Charging Multiple 3S Lithium Ion Packs Together

Question

This is my current situation, just to provide background:

I wanted to build these small battery pack modules that were 3S2P as a modular system, where a pack could be used with many different purposes like powering a power bank or a light or powering my Segway or acting as a power all during a power outage. I’m planning to add a BMS/Balance board to it that is 25A. The cells I will be using are the Panasonic NCR18650Bs. Their charging current is 1.6A and I have 2 in parallel, so that would be around 3A just to be safe.

Now onto the part I’m having trouble with:

How do I charge these? I want to build a box on the wall that has slots for these packs to fit into. When I stick in a pack, I want to it automatically charge up at the right speed (so 3A I think). I also want to be able to add and remove packs anytime I want, without the other packs killing a pack. I also want this panel, regardless of how many packs there are in it, to be able to work together to power something if the power goes out. I understand that I might be asking too much of these batteries, so if making them work together to power something is not possible, then I would like this panel to just be a large charging station.

Another thing I want to add in the future is a solar panel with a solar charge controller that will be charging 24V batteries. These will also act as a backup when the power goes out and I also want this to charge the other packs, if that’s possible.

Also, since I want to be able to just stick these packs in without messing around with plugging in wires, would it be better to use XT60 connectors built in? Are there other connectors that will be better?

I know this is a lot and I might not have explained everything that well, so if you need any info or I’m not clear on something, just ask, I don’t really have a lot of experience building and working these battery packs.

Thank you for reading this and any help you can provide would be greatly appreciated!

Answer 1

The first thing I'll warn, is that this is going to be rather complicated to do safely.

Here's one way you may be able to do it:

1. Make each "socket" an independent multi-cell balance charger. Ensure that these will not be back-powered if power to the charger is lost while the battery is connected.

2. Make the sockets also branch off power from each battery through a diode to a common bus. Whatever battery is highest will power the bus until they are similar.

3. Pull power from this bus to power things in blackout conditions. Never draw more than 1 battery can handle.