Running Lithium Battery Packs in Parallel?

[SquidBonez]

I've found it quite difficult to find a singular battery pack that meets the requirements for my plan. My idea is to run two battery packs in parallel, which I have seen done before on Youtube but I'm unsure of the logistics. How do you run two battery packs (each with their own BMS) in parallel without damage? My concern is that if one pack dies before the other, that the load will all go to that one pack and destroy it. I'm assuming you can just have a fuse in between each pack and the controller to prevent it from pulling the full load from just one pack? And what about charging? Do you need to charge each pack separately or can you just charge 1 pack and that will also charge the other?

 

[Functional Artist]

I have been running (2) 48V 50AH Lithium modules, (from Chevy Volt) connected in parallel, to create a 48V 100AH battery pack on my ElMoto (electric motorcycle) for a couple of years now without any issues.

If they are "matching" packs (Voltage, AH, age, condition etc.) they should "drain" evenly when connected in parallel
...& for re-charging they should "refill" evenly too.

Um... "I've found it quite difficult to find a singular battery pack that meets the requirements for my plan."
...."to prevent it from pulling the full load from just one pack?"

Are you "hoping" (2) packs will provide more amperage or load capacity?

Putting battery packs in parallel does not usually increase the "load" (or power capability of a pack)
...it increases/doubles the "capacity" of the pack. (think, depth of the pack)

* Example, when I ran my ElMoto, on (1) 48V 50AH pack , my range was ~25 miles
...& now with (2) packs, my range doubled to ~50 miles
...but, my available power "level" did not increase.

 

[benaj]

We run two 48v Li-ion battery packs in parallel on my son's electric conversion kart. They are repurposed computer server 13s4p packs made with 18650 cells, 13s8p when together (~12.8AH total capacity). Both packs were identical in terms of cells, BMS, construction, age, etc.

Each pack has it's own BMS and are connected together by a xt60 splitter outside the packs to give me flexibility to run the kart with a singular or multiple packs. I originally designed the kart's battery tray to hold total of three packs but two gave him more than enough run time so never wound up building that third battery.

Both packs were fully charged up individually from the start, double checked that the top charge voltage was equal before connecting the parallel cable to join them together. The combined packs are connected to the Kart or charger as one unit and have stayed that way.

The BMS on each pack should take control of power management and pack protection if one pack goes down. Just make sure each individual pack (ie batt cell and BMS amp draw ratings) can handle the amp draw of your kart if one pack suddenly goes offline during use, otherwise the BMS of the remaining pack may trip and shut everything down.

Although using multiple packs in parallel is mainly done to increase your total capacity or run time, there is also an added benefit of less stress on each cell due to increase in shared current draw capacity. For instance our packs are built with 25a rated cells, so each pack is rated to provide 100a max current outpt (when paired with the right BMS). Theoretically we could push 200a (!) with the two packs in parallel (assuming the right specs for wiring, connectors, etc). On our kart the 2000w motor and controller can only pull up to 34A draw. When running our two battery packs in parallel that 34A max current draw is now spread out among 8 cells or 4.25A max pull per cell (vs rated 25A max per cell). Less stress for each cell, cooler cells, happier and longer lasting cells, less Kaboom potential.

 

[SquidBonez]

Thanks for the responses guys. My idea was to pull 200 amps from two 100 amp-capable packs, so that is why I was concerned about fusing. I guess it would be better to just get a singular pack that is capable of pulling 200 amps rather than pulling from two separate packs close to their limit, yes? Still trying to learn more about EV mechanics before they become more mainstream lol

 

[benaj]

Advantage of pulling 200A from two 100A packs is that you pull less current through each set of packs so wiring gauge/connector size/BMS rating, etc doesn't have to be so beefy/$$ because each set only pulls half the load (at least until you get to that last connector before the motor controller).

The disadvantage is that if one of the two packs goes offline while in use, you only get 100A from the remaining pack. The BMS should take care of protecting the each pack so if done right there should be no worries about the other pack melting down if one goes down when the motor is running. You will either see a decrease in performance of your motor controller or you may totally shut down depending on what the BMS is programmed to do.

Pulling 200A from a 200A pack vs pulling 200A from two 100A packs is still pulling packs to their limits but I would assume going the singular 200A pack route will be more costly in the end. Ideally with lithium batteries, you want to overbuild the specs so that you understress/minimize heat soak to the battery cells. Just like ICE, heat is the enemy to performance with EV's.

 

[SquidBonez]

Advantage of pulling 200A from two 100A packs is that you pull less current through each set of packs so wiring gauge/connector size/BMS rating, etc doesn't have to be so beefy/$$ because each set only pulls half the load (at least until you get to that last connector before the motor controller).

The disadvantage is that if one of the two packs goes offline while in use, you only get 100A from the remaining pack. The BMS should take care of protecting the each pack so if done right there should be no worries about the other pack melting down if one goes down when the motor is running. You will either see a decrease in performance of your motor controller or you may totally shut down depending on what the BMS is programmed to do.

Pulling 200A from a 200A pack vs pulling 200A from two 100A packs is still pulling packs to their limits but I would assume going the singular 200A pack route will be more costly in the end. Ideally with lithium batteries, you want to overbuild the specs so that you understress/minimize heat soak to the battery cells. Just like ICE, heat is the enemy to performance with EV's.

So it is possible to get 200A to the motor with two 100A packs. Ideally I would find packs that are capable of a bit more than 100 just to be safe, but it was just an example. Still, the question of fusing remains. Would it be worth putting said fuses between the packs/controller just in case the BMS is not set up with such a fail safe? I'm planning on using e-bike batteries (many of which are Chinese, so quality control could possibly be an issue).

 

[benaj]

You could run fuses but at that level of current you should probably run a circuit breaker. Our kart has a marine grade circuit breaker from Blue Sea Systems iirc. It works well doing duty as a master off switch in addition to current overload protection.