frankvw Posted June 30 Share Posted June 30 (edited) My LiFePO4 battery is spec'd as having a service life of 2000 cycles @100%DoD, 3000 cycles@80%DoD and 5000 cycles @50%Dod. Which is pretty normal for this type and price range of battery. However, this assumes that the battery is recharged to 100% after each discharge. What happens if the battery is recharged only partially? Let's say the battery is discharged to 80%DoD and then recharged back to 30%DoD. This moves as much energy in and out of the battery (50% of its total capacity) as it would while keeping it between 100% charge and a 50%DoD. The latter should give me 5000 cycles according to the specs, but the charging and discharging now takes place in a different section of the charge/discharge curve, so that the battery is drained to 80%DoD (which should give me 2000 cycles). So: is the "wear and tear" on the battery mainly a factor of how much energy moves in and out of it (which would mean that keeping the charge between 70% and 20% should give me about the same service life as keeping it between 100% and 50% charge) or is it more a factor of what the minimum charge level is (in which case a partial recharge would be worse in terms of service life reduction than a full recharge)? Any insights on this would be extremely valuable! // FvW Edited June 30 by frankvw Quote Link to comment Share on other sites More sharing options...
Scorp007 Posted July 1 Share Posted July 1 On 2024/06/30 at 12:18 PM, frankvw said: My LiFePO4 battery is spec'd as having a service life of 2000 cycles @100%DoD, 3000 cycles@80%DoD and 5000 cycles @50%Dod. Which is pretty normal for this type and price range of battery. However, this assumes that the battery is recharged to 100% after each discharge. What happens if the battery is recharged only partially? Let's say the battery is discharged to 80%DoD and then recharged back to 30%DoD. This moves as much energy in and out of the battery (50% of its total capacity) as it would while keeping it between 100% charge and a 50%DoD. The latter should give me 5000 cycles according to the specs, but the charging and discharging now takes place in a different section of the charge/discharge curve, so that the battery is drained to 80%DoD (which should give me 2000 cycles). So: is the "wear and tear" on the battery mainly a factor of how much energy moves in and out of it (which would mean that keeping the charge between 70% and 20% should give me about the same service life as keeping it between 100% and 50% charge) or is it more a factor of what the minimum charge level is (in which case a partial recharge would be worse in terms of service life reduction than a full recharge)? Any insights on this would be extremely valuable! // FvW My take what I have read would be that the 20-70% will give a lot more cycles than 50-100%. I have not seen too much on this. Just my feeling based on a few very informative articles read. frankvw 1 Quote Link to comment Share on other sites More sharing options...
zivva Posted July 2 Share Posted July 2 10 hours ago, Scorp007 said: My take what I have read would be that the 20-70% will give a lot more cycles than 50-100%. I have not seen too much on this. Just my feeling based on a few very informative articles read. Batteries should reach 100% SOC on a regular basis for cells to be balanced that typically occurs after 3.4V. When only charging to 70%, cells will remain around 3.25V then never be balanced. Expect more & more unbalance in the long run ... What matters is the DOD : from 20% to 70% or from 50% to 100% doesn't impact battery health but cells balance. The less DOD, the longer the battery life. 10 years of cycles life vs 15 years at best. To put in perspective vs how much would you save by using 100% of the energy stored vs only 50% ... P1000 1 Quote Link to comment Share on other sites More sharing options...
P1000 Posted July 2 Share Posted July 2 Unfortunately, pretty much all the BMSs require the battery to be charged to balance the cells, so if you don't charge to 100%, you are reducing the battery life. frankvw 1 Quote Link to comment Share on other sites More sharing options...
FixAMess Posted July 2 Share Posted July 2 Also remember that it's not ideal to keep a battery at 100% . So charge the battery, let the cells equalise and then discharge to 90 - 95% SoC. frankvw 1 Quote Link to comment Share on other sites More sharing options...
zivva Posted July 2 Share Posted July 2 30 minutes ago, P1000 said: Unfortunately, pretty much all the BMSs require the battery to be charged to balance the cells, so if you don't charge to 100%, you are reducing the battery life. Nothing unfortunate but lifepo4 chemistry: unbalance occurs at the end of cycle. Balancing @ 3.25V is pointless. Quote Link to comment Share on other sites More sharing options...
Antonio de Sa Posted July 2 Share Posted July 2 6 minutes ago, zivva said: Nothing unfortunate but lifepo4 chemistry: unbalance occurs at the end of cycle. Balancing @ 3.25V is pointless. @zivva Agree with you, and I have three years of archived data to prove it. In my case LBSA, 5.1 KWH, 100AH battery I can clearly see that when charging it to 100% @ 56 V as per OEM recommendation the individual cells reaching 3,5 V and the BMS goes into equalization mode. This is done basically every single day. Also find out that once the battery is fully charged the best "resting cell voltage" in my case is at 3,23 V, to achieve this floating voltage is set at 53.3 V OEM recommends 54 V I run my battery on voltage settings and from my archived data is pretty obvious that the BMS does what it is designed to do. Three years of use 1540 cycles and still show 100% health. Charge every day to 100% and discharge it over night to 30% to 20% Scorp007 1 Quote Link to comment Share on other sites More sharing options...
Scorp007 Posted July 2 Share Posted July 2 1 hour ago, zivva said: Nothing unfortunate but lifepo4 chemistry: unbalance occurs at the end of cycle. Balancing @ 3.25V is pointless. You are spot on with getting cells to balance at over 3.4V per cell. Would there be any problem if the 100% only happens say every 14 days instead of every charge? Some BMSes around one can actually set the point of starting to balance like the JK which I use for my DIY. May be we can accept that different way of charging even using voltage like @Antonio de Sa is doing and still has a 100% state of health is one of the methods. Have we not seen how many topics we have where the BMS did not do what it should do. Swollen cells. Quote Link to comment Share on other sites More sharing options...
Antonio de Sa Posted July 2 Share Posted July 2 (edited) 8 minutes ago, Scorp007 said: You are spot on with getting cells to balance at over 3.4V per cell. Would there be any problem if the 100% only happens say every 14 days instead of every charge? Some BMSes around one can actually set the point of starting to balance like the JK which I use for my DIY. May be we can accept that different way of charging even using voltage like @Antonio de Sa is doing and still has a 100% state of health is one of the methods. Have we not seen how many topics we have where the BMS did not do what it should do. Swollen cells. @Scorp007 I would like to add that so far, my battery has supplied me with just over 2 MWh of power, not bad at all. I charge it to full every second day, sometimes every 5th day Edited July 2 by Antonio de Sa Scorp007 1 Quote Link to comment Share on other sites More sharing options...
zivva Posted July 2 Share Posted July 2 1 hour ago, Scorp007 said: You are spot on with getting cells to balance at over 3.4V per cell. Would there be any problem if the 100% only happens say every 14 days instead of every charge? Some BMSes around one can actually set the point of starting to balance like the JK which I use for my DIY. May be we can accept that different way of charging even using voltage like @Antonio de Sa is doing and still has a 100% state of health is one of the methods. Have we not seen how many topics we have where the BMS did not do what it should do. Swollen cells. No problem reaching 100% from time to time only, the less you do it, the more time it takes for cells to get balanced. Not a problem with an active balancer, might take days of cycling with a passive one ... Most BMS allow to set balance start V at whatever value you want, not only the JK. It's not a reason to do non-sense and start balance @ 3.2V I have no idea how BMS calculate SOH or what criteria are used to determine it. Got a pack with crappy cells, 1 that couldn't reach 3.4+ V but is the 1st to reach 2.8V when others are still @ 3.2V : Seplos happy with that, 100% SOH Quote Link to comment Share on other sites More sharing options...
Scorp007 Posted July 2 Share Posted July 2 1 hour ago, Antonio de Sa said: @Scorp007 I would like to add that so far, my battery has supplied me with just over 2 MWh of power, not bad at all. I charge it to full every second day, sometimes every 5th day I'm sure with your lower number of charges to 100% will give you gr8 service. We will hopefully follow it over the years. Quote Link to comment Share on other sites More sharing options...
Antonio de Sa Posted July 2 Share Posted July 2 14 minutes ago, zivva said: No problem reaching 100% from time to time only, the less you do it, the more time it takes for cells to get balanced. Not a problem with an active balancer, might take days of cycling with a passive one ... Most BMS allow to set balance start V at whatever value you want, not only the JK. It's not a reason to do non-sense and start balance @ 3.2V I have no idea how BMS calculate SOH or what criteria are used to determine it. Got a pack with crappy cells, 1 that couldn't reach 3.4+ V but is the 1st to reach 2.8V when others are still @ 3.2V : Seplos happy with that, 100% SOH My battery also from day one, 1 of the 16 cells only charges to 3.45 v, all the others go up to 3.5 V, I think it's due to a slightly difference in internal resistance, I've contacted the OEM namely LBSA, they were willing to change that cell, provided I bring the battery over to the workshop. I've decided to live with it, seen that I still get to 102 A/h instead of 105 amp/h Quote Link to comment Share on other sites More sharing options...
JayMardern Posted July 2 Share Posted July 2 1 hour ago, Scorp007 said: Would there be any problem if the 100% only happens say every 14 days instead of every charge? The AM-5 warranty (also a LiFePO4/LFP) only requires a 100% charge every 7 days (and I'd imagine that's being conservative). My own bank hasn't been to 100% since the weekend... From the warranty conditions: Quote The Installer/Client is required to ensure that the Batteries are fully charged at least once every 7 days to enable the BMS cell balancer to activate and perform cell balancing. This is a necessary step to prolong and protect the cells life. The cell balancer only activates on 100% charge. If the balancer does not activate for an extended period, then the cells could eventually become unbalanced and can cause undercharged cells to degrade in performance and could result in eventual cell cycle life being greatly reduced. Cell cycle life or performance in these scenarios cannot be warranted and the warranty will be void. 37 minutes ago, zivva said: I have no idea how BMS calculate SOH or what criteria are used to determine it. Got a pack with crappy cells, 1 that couldn't reach 3.4+ V but is the 1st to reach 2.8V when others are still @ 3.2V : Seplos happy with that, 100% SOH I asked about this during a firmware update of mine (AM-5's) and they said SOH is calculated as the 'Stored Capacity' divided by 'Designed Capacity'. So if the BMS detects a full charge is storing 97Ah and the battery has a specified capacity of 100Ah, then the SOH would be 97%. This might differ from one battery/BMS to the next though. Quote Link to comment Share on other sites More sharing options...
frankvw Posted July 2 Author Share Posted July 2 7 hours ago, zivva said: Batteries should reach 100% SOC on a regular basis for cells to be balanced that typically occurs after 3.4V. When only charging to 70%, cells will remain around 3.25V then never be balanced. Expect more & more unbalance in the long run ... What matters is the DOD : from 20% to 70% or from 50% to 100% doesn't impact battery health but cells balance. The less DOD, the longer the battery life. 10 years of cycles life vs 15 years at best. To put in perspective vs how much would you save by using 100% of the energy stored vs only 50% ... The PV panels charge the batteries regularly to 100% so cell balancing should be taken care of. It's only during the night when the charger kicks in to provide additional oomph (ideally I should have a bigger battery but the ship with money was hijacked by Somalian pirates) but that doesn't recharge the battery to 100% since that isn't needed. I'm keeping the DOD above 30% (barring emergencies) in order to get a decent service life out of the battery. I just wasn't sure that I'm not doing something stupid here. Which doesn't seem to be the case based on the above. That's heartening at least. Quote Link to comment Share on other sites More sharing options...
frankvw Posted July 2 Author Share Posted July 2 Wow. Lots of great advice here. Thank you all!! Based on what I read here I'm at least headed in the right direction here. During even mildly sunny days (it's a horribly grotty, heavily clouded winter's day right now) the battery gets charged to 100% (first bulk, then absorption, then float) and appliances pretty much run straight off the PV panels. So cell balancing shouldn't be in jeopardy. Normally the battery drops to about 30% remaining charge around midnight, at which point the charger kicks in to provide additional oomph for the appliances and recharge the battery to about 80%. I cobbled my system together from what I had (starting with a Victron Multiplus inverter/charger) which means I can't balance the appliance load between the battery or the mains - all I can do is to put an additional 15A on the battery which is then used for both charging and appliance load. Not ideal, but my budget is severely constrained and beggars can't be choosers. That said, from what you all write here I understand I'm not killing my battery in short order by doing this, and yet I'm able to achieve a considerable mains power saving. So I'm happy. Thank you all so much for responding - it's greatly appreciated! Quote Link to comment Share on other sites More sharing options...
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