diy solar

diy solar

Solar for a while. New to LifePO

Stuman

New Member
Joined
Nov 26, 2021
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3
Hi. I have been solar at my home for about a decade with a lead acid battery system that I assembled myself, so I have a good basic knowledge. My battery bank on this system is just now weak enough to need replacement, but that is a separate thing from what I am doing today. What I'm looking for is information on the nuances of LifePO. I'm putting together my second system...independent of the first. It is...

3060 watts of panels, wired for 120 volts or so (nine 340 watt panels, wired in threes)
Midnite 150 controller (capable of 83 amps...64 amp is nominal at 48 volts for 3060 watts)
100ah LifePo battery
Chinese 6kw PSW 220 inverter (has charger too)
Transfer switch to switch to grid when depleted...maybe, if I can this undocumented transfer switch to work for me.

What I'm wanting is some education on the nuances of setting up a new 48 volt lifepo cell bank. The cells are brand new, and measure 3.26 to 3.28 volts, with the majority of them measuring 3.27. The BMS is included, but very basic. I have no external control over it, and must simply trust that it is setup acceptably (I may "graduate" later to one I can control with a computer).

My main question at this time is...do I need to parallel the cells for a while to equalize them with only a 0.02 volt difference? If so, how long should it take?
 
I presume a top balance is just leave them parallel until they each measure precisely the same voltage. Correct me if I'm wrong.
Ok...I have a better grasp now. I have ordered an adjustable power supply to top balance them properly. I want maximum use from this purchase.
 
Try these:


 
I see you Already knew top balancing.

So, for your BMS, here’s some lessons learned form a 24 volt BMS I use that does not have any adjustments:

-Inadequate Low Temp cutoff. The BMS cuts off at several degrees below freezing, but my batteries require a low tem cutoff of freezing. Bad news if they charge below freezing.
-High Voltage cutoff too high: Cells max rating is 3.65 VDC per cell, and my BMS goes to 3.65
-No way to monitor the cells: If I could monitor,, at least I could check each cell’s health.

It’s not that you should not use your current BMS, but know what it does not do. For low temp, I am in sunny AZ and my battery pack will never see freezing, so that is no issue. For the cutoff of 3.75, I set my charge rate to 3.45, which is about 90% capacity. With this smaller battery I am using with these settings, I’m confident that I do not need to monitor. If you have ways to keep the batteries in specs, then its fine.

Can’t always assume the BMS that came with the battery will work with it. That is mostly for a low temp cutoff. SOme batteries do not have a low temp cutoff in the BMS, so using that below freezing will ruin the battery,
 
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