voltage vampire???

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earthwormjim
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voltage vampire???

Post by earthwormjim »

quick question.
I have a made up 2s battery pack.It works and charges fine,but it drains slowly.The +ve has a switch connected,but the -ve connects straight into a regulator,and on around the robot.Where is the drain occuring,is it possible that there is a "leak" within the battery,or is voltage being sucked out through the -ve,by the regulator,or another scource?????
Cheers,Jim. :)
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BeligerAnt
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Re: voltage vampire???

Post by BeligerAnt »

Charge can only leak away if there is current flow. Current can only flow around a circuit, from the battery through a load and back to the battery.

If you have an open switch in one battery lead, the only possible leakage path is within the battery.

LiPo cells do "leak" (charge) when they are damaged. Can't remember the exact failure mechanism off hand, but it's something to do with the internal separator breaking down, meaning that the cell will not hold its charge due to an internal leakage path.

It is possible to damage cells with heat during soldering. It's tricky to get enough heat into the tab without getting too much into the cell! Far easier to buy pre-assembled packs if possible :)
Gary, Team BeligerAnt
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earthwormjim
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Re: voltage vampire???

Post by earthwormjim »

ah,yep.Fair enough! One of the tabs was a slippery little sucker.Took longer than I was hoping to take,and in the end it took 3 or 4 attempts!!!!
Thanks Gary. :)
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earthwormjim
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Re: voltage vampire???

Post by earthwormjim »

I had a look,and for easy soldering,I had the regulator in the circuit before the switch!
Not a good idea,I think.I replaced the battery with a standard 2s lipo pack,and 2 days later,the pack is down to 1 or 2 v.
I will re-do the wiring!
Cheers for the help,Gary. :)
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BeligerAnt
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Re: voltage vampire???

Post by BeligerAnt »

Yes, the regulator will have a standing current even when there's no load on its output.
Even if the standing current is only 1mA, it will flatten a 300mAh battery in 300 hours (or less!).

The worst thing about having anything connected to a LiPo pack without a switch is that it will happily discharge the battery below 2.5V per cell, at which point the battery becomes landfill :(

Battery packs for phones, laptops etc get around this problem by having a protection circuit built into the battery pack. This prevents over-charging and discharging of the pack.

Since the LiPo packs used in antweights (and pretty much all R/C models etc) do not have a protection circuit it's very important to ensure that the switch is connected directly to the battery - or remove the battery altogether when the robot is not in use.
Gary, Team BeligerAnt
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peterwaller
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Re: voltage vampire???

Post by peterwaller »

Yesterday when I tried to run one of my robots at the Birmingham event I found I had left it switched on from when I tested it a few days before.
Having swapped out the battery I have just been trying to recover it and it seems to work even though it was down to 2.6v for two cells.
I don't for the minute recomend this (unless you like fires) but by connecting it to a variable PSU I gradually increased the voltage, making sure the charge current never went above 0.5A, until the battery was up to 7v. I was then able to connect it to my charger which so far has got it up to 7.87 V and still charging. I have just checked the balance as I am charging unbalanced and the cells off charge are at 3.82 and 3.81 so it is looking good.
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BeligerAnt
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Re: voltage vampire???

Post by BeligerAnt »

Yes, you can (sometimes) recover cells from over-discharge. The trick is to do it very slowly, otherwise the flames escape :o
Some chargers will do this automatically, others will simply refuse to charge cells if the voltage is too low. Sadly I suspect most will just fry the cells for you... :(
Gary, Team BeligerAnt
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