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Aug 23, 2020
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Hi, I have a question about battery safety , based on internal resistance.
Have you checked internal resistances? I recently checked a couple of batteries and ISDT charger shows very different values per battery and cell. The video is saying if one of the cells shows different values, it’s time to dispose because it maybe dangerous and also damage other lipo batteries when doing parallel charge. Thus I would like to ask 3 questions;

1 - how do you decide to dispose a battery , due to the gap between cells' resistances? For instance, if one of cells shows more than 10m ohm difference, do you dispose battery?

2 - how do you decide to dispose a battery , due to the average of internal resistances? For instance, if average of internal resistance hits 20m ohm, do you dispose battery?

3 - how do you decide to not take a battery for parallel charge because it might damage other batteries? For instance, one of battery has extra 20m ohm and/or one of cells has more/less 3m ohm, then we should stop do parallel charge, but still charge the battery one by one.



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20ohm is a lot of difference. I'd definitely balance charge it separately first before parallel charging it if it was me.
My smaller batteries (350-450) tend to have much higher resistances than my bigger batteries. If I find big differences in the cells I balance charge them and they usually come back. If it's a lot I mark them and keep an eye on them.

Just don't charge your batteries unattended and make sure they're in a safe place when you do incase something goes wrong(like not on a pile of papers next to your laptop)
Ive been researching this topic too... and also monitoring my batteries very carefully....

the conclusions Ive come too are :

check batteries for swelling and physical damage.

check for cells that have 100% over other ( I think once you get over smaller) IR,
I think particularly after they have been balance charged - as they should be even after this!

check for batteries getting warm/hot during charge, if you are charging at 1C they should remain cool.

I read cell resistance increased with age/wear/use, a general guide is something like:
cell IR < 10mOhm = good/new (they seem to start between 3-5), 10-20 = used/ok , 20+ getting time to retire.
(of course, how you use, will determined how quickly they 'deteriorate')

look for cells not charging fully, or not holding charge - so unbalanced voltages.

if you see imbalances, or are concerned about a battery, you can try **balance** charging at a low C (0.2C - 0.5C) (on it own not a parallel board)
keep checking its now swelling, not getting warm... if it gets to full charge, then fly it **gently** to 3.8v, see if its remains balanced.
if its getting unbalanced, or not holding charge - time to bin.
if it looks ok, do the same for a few cycles to see if its 'recovered'.

generally my understanding is do not parallel charge, if there are large differences in cell voltage or internal resistence across a single battery, and also do not parallel charge batteries that have big differences in voltage/ir
basically view all the cells across all batteries as one battery... and you do no want large differences in IR/voltage across any cell.

I personally have decided against using parallel board - I know they can be used safely, but given number of batteries use, Id prefer to play it safe.
Ive got a SkyRC dual charger D100, but think Im going to buy a D200 quad charger.. as I usually fly 4 packs per session.

my favourite article on this (and many things) is by oscar liang

my main take away, is don't take risk , for the 20-30$ you get a brand new/better battery - and importantly you dont risk a fire!

of course, someone on the internet will tell you, they dont take care, and never had a fire... im being too cautious.
but each person has to decide on the risk based on thier own assessment.
(we all have different circumstances/risks - it may be they charge in a fire proof box in an out building, so one popping may be not a big deal - vs, someone else charging in thier apartment on the 15th floor)

I'd love to now others thoughts....
from what Ive read, its not an 'exact science', we know when a battery is more likely to 'explode', but its not 100% predictable, so its more about precaution than solid rules.

one final tip, which someone told me, and I think is a good one..
keep a rough log of your batteries (from new onwards) of things like IR, charge/flight current, flight time

it can be really quite 'rough', not every fly, not precise numbers
the idea is simply, to have a rough idea of a batteries health, and how its deteriorating over time.
this will help you know, when its time to replace... as you'll not only see it might be damaged, but you're getting worst performance/flight time.

seems like a bit of a faff, but a pretty good idea.
Last edited:
Thanks for quick response, mrmud ! Thanks for sharing deep knowledge, thetechnobear!!

So far, I got a trouble once. When plugging battery to happymodel drone, circuit was short and got tiny spark and chemical smoke came out . I think problem is old charger was not good quality to make balance charging. Since using new charger ISDT, I have not got any problems during charging at all.

Before a balance charging, I always check each voltage of battery. If voltage difference is more than +-0.1v, I separately charge it.

A store person recommended to use single charging , not parallel charge.

I've never pay attention to resistance because of so far no heat-up, no internal warning, no problem. However, I will retire 20ohm's battery to avoid any potential risks.

Best regards,

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