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rodeo avatar image
rodeo asked

8 x Winston Cells = 400ah in series (24v) – separate Batterybank – Different Cable lengths

Hi everybody,

first of all, sorry for my english.

Im Gian from Switzerland, building an Expeditions-Mobile of a Bucher Duro (Ex-Swiss-Military-Truck).


I bought 8x 400ah Winston Cells with REC BMS which is compatible to all the rest of my Components which are from Victron: 5000W Multiplus II, Orion, Buckboost and so on.


Due to space limitations, i would like to separate the batterybank in 2 blocks.
The cells will be connected in SERIES to achieve 400ah on 24v.


Someone told me to look after exact same cable lengths and that its not good to separate the cells in two blocks because of the much bigger Reststance between block 1 and 2.

I then tought its only a problem if wired in parallel but he told me NO. Its the same issue when connected in series.


So whats correct? I dont wanna risk anything.
The two blocks would have a distance of about 70cm (27.5").

The the cells will be connectet with the original winston linkbars which are very short. Except of the two blocks which are connected then with a 70cm long cable.


Is this a Problem or not?

If not, what cable size for the 70cm should i use?


thank you very much for your help.

Gian

Lithium Batterybattery connection
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3 Answers
klim8skeptic avatar image
klim8skeptic answered ·

The two blocks would have a distance of about 70cm (27.5")....The the cells will be connectet with the original winston linkbars which are very short. Except of the two blocks which are connected then with a 70cm long cable.

Interesting, given that all the cells are in series, one would think that the current flowing through each cell would be equal. It is.

No real problem having an extra long cell interconnect in the middle of the battery stack??

Well there is a bit. Voltage drop along the long cell interconnect will affect the BMS voltage reading for the middle 2 cells, and balancing current will be a little off.

Does this small difference really matter? Probably not if batteries are fully charged and balanced regularly.

I actually placed a fuse mid pack, and I only see a few mV difference between the mid pack cells @ absorption stage.

For reference the cell interconnects that were supplied with my Winston 300Ah cells measured 2.5 x 30mm. =75mm sq.

You could use single 95mm cable for your inverter neg & pos (up to 5m). And use 2 x 95mm cable to interconnect your battery halves..


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rodeo avatar image
rodeo answered ·

Hi,

thank you for your help. But just double up two 95mm2 cables to have a massive connection between the two blocks, is not the proper way im looking for... there is even more resistance then.


A friend which calculated resistance of the original winston linkbars to find a cable thickness to mach the same resistance at a length of 70cm is anywhere near 520mm2... crazy!


So it seems, it just does not work if you do'nt want to risk unbalanced cells then.


Anyone with more/other experience here?


thx,
Gian

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klim8skeptic avatar image klim8skeptic ♦ commented ·

But just double up two 95mm2 cables to have a massive connection between the two blocks, is not the proper way im looking for... there is even more resistance then.

If you double the copper area, you halve the resistance. Have a look at Wiring Unlimited.

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kevgermany avatar image kevgermany ♦♦ commented ·
Sounds like some confusion here.

@klim8skeptic is perfectly correct.

We have no idea what your friend is basing his calculations on, but read on.

Voltage drop between the banks is dependant on current passing through the cable connecting the banks.

BMS will only balance cells when charging. So max charge current is the governing factor for cable size and balancing effects. And in practice it's usually less.

BMS will only balance cells near the end of the charge cycle when charge current drops.

BMS balances cells using low currents. Usually by taking charge out of the cell with the highest voltage.

So long as the BMS cable connections to the batteries are identical in length and guage, and charge current is normal, balancing will be good.

If you download the Victron toolkit, there's a voltage drop calculator there. A rough check shows that a 90mm2 cable, length 1M and a current of 30 Amps gives a voltage drop of 0.1% at 12V or 0.012V. In reality using the double cable suggested will halve that to less than 0.006V when you factor in the improved performance of 95mm compared to the 90mm offered by the calculator. A BMS will not perform that well.

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rodeo avatar image
rodeo answered ·

thx guys, as im not a pro in electrics, not at all, i can just tell what has come to my ears.


So you think im absolutely fine with 2x95mm2? i could even go thicker to 2x120mm2.


it would solve so many problems if i could separate the bank.¨


gian

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klim8skeptic avatar image klim8skeptic ♦ commented ·

I would leave it as 2 x 95mm cable/interconnects.

winston-absorption-20-min.png

My 300Ah Winston cells today.

At the start of absorption (3.5v/cell) the battery was charging at 46a, with a cell voltage deviation of ~10mV.

After 20 minutes of absorption, the charge current had dropped to 3.6a, with a cell voltage deviation of ~30mV. The charger then switched over to float.

The point I am trying to make is that charging is completed in the matter of minutes, and the charging current drops like a stone. The ~30mV deviation I see between cells is unlikely to engage any meaningful balancing currents.

YMMV, and others should also comment.

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winston-absorption-20-min.png (56.0 KiB)

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