24v battery configuration

Diesel Pro

Solar Enthusiast
Joined
Jul 21, 2021
Messages
107
I'm in the process of building an 8s 280AH battery for my off grid cabin. I have the cells here top balanced and assembled as 2 12v batteries that I was planning to assemble into block configuration. On the one hand I can build the 8s 24V block configuration into a Milwaukee Packout case. On the other hand I could use a pair of the custom battery cases that resemble a conventional battery.

It has always been my opinion that the battery configuration is best to match the charge voltage. In other words a 24v charge source matched to a single 24v battery/BMS. My reasoning is balancing in that if one battery gets too far ahead of the other and finishes charge and goes offline the other battery is left behind.

I'm curious though now with the new active balancing BMS is this a non issue? I have a JK 8S 2A 200A BT BMS coming. Not sure if I can run this as a 4S and add another same model BMS?
 

Martijn

Solar Enthusiast
Joined
May 6, 2021
Messages
161
I'm in the process of building an 8s 280AH battery for my off grid cabin. I have the cells here top balanced and assembled as 2 12v batteries that I was planning to assemble into block configuration. On the one hand I can build the 8s 24V block configuration into a Milwaukee Packout case. On the other hand I could use a pair of the custom battery cases that resemble a conventional battery.

It has always been my opinion that the battery configuration is best to match the charge voltage. In other words a 24v charge source matched to a single 24v battery/BMS. My reasoning is balancing in that if one battery gets too far ahead of the other and finishes charge and goes offline the other battery is left behind.

I'm curious though now with the new active balancing BMS is this a non issue? I have a JK 8S 2A 200A BT BMS coming. Not sure if I can run this as a 4S and add another same model BMS?
That BMS is a 8S 24V BMS, so you have to run the 8S configuration.
 

Diesel Pro

Solar Enthusiast
Joined
Jul 21, 2021
Messages
107
OK: Configure as 2 4s batteries with 2 separate and proper BMS. I assume I would not want to try to put the BMS in one box and rung balance wires to the other box?
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,405
OK: Configure as 2 4s batteries with 2 separate and proper BMS. I assume I would not want to try to put the BMS in one box and rung balance wires to the other box?
I think you mean buying teo empty battery boxes like this:
52DF55CC-49FA-4696-AD5C-ECE71CD3CEB9.jpeg
Putting 4 cells in it with a BMS, and then putting the other 4 cells in an identical box?
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,405
Is the intent to get these batteries to be easily removable? Kind of like not wanting to leave them unattended in the heat or the cold?

The cheapest I’ve seen the boxes from China on Ali Express may have been $35 each plus shipping. In stock from a dealer in the states as much as $100+ each.

If the cost and waiting is not a factor, then you may be able to build something, but I don’t see how you could do it without drilling more holes for the wires. With the drilling of these battery cases, I would think your Milwaukee case is better. Plywood would work, but its not cheap anymore and there’s Something to be said about plopping the batteries in something that fits that takes little other work.

I spent a considerable amount of time looking for an 8S case for my 280 cells, and found none. I know others have looked, and they found none also. I even considered welding a case, but that would be too much for my first welding project, and would probably cost as much a the battery itself to toolup.
=========
I think 24 volt is a better configuration to start with if you have no 12 volt items already built in. If you do, the 12 volt can be run off a DC converter. To me 24 volts is built for power. You can easily run a single 15 amp 120 VAC high wattage appliances without getting into amperage problems for a good length of time. My bet is the cabin will likely get expended Needing more energy rather than less.
 
Last edited:

Diesel Pro

Solar Enthusiast
Joined
Jul 21, 2021
Messages
107
The batteries stay put. Heat set to freeze protect in winter and stays there so long as the pilot does not go out which only happens if it is VERY windy or if we have a long winter and I run out of propane in spring.

Right now I have a huge poly box and my 4 pcs 430ah lead acid. I'd have liked a single 8s case but apparently people are concerned about a 100# battery. two 4s cases would also be nice and compact. The Milwaukee box is way smaller than what I have which is nice to save space. It's still a lot larger than what I need and I guess that (OCD) bugs me. I'm not sure if I will do a permanent install in the Milwaukee box or just nest in there and figure a way to leave open. I don't know that we'll be expanding all that much but I do have a portable (now built in) AC unit and a microwave to run.

Here's a picture I took when I initially cobbled the system together.
 

Attachments

  • IMG_0949.JPG
    IMG_0949.JPG
    695.9 KB · Views: 12
Last edited:

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,405
I’d like to see what you come up with. I cannot figure out why an 8S case is not available. There’s similar heavy lead acid batteries for sale on Aliexpress, so I would think someone would make one.

All that looking and planning for a battery box while I was waiting, I came up with a plan for the actual battery and fuse placement, but didn’t think enough about the box. This is the plan I came up with:

8621AC53-D6F7-4134-B25B-C75A0C513233.png
I built it that way. I had been hoping to get something easily removable, but once put together, this is what it looked like:
63131058-49E3-444C-9987-7B6C175F44C6.jpeg
That is not being removed from my trailer in the summer. The cels will stay in the RV. The bus bars and fuses took up more room than the diagram made it look:
EABF3D97-6F42-42F1-BC54-675A2EAD339F.jpeg
 

Diesel Pro

Solar Enthusiast
Joined
Jul 21, 2021
Messages
107
I am curious about all the switches and fuses etc. I guess I kind of get it, but a lead acid battery typically has none of this. My inverter has a breaker on it. No automobile, boat, powersports, etc has a battery fuse. None was designed into my current lead acid bank. Maybe a fuse right at the positive terminal of the cell bank, but beyond that seems you are just adding more terminals and unnecessary bulk and cost.

My cable run is short so I'm just going with 2 ga cables.
 

SettingSun

New Member
Joined
Nov 1, 2019
Messages
2
Look at Andy's Off-Grid Garage battery build#2 and the comments he got using an 100A ANL fuse. He switched to a class T fuse after someone comment that when LiFePO4 battery shorts - it can generate 10,000A and the ANL fuse would just melt and allow the current to go thru. LiFePO4 batteries are very different than Lead Acid. They have very low internal resistance so that current passes thru them very easily. The fuses are to protect the wire first and then the device. Good luck in your build. I have the same cells in 8s 24V configuration split as 4s that are connected in series with one BMS (500A Daly) because as you mention 8s in one box is way to heavy for me to move around and with 24V it decreases the wire size needed. Most people don't use a large enough BMS. It should be rated by your largest load - usually your inverter. So if you have a 3000/6000W inverter - 3000/24= 125A. but if you have an inductive motor and use the surge on the inverter - it amounts to 250A. Granted that the surge is only a few seconds but why take a chance on the BMS and the wires??
 
Top