SparkFun Forums

[Tsaven]

Hey guys, two part question.

tl;dr - 2x 7.4v 700ma batteries. Can I hook them up in parallel? Is there any power filter/smoothing/protecting stuff I should put into the circuit?

I have two 7.4v 700ma aircraft batteries that I'm going to use to power my version of the Chopsticks bot that I saw on Make's website, it's an eight-legged walking robot using 24 mini servos for the three-jointed legs. It's all being controlled via an Arduino Mega 2560.

First off, is it okay to hook two of these batteries up in parallel as a power source, assuming they are equally charged? And if I do this, is there any kind of protection circuitry that I should built to protect the batteries/Arduino/Servos from when I inevitably do something really stupid?

Disclaimer: I'm still very new to electronics and my understanding of electricity is only slightly better than "Voltage is like water pressure, amperage is like water volume!" Pretend you're talking to a fifth-grader and I should understand you fine

[jremington]

LiPo batteries are tricky. They can be extremely dangerous if overcharged and quickly destroyed if discharged too far. I recommend that you spend some time with google. There are several RC forums and websites that discuss in detail serial/parallel connections, charging, protection circuitry etc.

BTW "700 ma" doesn't make sense. Your batteries may be rated as 700 mAh (milliAmpere hours, which means they have the capacity to deliver on average 700 mA for about one hour total).

[stevech]

Batteries in parallel, no matter the chemistry type, is not done. Reason: They will tend to equalize, with the lesser one consuming current produced by the other.

One can "OR" them together using two diodes, at the expense of the forward voltage drop (about 0.6 V).

[Duane Degn]

If the batteries have the same chemistry, capacity, state of charge, then yes they can be parallelled. SparkFun sells a pack with three LiPo cells in parallel.

The parallel pack SparkFun cells does have a PCB, but IIRC the protection circuitry is between the cells and the connector. The batteries themselves are directly joined together. (I've taken one of these apart to change it from parallel to serial.)

There's a good video on YouTube about charging LiPo packs in parallel.

[chartle]

Any reason not to get a bigger 7.4 v battery?

There is one here that's 2200mah PRT-10471 but you should easily find one in the 1,400 1,500 mah range.