Simplified lifepo4 charging and care

Lifepo4 charging and care is actually very simple. In this example we are going to use a 12v battery made up of 4 cells, aka a "4S" configuration.

We'll assume that you have purchased a pre-built battery from a reputable manufacturer a 4S configuration of quality CALB, GBS, Winston, Sinopoly, Hi-Power large prismatic cells, and NOT some random collection of RC modeling cells. Pre-built means that they have connected up all the cells with links, and have strapped the battery together for physical stability. Each cell is typically pre-charged to about 50% SOC before shipping.

You'll need to babysit the very first charge, and accordingly have a voltmeter of decent quality.

1) Upon receipt, measure the voltage of each cell. They should be relatively close to each other, somewhere around 3.2v. What we are looking for here are wide swings in voltage, like one cell reading 2v, and another reading 3.4v. Either that low cell is bad, or it just didn't get charged properly before shipping. Anything below about 2.6v upon receipt is a red flag. Having cells that read close in voltage upon receipt does NOT mean you have a good balance since during the flat part of the discharge curve they mean almost nothing. What we are looking for are wide variations for obvious signs of dead cells, or an indication that you may have received just a random collection that was put together.

2) Set your AC charger or solar charge controller (disable any temp comp!) anywhere from 13.8 to 14.0v. There is NO NEED to set the charge voltage any higher. Those that do are trying to drive external balancing circuits, which can be additional points of failure. If you follow these precepts here, and of course are not using used/abused trash, but quality cells, there is no need for constant balancing. But don't tell them that.

3) Apply the charge, and just make sure that no individual cell exceeds 3.6v during charge. If the cells are high quality like those above, and have been charged to about the same amount prior to shipping, ideally each will read very near 3.5v at the end of charge. If they are slightly out of balance, they might read anywhere from about 3.45 to 3.55v.

So what is "end of charge"? Technically, that would be when the current to each cell drops to about 0.05C. But because we don't have to worry about sulfation, there is NO NEED to ever complete a charge to full! YOU decide where end of charge is, as long as it provides the capacity you need to get the job done without going beyond the low voltage limit upon discharge. This makes lifepo4 ideal for solar where obtaining a TRUE full charge is often not met. It has no problem with partial state of charge operations, and in fact prefers it.

There is no need to go nuts over a small SOC voltage difference like this. No cell is above 3.6v, and due to minor differences in a cell's actual manufactured capacity and internal resistance, it is not uncommon for the SOC voltages to differ a little bit. Assuming you have quality cells, you'll still be able to achieve 80% DOD without suffering any major imbalance at the bottom end.

4) Don't over-discharge. Set your alarm for a pack voltage of 12.8v under load. Disconnect them if they reach 12.6 to 12.7v under load. (3.15 to 3.175v per cell). This will be approximately 80% DOD, and you won't be travelling too far down the steep discharge knee. Because we are not going well beyond 80% DOD, small imbalances just before the knee drop are not that big of a deal. If you want to be conservative, just use 12.8v under load as a disconnect. Remember this: as solar users, we still should only design our systems not to use more than 50% DOD even with lifepo4. So go ahead and be conservative here.

Optional: if you feel you must, or if your cells WERE somewhat charged hastily prior to shipment, and one cell wants to rise to 3.6v or more before the charge is finished, you can easily discharge that individual cell a little bit and check it again on the next cycle.

In the case of my 20 and 40ah GBS batteries, this was nothing more than an RV incandescent brakelight bulb fixture. I "top balanced" them just for fun, even though it wasn't strictly necessary. When one cell was higher than the others, and when OFF CHARGE, I applied about 30 to 60 seconds of discharge to that cell. The battery quickly came into line after about 2 or 3 individual cell cycle/discharge compensation attempts. BUT note that it didn't change or improve things operationally! I was still stopping at a max of about 80% DOD, so nothing was really gained.

What I am saying here is that upon receipt, with good quality cells and a decent charge from the factory, you may never have to "balance" at all!

Is it that simple? Yes. Of course, most will want to use automation / relays for a low-voltage disconnect, like any other serious battery installation, but here I'm just showing how easy the majority of charging and care really is. In our application of being "fractional C", that is, low voltage and low current, there is no need to go nuts over balance as long as each cell rises no higher than 3.6v at the top, and just don't let any cell go below about 3.15v during discharge. That's IT!

Individual cell monitoring? Sure, good stuff. But I consider that excessive since I don't (actually can't!) do that with my lead-acid batteries. If I suffer a dropped cell or other signs of poor performance, I'll be taking that up with the dealer / manufacturer much as I would with lead-acid, rather than hanging a rat's nest of wiring all over the top of my batteries - or worse yet letting an external balancing system fail or mask an issue that needs to be dealt with sooner than later.

The overarching moral to the story is that we are NOT EV'er or RC modelers. OUR needs allow us the headroom to use lower charge voltages and conservative discharge depths. If you don't keep this in mind, you can easily be led astray by by those who operate in entirely different application scenarios.