4s to 3s battery conversion help

[Abba]

Hello, so today I had crashed my quad and I guess the battery got punctured cause I started to smell a sweetish epoxy like smell coming from it. So I then checked the cells for damage, and turns out the 4th cell was completely dead.

I then carried on to dissect the battery and desoldered and snipped out the dead cell. Everything seems to be functioning as anticipated, battery charger is detecting 3 cells and reading 3.9v each.

SO, considering that I had an 1800mAh 4s (14.8v), and took out 1 cell to make it a 3s (11.1v), would the mAh also go down with it?

I was thinking since its 1800mAh/4cells =450mAh per cell right? So 1800-450 = 1350 new mAh capacity?

Or does it only drop the voltage and leaves the mAh capacity in tact?

I know it is a pretty dumb question to ask, but if anyone has input, I'd greatly appreciate it thanks

 

[ElectriSean]

Each cell is 1800mah, so you only dropped voltage, your capacity is the same. In a series circuit, voltage rises and drops are additive, current is the same. In a parallel circuit, voltage is constant and current/capacity is additive.

 

[PsyBorg]

Wouldn't that effectively drop the C rating after losing that cell to draw from as well? In any case that is a crashed battery pack that took damage. You are taking a huge risk even charging that battery even though you checked for damage. Though not punctured the other cells could have crush damage thus changing the bond inside the packs and potentially raising internal resistances. Yes you CAN use it but should you and are you willing to take the risk.

 

[Snarls]

I don't think the C rating will change. Packs are built by selecting cells with similar qualities like IR and C rating and placing them in series. In a series configuration the amp draw from each cell is the same, so each cell will have to withstand the same amount of current. Remove one cell while keeping the amp draw the same and each cell still has to endure the same current as before, there's just less mAh to supply. Of course with one less cell the amp draw will probably be different than before.

Yes be careful with the repair battery. Make sure you monitor it constantly while charging and even keep an eye on it when you aren't charging for a few days. I have a 4s battery with one bad cell that is close to 0V, and the other three still perfectly at storage voltage (I haven't touched it in months). I plan to reduce it to 3s but I will be very careful.

 

[commandosqueak]

As someone who has made battery packs before I can tell you from experience it isn't worth it. Safely dispose of it and buy a new one. It really isn't worth the risk. If that catches fire you basically have a flying fireball. Now the rant is out the way...

A battery pack wired in parallel would keep the same voltage but lose capacity, A pack wired in series as yours is would lose voltage but not capacity. So it would now be a 1800mAh 3s (11.1v)

 

[Abba]

Thanks for the replies guys, if i lived in the states and had access to 3 day shipping/ hobby stores that have lipos readily available, i would most definitely toss that battery.
But due to the fact that i live in Japan, it is very difficult to ship them here, also takes up to a month or more for them to get here. All of the hobby stores around here have batteries made specifically for premade store drones which have terrible C ratings. Ive seen very few of which i would use, but those were approx 200 USD and i kinda dont really want to drop a fortune on those crap batteries that may not even hold up.

 

[commandosqueak]

It sound like your between a rock and a hard place. I would proceed with caution as you don't know what damage has been done to the other cells. To be safe I would fully charge and discharge 3-4 times in a safe area with a bucket of sand nearby. FT did an episode on a battery bunker, that would be ideal for this.

What ever you do DO NOT charge it inside!!!!!!!

 

[ElectriSean]

A battery pack wired in series would keep the same voltage but lose capacity, A pack wired in parallel as yours is would lose voltage but not capacity. So it would now be a 1800mAh 3s (11.1v)

You are completely backwards, though your end numbers are correct. The voltage drops, not the capacity. His pack is wired in series.

 

[commandosqueak]

You are completely backwards, though your end numbers are correct. The voltage drops, not the capacity. His pack is wired in series.

Thanks I corrected it!