Newbie building 4s8P

jbesedic

New member
Joined
Oct 24, 2017
Messages
4
Hello everyone. I am in the process of building my first battery pack and need some help.

I've used the (pack building page) on your site to line up my 18650 in 4s8p configuration but for some reason I can visualize how the positive and negatives should be laid out.

Also how would I wire up the BMS to this.

I have a 12v 2000 watt inverter that I want to power with these cells so eventually when I get more and more cells will want to expand but for now this is a start for me.

If you are curious what I will be powering... It will be running an amateur radio station and a vhf repeater.

Hope this question does not sound dum. Like I said I'm a newbie.
 
Welcome!

First of all, 4S lithium isn't suitable for a 12V inverter. From our FAQ which is in the process of being set up:

5. What lithium configuration do I use for 12V? 3S or 4S?

Neither. This is the major case described under No. 4 where the chemistry prohibits the effective use within the 12V system:
3S lithium batteries have a range of ~9.0V when empty to 12.6V when fully charged.
4S lithium batteries have a range of ~12.0V when empty to 16.8V when fully charged.

The 12V system usually ranges from ~10-11V to ~15V. 3S is too low, you will have early cutoffs and can't use the full capacity of the battery. 4S is too high, when fully charged you will force some devices into their overvoltage protection or simply destroy them.
Lithium cells fit the 24V (7S) and 48V (14S) systems nicely so 24V is the minimum you should use. If you need 12V, you can either use a DC-DC converter from 24V to 12V or use lithium iron phosphate cells (called LiFe, LiFePo or LiFePo4). They have a lower nominal voltage of 3.2V and therefore a 4S configuration will fit the 12V system.

Also, 2000W is a bit much for eight cells. Assuming that your setup works, which it doesn't really, if you are going to use the full 2kW then you draw 20A per cell. You need new high drain cells for this to withstand such a discharge.

So, this isn't dumb, but you need to seriously rethink your approach :)

Regarding your initial question, you need 32 cells and build four packs with eight parallel cells each. That means eight cells with all of their positive terminals connected to a common point and the negative terminals as well. And you need four of them which are then connected in series with their positive and negative terminals together. The positive terminal of the first group is the most postive point of your battery and the negative terminal of the last group is your most negative point. These two will be the main terminals for the battery. The BMS or balance wire is connected to the most positive and most negative terminal and the three -/+ connections in between.
You can also build it, because it is very small, into one package by putting the cells in the holders and alternating their orientation so you can easily connect their positive and negative terminals for the serial connection.

Take this as a theory lesson because, as I said, what you want to do will not work properly. Don't build it like that.
 
Welcome,

for a 4s8p setup you need 8 cells all with the positive and negative connected together, this is a pack.
you then need to connect 4 of those packs in series ( positive of first pack to the negative of thesecond pack andpositive of the second pack to the negative of the third pack....and so on.)
This gives you a voltage of4x3,7 = 14,8V nominal.

The bms is connected on the total positive and total negativeand on every connection in between....

This Daromer guy
explains it very well... you have to watch all of his videos... no way arroud this :D

Edit:
Hola, Darkraven types faster then me.... ;)
 
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