Building Li-Ion Battery Pack for Field Charging - Safety Concerns

[Andy-H]

I’ve been reading up on this forum about assembling a battery pack from li-ion cells for field charging, say 6S5P to give me 24v 30ah capacity or 6S8P for 40ah. I can understand why they are preferable and potentially safer than using big lipo packs. However, my concern is I believe e-bike batteries use this type of battery and there’s lots of press coverage about them catching fire. The alternative might be to use a couple of LiFePO4 leisure batteries, but they would be heavier. 
 

Is the difference that e-bike batteries tend to be fast charged, which is when problems can arise,  whereas a battery pack for field charging use is likely to be slow charged ? 

 

 

 

 

 

[Frank Skilbeck]

I believe on the e-bike batteries it's more the aftermarket batteries, either substandard or not having built in over charge protection.

[Phil Green]

My electrics are smaller models, typically with 500 to 1000mAh packs so I wanted a portable charging pack to suit. Shaun kindly gave me a bundle of ebay 'used' A123 M1 LiFePo4 cells and I made a 4S5P pack which is 12v 11.5Ah, plenty for me at an Old Warden weekend and also handy for general use such as camping and winter power cuts. I've also made several ebike packs 10S4P 18650s, and a huge pack of A123s for a racebike project, so I'm used to the odd spark or two 😉
To answer the question, its a personal thing but from a safety aspect I'm much happier with LiFePo4.  I'd happily have the LiFePo4 packs in the house but I'm more wary of the large Lion 18650 packs.

The thing is, contrary to advice given on this forum by a serving Fire Service Officer, very, very occasionally they can and do spontaneously combust - though I'd suggest the media like to hype-up the few occurences, and given the average flyers knowledge & respect for lithium I think we're probably safer than the average joe public :)
I think most ebike fires happen during charging, with daily use it could be frayed charger wires etc, faulty charge plug/socket, or a bad (or absent!) BMS, you never get to know the actual reason. 

The pack shown here doesnt have a BMS, but I balance charge it just like any other 4S pack.

Cheers - Phil

 

Edited November 19 by Phil Green

[Peter Christy]

I just put an old car battery in the boot! As long as its charged regularly, I get plenty of charges at the field...

 

--

Pete

 

[john davidson 1]

2 hours ago, Andy-H said:

I’ve been reading up on this forum about assembling a battery pack from li-ion cells for field charging, say 6S5P to give me 24v 30ah capacity or 6S8P for 40ah. I can understand why they are preferable and potentially safer than using big lipo packs. However, my concern is I believe e-bike batteries use this type of battery and there’s lots of press coverage about them catching fire. The alternative might be to use a couple of LiFePO4 leisure batteries, but they would be heavier. 
 

Is the difference that e-bike batteries tend to be fast charged, which is when problems can arise,  whereas a battery pack for field charging use is likely to be slow charged ? 

 

 

 

 

 

My E-bike takes six hours to charge, so just a trickle and I don't think there is a problem. Still charge out doors though, even though the pack is removeable for charging and can then be put in my  purpose built  metal box station for plane lipos

[Paul De Tourtoulon]

I took one of my Ebike battery's to pieces, it was just an ordinary lipo in a case with a few wires and circuit boards in it.

[Andy-H]

Thanks all. Will mull it over - on the one hand a diy pack Li-ion pack would be cheaper & lighter, but with risks associated with soldering 30+ battles together. On the other, it would be simple to just buy a couple of 30ah Life batteries which are fairly small. 
Very interesting to hear others’ views on the subject. 

[Geoff S]

2 hours ago, Paul De Tourtoulon said:

I took one of my Ebike battery's to pieces, it was just an ordinary lipo in a case with a few wires and circuit boards in it.

 

I've taken a couple apart and they were both Li-ion.  There is a charger circuit powered from a simple nominal 42v max rough dc mains supplied charger.  I've converted several pedal cycles to electric assist with a powered front wheel and a special rear pannier to carry the battery that works well.  My wife's bike battery had an intermittent problem somewhere so I bought a new one (including pannier etc) and stripped the old one.  I'd be surprised if your battery was any different.

 

 

[Don Fry]

I’ve got a 4s LiFe pack I soldered up, in a custom built ply case. It powers the winch wot lifts my 1500 kilo boat onto its trailer without sweat. It also powers chargers at the flying field. Simple with a blade fuse. 
There it gets to be a shared unit. Nice battery. 25Ah, £50 from memory. Good value, 4-8 years ago.

[Andy Stephenson]

I think it's a good idea to put Kapton tape between cells where they touch and where the cans are at different potentials because PVC heatshrink sleeve can melt through and short-circuit if they overheat. Kapton is good to 400°C.

[Philip Lewis 3]

Or better still use comercialy available battery cell holders, that also allows airflow between individual cells, also it's better to spot weld than solder using  pure nickel strip.

[Andy-H]

Both good practical suggestions, thank you. I have ordered a spot welder as mentioned above, which seemed less likely to cause heat damage. 

[Philip Lewis 3]

If anyone else is interested this is a good spot welder, three pulses per weld, first heats up the strip, second welds and third aneals, well thats the theory anyway, and with twelve mosfets it's got a lot of power, you can set the delay and power level for each pulse. 

[Phil Green]

I use one of these, works very well. Shaun has a posher one with an OLED display.

https://www.aliexpress.com/item/1005006226735630.html

Comes with a good reel of nickel strip. 

Mcuh quicker & easier than soldering, although I soldered the negs as they already had tags.

Remember A123 M1's are polarised the other way around to what you'd expect - case pos, button neg.

The Kapton tape thing is a good point, mine are assembled with strips of thin paxolin between rows.

 

[Neil R]

As mentioned in Duncan Disorderly's original thread on here, I built myself one of these field packs - I attempted to make a direct copy of his. It's had a big impact on the number of flights I've been achieving, but next time I'd do one thing slightly differently. 

 

My conscience still occasionally bothers me about the +ve end of each cell. Apart from the small positive button itself the entire remainder of the cell casing is negative. This includes the casing overlap at the +ve end of the cell. Connecting the cells with any sort of flat strip, or soldered wire in my case, doesn't leave much margin for error in not damaging / relying on that small amount of plastic protection that the cells come with - failure of such would cause a dead short. 

I see in the photos above that Phil has added some additional red heat-shrink at this critical location (it's the -ve end in his A123 example), plus I can maybe see some slight arching of his strips. Both strike me a good practice, and is what I've done when going on to make actual Li-ion flight packs. (Noting that the normal types of nickel strip don't carry enough amps in that application). You might also consider buying some additional stick on insulator rings - something like these: 200x 21700 Battery Insulation Gasket Adhesive Cardboard Rings For Battery Green | eBay.  

 

[Phil Green]

Thanks Neil, its actually thick-wall silicone tubing rather than heatshrink.   I soldered the negs (button ends) as they already had good tags.  The pos/neg thing can most confusing when you use both Lion and LiFePo4!  😉 

[Frank Day]

Combined IC and electric box. Stanley wheeled 3 tier tool box bottom section has a deep discharge 12v caravan/boat battery, linked to a lipo charger good for 15plus 5000mah 6S charges or so. Jack plug for charging and a power panel with inbuilt fuel pump, starter etc. Top section has 5l fuel tank with external overflow. Centre section has plenty of room for tools and batteries. USB phone charging point. Not pocket sized but good for one particular site.