Tony Swash Posted March 9, 2019 Share Posted March 9, 2019 Hi All, I have an Axpert 5KVA MKS 48VDC running from 20 x 270 watt panels connected to provide 135VDC. I need some advice please as I have removed the LA bank and purchased a SOLAR MD 7.4Kw LIFEPO4. The spec says bulk charge at 55.4 VDC and float at 54 VDC. If I raise any of these values then I get a voltage too high LED light up on the BMS. My question is this. Is it normal after a full charge solar/220VAC for a day for the battery to "rest" around 54 VDC. We have had 4 cloudy days so have had the mains charger running at 40AH. We have a load of around 300-350 watts per hour from 6pm until 8am . The battery then shows a voltage of 51.5 VDC. When the battery is being "float charged" it pulls anything up to 6 amps. We have constant loads. I have read that when you change from LA to Lithium you must forget all you learnt about LA and dont worry about 100% SOC. However these tiny voltage drops are not very convincing. Also does a LIFEPO4 battery need "running in" ? I look forward to any answers from the board. Rgds Tony Quote Link to comment Share on other sites More sharing options...
Youda Posted March 9, 2019 Share Posted March 9, 2019 Hi Tony, T: I need some advice please as I have removed the LA bank and purchased a SOLAR MD 7.4Kw LIFEPO4. The spec says bulk charge at 55.4 VDC and float at 54 VDC. Okay, it looks like it's LiFePo4 16S battery pack and the manufacturer added some extra safety by setting the BMS to ballance at 55.4/16S = 3.46V per cell. Normally, it's okay to start ballancing at 3.5V per cell, while some cheap chinese BMS are balancing even higher, at 3.6V Can you please double-check whether it's really a 16S or 15S wiring? T: If I raise any of these values then I get a voltage too high LED light up on the BMS. That's good. Means that internal BMS is working okay. Please, do not go above 56V DC, because the higher voltage can damage the cells really quickly. Once they will start to physically inflate, the damage would be irreversible (Did you ever saw an inflated cellphone's battery? So, this is the same process.) Also, since there is an integrated BMS, any charge above 55.4V will be immediatelly burned in the ballancers. Basically, while the balancers will be in a good shape, you will be unable to overcharge the battery. T: My question is this. Is it normal after a full charge solar/220VAC for a day for the battery to "rest" around 54 VDC. For LiFePo4 batteries, it's common to charge them using 2 stages: - first C.C. to 3.5V per cell - then C.V. at 3.48V per cell Looking at your numbers, you have C.C. to 3.46V and C.V. to 3.37V. To me, the voltage difference between these two levels is too high. In other words, once the inverter switches to C.V. stage it will start discharging your batteries. The "charging current" will be negative, actually. Can you check if this is really happening with a DC Clamp Ammeter, please? Personally, if the manufacturer says "bulk" = 55.4V, then I would go for "float" = 55.0V. Can you double-check both values recommended by the manufacturer, please? If 54V seems too low to me. T: We have had 4 cloudy days so have had the mains charger running at 40AH. We have a load of around 300-350 watts per hour from 6pm until 8am . The battery then shows a voltage of 51.5 VDC. If I get it right, your loads were running from the batteries overnight, right? And in the morning, batteries are reporting 51.5V, okay? If that's the case, then the batteries are almost empty. For the LiFePo4, there's not an easy correlation between the voltage and the SoC. That's the main difference to learn when switching from LFA to Lithium. 51.5V translates to 3.2V per cell, which might be 30% SoC, but it might be 10% too. The discharging curve is very flat, till the end and then there's a huge voltage drop. The only way to determine SoC is to count amphours - and your BMS is doing this for sure. Is there a GUI available or SoC LED indicators on the box, at least? T: When the battery is being "float charged" it pulls anything up to 6 amps. We have constant loads. This sounds really strange to me. If you charge the battery up to 55.4V and then "float" at 54V, the battery has a higher potential then the charger. Therefore, it cannot "pull" more amps. The only scenario acceptable to me would be that the charger is switching to the float too early. T: Also does a LIFEPO4 battery need "running in" ? The individual fresh cells have to be formatted. But when the battery is comming with the BMS, they are already formatted, because formatting cannot be done with BMS attached. The first 20 cycles should be just gentle, without any deep discharges. Otherwise, you will loose some capacity for ever. LiFePo4+BMS = easy life. Just set a correct charging voltages, have a decent cables, check the SoC from time to time and that's all. No voltage check, no acid refills, no worries Quote Link to comment Share on other sites More sharing options...
Coulomb Posted March 10, 2019 Share Posted March 10, 2019 Welcome, Tony. 11 hours ago, Tony Swash said: Is it normal after a full charge solar/220VAC for a day for the battery to "rest" around 54 VDC That's not all that unusual, but as @Youda mentions, it might mean that the charger has gone to float prematurely. It's probably at 54 V because your float setting is 54 V. I don't believe that the absorb (misnamed bulk, sometimes called "C.V") or float settings are bad. My system, which is 16S, uses 55.2 V absorb and 53.7 V float. Yes, the battery does discharge a fraction of a percent when going to float, so it might sit at around 99.2 % SOC during the day, assuming that there is enough solar power to cover loads. It's better to stay away from high battery voltages (> ~54.0 VDC) as long as possible. 11 hours ago, Tony Swash said: When the battery is being "float charged" it pulls anything up to 6 amps. We have constant loads. Assuming that this is during the day with intermittent (solar) charging, this is normal, especially on a cloudy day. A cloud will come over, momentarily the load will exceed charge power, and the battery discharges a little. The cloud passes, and the solar charger brings it back to 54.0 V, replacing the charge lost, by charging at up to 6 A. In better weather, it could charge a lot more than 6 A for a minute or so, depending on how much sun there is, and how much charge was lost. However, when you have constant sun for a while, enough to cover the load, then the battery current really should be nearly zero. If not, then the battery isn't fully charged. The factory firmware in Axperts have since inception had two premature float charge bugs. These are serious, and can under-charge your battery, causing higher than necessary discharge, lowering life. It's especially bad with lead acid, since for maximum life, you really need minimum depth of discharge. It's so serious, a colleague and I went to the trouble of patching the firmware in some models, initially only to fix these bugs. We also provide a "LFP flavour" that has much more LFP-friendly voltage thresholds. If yours is a 4 kW model (PF0.8), or a 5 kW model (PF1.0) with the 64 V option, you can use patched firmware to fix these problems. See the AEVA thread index for details. Finally, I have to gently tell you that you got three out of four units wrong in your first post. (I have to do this, because I'm a pedant, I have no choice ). Your battery has 7.4 kWh of capacity, not 7.4Kw. (The "k" in "kilo" is always lower case; the W in watts is capitalised for units that are the names of people, and not for the unit written out, as in "watts".) Your mains charging was at 40 A, or 40 Ah/h (40 Ah of capacity was added to your battery every hour). Your constant load is 300 - 350 watts, not watts per hour. But "6 amps" was correct These unit errors are very common, especially with beginners, so don't feel too bad. Youda 1 Quote Link to comment Share on other sites More sharing options...
Tony Swash Posted March 10, 2019 Author Share Posted March 10, 2019 Thanks guys for the quick response. I enclose a screen shot of the battery being charged this morning from utility at 40A. It seemed to charge for around 2 hours before dropping down to 54.3 which is the float charge voltage. I increased it to that voltage as a test but the BMS flashes an orange voltage error, so set it back to 54. So should the battery have stayed at 55.4 volts for a while or is it the float voltage that brings it down and if so is it good to do this? When I try to adjust the float voltage up then the BMS complains. Rgds Tony Quote Link to comment Share on other sites More sharing options...
Youda Posted March 10, 2019 Share Posted March 10, 2019 Tony, do you have a communication cable between the ICC and the battery? Looking at these charts, I assume that you do... Quote Link to comment Share on other sites More sharing options...
Tony Swash Posted March 10, 2019 Author Share Posted March 10, 2019 No I just read this from the axpert via a usb cable Quote Link to comment Share on other sites More sharing options...
Youda Posted March 10, 2019 Share Posted March 10, 2019 Okay, then do not take SoC reported by the Axpert as correct one. On the other hand, the voltage levels from Axpert are accurate enough (when not under heavy load). That immeadiate drop in the battery voltage is caused by the inverter going into the float stage. Although Axpert is reporting 100% SoC, it might not be true. The only way to determine a real SoC would be to read the SoC data from the BMS. IMHO, that's something you should focus on... Quote Link to comment Share on other sites More sharing options...
Coulomb Posted March 10, 2019 Share Posted March 10, 2019 10 hours ago, Tony Swash said: So should the battery have stayed at 55.4 volts for a while or is it the float voltage that brings it down and if so is it good to do this? That's normal and expected. That charge looks pretty normal to me. But of course, that's utility charging, and 40 A is more than a fifth of the maximum charge current. Presumably, you normally want to charge from solar. As soon as a cloud goes by, such that you generate less than a fifth of maximum charge current (setting 02), and the battery voltage is not much lower than the float setting, it will terminate the charge. It won't restart the charge until the battery goes below 50 V (if the float setting is at 54 V), which is unlikely, even with a large load, unless it's real state of charge is quite low. Factory firmware; you're welcome Quote Link to comment Share on other sites More sharing options...
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