How Low Do You Go (When To Recharge)

[Ronno6]

Originally Posted by Volt_Ampere

Not sure what you mean by battery switched off. It's not connected to the battery if switching it off disconnects it.

The BigBattery 48v HSKY has a power button.
Push to turn on; push to turn off.....
When off, the digital readout on the battery will eventually go blank.
The charger is always connected to the battery.
I woulda thunk that turning the power switch off while charger is running would show the charger voltage on the readout.....maybe not.
I'll have to see what the voltage readout shows when charger is connected and running, and see if it reads differently when the power switch is on vs. off..........

[Ronno6]

Now, Total Confusion Reigns..............
I don't know whether to trust the BB battery voltage readout or my DVM

With the charger disconnected, I again measured the output voltage with my DVM. Today it read 60.1, after I had adjusted it to 58.8 2 days ago...
After AGAIN adjusting it to 58.4, the BB readout showed 54.1 with the battery switch OFF.

Another interesting measurement: The DVM measured the battery voltage as 56.5, whereas the BB meter showed 53.1.....
Dunno which one to believe at this point.

FWIW, the readout on the battery is typically within .2v of the readout I have in the dash....

MAybe I should test with the DVM at the ring terminals of the cable rather than the parallel charger connector. Still puzzled...

I think I'm gonna hook up my Renogy 500 to add yet another data point.
Maybe it will agree with one or the other.....

[Ronno6]

So I purchased a new Klein DVM.
It is better than the Craftsman that I had been using.
However, I still cannot get the charge voltage to match...DVM with the battery display.
The new DVM agrees with battery display on current voltage while not charging.
But, when I disconnect the charger and adjust the output to 58.4, the plug it back in to the system, the battery (switched OFF) shows 53.5....... then, when switched on, it shows the 52.5 current voltage, and begins climbing.

I would THINK that the battery display would read 58.8 with the battery switched off..........

I have attempted to contact BB, but the phone system has a problem and hey are not responding to online chat.........may be at lunch now....

BB tech says that the disparity puzzles him as well, but to let 'er charge up and see what happens...............MOST REASSURING...............

[Ronno6]

After the battery reached full charge, the meter shower 58.4v........

PERFECT !!

Now I can divert my attention to something else.......

[Pat911]

Hi Ronno,

Your original multimeter may have a flat battery, hence the varying voltage.

What you are observing seems to be the BMS switching on/off. Whilst charging is taking place, you will be reading the battery voltage. If the BMS disconnects for any reason you'll be reading the full charger voltage of 58.4v. With a charge voltage of 58.4v, your BMS will switch off on high cell voltage of 3.65v unless your pack is PERFECTLY balanced to the millivolt, which no pack ever is.

Here's another curve ball for you, to prolong battery life, drop your charge voltage to 56v (3.5v per cell). Your BMS will stay connected, you'll still get above 99% charge and your batteries will be happier.

Cheers
Pat.

[Ronno6]

Originally Posted by Pat911

Hi Ronno,


Here's another curve ball for you, to prolong battery life, drop your charge voltage to 56v (3.5v per cell). Your BMS will stay connected, you'll still get above 99% charge and your batteries will be happier.

Cheers
Pat.

:confused1 :

[Pat911]

HAHA,

LiFePo4 is a funny chemistry and voltage does not correlate with state of charge, no matter what Tom tries to tell you

If you charge with 58.4v, which is exactly 3.65v/cell, one cell will hit 3.65v first as they will never be perfectly balanced, The BMS should have its cell high limit set to 3.65v so as soon as that first cell hits 3.65v, the BMS will disconnect.

You can practically fully charge LiFePO4 with as little as 3.45v per cell as long as you allow it to saturate for a few hours once the cell voltage does get to 3.45v.

Cell life is shortened when you charge with/to a higher voltage. Micro cracks occur in the cell at every recharge and it are these microcracks that degrade the cell. Less/smaller microcracks occur when charging with a lower voltage, lengthening the life of the cell.


Cheers
Pat.

[Tom47]

Originally Posted by Pat911

HAHA,

LiFePo4 is a funny chemistry and voltage does not correlate with state of charge, no matter what Tom tries to tell you

And you can take that to the bank!!!

I'm outta here:

[Ronno6]

I adjusted the charger to 56.0V and, after a short run to drop the battery voltage a tad, I put the charger on for 3 hours.
The display ran up to 56v and stayed there.
Then, when I disconnected the charger...it STAYED at 56v......
Never done that B4......
After sitting for 3 hours, I powered it up and it was STILL at 56V !!!
So I did a 2 minute run and it dropped to 53.2v........pretty much like always..

But, now everything agrees.

I ordered a power cable to hook up my Renogy shunt.....

Thanks, Y'all !!

[Tom47]

(Don't tell Pat that I said this. He might think I lost my mind.)

If you want to be a happy camper, never look at the cart voltage again.

Just use the Ah readout to view your consumption. Use the percentage readout to see when to recharge. Recharge between 20 and 70% and your BB will last forever.

But, ignore that voltage reading before, during and after a trip. It is almost meaningless.