Battery on battery charging

Question

Can one Li polymer 18V 3100mAH charge another? If so, contact to contact ,or induction? Drawbacks? How long to charge? Thanks for your help

jw0752 · Accepted Answer

Hi Randal, Yes if you are supplying votage to the charging circuit of a battery you will need a higher voltage than the battery in order to charge it. For example: if we could charge one battery with another battery of equal voltage and one was fully charged and the other is at its minimun the best we could hope for would be 2 batteries with half a charge. I suspect that 2 times the voltage might be a little more than needed depending on the design of the battery and its charger but yes that is the right idea. John

e14 Contributor · Answer

Thanks John

supper_slash · Answer

Randal, what are you working on? what type of battery, what type of chargeing system, ect. Think about amp hours, i have a Lio batt, with 1.5 Ah capacity. If it was drained, and i was to connect in parallel, another identical battery that is fully charged, Some of the charge of the charged battery will charge up the dead battery. But most battery's are not designed to charge this way. Now, assuming that the battery's would not destruct, the best you could hope for would be a 50% charge of the dead battery, and a 50% drain of the charged battery, however. In reality, that would not be the case, because you will loose some in heat, so it would be closer to 1\3 would be given off in the battery's getting hot., thus, 66% left, divided into 2 battery's, leaves us at around 33% charge at both battery's. Problem number 2 is battery's are not meant to be charged this way. They use other charging circuits to charge them, because there life would be dramatically reduced. Problem number 3, is, it would be relay hard to achieve a full charge anyways. Because no matter what the charge of the Dead battery is, the best you could hope for is half the difference of the two battery's. Example. : 0%+1/2 of 100%=50%... 50%+1/4 of 100%=75%.... 75%+1/8 of 100%=87.5%... 87.5%+1/16 of 100%=93.75%... 93.75%+ 1/32 of 100%=96.875% Now, that is 5 times that you use a fully charged battery to charge a dead battery, just to get over 95% charge. And that is assuming no loss in heat, or anything else. So, it is not recommended to charge a battery that way. That is why we are not suppose to mix dead battery's, and new battery's in a flashlight. if you use it for any time, the battery's may leak, explode, short out, and if nothing else, go dead very quickly. If i have miss understood your question, i am sorry. But i hope this helps.