PF Battery Box with Li-Poly Rechargeable
Download PDF formatI decided to modify a Lego Power Functions Battery Box and fit with an 11.1 Volt Li-Poly Rechargeable Battery Pack & use a Regulator I.C. to provide the regulated 9Volts required by Power Functions type Devices. The modification was easy, but time consuming to remove the excess plastic in order to get the Li-Poly Rechargeable Battery Pack to fit inside the battery box.
Power functions Battery Box.
Front of PF Battery Box fitted with 11.1V 900mAh Li-Poly Rechargeable Battery Pack & the 9Volt Regulator I.C. with Heatsink attached at the top of photograph.
Back of PF Battery Box fitted with 11.1V 900mAh Li-Poly Rechargeable Battery Pack, the 9Volt Regulator I.C. with Heat-sink attached at the bottom of photograph & also showing the Li-Po Pack Charger Connection Plug.
The whole installation is rather fiddly to get the 11.1V 900mAh Li-Poly Rechargeable Battery Pack and the 9Volt Regulator I.C. with Heat-sink attached to fit comfortably within the Power Functions Battery Box. This said, it was definitely worth the hour spent modifying the battery box. The finished Power Functions Battery Box works very well and drives two Large Power Functions XL Motors with ease. The result is also considerably lighter at 120Grams, when compared to having the usual 6x AAA's Batteries as the power source in the Battery Box.
I could have went out and purchased a Rechargeable Power Functions Battery Box, but I wouldn't have had a 9volt output and the commercial one is harder to mount to Mindstorms & Technics models. My modified PF Battery Box, cost a total of less than $20.00 including the Li-Po Battery Charger, which is considerably cheaper than buying the Rechargeable Power Functions Battery Box here in Australia. All that's left to do is convert another Power Functions Battery Box, so I have a charged spare ready when required.
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Hey Ray,
Nice hack! Aren't you worried that the heat from the regulator could cause the LiPo to get damaged and catch fire? From what I hear, anything over around 80C can cause them to swell up and die a horrible fiery death.
- Xander
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Hopefully all will be well. There’s only a 2V drop across the 9V regulator & wth 2x PF XL motors running, the Heatsink is basically room temperature still.
I did dream off and cut the Li-Po’s Power Connector off in one hit which did create a noticeable hiss & flash. It was hard to tell if the old side-cutters suffered, as they have cut a few hot 240V a.c over their 20+ year history. “Great way to check for a faulty fuse!”
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You can get Low Drop Out (LDO) regulators that would only drop the voltage by 1V, leaving you with a lot more wriggle room. You may need special circuitry to also prevent the LiPo battery from being over-drained or it can get damaged. LiPos are super light but require a fair bit of care to stop them from being damaged. LDOs can be a bit more expensive than regular regulators (sorry). You can find them quite easily by searching for "LDO 9V"
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Thanx Xander, I’ll keep that in mind.
I paid $10 for two of them from China to play with. The Li-Po’s cost not much more than it did for 20x 9v regulators.
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Why you didn't try 14500 li-po batteries before crashing the box?
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