Share your knowledge, electrical guru's
 

Newish to the hobby, well lets say newish to the new stuff but a question about a y connector for nimh batteries. I want to connect my different capacities nimh speedstick batteries (900 & 1200mah) on a y "lead" to run other vehicles. I want to know if it will run the 900mah too low vs the 1200 when they are on a y connector or if the mah "combine" to run down the same voltage generically speaking to keep it simple at this point. I know enough in life to make me at least dangerous in most things and humble myself to those that are specialist's in specific topics.

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Re: Share your knowledge, electrical guru's
 

I’d assUme it’d work. The total mAh would just combine...?

If you’re serious and sticking around you may as well invest in lipo batteries.

Re: Share your knowledge, electrical guru's
 

When you run batteries in parallel (and series I would imagine) you should use the same capacity and C rating for batteries. If not you many drain one more than the other when you run it down, and the general idea is to have them balanced so they both do the same amount of work and drain evenly.

Also keep in mind wiring them in parallel will have the same voltage with higher capacity. Wired in series will give higher voltage and the same capacity.

Re: Share your knowledge, electrical guru's
 

C rating. Forgot that aspect. The discharge rate is vitally important.

Re: Share your knowledge, electrical guru's
 

Remember that Lipos are made up from individual 3.7v cells. A 2S is two cells in series, a 3S is three cells in series and 4S is four cells in series and so on. The "S" stands for series.

Lipos in series can only provide the lowest C rating of the pair while the voltage is added together if you disregard the small internal losses. Thus putting a 2S 40C in series with a 2S 75C gives you very close to the equivalent of a 4S 40C.

In parallel the C ratings add up so that the same two lipos above will give you close to a 2S 115C. Either way the lower C rated battery is working harder than the higher rated one which is why no manufacturer uses different rated cells to make batteries.

Even with fairly well matched cells after some use we see different voltages in each cell. This is due to manufacturing tolerances and lower tolerances cost more money which is why there is a wide range of prices for similar rated Lipos. The wider the tolerance the quicker some cells will degrade. This will happen when you use different rated Lipos in series or parallel, more so in series than in parallel.

In parallel the lower rated battery will show more voltage drop under load. The higher rated battery will be giving more current under these conditions so it tends to even out somewhat.

Additionally your Lipo cutout stops at a certain voltage. With Lipos in series one battery may drop below the safe voltage damaging the cells.

This also applies to other chemistries except for the Lipo cutout.

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Re: Share your knowledge, electrical guru's
 

Related/Unrelated, can I ask here if 1) 2x identical 2S 2200Mah 40c in a series connection maintains a 14.8 discharge for 4400Mah of discharge? Or all things being equal, in layman’s terms, at full throttle, vs. a single 2S 2200Mah 40c battery...roughly the same amount of run time, and what we’d expect running a 4S battery with ____ Mah. Maybe that oversimplifies everything, but I’m stretching my head around ballpark figures, I guess.
2) and if we show the same 2X 2S in parallel, does that, again in layman’s term, work out roughly to twice the run time, like I might find in a 2S 4400 Mah battery?

Re: Share your knowledge, electrical guru's
 

In series, assuming similar lipos, the Mah rating is the same as one of the batteries. In parallel you get double the rating or twice the run time.

So, as always, nothing is free. You can double the voltage or the runtime, not both.

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Re: Share your knowledge, electrical guru's
 

As long as they are both fully charged when you start, and the same voltage of pack, they will parallel just fine. I did this frequently on my RC cars growing up. The 900 will not dump first because the voltages of the packs will be the same, while the amp draw will not. Do not charge in parallel.




2x packs in series will have a higher voltage but same mah. 4s 2200mah. you must also use identical cells. Any smaller MAH pack will get damaged if put in series with larger cells, the opposite of running in parallel.

in parallel, it would be 2s 4400 mah.

Think of runtime being represented by the physical size of the pack. If you doubled the voltage with two packs in series, but then halved your gearing or kv to keep wheelspeed the same (and thus wattage needed), you now have twice the battery pack and almost twice the runtime. Additionally, if you doubled the mah with two packs in parallel, you would have nearly twice the runtime. Only nearly, because the extra battery weight increases load slightly.

Re: Share your knowledge, electrical guru's
 

Not only because of weight but also because of slightly lower efficiency of less voltage as well. Lower volts equals higher current, higher current equals higher losses due to heat ie. Resistance.