Hello all,

I have four 51.2V batteries in parallel (3x Revov R100, 1x Revov R200). All four are connected to a busbar. Three are connected with 50mm^2 cable. One is connected with 35mm^2 cable.

Ignoring the R200 battery (because it kind of acts like 2x R100s), when the batteries connected with 50mm cable charge with 18A, the 35mm battery only charges with about 12/13A.

The only difference I can see is the cable size. So, I was wondering if it would make a difference to switch one of the 50mm cables with a 35mm. That is, swap the +ve 50mm from battery_1 with the +ve 35mm from battery_4.

I am probably going to do this anyway, just for the fun of experimenting, but thought to post and find out:

a) If anyone sees any concerns with doing such?

b) If anyone has done similar in the past?

 

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**Additional Info**

- The inverter charges with a max of 80A

- This is a temporary setup until I can get back into a shop that sells 50mm^2 cable

- The busbars are 30x10x150mm copper

- From busbar to inverter there is 70mm^2 cable (excessive, but it is 2.5m run, and I hope to upgrade the inverter system in future)

- I am aware the biggest danger is if all 50mm batteries charge before the 35mm, and the 35mm tries to take the full 80A.

Edited November 10, 20232 yr by ShaunH
HTML cannot be rendered in posts :(

Say for instance you have 2 batteries same chemistry diffrent capacity in parallel they will both charge and discharge at the same rate but diffrent charge/ discharge currents. The same goes for your 3 batteries the higher capacity will take the most current the current split between the other 2 batteries. Measure the charging currents between the 3 branches and the sum will be your total charging current. The 200ah will charge at 40 amps the 100ah will charge at 20 amps each. The cable size between the batteries are not the problem but always better to stick to same size cable.

Edited November 10, 20232 yr by TaliaB

> The 200ah will charge at 40 amps the 100ah will charge at 20 amps each

Right. That is why I mentioned to ignore the 200Ah battery for the thought experiment. What I am seeing is the 2x 100Ah 50mm batteries are charging with more current than the 1x 100Ah 35mm battery. I understand the 200Ah taking more, but not that there is a difference between batteries 1 and 4.

There's a bit more to consider... the battery with 35mm^2 cables may well not be as discharged as the others, since a higher resistive path on the thinner cable would work both ways, hence on the recharge end, it would consume less as well...

@Kalahari MeerkatThe 35mm battery is almost always more discharged - all cells have a lower voltage than those of the other packs.

12 minutes ago, ShaunH said:

all cells have a lower voltage than those of the other packs

were any of these 2nd life and same age or older? lower Voltage being what vs the other battery and what do the BMS' report as SOC in all the batteries?

2 hours ago, ShaunH said:

- I am aware the biggest danger is if all 50mm batteries charge before the 35mm, and the 35mm tries to take the full 80A.

as for this. the BMS would limit the current to the cells and not just allow unimpeded current flow., so this is something you can probably safely ignore, the BMS will only allow the current flow that it has been programmed for and if the battery is already at 90% SOC or so, I'd think the current would already be limited some, by the BMS and the cells would not want to allow the higher current to flow, some people seem to think that current will flow unimpeded, but that is not true the cells will allow less and less current to flow into them the higher the SOC is and the BMS will also control this and reduce the current flow...

2 hours ago, Kalahari Meerkat said:

the BMS would limit the current to the cells and not just allow unimpeded current flow., so this is something you can probably safely ignore

The issue is the not battery, it is the cable. The BMS will happily charge at 100A. The cable is for a 12V 1000W system.

 

2 hours ago, Kalahari Meerkat said:

were any of these 2nd life and same age or older?

All of them are 2nd life. They are the same age (from when I bought them, I do not know the cells' previous lives).

 

2 hours ago, Kalahari Meerkat said:

lower Voltage being what vs the other battery and what do the BMS' report as SOC in all the batteries?

I do not have a picture of them charging, but I do have of them discharging:

The behaviour is similar, though. Even though battery_4 is more charged, it discharges more slowly than the other 100Ah batteries.

3 hours ago, ShaunH said:

I do not have a picture of them charging, but I do have of them discharging:

Ok, which one has the 35mm^2 cable?

The cells are not all that nicely balanced...

Pack #1 52mV between lowest and highest cell
Pack #2 31mV between lowest and highest cell a bit better than #1
Pack #3 seems to have larger divergence based on the average and highest
Pack #4 seems to have the best balance, I'm guessing based on highest and average...

If they are all in parallel, I would have expected Pack #3 to provide more current than any of the other individual 3, how long are your cables between each battery and the busbar?

Why are #3 and #4 both delivering significantly less current that #1 and #2, when #3 should be current king... where is the takeoff on the busbar toward the inverter, closest to #2 perhaps? followed by #1 then #3 and finally #4 possibly?

Based on the BMS cycles #1 and #2 have done 43 cycles and #4 40 and #3 only 5 so far, but that is the cycle count from the BMS since its count was reset, it could have done many more and the cells certainly probably many more yet, presumably in a different setup with a different BMS.

All you can do is see that you have as low a resistance as possible along every link of this DC chain. 35mm^2 cable is rated for over 100A, so yes, it probably has a little bit more resistance than the 50mm^2 cables, a longer length, of course, will have a more noticeable impact yet, but since the battery to which the 35mm^2 cable is connected, is a 100A current rated battery, the cable should be fine, but ideally the cables for all 3 100A batteries should be the same length and diameter and the 200A battery should also have the same length but double the diameter, to balance things out as well as possible.

Edited November 10, 20232 yr by Kalahari Meerkat
spelling & grammar

25 minutes ago, Kalahari Meerkat said:

Ok, which one has the 35mm^2 cable?

#4

25 minutes ago, Kalahari Meerkat said:

how long are your cables between each battery and the busbar?

Every cable is 30cm

26 minutes ago, Kalahari Meerkat said:

where is the takeoff on the busbar toward the inverter, closest to #2 perhaps? followed by #1 then #3 and finally #4 possibly?

Closest to #2 and #3. Config is: [#1, #2, I, #3, #4] (where I is inverter)

 

As in my first screenshot, #3 usually takes the most, because it is essentially 2x 100Ah batteries, but I cannot figure out why #4 is consistently lower. If there is more to this than cable gauges, then I will investigate and make a separate post. Otherwise, I will see what moving the cables around does.

9 hours ago, TaliaB said:

Say for instance you have 2 batteries same chemistry diffrent capacity in parallel they will both charge and discharge at the same rate but diffrent charge/ discharge currents. The 200ah will charge at 40 amps the 100ah will charge at 20 amps each. 

Always interesting to compare the drivers and theory to actual measured values. 

This we can see clearly by the values quoted by @ShaunH

As far as battery 4 is concerned the extra resistance of the 35mm cables can be seen with the lower charging values if compared to batt1 and batt2 being the same size. 

Edited November 10, 20232 yr by Scorp007

It would be interesting to see the measured results on the graphs should you swop battery #1 with battery #4 leaving the 35mm² cable in place.

My reason for the statement that the theory and in practise do not always match but there should be an explanation for it. Some months ago when I got a 2nd hand DIY set of batteries I used it on a table 1m long cables and parallel to another make but same tech in my battery box. The DIY worked fine with it's BMS and when happy I moved them inside the box. At this point during low loads they were providing about 25% of the inverter load. 

I then proceeded to move them inside my battery box. I then had a cable for each polarity of about 400mm and also doubled up. I expected to see this DIY now providing a much higher % of the load but it only went up to 33%. This under light load. It's been running like this for 5 months. 

When the system is under high load of about 65A from the combined battery the DIY provides about 66%. This I suspect is because of each parallel bank having different abilities(internal R) at a high discharge rate and less attributed to the battery cables. Both are 100Ah units. 

1 hour ago, Scorp007 said:

When the system is under high load of about 65A from the combined battery the DIY provides about 66%. This I suspect is because of each parallel bank having different abilities(internal R) at a high discharge rate and less attributed to the battery cables. Both are 100Ah units

Great observation @Scorp007 if the resistance of the cable is less than the internal resistance of the battery cells series string then the current will be limited to the highest resistance( impedance of the cells)

On 2023/11/10 at 10:13 AM, ShaunH said:

If anyone sees any concerns with doing such?

Any cable 25mm² and more is well sized for such application. Forget about resistance differences between 35mm² and 50mm², the differences of internal resistance in the batteries are likely to be way higher.

7 hours ago, TaliaB said:

Great observation @Scorp007 if the resistance of the cable is less than the internal resistance of the battery cells series string then the current will be limited to the highest resistance( impedance of the cells)

Because my clamp meters home is right next to my inverter and my battery to inverter cable has a loop made it makes measuring the total current easy and quick. Also handy to be able to measure the total current going into the 2 battery banks and  subtract the JK BMS value. Charge/discharge values at a glance. 

Update: 14/11/2023

On 11/11/2023, switched 35mm^2 cable to battery_2:

There still appears to be a discrepancy in battery_4. So, it seems the most likely culprit is battery_4 having a higher internal resistance than the other batteries.

 

11/11/2023

14/11/2023

 

To finish the discussion about mismatched cable gauges, though, I will put one 35mm^2 cable on battery_1, and only 50mm^2 cables on battery_2. Seeing as #1 and #2 are most similar in charging behaviour, this should allow us to see if there would be any noticeable difference at all.

7 hours ago, ShaunH said:

There still appears to be a discrepancy in battery_4.

It could also be that battery #4 is actually not 100Ah, maybe, and hence it seems to deliver less power/seem to be the 1st to be near empty... I would still have assumed that all 4 battery banks should discharge at the same % rate, but that does not seem to happen, 22.6%... 24.7%... 15.0%... 16.6%... in this scenario even battery #3 seems dodgy...

3 hours ago, Kalahari Meerkat said:

in this scenario even battery #3 seems dodgy...

Battery #3 is 200Ah. So, it is effectively two of the other batteries using the same terminal. It should be charging/discharging about double the other 3 (up to a point).

 

3 hours ago, Kalahari Meerkat said:

It could also be that battery #4 is actually not 100Ah

Possibly. But, I would have expected the BMS to recalibrate once the battery stopped charging (reached 100%), which it does a few times per week. So, following this theory, the only explanation is the BMS is faulty and cannot accurately measure the voltage.

8 hours ago, ShaunH said:

It should be charging/discharging about double the other 3 (up to a point).

Yes, in terms of current, but not percentage wise, hence it sitting at 15% and #4 at 16%, whilst #1 and 2 are still well above 20%, points to something else...

8 hours ago, ShaunH said:

Possibly. But, I would have expected the BMS to recalibrate

I suspect the BMS does not decide what the Ah/capcity rating is for the cells, but it has been told what it is and that number is wrong, else surely the 100Ah number for #1,2&3 just seems too nice a number, it should possible be 94Ah, 100Ah and 85Ah, or so, for each of those 3 batteries, these are numbers I'm totally guessing/sucking out of my thumb, but the point remains, you state that the BMS should, based on discharge/charge data decide what the capacity of the battery bank is, but I seriously doubt this, I'm sure there is an entry on the BMS, where it has been told what the capacity of the battery is and this would not change, unless you actually update this with a more realistic/honest number.