Upgrading to Eve MB31 Lithium Cells: Real-World Capacity and Charging Insights

[Winston-ClassB]

Well Booster, there will be no 10th Year Annual Lithium Report. After losing 30% of our 500ah GBS lithium pack capacity over its 9 year life, we decided to replace these 20-100ah cells with 8-Eve MB31 cells (314ah each) for a new pack ‘theoretical’ capacity of 628ah.

We are continuing to use our original Elite Power Solutions BMS, now reprogrammed for the higher pack capacity and fewer cells. One issue, however, is a difference in maximum charge voltage - - 14.2v for the GBS and 14.4v for the Eve cells. As our Elite BMS balances at the 14.2v level, we shall continue using the 14.2v when balancing our new Eve lithium pack. And since our Eve pack retains 80% SoC when charged to 13.4v (leaving 500ah usable - - the size of our original GBS pack when fully charged 9 years ago), this will be our normal target charging level. Only if venturing into the unknown boonies for an extended period will we have need to bump this charge level by 0.1v or 0.2v to recapture that last 100+ah of capacity.

In anticipation of this lithium project, we purchased a new ‘gadget’ - - a FNIRSI HRM-10 battery voltage and internal resistance meter. Booster, you’ll be jealous - - we have no idea its actual accuracy but it purports to provide cell voltage resolution to 0.1mV! Even if inaccurate, we assume it will give us 2 more decimal places ‘comparative’ resolution over our Fluke 177 (with its 10mV resolution).

Our new Eve cells clocked-in at ~0.22mΩ (the spec is for 0.18mΩ +/-0.04mΩ). We tested our old GBS cells - - expecting extremely high values - - but discovered they were only ~0.88mΩ. Considering that these old GBS cells were only 100ah (as contrasted to the Eve cells at 314ah), we assume, even when new, that they would have had a correspondingly higher internal resistance.

Finally, we performed our “Zero’th Year” capacity test. We charged (and mostly balanced) the pack to 14.2 volts. Interestingly, Eve specs the full discharge voltage to be 2.5 volts/cell (10 volt pack voltage). This number is considerably below the 2.8 volts we’ve been using over these past 9 years when measuring our GBS cell capacities. So our ‘new capacity’ tests might be applies-to-oranges when compared to our past efforts - - we charged to 14.2 (not Eve’s 14.4 spec) and discharged until the lowest cell reached 2.5 volts (again, not 2.8v as in the past). The result: 643ah capacity.

 

[booster]

Sounds like you are on your way, and I particularly like the 80% top charge, as you already know because it is what we also use. What approximate SOC are recharging at typically?

Our SOK batteries are 14.4v balance and charge point so if needed we would have to go that high to balance, but haven't had to yet. They tested at about the same amount over rated when new.

That is a very reasonably priced meter. It will be interesting to see how it all works out with it. My Bryman was in the $150 range and also measures to .1mv. Accuracy is defined as a % plus some significant digits, but exact reading within reason isn't a concern, as you say repeatability is what really matters for this stuff. I found that taking repeated measurements of the cells yielded repeatability of +/- .0001v of variation on any given cell between readings.

 

[Winston-ClassB]

What approximate SOC are recharging at typically?

Not sure we understand your question. Recently we've been doing a lot of 14.2 volt charging in an attempt to get our old GBS cells balanced. But after 9 years, we had at least one cell that wouldn't come-up to the so-called balance threshhold of 3.55 volts (the point at which the balancer drops a 0.5 amp resistor across the cell). In the past, before balancing become troublesome . . . our target was 13.4 volts (you may remember, actually, 13.36v, but neither our Midnight Solar nor our Magnum had 0.01 volt resolution so we had to pick the nearest 0.1 volt). Currently, when we're in 'space dock' (our hangar), we're going to lower the resting voltage to 13.3 volts and bump it back to 13.4 volts when we hit the road. But remember, our charger is a "power supply" - - it's connected to the lithium pack continuously. :~))

 

[booster]

I was not referring to the setup charges, but toward how low do you let the SOC get before doing a recharge on the road. We normally would do an alternator charge on a drive if we get under about 35% SOC, but that rarely happens if we have good sun. We generally will leave home at about 60% and solar on but sometimes at 80%. Last year we didn't have to alternator charge at all in two week trip to Custer State Park.

 

[Winston-ClassB]

While on-the-road (or sitting at a campsite), our solar will be enabled and set to the 80% SoC (13.4v) target level. Our 930 watts of solar is generally sufficient to maintain this 80% charge level. Yet, we do use more power than most . . . in the order of 4 kwh/day. So, in the absence of sun, we will be getting nervous on day two. This past year, with our old GBS pack down to 350 ah (and when 'not driving'), we started to run our 2nd alternator, not to charge the pack, but to minimize discharging when operating the induction stove, coffee pot or other high current appliances. Hopefully by restoring (indeed, increasing) our capacity we won't have to rely on the 2nd alternator. But the beauty of a 2nd alternator is its ability to generate over a kilowatt - - to power high current applicances or quickly recharge the batteries - - while the van just sits there, idling.

Not sure we can answer your question as we, you and I, have different 'operational' philosophies. You charge your system to 80%, then turn-off your chargers allowing - - indeed, forcing - - your batteries to supply all required power (either 12VDC or 120VAC through your inverter) until your batteries have discharged to 40% at which point you re-enable charging to again reach the 80% SoC level. So, for you, the answer to your question is obvious - - it's 40% SoC. But using our approach, we don't disconnect our power supplies (chargers) at 80% SoC. We keep our chargers 'on' in an attempt to maintain our target 80% SoC. So, for us, the answer to your question is 80% SoC. Of course, if there's no shore power and the sun sets - - well, that's why we have batteries. Under such circumstances we're forced to allow our batteries to supply all required power. But, if shore power becomes available, or if the sun rises, we're not going to wait for our lithium pack to 'sag' to 40% - - we're going to immediately apply whatever charging is available to maintain our pack at 80% SoC. Yes, we hear you . . . we've kinda (deliberately) ignored our 2nd alternator and we suppose you may really be asking at what SoC, Winston, will you panic and fire-up your Promaster with its 2nd alternator? Hopefully, Booster, with our enhanced capacity, we'll have enough capacity to avoid having to answer this question. We're assuming that, at some point before the batteries hit 0% SoC, the sun, shore power, or the need to drive somewhere will intervene, enabling our return to 80% SoC without having to prematurely engage the 2nd alternator.

 

[booster]

Thanks, that filled in what I probably forgot about your system in that you have a lot more solar than our 300 watts. We also have substantially less use than you do. We use ours the same way you do by leaving the solar on full time, once we get down off 80% a bit and it will bounce around 40-60% most of the time unless it unusually cloudy or sunny. If we get full sun all day, we gain charge, if it is cloudy we lose charge but on average close to breaking even. On our fall trip to the U.P. last year we hit 4 days straight of cold and rainy so used more power for the furnace and got nearly no charging from the solar. We left early as it was going to be cold and rainy the rest of our week and we were at about 40% at that point. We have a similar size bank to your new one at 618ah.