When I dug into the battery I discovered that nearly every bolt holding the busbars to the battery terminals had become loose. The negative "S" shaped bus bar was so loose it literally wiggled up & down freely.
Once the case was apart I discovered that the lower three-quarters of all the cells were bulged & swollen. The only cell compression was via that one gray colored sheet metal bar near the top of the case. The gray metal plate behind it, with two vertical ribs, is not physically attached to the bottom of the case and is literally free floating. Nearly all LiFePO4 prismatic cell manufacturers require full compression end plates of the cells.
The case design on this $10,000 battery simply does not satisfy a cell manufacturers requirement for adequate cell compression, especially if your design includes regularly pushing the cells into the dangerous upper-knee. This design lapse, and I can say that because Mastervolt has admitted to me & recognized this was an issue, allowed the lower bout of the cells to bulge outwards when being charged at 14.6V, and then that voltage held/maintained there for far too long.
"But RC the cell maker suggests up to 4V per cell is safe and 3.65VPC is a good charging voltage?"
Let's stop for a minute and discuss the three things that are misleading about most LFP cell makers specification Sheets;
Max safe voltage - Maximum brief safe voltage for the cell
Max Constant Voltage DURATION - Rarely if ever specified or discussed
Charge Current as related to Ah capacity and Considering CV DURATION - Yet to see this in any specification sheet.
Misunderstanding #1: "These cells can't be fully charged until 3.65V / 14.6V or in this case 29.2V".
This statement is actually untrue. I can charge LFP cells to the same exact Ah capacity, (tested using lab grade equipment) as I get at 3.65VPC / 14.6V at just 3.4125VPC / 13.65V. The only difference being the CV (constant voltage) duration. I have tested this repeatedly on numerous cells including multiple brands of prismatics (LiFePO4 & LiFeYPO4), and 18650 LiFePO4 cylindrical cells. All of these cells can be charged to 100% Ah capacity at just a tick over 3.4V the only difference is we can now hold the CV duration longer if charge current is in the .3C range. With very low charge current the battery is near fully charged as the voltage approaches 3.4125VPC and the CV duration is extremely short even at this very low voltage. the rate at which you charge the cells also matters. Typically speaking if you charge to 3.6VPC you'll want to stop charging once you hit that voltage. In a perfect world you could allow the current to drop/taper to .03C at 3.6V but again we are dancing on the head of a needle at this voltage so teh safe bet is to just stop at 3.6V per cell.
Misunderstanding #2: "3.60V / 14.6V / 29.2V is perfectly safe, the spec sheet says so."
This is only quasi-true and it leaves out a lot of critical information. The critical things left out, that really matter, are charge current and the duration the constant voltage is held. What? Huh?
The max safe recommended charge voltage is not just about the target voltage with LFP. The decision on when to stop charging needs to include the charge current rate and the duration the voltage is held at the max safe level. In other words, in order to recommend a maximum safe charge voltage for LFP it must include the charge rate and the max CV duration at the charge current being used.
Lead acid charging requires multiple hours at the constant voltage target, but LFP does not. LFP can not be held for multiple hours at 3.65VPC / 14.6V / 29.2V yet this is the part that is almost always ignored when discussing LFP charging.
Most boat owners are using lead acid chargers that offer anywhere from 1 hour to 4+/- hours of CV duration or unlimited hours at constant voltage in the case of stock alternators or cheap charge sources. Holding the constant voltage duration, at the 3.65VPC that Mastervolt used to recommend, and then holding it there is what over charged and caused these cells to swell and lose considerable Ah capacity. We have tested, in our lab, 35-40 different LFP cells charged at 14.6V, using lead designed acid chargers, and the capacity is diminished rather rapidly when CV duration is held.
MAX SAFE CHARGING VOLTAGE IS A USELESS SPEC IF IT IGNORES CHARGE RATE AND DURATION AT CONSTANT VOLTAGE
What the Chinese spec sheets / manuals fail to tell you is the "rest of the story" and the rest of the story must always include:
Charge Rate & Constant Voltage maximum duration, at XX charge rate, yet it rarely does.
For example at a charge rate of .5C or 50% of Ah capacity (50A for a 100Ah battery) when charging to 3.65VPC / 14.6V / 29.2V you can only hold that voltage for a few minutes once 14.6V or 3.65VPC has been attained or we now start over-charging the cells. Repeat this on every cycle and you can kiss your investment & long cycle life you paid for bye-bye, as this owner did.
In contrast to the max CV duration at a high rate of charge, if we charge to 14.6V with a low charge rate, such as a solar system, of just .05C or 5% of Ah capacity or 5A for a 100Ah battery the cell is going to be full well before you even hit 3.65V / 14.6V / 29.2V and anything beyond this point is unnecessarily damaging/stressing/overcharging the cells.
This battery had a Mastervolt suggested charge voltage of 29.2V for a 24V bank. Holding absorption at 3.65V is what will cause the over-charging and cell swelling or bulging, like these cells suffered from. Cell swelling is considered normal in most prismatic cells and this is why prismatics need "cell compression".
The Chinese manufacturer of the cells in this Mastervolt battery insists on cell compression using "jigs & straps". The "jigs" are metal end plates and the "straps" are meta straps that join the "jigs" and keep the cells in compression so case swelling can not occur during normal use.
If you know me you also know I believe any voltage over 14.2V is simply unnecessary for a marine house bank and 14.2V is using a "stop charge" protocol meaning when it hits 3.55VPC / 14.2V / 28.4V at .3C you simply stop charging. If using a low charge rate the stop point would be a bit lower than 3.55VPC / 14.2V / 28.4V.
Interestingly enough, Mastervolt now recommends 14.25V or 28.5V as the new "recommended max charge voltage". This may help minimize cell swelling, due to chronic upper knee charging and over absorbing, but is still a bit on the unnecessarily high side, especially if you are using charge equipment designed to hold the CV duration..
When the Chinese cell makers, Winston/Thundersky in the case of this Mastervolt battery, say 3.65V per cell is okay this means it is "okay" to charge to 3.65V then STOP CHARGING. This does not mean to charge to 3.65V then hold that voltage for an "absorption period" like typical lead acid charge products do. it also fails to include for low charge rates and the over charging it can create..
Charge to XX.XX volts and stop is NOT how any of the CC/CV charge sources we use in the marine environment operate. To get around this and keep our cells safe from over-charging we can lower the CV charge voltage and adjust the absorption duration to as short as you can so as to not over charge the cells or to hold them in teh upper knee range any longer than is absolutely necessary.
In my experience, which I gained through actually bench testing of LFP cells, the Mastervolt engineers should have picked up on this far sooner than they did. Today, a bit too late in my opinion, many LFP makers are finally recommending lower charge voltages, including Mastervolt.