the box.
 
#27
I will draw you a diagram of how it hangs together, the MPPT controller tells me what is going into the battery and the shunt tells me what is coming out of the battery. Yes it's all fully self contained in the box.
Ah - so it is are single direction shunt, which is what i had in my first diagram :)

Code:
         +------------------------------------BOX-------+
[SOLAR]--|--[MPPT]--[BMS]----[CELL1]                    |
         |                      |        +--[DISPLAY]   |
         |                   [CELLn]     |              |
         |                      |        |              |
         |                   [SHUNT]-----+-[24V/12V]----|--[12OUT]
         |                               |              |
         |                               +--[INVERTER]--|--[ACOUT]
         |                                              |
         +----------------------------------------------+
 
#28
Well - all of my parts are ordered! Once they arrive i need to pull apart my Laptop battery packs and test their capacity. Hopefully they will suit, as the price of 18650's in Australia is astronomical (AU$10 ea!).

I'm a bit excited about this! :D

I'll update the build as i go along.
 
#29
$10? try $25+ for high capacity 18650s, yes harvesting is the best option but make sure you fuse each individual cell when you aren't sure of the cells and make sure you balance the cells so each string is roughly the same capacity. When you do you first change and discharge test, let the cells sit for a couple of weeks and see if any are self discharging if they are bin them or use them for torches etc. Also check they aren't getting hot when charging.

Safety first when dealing with harvested cells, things can go wrong quickly if they aren't treated with some respect. If you take some sane steps it's perfectly safe.
 
#30
make sure you balance the cells so each string is roughly the same capacity. When you do you first change and discharge test, let the cells sit for a couple of weeks ....
yeah - a quarter of my costs so far are for a 4-cell tester and charger. I got the Opus BT-C3100, which seems like a fairly common one. I recovered a number of laptop batteries from work, which should be all good quality, but will test and rest. There is also a website that will tell you wwhere to put each battery to best balance the pack.

I'm going down a battery holder route (4P), instead of spot-welding. This should give me a lot of flexibility in how everything goes together, and make it easier to build the pack with standard tools

I have also opted against the shunt, and am just using a digital capacity meter, which essentially just measures the battery voltage to determine the capacity. Not the best, but cheap and should be fairly effective for at a glance understanding of remaining power.

Thanks for the inspiration - i'm sick of my Lead Acid battery!
 
#31
The 3500mAh cells in [7s]8p give me 28AH total, being lithium you can safely discharge them to 80%.
I noticed other references to 18650 cell packs were much lower (eg 7S10P 25Ah @25.4V = 648Wh total).

My next build is using reclaimed Cells that are coming in at an average of 3500mAh's after testing.
Extremely selected cells? This matters because the designed pack capacity (22Ah, 570Wh available) is pretty reasonable for a light camp with a fridge but if 2500mAh cells were used it would be more like 2/3 of the capacity. Fortunately, pack size can be increased fairly easily.

18650 Lithium Ion Battery Identification Reference.pdf
 
#32
I noticed other references to 18650 cell packs were much lower (eg 7S10P 25Ah @25.4V = 648Wh total).
Well it depends entirely on the capacity of the 18650 cell you are using, if you look at the reference you listed my LG MJ1's are 3500mAh. So it's just simple math from that point on. If you use a 2000mAh of course it will result is an overall lower capacity battery pack.

And yes when using reclaimed cells you are very selective and you make sure you balance the cells so pack sizes are of equal capacity, there are tools we use like https://secondlifestorage.com/repackr.php to get the balance right.
 
#33
I'm going down a battery holder route (4P), instead of spot-welding. This should give me a lot of...
... grief, especially in a vehicle. The cell contacts are reportedly inadequate, generating heat, with all sorts of undesirable outcomes. Only real alternative to spot welding seems to be soldering which has it's own issues.

I used Pure Nickel strip with a home made spot welder to build the pack and then soldered the BMS up to it.
On reclaimed cells where I don't know the origin I fuse each individual cell as well
Since this is the DIY section a bit more harvested cell build info would be good: design amperage, nickel strip used, how fused, BMS on the battery pack and wrapping. Any other considerations? A few pics of a harvested cell battery pack out of the box would be nice.
 
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