Is my DIY battery pack safe to use?

[haulme]

Hi all

I was dismayed at the lack and expense of Li-Ion battery packs (lots of Li-po) so I thought of DIY'ing a solution using 2x21700 in series to give a capacity of 4000mah and 7.4v.

The battery is a 40T Samsung with a 35a peak discharge rate.

Using my 18650 charger I charged both to full and measured 8.2V wired in series.

I can remove each battery and charge them hence I am forgoing a BMS and the subsequent wiring.

Other than soldering an XT60 wire in place of the current one is this good to go or should I look to add a fuse in line to the Heng Long TK7.0 board?

[Ecam]

Series or parallel? I always get those mixed up.

Both positives hooked together and both negatives, gives same volts twice the amps. Connecting the hot from one to the ground on the next gives twice the voltage and standard amps.

I do the double amps all the time, and I don't use a fuse without issues thus far. Fuse isn't a bad idea, but would have no idea what to choose. The system will only use/take the amps it needs, just would have a longer run time.

Essentially you are making the power bucket larger, but the hole is still the same size as opposed to making the hole larger.

My larger DUKW will likely use 4 packs wired the same way. The smaller duck uses two packs but the model is quite heavy and for a reasonable run time it is needed. Having power dry up in the middle of the lake is inconvenient. The big one I am thinking of separated power packs for drivetrain and the propeller.

[Rad_Schuhart]

Those wires are way too thin.

Also using the case is not OK. Solder the cells and use thick wires. Otherwise it won't work good and it is even dangerous.

[haulme]

Thank you for confirming....Ill swap out for 16AWG but I cant solder the cells unless I spot weld them and wire up a BMS...is that the only option or can I charge each cell individually and use them in series like above?

[Rad_Schuhart]

Thank you for confirming....Ill swap out for 16AWG but I cant solder the cells unless I spot weld them and wire up a BMS...is that the only option or can I charge each cell individually and use them in series like above?

16 AWG seems fine, as well as the XT60 connectors, which are my fauvorite.
Yes, you can solder the cells with a normal 2 eur iron solder. use some solder flux paste and it will be easy and fast. I did it many times. You can also wire a balance plug for easy of charging with a smart charger, no BMS needed at all.
The amp draw using the case will probably be nowhere near what your toy needs so it will reset itself as soon as you hit the throttle.

Happened to me many times at the beginning of my experiments, lol. I use my own hand made packs in most of my gadgets, tanks, cars and even in the radio.

Anyway, an already wired 2S li ion 3.500mah should cost like 15 eur and will save you all the trouble.

[haulme]

OK Ill give it a try...I got the flux...so just put some flux on the contacts, a blob of solder, then heat solder and put the nickel strip on the heated solder?

[Rad_Schuhart]

OK Ill give it a try...I got the flux...so just put some flux on the contacts, a blob of solder, then heat solder and put the nickel strip on the heated solder?

If images are worth more than 1000 words, imagine a video, lol.

You have plenty of videos in youtube that can explain it much better than me. The optimal tool would be indeed that spot welder, but there are many tutorials with soldering iron too.

[Jimster]

I made some batteries per Rad’s directions then he turned me on to these little jewels.



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[HERMAN BIX]

can you get those with Tamiya type plugs Mr Jimster ?

[haulme]

You certainly can:


https://www.motionrc.eu/collections/adm ... 2s)%207.4v

I did look at those but its the postage(import fees) that add up. Had I bought my challenger 2 from them I would have ordered one at the same time but it was about £110 cheaper from China.