DIY Lithium battery

[Skxawng]

My 110ah leisure battery has died and looking at replacements. 

I was thinking, lithium technology is far better suited to our application than lead acid: 

-You can use 80% of the charge  without affecting the lifespan as opposed to 50% with lead acid.

-Lithium has much longer lifespan (around 3x more charge cycles) making it a better investment.

-It has a much higher energy density so will take up less weight/space for the same capacity.

-Charge times are much faster.

-Lower self discharge.

-Orientation doesn't matter.

So why do we all use lead acid? 

I looked for an equivalent 12v 110ah battery on ebay. Same form factor as mine but lithium and 1/3 of the weight.

£800!...

So I thought about building one. 

Electric cars like Tesla and nissan leaf, laptop batteries (as well as solar power walls) use an array of 18650 Lithium cells. They look like AA batteries but a bit bigger. They typically have about 3Ah capacity and cost a few quid each.

You can buy grids that they fit in, build it to your own specifications of volts and capacity by the number of cells in series and parallel then a £3 BMS board controls the charge/discharge and monitors the cells. There's plenty of vids on utube and it's easier than it sounds to build with very little electronics knowledge.

I'm going to have a go building one. Just wondering if anyone else has thought about using these 18650 cells?

 

[BigFoot]

I have an interest in this sort of thing as I plan on building a stealth off grid camper van and am keen on various battery storage etc. 

 

[spruce]

My 110ah leisure battery has died and looking at replacements. 

I was thinking, lithium technology is far better suited to our application than lead acid: 

-You can use 80% of the charge  without affecting the lifespan as opposed to 50% with lead acid.

-Lithium has much longer lifespan (around 3x more charge cycles) making it a better investment.

-It has a much higher energy density so will take up less weight/space for the same capacity.

-Charge times are much faster.

-Lower self discharge.

-Orientation doesn't matter.

So why do we all use lead acid? 

I looked for an equivalent 12v 110ah battery on ebay. Same form factor as mine but lithium and 1/3 of the weight.

£800!...

So I thought about building one. 

Electric cars like Tesla and nissan leaf, laptop batteries (as well as solar power walls) use an array of 18650 Lithium cells. They look like AA batteries but a bit bigger. They typically have about 3Ah capacity and cost a few quid each.

You can buy grids that they fit in, build it to your own specifications of volts and capacity by the number of cells in series and parallel then a £3 BMS board controls the charge/discharge and monitors the cells. There's plenty of vids on utube and it's easier than it sounds to build with very little electronics knowledge.

I'm going to have a go building one. Just wondering if anyone else has thought about using these 18650 cells?

No, but don't the Telsa battery packs involve a necessity for cooling as well to prevent them overheating. This may not be an issue for us drawing power as our demand rates are much less, but how would you recharge them?  Wouldn't heat be a problem with fast charging?

 

[Skxawng]

No, but don't the Telsa battery packs involve a necessity for cooling as well to prevent them overheating. This may not be an issue for us drawing power as our demand rates are much less, but how would you recharge them?  Wouldn't heat be a problem with fast charging?

This is a picture of a typical DIY battery pack: 



Basically a plastic grid that comes in squares that snap together so you can build it in any configuration. It's spot welded with nickel strips to connect the cells and a circuit board a 'bms' is glued to the front and the strips attached in the 4 corners.

This keeps the temperature down with spacing between the cells, also the cells are monitored and used evenly by the BMS. Also there is over current and temperature protection as well as over charging protection and it cuts power when the voltage drops too low to avoid damaging the cells. So you shouldn't be able to draw enough current to cause problems at least at this scale. It can still provide 25A of current at 12.6v though.

There are 5 rows in parallel of 3 batteries in series in this one in the picture making it a 3s5p but I would make a 3s10p which would look the same but twice as long. You can make it as big as you want- the longer the more capacity.

I'm having a little trouble figuring out how people charge them. I think it's pretty straight forward but I haven't worked out the details. You do at least charge the whole thing as a single battery though, you don't have to charge the cells individually. I think with this one you just use a standard lithium battery charger on the main wires and the BMS doesn't interfere except it will cut the power if it detects the cells are getting overcharged.

Edit: just seen you can get a solder less kit too:

 

[Nudel]

Now this is interesting @Skxawng. I was about to write a lengthy warning about exploding li-ion batteries and dangers of overcharging etc. but this seems like a proper solid solution. Just don't get the cheapest cells and charger.

Might have a go at this. I just bought a 12v battery pack for a trolley system I'm building for a friend.