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LiFePo for noobs

Posted: Wed, 23 Nov 2011, 23:18
by gholm
OK, just wanting to clarify something about LiFePo batteries.
(Just for the numbers, my pack is 45x LiFePo 180AHA CALB cells.)

Hypothetical situation here as an EV driver
1) I am armed only with a voltmeter/ammeter to measure total pack voltage/amps.
2) I have no prior knowledge of how much charge is in my pack.

Is there any "good" rule of thumb to follow to determine my total remaining pack capacity using my voltmeter and ammeter only?


LiFePo for noobs

Posted: Thu, 24 Nov 2011, 00:40
by weber
gholm wrote:Is there any "good" rule of thumb to follow to determine my total remaining pack capacity using my voltmeter and ammeter only?

IMO, the short answer is "No".

The slightly longer answer is "You can tell 0% and 100% but not much about anything in between, unless the cells have been rested for a day or more". Integrating rapidly repeated current measurements to measure charge (often called "coulomb counting") is the only reliable method.

For CALB, if you put in a current of around 0.05 C (9A for a 180 Ah cell) and the voltage goes over 3.5 V then it's full. If you pull out around 0.05 C and it goes below about 3.0 V then it's empty.

LiFePo for noobs

Posted: Thu, 24 Nov 2011, 22:58
by T1 Terry
I have 720Ah of Winston cells @ 12v nom. as my house power supply. Yesterday morning the state of charge meter read 17%, there was a 25 amp load on the batteries and the terminal voltage was 12.9v, when the fridges cycled off the load dropped to 2.5 amps and the terminal voltage immediately climbed to 13.1v. At a 60% state of charge with a 2.5 amp load the terminal voltage is 13.2v, I go along with webers answer but add, if the cell voltage is 3.45 volts or higher rested, it's 100% full, if it's below 3v rested it's virtual 0% full, nothing short of guessing with any voltages in between.

T1 Terry
EDIT: fixed typo, 2.45v to 3.45v

LiFePo for noobs

Posted: Sat, 26 Nov 2011, 21:32
by gholm
Fair call and a healthy warning.

So what IS going to happen to my battery pack if my EV gauge (X-pert Pro) fails silently, or my charger just doesn't charge my van properly, and I head out unwittingly to drain the battery.

Is it that I'll be just cruising along, slowly the ooomph starts to fade, and VOILA, I'll killed a $14k pack, stone dead?

In other words, do I always (ie: AT ALL TIMES) need to keep a mental snapshot of how much charge is in my battery?

LiFePo for noobs

Posted: Sat, 26 Nov 2011, 22:20
by Simon
If you just get into the habit of keeping an eye on your battery voltage you should know what is normal for different situations.
For example what your battery Voltage is fresh off the charger after just shutting off confirms you have recharged.
Or voltage after recharge and sitting overnight will be a lower normal.

Other than that I think watching how much the voltage sags under load will show you are not "empty" but not much else.

LiFePo for noobs

Posted: Sun, 27 Nov 2011, 07:32
by evric
gholm wrote: Fair call and a healthy warning.

So what IS going to happen to my battery pack if my EV gauge (X-pert Pro) fails silently, or my charger just doesn't charge my van properly, and I head out unwittingly to drain the battery.

Is it that I'll be just cruising along, slowly the ooomph starts to fade, and VOILA, I'll killed a $14k pack, stone dead?

In other words, do I always (ie: AT ALL TIMES) need to keep a mental snapshot of how much charge is in my battery?


Your X-pert Pro and your charger will work together. After the charge is finished the X-pert Pro will flash "FULL" (That's a good indication that the meter is working well. You can then check the voltage on the X-per Pro. You'll get to know what this "fully charged " voltage is. Use you odometer to check range and you'll be able to keep track of what are normal readings "% etc" on the X-pert Pro.

The X-pert Pro meters are very reliable. Mine runs continuously.

LiFePo for noobs

Posted: Sun, 27 Nov 2011, 16:49
by Tritium_James
The only way to guarantee you don't destroy cells is to have a per-cell BMS running. Anything else that's only measuring the total pack voltage WILL eventually end up with cells out of balance and start killing cells.

eg: balanced pack of 4 cells: 4.0V, 4.0V, 4.0V, 4.0V = 16V
about to be destroyed pack: 2.5V, 4.5V, 4.5V, 4.5V = 16V

Start driving and that 2.5V cell goes below the minimum threshold, but your total pack voltage still looks entirely normal.

It works the other way too, with one cell that's higher than all the others, and it's going to get hot/vent/smoke/etc when you charge, but the total pack voltage still looks perfect.