Quick Lipo Question
 

Im looking for few lipos for erevo and have few questions:
There are two different combination.
2x 4000 4s 20c packs in parallel
2x 4000 2s 40c packs in series
I know first combination will last longer, but will they both have same punch. Which one would you run.

Also, is 2s easy to maintain and balance compared to 4s packs.
I mean is it better to run 2s packs in series than 4s packs in parallel.

Does C adds up and remains same when packs are connected in series or parallel.

Thanks

Assuming you have enough charger power to charge at a full 1C rate, there is no diff in balancing a 2S or a 4S, so its not a matter of one being easier than the other, its basically the same process whether its 2cells, 4 cells or 6.

Wiring in series cause the voltage to add, so two 2S lipos in series becomes a 4S equivalent with same capacity. The C rating does not change. If we start with 2S 5000mah 30C lipos, in series we have a 4S 5000mah 30C equiv. The discharge current (5000/1000 x 30) remains at 150amps.

In parallel, the voltage stays same, but capacity adds. The C rating is unchanged. So this pr of 2S wired in series would become a 2S 10,000mah 30C lipo equivalent. Discharge amps would change because the capacity changed, not the C Rating - 10000/1000 x 30 = 300amps. It doubled because we doubled the capacity, the C rating did not change.

The combos you propose would both have 160amps discharge, they would be no diff electrically as far as disch amp are concerned. Physically, its easier to run two 2S in series since they're much smaller and much lighter. The diff is in the run time, the 8000mah from the paralleled combo will generate roughly 2x the run time. The decision for me would be is the extra run time worth the extra weight and does the RC physically have the room to accomodate the size of two 4S pacs. You'd also need to adj suspension as well to deal with the extra weight. I would be running in series.

Thanks for the info.
Also, what about if both of those packs have same C, ie 40C.
I know that the 4s packs in parallel with give more amp discharge.
Is it worth to have double the punch than 2s packs in series to disregard the weight 4s packs have. The total output will be 4s for both setup, but 4s 40C packs will have 320amps compared to 2s 40C packs in series with 160amps discharge.

Will 160amps more discharge rate is enough to oversee extra weight of the 4s packs or will they have little gain.
Thanks again.

Is it ok to charge Lipo batts at more than 4.2V per cells? I know most charges will set it to a default 4.2V as CV, but some chargers out there can be set to like 4.2+V or even 4.3V per cells.

The prob with this reasoning is that the motor needs or can even use this kinds of amp levels. The data logging I'veseen shows avgs up around 65amps for large motors with amp spikes of 140-160amps. The amp spikes are within the lipos burst ratings. So theoretically, the increased amps may make up for the weight, but the typical motor doesn't pull those kind of amps. You'd wind up heavy and way over batteried with a sluggish, long run time RC. Now if you're running some kind of giant scale plane motor, that it would be diff deal.

Not sure what your obj is behind the ques, but you may be better off with a HV setup, something like Castles new car ICE controller running 8S when its released.

The is a quote taken from ROAR -

8.3.2.7.5.2 Overcharge – Lithium Polymer cells are extremely non tolerant to an overcharge condition. A standard charge profile is CC/CV to 4.200V. Drastically overcharging a cell just once is a sure way to send a cell into thermal runaway. Overcharging a cell slightly but repeatedly is also extremely detrimental for a cell. For example, it you charge a cell to 4.300V, the lithium ions start plating on the electrodes forming lithium metal. Lithium ions are not flammable, but lithium metal is. Every slight overcharge cycle will plate more and more lithium metal resulting in a battery that is very prone to igniting. The best way to prevent overcharging is to charge through a balancer and to avoid chargers that do not charge with the standard 4.200V CC/CV charge profile.


Unless you are such a high level racer a few hundredths of a volt will not mean anything practically. You're not going to gain anything significant from overcharging your lipos - you're just degrading them every time you overcharge them. Only a few chargers will let you do this and I believe to only a few hundreths of a volt over 4.2v/cell, like 4.24v/cell.