I am about to build a battery box for my tablet but I would like to post this and see if there are any problems with it….like maybe starting a li-ion fire by doing something wrong :bigsmile:
My tablet is an Asus TF300T and has a custom 15v USB charger. Here is what I’m building:
Ingredients:
4x 18650 battery box in series with leads
12v car adapter for my tablet that outputs 15 volts
12v female car connector with leads
I plan to solder the 12v female car adapter to the battery box and plug the 12v tablet adapter into it to charge my tablet. The tablet adapter takes up to 16 volts input so 4 18650 batteries in series should output the correct voltage.
My question is: If I draw all the 18650’s down to dead, will this damage them or cause a fire?
if you deplete the 18650s past 2.8v it will cause capacity/life cycle loss… below 2.5 volts will damage them even more
will they cause a fire? i dont think so, but if you go to charge a pack of batteries that has been discharged to ~2.5v then you will have a high risk of an explosion/fire etc.
you might want to look into including a pcb protection circuit that would cut off the packs voltage at 2.8v
i have not built packs like this myself, but many other members will chime in here
Well first of all you need to use protected cells. Second of all, these will need to be balance charged which means you will need to buy a hobby charger. (Or just charge them all separately)
Thanks for the quick replies. I have a 4 channel charger for charging my 18650’s. All my 18650’s are laptop battery pulls so they are all unprotected. I guess I will have to use protected 18650’s for this unless someone else has other suggestions for modifying my project.
Yes.
Edit: Here it is charging my tablet. If I didn’t suck at soldering I’d have it all hacked together by now. I am using unprotected 18650’s from an old laptop battery. They get a bit hot while charging my tablet so I hope they don’t explode
How many Wh does your tablet have?
How much charge do you plan…?
I have some IPads and with 4 18650 you can charge it 100% maximal…
Edit: Ok I checked the asus has only 20Wh so you charge it up 2 times with good cells and high efficiency circuits…
For the unprotected cells you could try the battery holders with build in protection circuit, these are available from fasttech…
Thx for the links…I have a question. I tested the voltage b4 hooking up my tablet and it was 16.9 volts. That’s 4 18650’s in series. I looked at those protection circuits and they seem to be for 18650’s in parallel. Is this correct?
