Dumb Questions Thread

RobertGreen

Solar Addict
Joined
Mar 15, 2021
Messages
302
I have some assorted dumb questions that don't really deserve their own threads, so I thought I would ask them here and invite others to ask their simple dumb questions as well. I'll probably ask even more questions as I think of them. Thank you for your patience.

1. Does it matter if the wires between cells in a 8s or 16s battery are different lengths? Will it cause problems? I ask because I am going to make a battery which will have to be transported by hand up a ladder; I won't be able to move a fully assembled 16s battery this way due to weight. So my options are either to completely assemble it in place or possibly make 4 smaller 4-cell sections that can be put into place and then connected in series. This would wind up with the 4s sections having slightly longer wires connecting one another than the wires used between the cells inside each section.
 
  • Like
Reactions: Dzl

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
3,540
Location
USA
I have some assorted dumb questions that don't really deserve their own threads, so I thought I would ask them here and invite others to ask their simple dumb questions as well. I'll probably ask even more questions as I think of them. Thank you for your patience.

1. Does it matter if the wires between cells in a 8s or 16s battery are different lengths?

No.

Will it cause problems?

Not really, provided you do it sensibly.

I ask because I am going to make a battery which will have to be transported by hand up a ladder; I won't be able to move a fully assembled 16s battery this way due to weight. So my options are either to completely assemble it in place or possibly make 4 smaller 4-cell sections that can be put into place and then connected in series. This would wind up with the 4s sections having slightly longer wires connecting one another than the wires used between the cells inside each section.

Helped a friend do exactly this with an 8S battery he didn't have strength or space to move it except in blocks of 4S.

This is not horribly different from stringing two 12V batteries in series. In that case as well, you want the interconnects to be pretty beefy.

Use suitably rated wire of equivalent or slightly larger cross section than the cell interconnects (bus bars or wires).

Worth noting that if you go undersized, it can influence the BMS cell voltage reading between the leads that include the interconnect. You can determine the significance by measuring voltage between at each end of the interconnect while under a heavy load. Should be a very small number.
 

RobertGreen

Solar Addict
Joined
Mar 15, 2021
Messages
302
Many thanks, friend.

Do I need to worry about heat buildup in a 16s battery made of 280Ah cells which are sandwiched together in a wooden box with a layout similar to https://diysolarforum.com/threads/my-diy-14-3kwh-eve-battery-build.32709/

The cells will be sandwiched together with no air movement around their sides and placed in a mostly enclosed box. They will be fused for 100A, or about 0.35c, although the normal operating current should rarely approach half that amount.

Is there a simple way to calculate the rate of heating of the battery due to load?
Is ventilation or cooling of the battery a common concern?
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
3,540
Location
USA
Internal resistance of the cells is very low, so heating is not a major concern. Assuming 1mΩ of total resistance, 100A current might produce 0.1W of heating.

Due to inefficiencies while charging close to full, you'll get a little more heating.

If operating in a mild environment, likely not a worry, but in a high ambient temp, you wouldn't want to trap the heat in.
 
  • Like
Reactions: Dzl

RobertGreen

Solar Addict
Joined
Mar 15, 2021
Messages
302
Is it oversizing the main battery fuse to use a 100A fuse on a system equipped with a 100A JBD BMS? (obviously conductors will be appropriate to match the fuse and reduce voltage drop) The system that I am planning with this would be a 280Ah eve cells, 16s configuration matched to a 100A victron charge controller and a 3000W victron inverter.
My real question here is: should I use a smaller fuse than the rating of the BMS so as to decrease the odds of damaging the BMS via overcurrent when the fuse is above its rating but not yet to the melting point?
 

Rednecktek

Photon Sorcerer
Joined
Sep 8, 2021
Messages
1,641
Location
On a boat usually.
Since the biggest flaw in most of the affordable LiFe batteries seems to be the lack of low temp shutoff, can't that be fixed with a soldering pen and a probe?
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
3,540
Location
USA
Is it oversizing the main battery fuse to use a 100A fuse on a system equipped with a 100A JBD BMS? (obviously conductors will be appropriate to match the fuse and reduce voltage drop) The system that I am planning with this would be a 280Ah eve cells, 16s configuration matched to a 100A victron charge controller and a 3000W victron inverter.
My real question here is: should I use a smaller fuse than the rating of the BMS so as to decrease the odds of damaging the BMS via overcurrent when the fuse is above its rating but not yet to the melting point?

No. The BMS can handle it unless you're doing it all the time. 125A fuse is good.
 

Zil

Just another ass on the web
Joined
May 27, 2020
Messages
1,759
Fuses protect wires. What size wires are you using?
 

RobertGreen

Solar Addict
Joined
Mar 15, 2021
Messages
302
1/0 AWG. Mostly sized so that I can avoid voltage drop, plus I had some sitting around :D
 
Last edited:

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
10,681
1. Does it matter if the wires between cells in a 8s or 16s battery are different lengths? Will it cause problems?

No.

Not really, provided you do it sensibly.

Yes. It will show up as differences in cell voltages.
Maybe not matter much, nor cause significant problems. But measurable.
I would imagine it can be dealt with by adjusting BMS to tolerate larger voltage difference.
Could also make short wires longer and/or long wires fatter.

If this had been lead-acid or anything else not monitoring cell voltages, or if BMS had 2 wires per cell (not sharing one between cells) then it wouldn't matter.
 

Cosmictraveler

New Member
Joined
Jan 9, 2022
Messages
5
I have my own dumb question. I am close to pulling the trigger on a brand new Winnebago Minnie FLX. It comes with a 320ah 4096watt hours of Lithium battery storage.

Here is the battery:


My question: is the only option to expand the stock battery capacity by buying another of the exact same battery's? Is there another way to safely add additional capacity? Thoughts?

Thanks!
 
Top