To clarify a lot of confusion
here:1.) cell = a single electro-chemical container, in the case of lithium cells considered here = 3.7 volts nominal2.) battery = (properly) multiple cells, in the case of lithium batteries here considered, any multiple of 3.7 volts, for example, a 7.4 v lithium battery3.) series = connecting cells (or batteries) so that the output voltage is additive. Preferably performed only with identical capacity, voltage, and type cells/batteries. Example two 1.5 v 3amp alkaline hour flashlight cells in series yields 3 v. at 3 amp hours.4.) parallel = connecting cells (or batteries) so that the output voltage remains the same as a single unit, but capacity is additive. Again, preferably done only with identical capacity, voltage and type cells. Example two 1.5 v 3amp alkaline hour flashlight cells in parallel yields 1.5 v. at 6 amp hours.5.) The above
can also be done together. In other words, serial batteries can be connected in parallel to yield increased voltage and amp hour capacity. So yes @enotar, it is
possible to connect
batteries in parallel to achieve 7.4 volts
. Assuming the batteries are both 7.4 volt batteries
. If you are saying additional batteries won't fit in the case, that's something different than saying it's impossible to connect them in parallel. Some people have modded cases, and some are considering external attachment.6.)
BUT (big but), being able to do something doesn't necessarily mean you
should do it. While the batteries most people are used to are either disposable, or relatively tolerant if rechargeable, lithium batteries and cells are not so casually used. Particularly, Lipo cells (the kind in soft cells found in the Eken) If you wire lithium cells together in combinations that the charge circuitry and protection circuitry are not designed for, you are asking for a damaged cell at the least and possibly an explosion and a house fire.Why is charge protection circuitry essential? Because Lipos can be damaged by either
under voltage or over charging, or also
over drawing its current limit. Once it is damaged, it is a potential fire bomb. The most common physical sign of damage of a lipo (other than a fire or explosion) is swelling of the cell. Once a cell is damaged
it should not be charged again. It should be removed from your house and recycled/disposed of properly. And that doesn't just mean tossing it in the wastebasket.Proper protection circuitry
senses when a cell is damaged and refuses to charge it again. That may be why a lot of people's Ekens appear dead, or refuse to charge. It may be that either one cell (of the two in series in the unit) is over-discharged, or is not balanced with the other in voltage beyond acceptable limits, or the charge or protection circuitry senses a charge current problem or cell voltage problem, or the charge sensing circuit is itself damaged.If you add an external battery to an Eken in parallel with the internal (series connected) cells, You may be defeating the internal cell protection circuitry (I say may because no one knows what that is exactly). and you may be overloading the power supply as well. Power supply burnouts appear to be epidemic in Ekens even without user added battery mods. These burnouts themselves can conceivably damage cells or even protection circuitry.In addition, it is unlikely you are adding external cells that match the internal ones in size, type, and even charge level. I suppose if you took the batteries from another same revision Eken and added them this way they would match in type and brand, though the charge levels would also likely be different. You would therefore need to balance charge them, in other words charge the two battery banks separately, not paralleled into the same charger. Most people will not do that unless knowledgeable about lipos, and in possession of a balance charger. The external battery should also have a protection circuit in order to regulate any backfeed from the internal batteries and vice versa if either set gets unbalanced. if you bypass the Eken's charge regulator and solder to the cells, you can conceivably back feed them at too high a rate, unless they have internal regulation. I don't know about that. Nor has anyone here said yet.In none of this am I saying that additional cells and batteries or replacement batteries are absolutely impossible or unsafe to install. I hope to do that myself at some point. What I am saying is that with the current state of a
lack of knowledge of the charge circuitry in the Eken, its protection method, and the state of its own internal cells in any individual unit, and the wide range of lithium batteries and cells people are proposing (based often on ebay price alone!) that if this continues
I do expect to see some serious mishaps here, just like on the RC group forums I participate in. A house burning or two in these pages might get people a little more serious about this subject, but I'd really like to prevent that by urging caution here.Personally I think any addition of cells in parallel with the the internal cells is potentially more problematic than replacing them. Others may disagree.Ideally, I think replacement of the internal cells with Li-ion rather than Lipo cells of known characteristics, and an appropriate battery protection pcb and/or possibly a 3 wire lead out to a genuine high quality Lithium balance charger set up to work with those characteristics is a safer way to battery mod. Possibly better even would be a removable battery that could be charged outside of the unit in a fire safe container. But all battery mods carry some risk which is very dependent on the abilities, knowledge and care of the builder.To each his own, I just hate to see people with no electrical experience, and no knowledge of the requirements of lithium battery charging proposing or performing modifications to the batteries and charge circuitry of this tablet. Or others saying it is simple. Yes, using Paypal to buy a battery and soldering it in place is simple. What you have, sitting on a nightstand perhaps a month later and smoking is not.