Parallel battery packs incl BMS

[Bazaki]

People,

I have many experience with paralleling etc with RC lipo packs no problems here.
But I was wondering if it also is possible to parallel complete battery packs incl BMS.

So just a stock plug and play battery 36v 10ah.

Can I put 2 of them parallel ? And what about charging, can I connect 2 chargers while the batteries are still connected ?
What about putting 2 of these batteries in serie ?
The factory claims max 20a discharge BMS, I tried to pull more from the battery but it seems that the BMS will limit it to 20A, that is good to avoid over discharge for the cells.
But can this bms handle the 20a for a longer period of time ?

 

[dogman dan]

The stock answer is to use diodes to isolate the packs, preventing one pack from sending current into the bms of the other. This would occur when one pack trips before the other. The pack that tripped off, would then start to see voltage entering the bms through the discharge side. That would fry some bms.

Series is different. One pack trips, the whole curcuit is now off.

 

[Bazaki]

So for a plug and play solution serie would be the answer. Thanks !

 

[dogman dan]

Another thread just yesterday asked the same thing about paralell.
Dunmum, who is much more knowlegable than I says paralell with no diodes in that thread. All I know is Ping used to recomend the diodes for paralell.

 

[dumbass]

So for a plug and play solution serie would be the answer. Thanks !

When you parallel 2 packs you increase the AH capasity. But when you series connect 2 pack you increase the voltage of the 2 pack. So no the solition is not to series connect them. This will fry your controller. You want to parallel the packs. No one can say for sure what will happen to your BMSs doing this without diodes. So for your BMSs safty it would be smart to install diodes.

Dogman was only explaining that if a person was intendign to increase his voltage by series connecting a set of pack it did not require diodes.

Bob

 

[Bazaki]

I agree, but since I have all kind of controllers it is not important of it is in series or parallel for this case. In series I also double the capacity (720w/h). For some situations parallel would be better ofcourse.

 

[dnmun]

you don't need diodes. if the two parallel packs are identical chemistry then you don't need to isolate them with diodes.

if you combine different chemistries, such as nicad and SLA then you have to use diodes to combine them in parallel.

BMS does not matter when combining, but is important to protect the pack from overdischarging or from overcharging.

 

[Bazaki]

A BMS can act strange I think, when a 36v 10ah battery is off and there is still 10v on the output !I wonder if this can give trouble.

 

[dogman dan]

I never was completely clear about having to have a diode. Ping may have recomended it, as a cover his rear answer. With diodes, nothing wierd could happen.

 

[magudaman]

I have parallel two separate packs each with independent BMS modules with no issues. One pack does shut down before the other and never had an issue. If I recall the Dewalt BMS modules had trouble with reverse current on certain pins but I can't seem to find much about that.

 

[cell_man]

This is my take on it.

If 2 identical packs are connected in parallel they should be ok for discharging, BUT only if they are connected together when they have a similar state of charge. Don't charge them with the discharge still connected in parallel. If 1 of the packs has a HVC on the BMS, that pack could continue to be charged via the discharge section of the BMS which in many cases does not go open circuit for HVC.

Another thing, just because the charger has indicated a full charge, it does not necessarily mean the pack has reached the full charge voltage. It could be that a cell group has hit HVC and the BMS has disconnected the charger. Plugging this imbalanced pack into a fully charged pack that is at the full charge voltage would cause charge current to flow into the former pack unprotected and could cause an issue. The more series cells that are being used, the greater the chance there is of a bigger voltage difference between packs. The bigger the voltage difference, the greater the current that could flow between packs.

So yes I do agree 2 identical packs with BMS can be discharged in parallel, but I am always reluctant to recommend it as there are some scenarios where things can go wrong. So additional diodes make a failsafe setup where human error is much less likely to cause an issue.

 

[dogman dan]

That's a good anyalysis of it IMO. Most situations it's fine, but there could be situations where diodes could be a good idea.

Bottom line, with diodes you wouldn't have any worries, and could be a bit less fussy about connecting differing states of charge.

 

[dnmun]

diodes are not needed if you pair up identical chemistry and voltage packs.

diodes are not needed

diodes are not needed


it is a mistake to put diodes in the current path.


diodes are only needed if you combine different chemistries such as nicad and SLA in parallel. but who in their right mind would do that anyway?

 

[Bazaki]

Well I hooked up 2 x 36v 10ah battery pack and it worked well. discharge and charge.
Just on the workbench so no real life experience on an Ebike yet

But the good think is, that the BMS inside can handle 20A, so 2 of them 40A and I have myself a 36v 40A plug and play battery for friends and family that I don't want to install potential dangerous RC lipo packs.

 

[Spacey]

I have been wondering about this as well as I want to fill my Cagiva Mito 125 ice bike with A123's when I get the cash.

I didn't like the idea of having to re wire the whole damn thing when I wanted to add more parallel packs for range/when funds allow. Adding another say 72V packs with its own BMS might be easier.