is there any issue doing this?

all batteries are the same.

looking to run two batteries in parallel.

I have three batteries that were purchased at the same time, and one purchased a few weeks later, but is the same specs/make/model.

the older batteries have had maybe one charge/discharge each.

they are turnigy 4000mah, 20-30c, 3s.

not that it makes much difference though

thankyou!

you shouldnt have any problems as long as both the battery's you are using all have good cells. they should all be around the same voltage. for example. if you have two, 2s 1p 30c lipo's. each cell should be about 4.20 volts. know if all your cells are running around there your're golden. dont panic if you have one battery that has one cell at 4.20v and the other at 4.17v. your still ok. but sense your running them inline i would make sure to balance charge them so each cell stays close to eachothers voltage as possible. if you have any more questions just hit me up.

O and never run two battery's together that have two differnt C ratings.

I'm running a pair of Turnigy 3S 1600mah 25C lipos in parallel in my Mini-E-Revo. Works really well and provides a ton of run time.

If you have a high amp draw setup, running different C ratings can cause the lower C rated lipo to overheat. If your amp draw is not that high, just watch the lower C rated lipo and make sure its not getting warmer than the other. The safest thing to do is use the same C ratings.

I figured it would be alright

That will give you an 8000mah, 40-60c, 3s pack. Keep an eye on motor temps during your runs. Don't want to melt anything else down...

It isn't any different than running them in parallel. Actually, it would be safer for the batteries if they were ran in parallel instead of series. Imagine you have a lake and you are trying to fill a swimming pool. With parallel wiring, it would be like running 2 hoses into the pool. With series, it would be like running a hose and connecting it to another hose to get it there.

As parallel, the batteries work separately and the one that can provide the electricity the easiest to the esc will do just that, whereas in series, they have to work together. The esc just pulls what is needed at the given moment so doing it either way is irrelevant in the end. If you really want to get technical, running 2) 14 volt batteries in parallel would be better than running 2 7 volt batteries in series.

Also, all lipos are made at 3.7 volts. They just series them internally to get to 7.4 (2s) 11.1 (3s) 14.8 (4s) etc....

...I am aware of how the whole series/parallel thing works.

I was asking because I know lipos can be rather unfriendly at times.

If it was any other battery chemistry, I would have just done it, no questions asked.

Running the lipo's parallel like this, do you have to charge them one at a time or is there a way to charge them both at the same time?

Seen it done both ways - charged together and charged separately. Charging is not the prob - you just use same parallel connector that you use to run them in parallel.

Balancing is the issue. For charger to 'see' all the individual cells, they have to be in series. You can charge them in series and balance them this way, then run them in parallel as you want to.

As long as the lipos start off close together charge and balance wise, there shouldn't be any probs.

I would never charge any batteries in parallel.

I use two 5S 3000mAh 20C LiPos in parallel with my 1/8 buggy and it work great but I always charge them separately with balancer.

Here is my parallel connector for direct connect to the esc(MMM):