Abstract : How-to “hack” a battery charger to achieve the initial charge (if not done, for instance, on a brand new battery newly delivered from China).
I recently ordered 2 « Litium Polymer » batteries from china.
My Lipo model
Both are 3S / 3300mAh…
For the record : it is too big to “fit in” a standard ZMR 250 frame build… 😯. It can be bound on the top, but will fill this stage, preventing other mounts…
Then, I will have to find out a way to “hack” the frame with an additional piece, to make it fit UNDER the first stage (but not on the ground of the frame)… Which is an exciting challenge. 😆 Ideally, more the center of the gravity is inside the plan defined by the four motors, easier will be the control (further if done by learning, natural stability will avoid random crashes).
I formerly bougth an IMax B6mini… Actually a genuine one (checked the hologram S/N on skyrc website). For the price it was a very good deal !
1 – Unboxing
So when received the batteries I tested them immediately using the charger, but also in-flight voltage monitors (there is a bunch of clones on the market, mine are 1-8S LiPo tester with buzzers and a LCD dispay, which beeps when individual cell voltage falls below a preset limit, see illustration below).
One of the LiPo tester implementation — many clones are available
All measures were converging, the diagnostic made no doubt : one of my battery was under-charged (overall under 7.5V, all cells under 2.5V), where the storage value should be around 11.2V – 11.4V (3.8V/cell).
The charger was OK to start a charge cycle on the “good battery” but not on the “bad one”. By the time I was not able to understand why…
2 – The time of questions
Nevertheless, the problem at that point was to find out if the battery was completly broken, or if something gone wrong in the process (like missing an initial charge after manufacturing ?)
I wrote to the vendor, but I found out chineese guy with a poor english level (which was quite bad at first, and but got better, upto medium, after i uploaded photos as proof -_-‘ ).
On his side he seemed not fully surprized nor embarasted, but proposed a silly deal : me to bring it to repair and he to pay for the fees. Why it is “silly” : first, the cost of the repair could reach easily the price of the battery, second, I am not aware that we have any vendor with such activity here in France… But maybe i didn’t search enough, maybe some toy/hobby vendors could help. Anyway, for a 18€ battery, I suppose the cost would be relatively prohibitive.
I decided that, if a random vendor would be able to achieve a charge, I should be able to do the same, armed with an engineering degree, my charger and the Internet. And I was right !
I found out that :
- First, as low as the voltage is dropped on a battery, you can still theorically charge it. The biggest concern is that internal resistance increase exponentially as voltage decrease. If one do a charge with high current on a battery with high internal resistance, it wil make the battery heat a lot. For a LiPo, it means (probably) setting it on fire.
- Chargers have a cut-off voltage for battery recharge : it allows more security, as chargers won’t allow to charge an under-charged battery and set it on fire dumbly. It a good point, but I needed to breakthrough/workaround this, as the cut-off voltage is typically 2.5V.
3 – The Time for the Solution
The solution came from a very good blog on RC toys, and this article (sorry for original author’s site, it was easier to find it again on Instructables).
You just have to set a Ni-MH or any other charge program that dont feature this cut-off. Program it with minimum current as possible to limit the battery heating and go.
initial charge : 0.1 A under Nickel – Metal Hydride recharge program / minimal current
I just let it charge something like 15/20 minutes. At first, the charger limited the current to the battery so it displayed 0.0A, but the voltage was increasing quickly on the display.
After a while the current reach the driving value (0.1A) and voltage increased slower at this point… Upto 9V, which was the program threshold.
Volatge after initial charge : it’s quite better !
At this point it was easy to go throught config menu and to start a LiPo cycle “STOrage” charge, since all cell are above 2.5V…
Normal LiPo 3S 1A storage charge
And that was it ! Now the battery is stored at correct voltage storage level in LiPo gard sack ;°)
Have fun with LiPos, and take care of them… And of you and your stuff.
PS : No heat was detected by touching the battery with fingers during the process. Just feeling like it was still at room temperature : 18°C.
I will never find out if my battery was “discharged” or “never charged”, but my belief is going to the latter.