Possibility of connecting multiple FW batteries for longer battery life

I dont know a ton about batteries, but seeing FW get picked up more and more by the DIY community (like DIYPerks) and also managing a small framework community of my own (and seeing people ask about battery life, im curious: How possible is it to (safely) use multiple FW batteries for greater battery life in a DIY enclosure?

Optimistically it would be nice if it was a relatively simple wiring job if you have the connectors and could DIY a splitter or something to connect two FW batteries to the one motherboard connector. but what little I do know about batteries tells me this is probably going to require some custom PCB solution and/or some kind of balancing circuit.

Ive had previous laptops that have dual batteries (one internal, one external/removable), so id be curious how they do it and if this could be mimicked by some framework-compatible DIY mod.

This is also similar to my other thread where i was thinking about trying to reuse those still-good batteries in my older lenovo laptop, which seemed to have more challenges as you mix voltages and capacities and stuff. Maybe using only FW batteries is easier?

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Having personally just torn into (CAREFULLY) an XPS 13 battery pack that no longer holds a consistent charge, I can say it’d probably be easier to parasite (for want of a better term) a larger cell pack onto the existing battery balancing circuit (the part that is inside the battery pack itself, not on the mainboard).

My plan, as I’ve said somewhere in this forum before, is to have cylindrical cells in a flat package in an extended bottom case, and simply tap into the existing battery circuit, probably minus the factory pouch cells. But then, I’m going for something monstrous, like 500+ Wh of capacity, so take my designs with a grain of salt. :grin:

If you connect two identical batteries in parallel, the BMS will see it as one battery with 200% Battery Health. The beauty of it is both batteries will be drained and charge at the same time, they will simply help each other out.

The key is to connect the batteries before the BMS

Here are some examples

I have done this on both my iPhone and Samsung phones, so they have 6700 mAh (1700 mAh original + 5000 mAh) and 12000 mAh (7000 mAh original + 5000 mAh) respectively, both saw a massive endurance uplift from the extra battery

You also need to make sure that the batteries have matching voltage levels. In general there are 2 types of batteries typically used in phones and laptops:

  • 3.7V cell, fully charges at 4.2V
  • 3.85V cell, fully charges at 4.35V

Those specifications are printed in the battery itself

If the original battery is a 3.7 type, its fine to add a 3.85 type. But if your laptop has a 3.85 type, you MUST also add a 3.85 as it would otherwise overcharge the poor little battery cell to 4.35V

In a laptop, the batteries are usually arranged in 3S or 4S layout, that is 3 cells in series, or 4 cells in series. So 3.7 type would have 11.1V (3S) or 14.8V (4S) printed on the battery. Meanwhile, a 3.85 type would have 11.5V or 15.4V

Remember to also always put identical battery in each part of the series. If you add 1000 mAh to the first cell, you must also add 1000 mAh on the second and third one, as the machine will simply pick the smallest one to determine the percentage

The possibility of starting a house fire goes up too!

or just plug a power bank into the type-c port…