newbie question about amps/loads

Hi,

I'm new to DIY powerwalls and wiring. If I have 2 24 volt batteries in parallel and my inverter pulls a 20 amp load, does this get split 10 amps to each bank?

Yes, pretty much.
If the batteries are in similar condition & similar capacity, it'll likely be pretty equal.
But if one has higher internal resistance eg because it's older or maybe much colder than the other it would be different.

Note also that even if they start out in similar health and roughly share the load, they typically won't remain that way as they age, so you'd need to periodically check how well-balanced they remain. Even batteries from the same batch can have highly diverging IR as they age, which can greatly disturb load balancing.

Wouldn't it also matter how the batteries are connected to the load?
one battery has the positive of the combined 24v battery and the other battery has the negative?

Later floyd

floydR said:

Wouldn't it also matter how the batteries are connected to the load?
one battery has the positive of the combined 24v battery and the other battery has the negative?

Later floyd

Click to expand...

Yes, but generally that is more dependent on higher amp loads. However, 20A is pretty significant, even for lead acid batteries. Though, the OP doesn't mention if it's lead acid or lithium. This also makes a difference of how things will maintain balance; especially if it's 24v lithium-ion based which does not 'really' fit 24v systems, but better than 12v

I'm running two batteries which contain 140 (7s20p) 18650 cells each, which each batteries having a BMS system. Specifically the jehu pcbs and bms. Each bms is rated for ~50 amps but the boards specify a 40 amp limit without modification. They are connected in parallel to copper bus bars which are feed by a solar MPPT charge controller which maxes out at 80 amps.

I typically have a load connected to this system and the system will feed to thr load before the battieries, so there is almost never 80 amps avaliable to charge between load and panel output.

In theory even if it did have 80 amps of charging it should split 40 to each bank? (assuming similar health and capacity).

I'm planning to add an 3rd battery in the future for more capacity and it seems like this would further spread out the charging amps.

Eddieb said:

[...] In theory even if it did have 80 amps of charging it should split 40 to each bank? (assuming similar health and capacity).

Click to expand...

In theory (by Ohm's law) the current would split perfectly halved if each pack has the same total resistance, which includes both the (total) pack IR (internal resistance), along with the resistance of the cables going to each pack (try to keep them the same length/type). But it's complicated by the fact that IR depends on temperature and SOC, which may differ for each pack (they may have different IR vs SOC/temp profiles if they are a motley mix of chemistries and models).

In this example there are 3 ways to connect batteries in parallel to loads.

As you can in the first one, the Pos and Neg main leads are connected to the same battery. This is generally considered a bad idea as due to Ohm's law, the second battery will see a higher voltage drop than battery 1. This is because the wire has resistance. The longer the run, the higher the resistance.

With option #2, Pos and Neg are on opposite ends of the parallel connections. This makes it so that current "must" flow the full length of the wire for "both" batteries. This works for 3 or more batteries as well.

With option #3, it is "kind of" a hybrid between the two. However, current "should" flow the full distance and both. However, if there is a somewhat loose connection on either battery, that will cause a slow down of flow to/from that one.

If all runs are equal, then all batteries should see the same current/resistance. In theory this is accurate. In a perfect work this is accurate. In practical application, there will always be a difference between batteries. And, with the higher the amp load, the higher the resistance and the higher the imbalance. This is partially why you should always have a heavy main buss wire to minimize resistance as much as possible.

^^^ For the record, the source of the above image appears to be How to: Connect two batteries in parallel, by Simon P. Barlow,on the site Caravan Chronicles - acaravan blog.This is thenumber one google (image) result if you search on that title.

There is much further discussion of that topicthere (which I have not read so I don't know how reliable it is).

@Korishan Given that the author has complained thereabout people copying his copyrighted imageswithout permission, it might be a good idea to get permission (or find an alternate image).

This is how they are currently connected. Once my isolator mounts come in I'll tidy up the large wires.

Bus bars

Those are decent bus bars!
Are the cables from the busbar to the batteries equal length?
Assuming the batteries are connected toward the "back" of the image and the load at the "front" there might be a small imbalance in voltage drops.
You seem to have "diagram #1" layout from Korishan's post above.
Maybe test with your multi-meter on mV, put one probe on the 1st batteries neg connection point on busbar & the other probe on the neg point for the 2nd battery.
Turn on a bigger load (or charge) & measure. If there's much drop it might be worth correcting.
If you swapped positions of eg the battery neg cables (keeping pos the same, carefully!) you would get "diagram #2"
You could also swap one of the "load leads" from the front to the back, this way future additions would be correct too.

I love the busbars! but I cringe picturing slipping with a wrench in my handand shorting across them. Maybe some kind or cover or barrier in the future? That's why I use a largemetal box (with a cover) andbus bars inside - its the left most metal box in my ID picture to the left

Redpacket, below is the link to a wider shot of the bus bars and leads. Are you saying move the red lead to the other end of the bus bar?

Bus bars 2

I also had a similar question to the OP's Post, So to add to the OP's question: A 7S80P pack using 3000 mAh cells, Assuming a 0.5C (1.5 amp) discharge rate

That would be 120 amps at 29.4(ish) volts = 3528 Max Watts
240Ah * 29.4 = 7.056 Kwh (not taking into account any losses.

Are these safe numbers, 1.5 amps seems like a lot from each cell, i have read where some are not expecting over 200-500ma draw per cell, but I assume this is why you build larger sized "P" packs.

Eddieb said:

Redpacket, below is the link to a wider shot of the bus bars and leads. Are you saying move the red lead to the other end of the bus bar?

Bus bars 2

https://imgur.com/a/CS0n5HB

Click to expand...

Yes. You could move either the black or the red to the other end.
To OffGrid's comments about insulating it, you could fit this duct or similar over it:
https://www.cabac.com.au/products/c...-pvc/slotted-cable-ducting-grey--pvc/19201505

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enzo86 said:

I also had a similar question to the OP's Post, So to add to the OP's question: A 7S80P pack using 3000 mAh cells, Assuming a 0.5C (1.5 amp) discharge rate

That would be 120 amps at 29.4(ish) volts = 3528 Max Watts
240Ah * 29.4 = 7.056 Kwh (not taking into account any losses.

Are these safe numbers, 1.5 amps seems like a lot from each cell, i have read where some are not expecting over 200-500ma draw per cell, but I assume this is why you build larger sized "P" packs.

Click to expand...

You don't use Ahr (current over a period of time) to calculate instantaneous current per cell, you use Amps
So if you are drawing say approx 3.5kW at 29V, total current is kW/V so 3500/29 = ~120A.
To get the amps per cell, it's # of cells / amps, eg 120p cell pack would be 120/120A = 1A per cell (still high)

For load power levels over about 2kW, you should move to 48V (nominal) systems to keep the currents manageable.

"You don't use Ahr (current over a period of time) to calculate instantaneous current " Correct, i was just doing a rough calculation on capacity. Thanks for your response and information, looks like ill need to start looking at 48v designs. What is the typical amp draw per cell that everyone shoots for ?