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Hi, I just bought 3x 3.7v 2200mah li-ion batteries at my local electronics store. And I connected them all in series, for 12v of course. Some of these batteries are pretty expensive, but i bought the cheapest, one for 4€. So I they costed me 12€. But if you have a broken laptop/Notebook, you can reuse those. I bought online some broken laptops for very cheap just to reuse their batteries and other stuff that might be usefull. Hope I could help you.