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DIY LiFePo4 On Boats Last edit;3/26/23:The base of this article was written a number of years ago (2010) but this does not mean the information here is outdated. We have been keeping it updated and have added to it when ever we had the time. This article deals
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Cell Balancing
Cell balancing is an extremely important aspect of LFP banks. When you have lead acid batteries in series they can be purposely over charged/equalized to a 15.5V pack voltage and they will, in a sense, self balance. With LFP banks this will not happen due to the knee ranges. As a cell becomes full the voltage all of a sudden skyrockets and the cells need to be in balance in order to charge and discharge at matched voltages.
TOP BALANCE vs. BOTTOM BALANCE:
There is much controversy over top vs. bottom balance mostly due to confusion over differing uses.
BOTTOM BALANCE:
A bottom balance simply means the cells are balanced at the lowest “safe” voltage and all cells will converge and match exactly at say 2.75 VPC. In the EV world bottom balancing is almost always the preferred method, and makes the most sense. With high loads, and frequent opportunities to completely drain the bank, a bottom balance is critical with an EV pack. In an EV the car is then brought back to the garage and charged with ONE charge source.
Bottom Balance:
1-Discharge cell using a 20-30A load to 2.50V
2-Let the cell rest at room temp for 24 hours and allow voltage to rebound
3-The cell will now be resting somewhere between 2.75V and 2.85V
4-Apply the load and stop discharging at exactly 2.65V
5-Allow voltage to recover for about 6 hours
6-Repeat load discharge to 2.65V until the resting stable voltage of each and every cell is 2.75V
7-As you get closer and closer to resting voltage of 2.750V a small resistor can be used as opposed to the large load.
Once all cells rest at 2.750V and stay there the cells are bottom balanced.
NOTE: A guy recently dropped off 4 cells he was having trouble “balancing”. He was attempting a bottom balance and intending on using these for fractional “C” use stopping at 70% DOD.. He had spent countless hours trying to bottom balance these cells, and he did.
So what’s the problem? The problem is that at a 14.0V pack voltage he had one cell at 3.65V and one cell still at 3.380V!!!! His cells tested at varying capacities and thus the cell with the lowest capacity was firing into the upper knee sooner than the rest, even at a pack charge voltage of 14.0V. These were cells with an absolute MAX cell voltage of 3.600V. With a bottom balance and used cells (I don’t suggest buying used cells) he was sending one cell into the dangerous upper knee even at just a 14.0V charge rate. I conducted a top balance for him and the cells now all remain well balanced at the upper charging voltage range. On the low end one cell will still fall off the cliff early, but at 70% DOD that does not happen.
TOP BALANCING:
On boats we have multiple charge sources, shore charger, alternator, solar, wind, hydro or even hydrogen fuel cells. Our risk of cell imbalance is more pronounced at the top end rather than the bottom end. We run a much higher risk of over charging imbalanced cells than we do by over discharging, like the electric vehicle (EV) guys do, but it can still be a risk.. For off-grid / marine use top balancing is quite often the preferred method so the cells converge or are in excellent balance at the top, when fully charged, rather than when dead or fully discharged…
In theory the BMS would always protect the cells at either the bottom or the top end but keeping the cells well balanced ensures an extra level of protection, just as keeping charging voltages out of the upper knee range does. Don’t discharge below 80% DOD and have a max charge voltage of 3.5VPC / 14.0V for a 12V bank, and your cells will be very happy.
Balancing – Wire The Cells In Parallel
2010: As mentioned, I first charged these cells, INDIVIDUALLY, to 3.75VPC and X current taper. The bench-top power supply allows you to set the voltage to 3.XX and let the cell become “full” at 3.XX VPC. For these cells, based on the data available at the time, late 2010, I held the voltage at 3.75V and allowed the current to tail off to 20A then stopped charging and moved onto the next cell.
Within seconds of wiring these in parallel only 0.59A was moving between cells which means the balance to 3.75VPC was pretty close.. Leaving them in parallel will get them in closer balance but this can take lots & lots of time.
Updated Cell Balance Process: Parallel Step-Method Top Balance
My goal when balancing cells is always the following: Keep the cells in the upper-knee for the shortest amount of time and still net a perfect balance
Trough testing and experimenting with numerous balancing processes I’ve found the “parallel step-method top balance” (PSMTB) has proven to be the absolute fastest method that also keeps the cells in the upper-knee the shortest. This means less upper-knee time for the cells. You will need a variable power supply capable of low voltage (3.6V) to do this. You will also want a model with the highest amperage you can source. Keep in mind that when we wire the cells in parallel the bank capacity grows tremendously. Four 400Ah cells become a 1600Ah 3.2V pack! Getting to 3.40V will take quite some time! The key with the PSMTB come from the fact that the cells are essentially full when you get to 3.40V and 0A. This 3.40V threshold is a perfectly safe voltage for the cells so no matter how long it takes to get there will not be causing damage to the cells. Once at 3.40V this means our steps to get to 3.5oV and then 3.60V are much, much shorter than the first step getting to 3.40V. The step to 3.50V is longer than the final step to 3.60V, which happens pretty quickly.
Parallel Step-Method Top Balance:
1- Wire the cells in parallel
2- Set the power supply to 3.400V and 80% or less of the rated amperage (80% to not burn it out)
3- Turn on power supply and charge cells to 3.400V
4- When current has dropped to 0.0A at 3.400V turn off the power supply & set it to 3.500V
5- Turn on power supply and charge cells to 3.500V
6- When current has dropped to 0.0A at 3.500V turn off the power supply & set to 3.600V
7- Allow current to drop to 0.0A (or very close) at 3.60V
8- Done, pack is balanced.
WARNING: Top each cell up, to a similar SoC level, prior to wiring them in parallel.
Balancing Via Parallel Resting Voltages???
Many often assume that by simply wiring the cells in parallel they will magically get themselves in balance. This is not entirely true, if you expect it to happen in a timely manner. When cells are wired in parallel, the the cell voltages attain a parity voltage rather quickly. Once a parity voltage is attained the transfer or movement of current between cells, in order to balance SoC, slows to a crawl. Ohms law is in control here and we are talking 0.0001A level movements of current. Attaining a true balancing, by letting cells sit in parallel, at a resting non-charging voltage, takes a very long time. You can let them sit for a week or more, but again, this may not be enough time. Balancing ideally requires a voltage differential to move current between or into the cells. When cells are at the same voltage this transfer of current = slow.
You can drastically speed the process by presenting the parallel wired cells with a charging voltage.. The PSMTB method is the fasted way we know of to attain a perfect balance. Once all cells are at the same voltage and no more current can flow into the cells they are all at the identical SoC.
Bench Top Power Supply
As I mentioned earlier I am a believer that if venturing into DIY LiFePO4 it should be done as a system. Part of that system should include funds for a bench top power supply and other equipment to test for capacity etc.. In my opinion a bench top power supply with variable voltage and current should be a pre-requisite for DIY LFP. Can you make do without? Sure, and I am certain Bode Miller could ski with only one leg, but why..? In the whole scheme of things they are inexpensive and they have multiple uses not just for charging or top balancing LFP.
The bench top power supplies I sometimes use are made by Mastech, specifically the Mastech EX series. We own a 3030EX and a 3050EX. These are not the fanciest or the most expensive power supplies but they work and they work pretty well, especially for the price. Years ago these devices would have run four figures each but today they are very reasonably priced. A Mastech 3020EX (30V X 20A) will run you just $219.95. It will save you $400.00 in your time fiddling with top balancing alone. You will be looking for a 0-30V and 0-10A or larger model. This is my 3050EX. The EX in the Mastech line signifies these units are specifically designed for charging batteries, usually Li batteries. The dial second from the left is EX knob or the over voltage protection dial. Set this dial and the power supply will protect itself.
TIP: When charging LFP cells or banks with a bench top power supply please dial the current back by about 20%. This will allow the power supply to run almost indefinitely and not cause undue wear and tear on the unit. I run my 30A model at 24A and my 50A model at 40A… I sometimes parallel them and charge at 64A when doing cycle testing.
Nothing makes top balancing easier than a bench top power supply:
#1 Charge individual cells to .05V below max top balance voltage and allow current to taper
#2 Wire cells in parallel and let sit, the longer the better.
#3 Charge cells to max top balance voltage Winston = 3.65V
#4 Allow current to go to 0.00A
#5 Turn off power supply and you’re done.