Adding a parallel battery to eBike

I already have a 48 V 20 Ah homemade (18650 Li-ion) battery hooked up to my BBS02.

Is it possible to connect another 48 V 20 Ah battery in parallel?

Would this also help with the voltage sag on my current battery?

I have been thinking about doing this. I have a 48V 11.6AH dolphin pack and am considering building a 13S1P pack for a bit more capacity. From what I have gathered from reading, and the short answer is yes, you can do it. There are some BUTs, however:

1. The two batteries should be the same type (chemistry) with same operating voltages
2. The trickiest part is when they are at different voltages and you hook them up in parallel. The higher voltage pack will discharge current into the other battery and if not controlled, may be too much current. The easiest way is to get them to the same voltage, hook them up, and keep them connected. This will ensure that they are always at the same voltage. I am guessing for both charge and discharge terminals. Otherwise, diodes or active "ideal diode" devices (info floating around here somewhere) can also be used to ensure that current only flows out of the discharge terminals of both batteries.
3. If the first battery has its own BMS that you rely on for charging and for LVC, then the second one should have its own too.

Would be nice if the more knowledgeable folks can chime in now ...

Yes you can run 2 packs in parallel which would give you almost double the capacity.
They do not really have to be the same type, I've read people running both lipo and lithium in parallel.
But if you are running 2 packs in parallel then they need to be the same voltage and ah.
Flyinbrick pointed out that if you have one battery at 48v and the other pack at 24v when you parallel them then the 48v is going to try to dump into the 24v.
Which would ruin the 24v pack. You can run them in serial like that but never parallel.

If you have a BMS on one then you want to get a bsm for the 2nd one as well.
You can however run 2 different capacity pack if you have both on BMS.
For example a 48v 20ah pack and a 48v 15ah pack. The only problem you would run into there is the 15ah pack will cutoff before the 20ah pack.

Yes and yes. If you hook two with bms in parallel, that's fine. If you hook one with and one without bms together parallel, then you still have to monitor the one with no bms.

Charge to the same voltage, then connect. You can disconnect and charge separately if you want. Or charge the bms equipped one, while still connected to the no bms one. The current will slowly flow into both, but pass through the bms pack first.

Two in parallel will result in a lower c rate discharge for both, which means you can wring a few more watt hours out of them combined than you could individually, and with less wear and tear on both.

This is very desirable, if your current 20 ah pack sags a lot under load. If it does not sag much now, then there will be less benefit from paralleling them, but still worth it to do so.

The only reasons not to do it, would be the voltage is different. Like one is NMC, and the other lifepo4, or one is 13s and the other 14s.

"48V" could be anything from a 12s NMC or Lipo that charges to 50v, to a 16s lifepo4 that charges to 58v. So make sure you get as close to the same type as possible, and the same cell count.

Good news!

The battery I have is a 13S 8P with a BMS. I plan to build the same exact battery pack to match the first pack.

Thanks for the help.

Can I charge in parallel with one charger as well? just connecting ++ and -- on the charge of the two batteries and charge it/them?

I was confused about this too for a while... the packs need to be the same voltage, but they can be different ah capacity. In a series they should all be the same capacity or the weakest one will break the series early, but paralleled they will act like one large battery even if one is smaller.

But if they are the same, it should give you a little more than double the capacity as each cell is only working half as hard suddenly and overall losses drop.

If you parallel batterys. Same voltage is only important. Even different chemistry is ok.
I have offgrid system with 16S Winston Lifepo4 and 14S Nissan Leaf and 14S 18650PF directly in parallel without any problems.
For safety install quick blow fuse for each battery.

There is a way to wire up two bms equipped parallel packs to charge together with one plug, by connection to just one of the batteries through the bms. Two bms, but able to charge through just one bms's plug.

But I'm not able to explain it. It is here, someplace, in a post by Dnmun. Happy hunting.

You could make a plug that bypassed the bms of pack one, paralleling it to the other. Or make the new pack without bms, and connect to the other before the bms. You could make the second pack so that it uses relatively fat wire to connect to the second pack at cell level. then the one bms will protect both.

I just charge separately, faster that way with two chargers. Then parallel the packs for the longer rides.

Hello,
Im about to do the same thing:
2 identical batterypacks that I want to connect in parallel on the e-bike. I was told that I need to use a diode in each battery. True?

manneokoko said:

Hello,
Im about to do the same thing:
2 identical batterypacks that I want to connect in parallel on the e-bike. I was told that I need to use a diode in each battery. True?

Click to expand...

If they're the same SOC (state of charge) there's no need, that I'm aware of, for any Diodes.

Ykick said:

If they're the same SOC (state of charge) there's no need, that I'm aware of, for any Diodes.

Click to expand...

Well, they are identical batterypacks but since they have been used they differ a little bit in capacity from wear. Should be OK without Diodes as long as both are fully charged when connecting?

A 48v 20ah pack made with quality cells shouldn't sag with a BBS02. And 40ah is starting to get big.

manneokoko said:

Ykick said:

If they're the same SOC (state of charge) there's no need, that I'm aware of, for any Diodes.

Click to expand...

Well, they are identical batterypacks but since they have been used they differ a little bit in capacity from wear. Should be OK without Diodes as long as both are fully charged when connecting?

Click to expand...

Both fully charged I wouldn't hesitate to parallel wire them together using no diodes.

How about nmc 4.2v and lifepo4 3.5 charge voltage.
10 x 4.2 = 42 volts cutoff 35v equals 3.5v per cell
12 x 3.5 = 42 volts cutoff 35v equals 2.9v per cell
They meet there at 20s and 24s ez math for me.
Or 10s and 12s in one battery charge to 84v ?

999zip999 said:

How about nmc 4.2v and lifepo4 3.5 charge voltage.
10 x 4.2 = 42 volts cutoff 35v equals 3.5v per cell
12 x 3.5 = 42 volts cutoff 35v equals 2.9v per cell
They meet there at 20s and 24s ez math for me.
Or 10s and 12s in one battery charge to 84v ?

Click to expand...

My problem there Zip is that Lifepo4 spends a great deal of it's discharge curve around 3.35V while NMC is fairly linear between min-max voltage. Very different discharge curves.

Also with cell level monitoring and protection NMC can be safely operated down to a little under 3V under load.

I was thinking of A123 20ah cell and a westart 20ah grephene cell. For use on ebike level @ the 90amp at the most at 84v full charge. I not going to do it I would get an all new westart 20ah cell @ 20s. But may try after I pull the string. Just need more info on the cell about fire hazard ?

Plus i have moved with high quality shit. As why almost and to the Limit cells ? My cells can take 220 amp build something and then not use it to its limits live at your limit ? Light fried cheese. Why . Cheese !!! Cheese balls. ?

Sorry for phone taking over