70 replies to this topic

[rando]

rando, can you revisit how you set your BMV and  MPPT controller to self-limit charging to 90% or so?

 

It is not that precise - when the camper is parked for a while, I set the absorption voltage to 13.5, absorption time to 10 minutes and float to 13.2V.  Based on these measurements from powerstream and my experience 13.5V (3.375V per cell) should get the battery to somewhere around 80 - 90% SOC.   This seems to be a better place to store the battery than 100% SOC.     

When we are using the camper, I up the absorption voltage to 13.8V, which based on the above measurements should get the battery to 99% SOC.     There is almost no advantage to going over 13.6V - 13.8V,  taking the battery higher just increases the likely hood of unbalancing the cells, 

[rando]

Thanks Lars. I had changed the absorption to 14.3 as seen below and the time at 30 minutes. I think I will increase the time to an hour. (kinda feels like I'm just jiggling the butterfly valve on the old chevy... see what happens!)

Attached the settings and the monitor this AM... Voltage still only 13.2

 

13.2V is the normal resting voltage of fully LiFePO4 battery, so that is what I would expect to see first thing in the morning. 

 

I don't really think there is any advantage in setting the absorption that high (see my last post), but if you want to see higher numbers is won't really hurt.   I would also raise your float to 13.3 or 13.4V.   Lithium batteries don't really want to float charge, but you do want the float voltage to be enough that any loads that come on during the day are drawing from the solar panel, not the battery.

[Vic Harder]

When we are using the camper, I up the absorption voltage to 13.8V, which based on the above measurements should get the battery to 99% SOC.     There is almost no advantage to going over 13.6V - 13.8V,  taking the battery higher just increases the likely hood of unbalancing the cells, 

 

OK, that's some interesting research.  I wonder why Battleborn  recommends that the absorb voltage be set to 14.4V (see post #2 in this thread).  I recall something I read on their site which indicated using a 30 min/100AH Absorb phase actually helped with balancing the cells.

[rando]

Many of the off the shelf battery management systems use what is called 'top balancing' to actively balance the cells.   Once the individual cells get above 3.6V (14.4V total) the BMS connect resistors to the high cells to bleed them down to match the lower cells.   I am guessing that is why Battleborn wants you to get the voltage so high, so their internal BMS can top balance the cells.

 

My BMS also has this feature, but because I never charge my battery that high, I just manually run an equalization cycle using a battery balancer every 6 months or so if the cells need it. 

[Stokeme]

StarkPower also recommended absorption V set to 14.4V & float set to 13.8. Top balancing as a term makes sense. I never considered any different setting but it makes sense based on active use vs non use. I am fine with my float V set for now.
My max absorption time is set for 6 hours. Is this excessive, ie “not a good idea”? Would also appreciate any comments, on my query, concerning the expected charging parameters one would experience with a drained Battery vs “topping” a mostly charged Battery. Thanks.

Edited by Stokeme, 03 September 2019 - 10:42 PM.

[rando]

The 'drop in lithium' battery assemblers face a little bit of a conundrum - their sales model is to offer a LiFePO4 battery that can approximate the characteristics of a lead acid battery and play reasonably well with existing lead acid charging infrastructure.   As such they need to publish specs that fit within the range of most lead acid battery chargers.  However the chemistry of LiFePO4 is completely different than lead-acid and while it will work at the listed specifications, they are not optimum.   

 

Low charging voltages lead to longer cycle life.  While there is much more data for lithium-cobalt (aka lithium ion) than for LiFePO4, given the similarity in the chemistry it is not unreasonable to assume the relationship between charge voltage and cycle life is similar for both chemistries.   The rule of thumb for lithium is that for every 0.1V you reduce the charge voltage you double the cycle life.  I have no idea if this relationship holds for LiFePO4, but I am sure there is a similar relationship.   

 

There is also a clear relationship between state of charge during storage and calendar life - after 1 year of storage at 100% SOC an LiFePO4 battery will irrecoverably loose 20% of its capacity, whereas one stored at 40% SOC would only loose 4% of its capacity.    Let your expensive lithium battery sit at 100% SOC is probably unwise. 

 

Any I am rambling - to summarize, yes you can charge your battery to 14.4V and float it at 13.8V, but it will last longer if you charge it 13.8V and float at 13.2V or not at all.   It is very likely that your battery will last long enough in either case, but if you have a choice, pick the lower numbers. 

[Vic Harder]

Thanks for the "ramble" rando.  Very helpful explanation!

[Vic Harder]

Just tested the lower settings rando.  Interesting.  Battery was fully charged and at 13.4v.  So when I set float to 13.2 the BMV showed the battery discharging.  Loads would have been the fridge and the truck battery.  The solar controller was not providing the power even though the camper was in full sun because float was at 13.2.

 

I bumped up the float setting to 13.4 for now, so now the solar panels are taking all the load during the day once the batteries are fully charged.  I guess the whole point here is to NOT fully charge them, right?

[ckent323]

For folks dealing with chronically partially charged batteries for extended periods I have read some encouraging information about the Lead/Carbon Foam batteries (Firefly Oasis).

I am still learning about these and finding independent data to back up the claims requires searching.

Price seems to be between high quality AGM and LiFePO4 (i.e. $400 - $500 for 12V 100ah). Apparently the marine boating community has been using them (based on my Dad's boat I understand the challenge of keeping a battery fully topped off so these are appealing for those who want to operate between 30% and 80% soc.

More info here:

https://oceanplanete...oasis-group-31/

From Battery University Site:

Firefly Energy
The composite plate material of the Firefly Energy battery is based on a lead-acid variant, and the maker claims that the battery is lighter, longer living and offers a higher active material utilization than current lead acid systems. It is also one of the few lead acid batteries that can operate for extended time in partial-states-of-charge. The battery includes carbon-foam electrodes for the negative plates, which gives it a performance that is comparable to NiMH but at lower manufacturing costs. Firefly Energy was a spin-off of Caterpillar, and in 2010 it went into bankruptcy. The company was later revived under separate ownership but folded again. Since 2014, the battery is manufactured in India under Firefly Batteries Pvt. Ltd."


Perhaps this should be the start of a separate topic so I will start one here:

http://www.wanderthe...-firefly-oasis/

Regards,

Craig

Edited by ckent323, 04 September 2019 - 08:56 PM.

[buckland]

This is interesting Vic I had the same thing happen. Even though the panels were in full sun I was drawing half the load off battery. I will bump up the float to 13.4 just for a look see. I did buy a temperature smart sensor from Victron and attached it to my setup (a bit tricky as the sensor has to stick to one paper covered cell). It is blue tooth to Victron app.