Simple LifePo4 swap sense check (part 2)

[stranded]

Having received invaluable advice and reassurance here I have now amassed all the hardware I need to swap out our agm house bank for a 460AH LifePo4. As part of this advice, some kind person (I think it was Kelpie?) suggested I come back here to check my proposed charger settings. My main charge source will be solar and I will connect that (and shore power charger, though we are currently on a mooring) when I do the installation this coming weekend. The dc to dc charger will follow, possibly Sunday if I have time.

Anyway, my goal is reasonable usability and decent but not necessarily world beating battery life. Below is a screenshot of the demo mode of the charger, with parameters that are my first stab to implement on the 3 x Victron SmartSolar mppt chargers. The one that is vexing me most is absorption time, as Victron suggest minimum 2 hours, but I am pretty sure I have seen more than one recommendation that absorption time be set to zero. So any thoughts on that (and given that tail current cutoff is also an option) would be particularly welcome.

Out of shot I have set max current as 15A (they are 75/15 chargers) and low temp cutoff disabled as battery incorporates automatic heating.

 

[lustyd]

With an MPPT right now it resets every day and starts from scratch so the absorption time will happen daily given enough sun. This is why some set zero time to avoid it trying to ram more power into a full battery. If the voltage is low enough though it won’t do a lot so I don’t think makes much difference. I can’t remember what I set mine to but pretty sure my voltage was 14.2. I’m sure others with better memories will be along shortly

 

[Lucy52]

I believe that unlike lead acid batteries, for long life they are best not kept fully charged, so perhaps there is no float charge needed? Someone who really knows may advise.

 

[stranded]

I believe that unlike lead acid batteries, for long life they are best not kept fully charged, so perhaps there is no float charge needed? Someone who really knows may advise.

Thanks Lucy - yes, that is my understanding too. My chargers don’t permit no float, so I hope by setting it below the battery resting voltage it simply won’t kick in. We’ll see.

 

[stranded]

With an MPPT right now it resets every day and starts from scratch so the absorption time will happen daily given enough sun. This is why some set zero time to avoid it trying to ram more power into a full battery. If the voltage is low enough though it won’t do a lot so I don’t think makes much difference. I can’t remember what I set mine to but pretty sure my voltage was 14.2. I’m sure others with better memories will be along shortly

Thanks Lusty. Fogstar recommend 3 or 4 full charges to start with to teach the BMS. After that somewhere round 14-14.2 sounds right. It seems like even higher will do no harm occasionally so it’s good to have a bit of additional capacity in hand for those cloudy spells. Though I’ve managed to get one of the new Victron 50A dc to dc chargers so that will be a more useful top up option than the previously planned 30A max.

 

[GHA]

Thanks Lucy - yes, that is my understanding too. My chargers don’t permit no float, so I hope by setting it below the battery resting voltage it simply won’t kick in. We’ll see.

13.3v works fine here. It just keeps up with any loads. Graph clearly shows how steep the knee is.

Purely from a charging perspective there's no point really going above 14v imho, very little power will go in, I use 13.8v & no absorption for bulk. Seems to be quite a common theme to try to cram every aH possible into LiFePo4, maybe a throwback to lead acid where it's essential for any kind of lifespan. And pointless really with LiFePo4, I`ve been down round 15% - 40% in the morning for months now, just not quite enough sun in the short winter days to push SOC all the way back up before a couple of cloudy days will pull it down again. Lead acid I'd be running the genny for hours every few days to get them really fully charged again. But LiFePo4 it just doesn't matter. Wonderful things!

There have been arguments that it's important to sit at high voltage (like 14+) so the cell balancers can do their thing, don't know about but sounds bit excessive holding the voltage up there every single charge cycle. No balancer on mine, top balanced xmas before last & still within a handful of mV of each other.

 

[Sea Change]

Kelpie here, new username.
AFAIK the reason you may wish to have an absorption phase is to allow balancing to happen.
Do you know what voltage your battery will need to reach to start balancing? It may be user configurable. Mine is set to 3.4v, which is quite low but allows me to be quite cautious with maximum voltage.

 

[geem]

If the boat is to be kept on a mooring such that you will have a daily solar charge cycle, I would suggest you use the expert mode in your MPPT to set a zero absorbtion time. Your bulk and float setting look fine but if the boat spends many days on the mooring without use, you might want to consider setting a lower bulk voltage, say 13.8v.
We are liveaboards and as such, we don't look to charge to full every day. Typically we will do a full charge every week or two to activate the balancer (set at 3.45v per cell). There is little benefit in having your cells at 100% SOC since this is proven to reduce the battery life. The cells are better for long term storage at 50% SOC but this is obviously not practical when you come to use the boat. You can set your own custom modes in the Victron. Let's call one mode 'storage' you could have this set with lower bulk charge. Another mode could be 'in use'. This could have a bulk charge set to 14v. These are easily adjusted when you leave and arrive at the boat. It would only be worth doing this if you planned to spend a few weeks at a time on the boat. Otherwise set the bulk to 13.8v.
If you left the batteries at 100% SOC for 5 years, you would lose about 20% of their capacity. With lead batteries this would be considered end of life. If you over size your lithium its not such a big issue but, if you oversize them, why not turn don't their maximum charge voltage such that you dont leave them sitting at 100% SOC when not using them. Then after 5 years you will have a less than 20% loss in capacity

 

[stranded]

If the boat is to be kept on a mooring such that you will have a daily solar charge cycle, I would suggest you use the expert mode in your MPPT to set a zero absorbtion time. Your bulk and float setting look fine but if the boat spends many days on the mooring without use, you might want to consider setting a lower bulk voltage, say 13.8v.
We are liveaboards and as such, we don't look to charge to full every day. Typically we will do a full charge every week or two to activate the balancer (set at 3.45v per cell). There is little benefit in having your cells at 100% SOC since this is proven to reduce the battery life. The cells are better for long term storage at 50% SOC but this is obviously not practical when you come to use the boat. You can set your own custom modes in the Victron. Let's call one mode 'storage' you could have this set with lower bulk charge. Another mode could be 'in use'. This could have a bulk charge set to 14v. These are easily adjusted when you leave and arrive at the boat. It would only be worth doing this if you planned to spend a few weeks at a time on the boat. Otherwise set the bulk to 13.8v.
If you left the batteries at 100% SOC for 5 years, you would lose about 20% of their capacity. With lead batteries this would be considered end of life. If you over size your lithium its not such a big issue but, if you oversize them, why not turn don't their maximum charge voltage such that you dont leave them sitting at 100% SOC when not using them. Then after 5 years you will have a less than 20% loss in capacity

Thanks Geem - that sounds eminently sensible - we will be 6 months mostly live aboard on the move, 6 months just a couple of days here and there on the mooring, so separate storage and use profiles must be the way to go.