Run two (almost) same battery packs in parallel

[PHIX]

Hello everyone,

I have two battery packs built with LG MH1 18650 cells which each have a 50A BMS (60V/30AH). One is a 2-3 years older than the other though.

I'd really like to run them in parallel to make use of the combined amperage but I'm still looking for the best/safest way to do this:

1. Charge them to the same voltage and just connect them. How do I charge the batteries then? And is this method safe in the first place?

2. Buy one of those 120A parallel battery "boxes" on Ali. Are they trustworthy? Has anybody tested them?

3. Build some sort of safety/charging circuit myself. Is that doable?

Cheers,
Philipp

 

[bananu7]

You're missing an option 4: use a dedicated, split BMS. In a perfect world, two identical batteries connected in parallel would see the same current and would always sit at the same SoC (same voltage is ensured by parallel connection).

So to understand why that might be problematic, you need to look at what conditions could make them diverge from each other, and what could potentially happen in those instances.

Looking at example dedicated device: BMS 4S 15A Daly Parallel module | OnVOLT - komponenty - magazyny energii - BMS - balansery - akumulatory
We can read that:

When using standard BMS, parallel connection of lithium batteries is not acceptable due to very likely damage to the BMS electronics (which may result in damage to the cells). This is the result of uncontrolled current flow between batteries due to the difference in voltage and internal resistance.

This situation occurs when several batteries with different levels of charge are connected, or when a cell is damaged during operation, in any of them.

Such a distributed BMS system can react in such a case preventing a runaway condition where the current starts flowing from one battery to the other.

 

[PHIX]

Do I have to wire this BMS in series with my other BMS's that are currently in my batteries? I can't really deduce from the pictures about how this is supposed to be wired

I really try to keep my batteries as stock as possibble so I don't get weird looks from the men in blue

 

[bananu7]

Do I have to wire this BMS in series with my other BMS's that are currently in my batteries? I can't really deduce from the pictures about how this is supposed to be wired

I really try to keep my batteries as stock as possibble so I don't get weird looks from the men in blue

No, it needs to be a system that supports it in its design. This module only works with Daly BMSes, there are offerings from other brands that support such scenarios as well.

 

[E-HP]

Hello everyone,

I have two battery packs built with LG MH1 18650 cells which each have a 50A BMS (60V/30AH). One is a 2-3 years older than the other though.

I'd really like to run them in parallel to make use of the combined amperage but I'm still looking for the best/safest way to do this:

1. Charge them to the same voltage and just connect them. How do I charge the batteries then? And is this method safe in the first place?

2. Buy one of those 120A parallel battery "boxes" on Ali. Are they trustworthy? Has anybody tested them?

3. Build some sort of safety/charging circuit myself. Is that doable?

Can you clarify what your goal is? Do your want to be able to discharge the combined pack at 100A, or do you mean you want to increase the range with the combined capacity?
To get 100A, you need to directly parallel using a parallel Y harness. If you just want the extended runtime, then you could use one of those battery blender boxes. Direct paralleling will give you both, but you need a lot more discipline in your charging routine, and options can vary depending on the type of BMS the packs use.
Using the blender is the safest option if you don’t need the extra discharge current. It creates no additional risk. I straight parallel. My charging routine is simple but dangerous, so I never leave my batteries charging alone.
If you do decide you want to go without the blender, it would be good to know the type of BMS each pack has, since some still offer protection whether the packs are connected in series or parallel.
You could always disconnect the packs, charge them separately to the same voltage then reconnect. That would be lower risk, but requires more steps. The blender avoids disconnecting the packs.

 

[PHIX]

I'm trying to get 100A for better acceleration out of my batteries. So direct paralleling is pretty much required.

My BMS offers some protection functionality(ref. attached picture) but I don't want to rely on just that.

 

[docw009]

I believe E-HP has covered it all.

Always safe til it isn't, We can hypothesize a leaking cell group in battery A. Left alone, it goes under 3V and It won't be allowed to recharge if the BMS is working. In parallel, it will be fed current from the other battery, stays charged amd who knows what will happen,

A battery blender would help here, if it's a good one. I've seen posts where some have just caught fire, Not easy to pass 75A thru semiconductors w/o heat,

 

[fechter]

I have a similar setup on my Sur-ron. Basically, I charged both packs full, then connected them. The second pack uses a single-port BMS and I charge through the first pack (which uses a two-port BMS). So if there is a fault in either pack, the BMS will shut it off and I can charge both from a single connection.

 

[lnanek]

At least for the cheap battery blender I got off AliExpress, it's always draining one battery or the other based on voltage measurements. So if each battery's BMS cuts power at 50A, I still have to set the controller max battery amps to 50A.

Setting the controller to, say 75A or 100A, trips the BMS on whichever pack is currently being discharged. The battery blender is good for paralleling packs for more range, but doesn't provide any additional max Amps that can be used at once for more acceleration. You just get additional Ah capacity without having to worry any battery will suddenly rush and try to charge the other like in a true parallel battery setup.

Theoretically it could be designed to say, have a battery at 48V, a battery at 47V, and the voltage drop from the load would bring everything down to 46V, then it would be safe to have the MOSFETs set to allow draining both batteries at once without one feeding the other. Unfortunately, that's not how the one I got works. Probably more expensive to build that way.

 

[Diggs]

If both BMSs are 2-wire then just wire together in parallel and when you charge one, obviously you are charging both. I run two 52v packs in parallel with one 6p pack several years older than the other 5p pack. No issues. A few years ago I tried this with one 2-wire and one 3-wire BMS pack. I only charged the 3-wire BMS and the 2-wire BMS pack would never fully charge. Charging the 3-wire BMS through its output port would have bypassed its BMS which is never good. I like the parallel packs. Little voltage sag and incredible range.