A Second Battery - Connecting permanently

Here's my take on connecting a second 48V 17.5AH battery to my Mate X ebike. Would love some input and/or discussion on pro's and cons of my idea/plan.

double post

wozzlegummich said:

Here's my take on connecting a second 48V 17.5AH battery to my Mate X ebike. Would love some input and/or discussion on pro's and cons of my idea/plan.

Click to expand...

What's the diode for ? Would certainly prohibit any regen. You can just plug the batteries in parallel with no additional circuitry using a cable like this one:

qwerkus said:

You can just plug the batteries in parallel with no additional circuitry using a cable like this one:

Click to expand...

Too simple and dependable. Insufficient potential points of failure. :lol:

I would be concerned in the beginning of pow each pack charges to top a charge and when you discharge what's the voltage of each battery you would have to check this time to time is specially a lot in the beginning to make sure this thing is going through and then I would never go to the bottom of my battery low voltage cutoff.
This situation would still I need you to properly monitor as a human BMS

If you're connecting permanently, then wire them in parallel when they're both fully charged. Charge them and discharge them together. No need to get more elaborate than that.

Diode protection makes some sense if you'll be swapping the batteries in and out, and running with a single pack or both together according to your ride. But in that case, I recommend using one of the MOSFET switched "ideal diode" devices that's available for the purpose.

I don't have much personal experience with it, but have read certain BMSs don't like being in parallel. Assuming that's not a problem, then diode free is better, otherwise the packs won't try to equalize voltage with each other. If they aren't perfectly matched units, one will have slightly less capacity and end up at a lower voltage during use, and the charger will get confused trying to charge the two different voltage packs, and I think it would be counting on one of the BMSs to cut the charge to one pack while the other one is still filling?

- I end up with a lot of functional packs with dead BMS, so do a lot of diode-less paralleled up bulk charging of slightly different recycled packs, with occasional manual balancing, so self equalizing during use between packs is important for me, but that's not the safest solution for most people.

The balance leads should be paralleled between batteries, allowing one BMS to manage the pair as a single whole pack.

And get rid of the diodes, what would be the purpose of those?

One plus of at least having plugs for the parallel-ing is you can separate the packs, and have two chargers, one per pack, and halve your charge time....

Here's one of my typical heap-o-recycled batteries setups.. 4 recalled Pedego packs, and 2 thrown away $900 Bosch packs that both got a drop of water on the BMS and failed
It's 3 modular 72v bricks, so can run one, two or three at the same time depending on range vs sporty needs, and up to 6 charger input .