You may add all the battery or solar panels that you want in SERIES. when you add them in parallel problems arises not all batteries or solar panel are created equaly. therefore the dominant will compensate for the weakness of another. example 1.25v another 1.22v one will try to stabilize to a nominal value while doing so it over discharge while the other is realy overcharging from the other. a diode in series or a very small resistor will try to limit these conditions.