29Ah+32Ah in Parallel?

[rg12]

I have two Li Ion packs that I need to connect together and was wandering what is the max capacity difference in percentage that is acceptable for connecting uneven capacity packs in parallel.

Let's say a system draws 100A from a 50Ah battery and I connect a 100Ah battery to the 50Ah.
I don't really see the problem as they share the same voltage and also more current will flow from the larger battery as it has less resistance.
But at some point if the capacity difference is very big the higher capacity battery will charger the lower capacity battery as they deplete unevenly, am I correct?

 

[j bjork]

Basically yes, but it is not really the capacity (Ah) of the batteries that dictates witch will take more load.
It is the internal resistance of the batteries. The bigger one likely have lower ir, but not necessarily.
(Like if you parallel a really stiff rc lipo to a battery built of worn out laptop cells)
The higher ir battery will charge the lower ir one when the load gets lower, but if the difference isnt huge I think it is unlikely to be a problem.
And if you parallel a very low ir battery to a very high ir one, the very low ir one will probably be able to handle the charging just fine.

 

[999zip999]

Tell us more about the batteries how old they are where'd you get them what's inside of them what chemistry they are They both new or used. Do they both have bms's what BMS do they have. we need this to know where to start. ECT.
Do you have a link for both batteries ?

 

[amberwolf]

You need to connect them when they are at the same voltage, regardless of their total capacities.

If they are the same chemistry, and the same number of series cells, and both healthy, then regardless of capacity differences (differnet numbers of parallel cells) they will drain at approximately proportional rates, so neither "charges" the other. (the lower resistance pack, which is usually the higher capacity one because it has more cells in parallel, or larger cells with lower internal resistance, will deliver proportionally more of the current),

If they are not the same voltage or chemistry or number of cells, then you won't want to connect them directly in parallel, but rather thru a battery-paralleling diode device so neither can feed back into the other.


If they are different states of health but same chemistry, number of series cells, then they may drain at different rates depending on load, and behavior would need to be observed in-system to know exactly what will happen.

 

[lnanek]

You can buy a y connector with ideal diodes in front of each battery if you really want to prevent them from charging each other:
https://a.aliexpress.com/_mq5EARQ

I've had a 13Ah 48V, 20A continuous discharge battery in there with a 22Ah 48V, 30A continuous discharge battery. Works fine if I set my controller's max current draw to 20A. Gets the combined range of both batteries and you can even shut one or the other battery off and everything still works.

Can't set controller max draw to 30A, though, unfortunately. The y cable disables whatever battery is at lower voltage, so sometimes the weaker battery is powering everything. Then if my controller tries to draw 30A the BMS disables the weaker battery until I turn it off and back on again.

 

[batteryGOLD]

at most cases is safe to connect batteries in parallel. as long batteries are the same type and volts & have same charge/discharge bms port (this is important)
even you can mix LiPo with regular lithium cells.

RULE: Never mix parallel batteries with different charge/discharge at bms! if one battery goes debalanced it could generate a fire at some later time point!

ALLWAYS check if bms has same charge/discharge port when doing parallel! Never do parallel with separate bms charge/discharge ports.

The best way to do a safe parallel:
Charge batteries at full charge (sure to be same volts batteries configuration example: 16S only match a 16S )
once 2 batteries charged, check volts should be almost the same. Than its safe to do parallel connection.

Advantages of parallel:
More life to both batteries, more current out, less stress to cells, more capacity, higher volts under load, more heat dissipation, etc

Consulting service.
Have a nice weekend.

 

[rg12]

Ok so more information...

The original pack is used (can be a year old (could be more could be less), li ion 60v 29ah 3c (could be 5c) (I believe it's 10P with 2.9ah cells (don't know which brand)).

The added pack is brand new li ion 60v 32ah 3c (10P).

The controllers are two of the small kelly controllers: https://kellycontroller.com/shop/kls-s/ and are open originally to about 50-70% of their power with the original battery.

Sometimes when the 2nd pack is added they will open the controllers to 100% (don't know how much battery current as kelly controllers are rated by phase current only).

The packs will of course be connected when they are both at the same voltage (usually charged to full), and will not be connected together at all if the original battery has barely any life in it but rather a new original battery or a used but still moderate life left in it.

About the charge port, I'm pretty sure the original has a common port bms (charge and discharge at the same channel, same as the new added battery) but will check that.
In case the charge port of the original battery is a separate input channel for charging only, then the charger will be connected to the discharge port of the original battery and in parallel to the new battery (which has only one port).
From what I know about how a BMS works is that charging through the discharge port is possible (just like regen) and the only issue that may be is that the max charge current limit of the original battery wouldn't work which is not a problem since the original charger now has it's current split between two packs making the charge current half of what it usually used to be charged at.

 

[rg12]

RULE: Never mix parallel batteries with different charge/discharge at bms! if one battery goes debalanced it could generate a fire at some later time point!

Didn't understand that one.
How can two packs connected in parallel go unbalanced? they share the same voltage constantly.

 

[amberwolf]

In case the charge port of the original battery is a separate input channel for charging only, then the charger will be connected to the discharge port of the original battery and in parallel to the new battery (which has only one port).

From what I know about how a BMS works is that charging through the discharge port is possible (just like regen) and the only issue that may be is that the max charge current limit of the original battery wouldn't work which is not a problem since the original charger now has it's current split between two packs making the charge current half of what it usually used to be charged at.

Charging thru the discharge-only port means that the BMS cannot ever turn off charging even if a cell is overcharged and about to start a fire.

Don't charge thru the discharge-only port; the BMS cannot protect the cells in this mode.

Regen thru the discharge-only port is different, because it is a momentary thing, and not typically being done when cells are already full. (cases where this might happen should be considered before allowing regen thru a discharge-only port).

 

[amberwolf]

How can two packs connected in parallel go unbalanced? they share the same voltage constantly.

Same *pack* voltage, but individual cell (group) voltages can be wildly different.

So you can have two 14s packs at 52v at their connection point, but have cells in one pack that are all around 3.71v (balanced) and the other one can have cells at 3.4v and 3.7v and 3.9v, etc. that all still add up to 52v.

In this case, paralleling the discharge-only connectors and then charging (even with separate chargers on non-paralleled charge-only ports) the packs with those discharge-only connectors still paralleled means that in the event of the unbalanced pack trying to stop charging because it has cells that are already full, current will then flow out of the discharge port of the one still charging (because it's a higher voltage overall than the other that's stopped charging) one into the discharge port of the other that's trying to stop charging and continue charging it anyway.

Now the cells that are already full will overcharge, and the BMS cannot do anything about it because it can't turn off current flow *into* a discharge-only port.

 

[rg12]

How can two packs connected in parallel go unbalanced? they share the same voltage constantly.

Same *pack* voltage, but individual cell (group) voltages can be wildly different.

So you can have two 14s packs at 52v at their connection point, but have cells in one pack that are all around 3.71v (balanced) and the other one can have cells at 3.4v and 3.7v and 3.9v, etc. that all still add up to 52v.

In this case, paralleling the discharge-only connectors and then charging (even with separate chargers on non-paralleled charge-only ports) the packs with those discharge-only connectors still paralleled means that in the event of the unbalanced pack trying to stop charging because it has cells that are already full, current will then flow out of the discharge port of the one still charging (because it's a higher voltage overall than the other that's stopped charging) one into the discharge port of the other that's trying to stop charging and continue charging it anyway.

Now the cells that are already full will overcharge, and the BMS cannot do anything about it because it can't turn off current flow *into* a discharge-only port.

I made a few phone calls for someone to test continuation between the discharge negative and charge negative and they beep, so it's a common port.
They also use the charge jack with a split connector for lights so anyway, no problem there

My initial question was, how much of a difference between two packs can still be safe?
I'm aware that there is IR and age of cells (different capacity than rated after a while with different IR) but let's say, identical specs and chemistries with one at 50Ah and the other at 100Ah.
I'm sure the 100Ah will feed the 50Ah a bit after every pull but I'm guessing it wouldn't be much as the 100Ah will provide more current as current likes to flow where it is easy for it to flow so only small differences will be fed into the 50Ah.

What do you think of that idea?

 

[lnanek]

> I'm sure the 100Ah will feed the 50Ah a bit after every pull

Ah is really irrelevant re which battery drains first. IR matters, not Ah. In my y connector, sometimes the weaker Ah battery has higher voltage than the stronger Ah battery, even though I start draining both from fully charged voltage.

 

[rg12]

> I'm sure the 100Ah will feed the 50Ah a bit after every pull

Ah is really irrelevant re which battery drains first. IR matters, not Ah. In my y connector, sometimes the weaker Ah battery has higher voltage than the stronger Ah battery, even though I start draining both from fully charged voltage.

IR is also affected by capacity, more capacity lower IR...

About your pack, you say they were connected in parallel together and didn't have the same voltage?

 

[lnanek]

They started at the same voltage, charged to full.

 

[rg12]

They started at the same voltage, charged to full.

Sounds odd as the higher voltage pack should charge the lower voltage pack if connected in parallel with no diodes.

 

[lnanek]

Certainly not. I already stated I have an ideal diode in front of each to prevent that. The MOSFET prevents it.

I'm simply pointing out the opposite battery is the one that attempts to charge the other from what you were expecting before that situation happens.