A compact, power-efficient way to parallel lithium batteries that can be individually hotplugged without interrupting power - we couldn't find it for sale, so we made one.

Takes two batteries, passing only the higher voltage to the load until the input voltages match. Then it draws from both batteries in parallel - so it's basically a parallel battery board. If you want to parallel lithium batteries, you could use it for only that, avoiding battery damage or fire from voltage imbalance and back-charging, and lengthening battery life and run time - but...

Its real utility lies in the hot-swap function. If one battery is removed or dies, the other is brought online instantaneously. You can power a mini-PC like the Raspberry Pi¹ from a pair of power tool batteries, swapping out dead batteries while it continues running uninterrupted.

The LEDs indicate which battery is online (not which has charge or not). You can still remove a battery without interrupting power, regardless of LED status, if the other has sufficient voltage remaining to power the load. The LEDs are paired to the side of the board that a given battery is connected at.

• Runs on 1 or 2 batteries (2 power sources required for hot-swapping function)
• Uses any kind of battery: alkaline, lithium, rechargeable, even mix batteries of differing chemistry, design, and voltage
• Outputs what you input² - 12V not required. Listed as 12V³ because it was designed for power tool batteries, but it will work down to 2.9V and up to 16V.

Battery backup and extended uses⁴:

You could build an alternative with a few resistors, but 5-50% of your power would be lost heating the resistors uselessly. It could damage your batteries, start a lithium fire, or significantly shorten your battery lifespan - and it won't run nearly as long.

The power this board uses is only a tiny fraction of total input, so your battery life will be significantly increased. With its intentionally tiny size to boot, it is ideal for mobile applications.

• Size - 22.86 mm x 15.24 mm x 3.25 mm (1.6 mm PCB, 1 oz. copper, 0.25 mm traces)
• Holes (Mounting) - 2.1 mm dia. holes @ 19.05 mm x 11.43 mm
• Holes (Connection) - 1 mm dia. holes @ 2.54 mm pitch

¹ We tested a different mini-PC, not the Pi specifically. Your load's sensitivity to the non-zero dip in power during battery swapping will depend on its details. This board does no voltage conversion - use batteries matching your voltage needs.

² The higher voltage of two sources, that is (or the source connected, if only one).

³ 10A max - see the LTC4353 datasheet, and datasheets for Fig. 1 components there. The LEDs and LED resistors are sized for a 12V circuit. They may be dim at lower voltages (not affecting any other functions).

⁴ Read the LTC4353 datasheet before building anything more complex than what we tested: two 12V rechargeable batteries and a 12V 3A mini-PC. The circuit diagram included in the images is for that use.

⁵ i.e. "an arbitrary number of" - within reason and component limitations. Read the relevant component datasheets.

ALL CLAIMS ARE ILLUSTRATIVE AND SUGGESTIVE ONLY. WE MAKE NO WARRANT OF FITNESS OR SAFETY FOR ANY PARTICULAR USE, NOR ACCEPT ANY LIABILITY FOR ANY OUTCOME RESULTING FROM USE OF THIS DEVICE. LITHIUM BATTERIES CAN BE DANGEROUS.