rolfbartels said:
26 - 28.2v "Bluk Charge" This is the max the Charger will accept"
27 - 28.7v "Floating Charge4.1v per cell"
29 - 21v "Low DC cut-off voltage (3.0v per cell)
Does this seem right ?
A typical lithium-ion cell cango from 4.2v (max charge)to 3.0v (minimum charge).
However, if you do this, the life (number of cycles of charge/discharge) will be *greatly* reduced.
Max v per cell considerations:
* If you allow charge up to 4.2v/cell (29.4v) then the7cells in series willneed to beperfectly balanced so that one or more cells (of 7 in series) don't go to 4.24v or 4.27v while another is at 4.1v - its not good / fire risk to go above 4.2v/cell.
* There is hardly any power between 4.15v and 4.2v / cell - so forcing things to 4.2v will greatly reduce life-span and not get you much power.
* Read this article -
https://batteryuniversity.com/learn...o make this feature user,charge to 4.20V/cell. and you'll see research in Table 4that says 4.2v = 300-500 cycle BUT 4.1v = 600-1000 cycles and 4.0v = 1,200 to 2,000 cycles. Clearly,a lower max voltage (along with higher min voltage) can add 1) safety to avoid any cell going over 4.2v and 2) greatly increase your life span.
>26 - 28.2v "Bluk Charge" This is the max the Charger will accept"
>27 - 28.7v "Floating Charge4.1v per cell"
Based on the above - these settings are perfectly good. You may even want to lower them to 4.0v/cell if you can afford less daily power but want longer life. Its up to you. I would measure the battery with a volt meter to see battery voltage thatresults from thesesettings... the battery itself could be up to 0.5v higher or lower depending on various factors. Let it get in to 'float' and then go out and measure the battery v... and then you can adjust accordingly if needed
Min v per cell considerations:
* Yes, cells can typically go down to 2.8v-3.0v *BUT* a little bit similar to the max voltage situation the life span will be reduced if you go to the bottom every day.
* The power curve - e.g. the power available in the cell based on voltage takes a sharp drop at 3.4-3.5v/cell. This means that there isn't much power after you get below 3.4v/cell (23.8v) but yet you risk shortening the life of the cells.
>29 - 21v "Low DC cut-off voltage (3.0v per cell)
Personally - I would suggest the 3.4v/cell (23.8v) or 3.5v/cell (24.5v) as a cut-off setting. You can actually make a chart of your battery bank by simply putting a steady load on the battery and measuring voltage at regular intervals. You will see voltage drop steadily per time until 3.4 or 3.3 ... and then voltage drop very rapidly. Then you can use this 'discharge knee' in the curve to guide your decision.
Discussion:
Perfectly OK toexperiment with 4.0 or 4.1 etc and 3.4 or 3.5 or 3.3... min/max settings and just measure / track your voltages vs power output. There is no 'one right answer' as long as you don't exceed the min/max specs for your cells - its just the lifespan that is affected the most. This voltage range is called the DOD (depth of discharge). If you look at Figure 6 on that Battery University link above - you can see a set of data showing the dramatic affect of life-span on hi/low voltage ranges.
For me - I'm 100% off-grid, somy goal is to consume all the PV power I create thru the night that I didn'tuse during the day - and I use my battery bank to store the excessdaily PV power. The larger the battery, the less DOD it takes. I have designed my system to use < 50% DOD on a daily basis... and after experimentation I've found that 4.0v max and 3.54v min works for me.