Voltage at cell 8 and 9 for a 16s DIY battery. Want to install a mega fuse.

[Dinobot248]

Need a safety / sanity check.

For a DIY 16s 280ah battery with two rows of 8 cells (see picture), what is the voltage at cell 8? I want to place a 100A mega fuse between cell 8 and 9.

I think the voltage would be half the total voltage or nominal 25.6v.

The reason that I ask is the mega fuse is rated at 32v, fits well versus the longer default bus bar when install 2 rows of 8 cells, and may provide protection before the expensive 200A class T fuse. I don't mind de-rating my DIY battery and BMS to the mega fuse limit of approximately 100A. My off grid solar setup charges and discharges my battery bank at less than 0.1C or about 20A max at 56-ish volts everyday.

Thanks in advance!

 

[robbob2112]

This has been discussed before if you search for it.

Now if I recall the details of that conversation the downside of Mega fuses is they get hot with the current flowing through them. This increases the temperature of the two cells they are between. The extra resistance of the 100a Mega fuse is 0.57milliohms... p = i^2 * r =100^2 * 0.00057 = 5.7 watts ... but inside a battery there isn't airflow to disipate that heat. If you run constantly the temperature of the fuse inside the battery case levels off around 75c (167f) ... Now there are assumptions made about heat disipation in a confined space and the material of the battery case ... so that is just an educated guess. ... this extra heat also added pressure to the insides of those two cells and could prematurely cause them to degrade.

Another issue is the Mega fuse AIC is to low to break current if there is a short outside the battery - it can arc and continue to carry current until it burns through the case...

The main issue is adding the extra resistance of the fuse in series with the cells - this means that cell 9 is not going to charge as fast as cell 8 does. A BMS with a balancer should keep them fairly close regardless.

Much better to just use bus bars in the battery and either a DC breaker or a class T fuse at the output.

 

[bolhuijo]

I have my class T fuse mounted in this location and it's been working fine. My system is lightly loaded, never over 30 Amps for any long period.
I have a couple comments though:
1. You can't cheat with the fuse location like this to use a lower voltage fuse. If there was a short the fuse would still have to interrupt the full 51 Volts of the pack.
2. I don't think the added resistance between call 8 & 9 will cause imbalance; current remains identical thru every cell due to it still being a series circuit.

 

[robbob2112]

I have my class T fuse mounted in this location and it's been working fine. My system is lightly loaded, never over 30 Amps for any long period.
I have a couple comments though:
1. You can't cheat with the fuse location like this to use a lower voltage fuse. If there was a short the fuse would still have to interrupt the full 51 Volts of the pack.
2. I don't think the added resistance between call 8 & 9 will cause imbalance; current remains identical thru every cell due to it still being a series circuit.

Do you have a YR1035 meter? Or the datasheet for your cells?

Work the problem as a series circuit with the impedance of each cell and the 2 connections for the bus bar between the cells. Now add in the fuse, holder and extra connection. Same series current, different voltage drop across the cells farther in the circuit?

Hrm.. now you are making me think and that is just not fair on a Friday night. I may have to work the problem tomorrow

 

[Dinobot248]

Thanks for the input.

I've decided to keep the configuration with the mega fuse in between cell 8 and 9. I already have a 200A class T fuse installed close to the positive terminal and by my re-calculations, this battery may only charge / discharge at 10A max.

I don't have a YR1035.
The cells are EVE LF280 V3.