Volt meter for Powerwheels

As many of us dads strive to be the cool dad and improve our kid's toys, we find that many many toys are in need of upgrades. My son wanted real headlights, a horn and a trailer for his F150 Powerwheels toy. I made the modifications and installed a dual battery power supply. The new setup lasts longer and fades slower than the stock set up but I was still always wondering how much was left on the batteries when out playing. Will we make it back without taxing the batteries at a low charge?

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