18650 Battery Case Project

[anth_85]

This is a write up of the project I have just completed.

I first started using 18650 lithium cells in flash lights on my bicycle for commuting over winter. I then read about how you can get good quality cells out of laptops, and that went on to buying lights and phone charging power banks all ran off the 18650s I had been collecting. I work in IT so get access to quite a few laptops before they go out for recycling. Around December time I counted up and had about 160 of these cells so started to think what I could do with them. Obviously I didn't need them all for lights and charging mobile phones in my caravan, (I camp off grid at dog shows so rely completely on batteries).

I had the idea of putting them together to create a case that anyone can plug stuff into to use at the caravan, AND for it to be lighter and have a bigger capacity than a 110 amp lead acid leisure battery (of which I already have 2).

I started to put them together in packs of 36 cells. Running in 3 serial and 12 parallel. I wrapped them in shrink wrap and put balance charge leads and xt60 plugs on them, the same way a RC car etc would, just on a larger capacity.

I have now made 6 of these. I have tested all the cells in them by charging them then leaving them for a couple of weeks then checking their voltage any drop from 4.2 to below 4.1v and they were discarded.
Cells do have a range of makes and models which is to be expected from where I sourced them for free. So I have guessed roughly on average of 2amp per cell, some will be more, some will be less, gives me a rough capacity of 144amps
I have 2 BMS 1 per set of 3 packs, I am planning on increasing this to 1 each, but this is a budget project. They are not plugged into the balance charge leads because I will be charging through my skyrc b6 mini they are only there to protect from over discharge.
I have installed 3 12v car plugs and 4 USB charge plugs.
I have also installed, a 150w inverter than I had spare and ran this to a plug on the outside to make it user friendly.

There are 2 switches on the front, 1 for the main on off for the unit, and another for the inverter, I don't want the inverter running all the time as it has a fan and wastes power when not in use.

I did some research and some people doing similar things were concerned about temperatures. So I have installed a 80mm computer fan, and a temperature controlled relay to automatically turn this on around 22C. I have also installed another temperature sensor with display on the top so I can keep an eye on it, both have their probes between packs. Finally there is a voltage display to I know the state of charge.

Internally my concern was amp draw for cable overheating. So a lot of the cabling has been doubled up. It is standard 13amp mains cabling so 26amp should be sufficient for now. The XT60 connectors are rated at 60amp. Both of these are higher than the BMS will allow before they cut off.

It will be use for the first time this weekend as it is my first weekend away in the caravan so I will see how well it performs. My aim is to not have to worry about the caravan lights and waterpump, the two leisure batteries can easily handle that. This case is going to keep our laptops, phones and tablets charged, run the beer fridge for the BBQ on Saturday and hopefully the projector (yes you read that correctly I have a projector in the caravan).

My original target was a something user friendly, lighter and larger capacity than a 110amp lead acid leisure battery. It is certainly more user friendly, you just plug in what you need. I haven't weighed it yet but I think it is around 15-17kg so it probably just over half the weight of a leisure battery and it should have a larger capacity.

Future plans, I have got another 40 cells so far so the next pack I make up will have larger copper plate connections and thicker wiring and probably xt90 connections. Eventually all packs will be migrated to that with some high gauge cable and some form of connector that I can plug into a 1500 or 2000w inverter so my other half can run her hair dryer off it, that is still quite a way off though and I have a few problems to work round for that.

I have read all about the risks of using lithium and I am aware it does look a little like a bomb :? If anyone has any thoughts of where I could improve the design or any input in general it is appreciated.

 

[Nobuo]

My respect you gained at first post . Man, this is top-tier DIY, my sincerely congratulations.

I have a couple of questions.
-What are the average capacity and brand of each cell?
-What are the overall Ah?
-I see that is working at 12V , how is the Voltage of the 150W inverter?
-what are the target stuff to feed with that?
- Are the BMS's connected inside each 3S pack?

All the batteries looks more or less like a bomb, but yours is a first price

 

[parabellum]

Is it an unisolated temp relay hanging over alloy inverter?
How are your cells connected? (spot welding?)

 

[anth_85]

My respect you gained at first post . Man, this is top-tier DIY, my sincerely congratulations.

I have a couple of questions.
-What are the average capacity and brand of each cell?
-What are the overall Ah?
-I see that is working at 12V , how is the Voltage of the 150W inverter?
-what are the target stuff to feed with that?
- Are the BMS's connected inside each 3S pack?

All the batteries looks more or less like a bomb, but yours is a first price

- most cells are samsungs, probably around 60% since we had a batch of 25 identical laptops that came with identical cells, I'm not sure what model they were but i think I googled one of the codes and they were 2200mah. 10% panasonic, 10% sony and the rest had no markings.
-overall Ah, like I say I'm estimating at 144Ah, but I haven't tested it, im not totally sure of the best way to do it?
-I haven't checked the voltage of the inverter, what's the best way to do? Stick a voltmeter in? I have tried running a 24" dell monitor and my macbook pro from it and they were fine.
-target stuff? I guess what I'm going to use it for? 2 macbooks, a 8" fan, projector, iphone chargers, ipad chargers, 55w powered cool box, probably only 1 or 2 at a time really.
-BMS are outside each pack. There are 3 packs in parallel with a BMS. This is mirrored on the other side.

Is it an unisolated temp relay hanging over alloy inverter?
How are your cells connected? (spot welding?)

Yes it is, schoolboy error, I've shrink wrapped it now.
Cells are soldered, I don't have access to a spot welder, it was very time taking, work was very quiet :lol:

 

[parabellum]

I have read all about the risks of using lithium and I am aware it does look a little like a bomb If anyone has any thoughts of where I could improve the design or any input in general it is appreciated.

Isolate/isolate/isolate.
I hope you did it well at soldering cells together (mostly at positive end).
Now avoid any pressure and rubbing on cell terminals, like the lowest pack for example, you should orient it the other way, unless horizontal is its normal position and anyway I would pad cell terminal sides with some foam.
Avoid and help to avoid human errors, they make those batteries to bombs in most cases, (laminate some user instruction or warnings on the case if some one else is authorized to use it)

Nice work!!!!!! I just noticed one aluminum case at home, doing nothing!

 

[anth_85]

I'm with you, I've got load of foam from stripping it out. Ill run that round, thats gave me the thought of mounting something to separate the batteries from the inverter altogether, an panel cut and bent into a U shape possibly. Probably out of some plastic.

 

[scouttster]

Nice looking packs! The only reservation I have and maybe I misunderstood??? Is about when you said you used cells with differing amp hour ratings? I was always taught that when making packs with the 18650 format you must use cells with the same identical ah rate? Still a noob here and maybe I learned wrong??? However, great job on the pack build

 

[999zip999]

I like a gasket on the pos. end. To keep the buss from touching the neg. case.

 

[anth_85]

I weighed it and my two leisure batteries before leaving.
110amp enduroline lead acid -25.5kg
110amp Hanfords lead acid - 29.8kg
Lithium ion battery case, 13.5!

It's 3-4kg lighter than I expected, really happy with that. It's been powering my powered cool or for 3 hours this afternoon with voltage dropping from 12.6V down to 11.8V under load then recovering back to 12.6V with it removed so I'd say that it's barely touched the capacity.

 

[anth_85]

Another update. I have finished recharging the packs one at a time. 5 had gone down to about 33-34% and one was still at 99% so I guess that had a circuit break it in somewhere which I will investigate.

I have put into the 5 packs, 18amp, 17amp, 15amp, 15amp and 14amp. So between those 5 packs it will give me a rough total capacity of 118.5amps. Once I get that 6th pack working properly I should be somewhere between 139 and 145amps.
I have been recharging at a low 1amp since this is the first recharge they have had in current form, I am also really pleased with how balanced the cells all are. None of the packs were more than 0.01V difference between them. So I feel more confident in buying a balance charge board to do them all in one go, not totally decided whether to do this or keep recharging individually, it's not that much of a hardship to go to the shed and switch them over once an evening for a week or so.

 

[anth_85]

Long time since my last post on this thread. I last used the case when I was away in the caravan in September last year. In the time I've been using it, it has been a godsend. The ease of use has even been mentioned when my wife took it to kendal calling for her Hen do. There was 15 women there and they all wanted to recharge their phones off it. Someone stupidly plugged their iPhone mains charger into the inverter though and ate loads of power, but that was just user error, and since I wasn't there to police it, I guess it couldn't be helped. It still laughed at the power draw and came back at 10.4v which I think is about 30-40% capacity left.

Over the winter I have pulled 3 of the 6 packs apart, an retested the cells on my imax b6mini. I have found the majority to be find and holding 2000mah+. But some where less than 1500mha. I am now in two minds what to do, I haven't managed to re-test all the cells in the pack I pulled apart. But I am about to start wanting to use it again. Ideally I'd separate all the cells, rank them by capacity, and build them back up in 3s12p configuration, going from all the highest capacity in one pack, down to the lower capacity in one pack, meaning when I got more cells I could chuck out the lowest capacity. The lowest used was going to be 1800mah. As it stands with only testing 50%, I am not sure whether to pull the remaining 3 packs apart and test them 1 cell at a time, and be without the case will I'm away in the next few months, or I can build up 3 pack from the best cells I've re-tested and then re-test 1 remaining pack at a time. Meaning I'll end up with less matched packs than i'd have liked. I don't have enough cells to make 6 packs of 3s12p purely of 2000mah cells tested between sept 2015 and today.