Ben Harper Posted April 14, 2022 Posted April 14, 2022 I've recently upgraded from a 3 kW Axpert VMIII to a 5.6 kW MKS 4. I have 4x 100Ah AGM batteries, which previously ran at 24V, but now I've got them all in series for 48V. I've had them for 2 years. For the first time ever, my system cut out today (after 2 hours of load shedding), because the battery voltage dropped below the cutoff of 42V. What I find really strange, is that my total battery power draw for this session was only about 500 Wh (due to a bit of solar still available). Also very peculiar, is how my battery voltage drops without warning from 50.8 to 40. Power draw is fairly low throughout this period, and the period when the battery starts to drop is not under abnormal load. It's about 350W all the way. Does this mean my AGM batteries are dying? I've had them for 2 years, and have treated them very well. I try to keep them under 30% DOD. Or could it be that it's my new inverter that's doing something wrong? My setup is: 5.6kW Kodak MKS4 (3 weeks old) 8 x 355W solar (2 years old) 4 x 100Ah AGM batteries (2 years old) Quote
Beat Posted April 15, 2022 Posted April 15, 2022 13 hours ago, Ben Harper said: Does this mean my AGM batteries are dying? I've had them for 2 years, and have treated them very well. I try to keep them under 30% DOD. Or could it be that it's my new inverter that's doing something wrong? I'm afraid your AGM batteries are getting old. Also 100Ah is rather low for 5.6kW inverter. Why not going Lithium? Quote
Coulomb Posted April 15, 2022 Posted April 15, 2022 14 hours ago, Ben Harper said: Does this mean my AGM batteries are dying? I've had them for 2 years It depends on how well they were charged. I see they start (in the graph you showed) at about 53 V, that's about 13.25 V per 12 V module. That seems slightly low for a float voltage, so it's possible that the bulk/absorb setting is a bit low too. There is also the premature float bug, proudly featured in even the newest Voltronic models. This can undercharge the battery, and chronic under-charging can truncate lead acid life to about 2 years. So what are your battery settings (bulk/absorb and float settings mainly)? Quote
Ben Harper Posted April 15, 2022 Author Posted April 15, 2022 Bulk charge is 56.4V, and float is 54.0V, which are the defaults on my inverter for AGM batteries. Quote
Coulomb Posted April 16, 2022 Posted April 16, 2022 Can you check if it typically does reach 56.4V and stays close to that for at least one hour, on partly cloudy days? If the premature float bug bites, it won't, which is bad for a lead acid battery. Quote
Tony Pereira Posted April 16, 2022 Posted April 16, 2022 Sorry to hijack this thread, I am also experiencing the same problem with my 2 x AGM 100ah batteries. I have gone back and checked my charging history and the batteries never reached the bulk charge, always sitting around the floating charge of 27v (26.99v). I have a Mecer Axpert Type 3000VA with 2 x 100ah batteries, no solar panels. The bulk charge setting is set to 28.2v, so what do I do to get the inverter to charge to 28.2v? Quote
Ben Harper Posted April 16, 2022 Author Posted April 16, 2022 When the sun goes does, it stays at the bulk voltage (56.4) for only about 10 minutes. After load shedding, it stays there for quite a while (eg 1 hour). Quote
Coulomb Posted April 16, 2022 Posted April 16, 2022 6 hours ago, Tony Pereira said: I have gone back and checked my charging history and the batteries never reached the bulk charge, always sitting around the floating charge of 27v (26.99v). That sounds very much like the premature float bug. 6 hours ago, Tony Pereira said: The bulk charge setting is set to 28.2v, so what do I do to get the inverter to charge to 28.2v? If you have timed absorb or equalisation settings, use those. If you don't have either of those, learn how to patch firmware , or get another inverter. Or at least another solar charge controller. Quote
Coulomb Posted April 16, 2022 Posted April 16, 2022 (edited) 2 hours ago, Ben Harper said: When the sun goes [ shines ], it stays at the bulk voltage (56.4) for only about 10 minutes. After load shedding, it stays there for quite a while (eg 1 hour). IF you're only using the battery for load shedding, that makes some sense. When there is no load shedding, the battery stays nearly full; you only have to replace storage drain and some losses. So the battery quickly gets to the absorb (bulk) voltage, and doesn't need much absorbing until it's full and the charge current falls below a threshold and float stage starts. After some load shedding, the battery takes time to even get to the absorb voltage, since there is significant charge to replace. It stays at the absorb voltage for an hour or so before it's full enough that the charge current falls below that threshold. So it doesn't sound like YOU are affected by the premature float bug, at least for most of the time. Edit: Which means sadly that the battery appears to be getting a full charge, which then points to the battery having lost capacity. You could attempt some equalisation charging, which risks losing water forever, but if they are going to get replaced anyway, it might be worth a shot. Start with short equalisation periods (e.g. 10 minutes), and not too high a voltage (e.g. 57.6 V), then ramp up the time and/or voltage to see if they last any longer before reaching that voltage cliff. Edited April 16, 2022 by Coulomb Quote
Ben Harper Posted April 17, 2022 Author Posted April 17, 2022 Thanks for the advice, much appreciated. Quote
neliuszeeman Posted April 18, 2022 Posted April 18, 2022 As stated above, 100a AGM is very low for a big setup like yours. Personally, learned from experience, the best discharge/charge rate for a 12v AGM is about 20 to maby 30 amps. In a 48v setup that means its about 1.4kw of draw max. Taking dc to ac losses in consideration, it will be about closer to 1.1kw load. That, with the floating bug, im afraid your batteries did come to the end of their life. Quote
Tony Pereira Posted May 25, 2022 Posted May 25, 2022 (edited) On 2022/04/16 at 2:55 PM, Coulomb said: That sounds very much like the premature float bug. If you have timed absorb or equalisation settings, use those. If you don't have either of those, learn how to patch firmware , or get another inverter. Or at least another solar charge controller. Thanks for the advice Coulomb, I will look into those settings. I do not have solar panels, I only use the inverter when there is load shedding. After studying my graphs I discovered that the inverter used to charge to 28.2v before: 2 February the battery discharged to 24.7v and when power was restored the battery voltage increased to 28.2v and stayed there for 90 minutes until settling down to 27.0v 18 May the battery discharged to 24.4v and when the power was restored the battery charge increased to 27.9v - 28.2v for 10 minutes then settled back down to 27.0v. No settings have been changed on the inverter, could this still be the premature float bug? Why would the inverter not charge like it did before? Edited May 25, 2022 by Tony Pereira Quote
Coulomb Posted May 25, 2022 Posted May 25, 2022 1 hour ago, Tony Pereira said: could this still be the premature float bug? Actually, I didn't realise that you don't have panels. Usually, the premature float bug doesn't show with utility charging, since the charging isn't intermittent. So: probably not. 1 hour ago, Tony Pereira said: Why would the inverter not charge like it did before? I hate to say it, but all I can think of is that some battery cells have collapsed, or at least are at a low state of charge. I would check the voltages of the two battery modules; they should agree to within about 0.1 V of each other. If not, try charging the lower voltage one with a car battery charger. You may need to add some sort of balancer to keep them in balance, once you get them close. Quote
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