How to be kind to my new Lifepo4

[jt_retro]

Hi Folks

I live in a part of the world where the sun doesn't really shine much. It's extremely inconsistent to say the least.

I've been using solar power for the past 2 years. My most recent setup from last summer is 6 x AGM batteries, 3 x epsolar controllers, and 12 Renogy 120W panels all in parallel (12v system).

Obviously, the inconsistent sunshine is not good for my agm batteries as they like to be fully charged always. I've been using a grid charger to keep my batteries topped up during periods of low sunshine. As such, my main issue is that on the days I have good sun, my batteries are charged too much to take the max current, and on the days I have little sun, my SOC drops low.

I've ordered a lifepo4 battery to replace my agms. My question is, do lifepo4 suffer if left at a low state of charge for long periods? Eg. Over the course of a week, 4-5 days hovering between 10% and 50%, then perhaps one or two days getting enough sun to get fully charged, before dropping low again.

It would be really nice if lifepo4 could handle my inconsistent charging patterns and not have to worry, but that seems like a miracle. Surely there must be some negative to lifepo4 (apart from high upfront cost)?

I do get enough sunshine to cover my loads, just not at the right time!!

Thanks!

 

[rmaddy]

LiFePO₄ is perfectly content to sit at 50% SOC for months. In fact that is the recommended way to store them long term.

As long as you don't try to charge the batteries while they are below freezing you will not have any issue with them sitting at 10%-50% for a few days.

 

[jt_retro]

10%-50% for a few days

That's great to hear! Is there a rule of thumb for how low is too low vs time?

Eg for the sake of argument, how long could I safely use the battery for hoovering between 10% and 20%?

That's an extreme example, but I'd just like to know where I stand.

Cheers

 

[MisterSandals]

3 x epsolar controllers, and 12 Renogy 120W panels all in parallel (12v system).

Are you running 3x solar controllers from the same array? Are these MPPT SCCs?

 

[fratermus]

It would be really nice if lifepo4 could handle my inconsistent charging patterns and not have to worry, but that seems like a miracle.

Crack open the champagne then, because that's really how Li behaves. With enough capacity you can timeshift charging and discharging at will.

 

[jt_retro]

Are you running 3x solar controllers from the same array? Are these MPPT SCCs?

Apologies, I wasn't clear in my OP. 4 panels per SCC. All three are MPPT (and the same model), and go into the same battery bank.

Thanks

 

[jt_retro]

Crack open the champagne then, because that's really how Li behaves. With enough capacity you can timeshift charging and discharging at will.

It just sounds amazing! LA is just so unsuitable for my area's sunshine pattern.

Cheers

 

[MisterSandals]

Apologies, I wasn't clear in my OP. 4 panels per SCC. All three are MPPT (and the same model), and go into the same battery bank.

Thanks

If you are experiencing low light conditions (i read this into your OP), then you will likely have more time when your parallel panels are not producing enough voltage to start charging. I would think that putting your panels in a 2S2P configuration, you would double the array voltage and start charging earlier, finish charging later, and charge more during cloudy or otherwise low light conditions.

Your panel Voc x2 would need to be lower than your SCC max input voltage of course.

 

[meetyg]

Lifepo4 does has a self discharge rate, but it's far less than LA.

You should consult with your Lifepo4 battery/cells specifications for just how much they will self discharge.
This is important if you leave them at 10% for a long enough time, they will self discharge below 0%, which could cause damage.

 

[mikefitz]

or the sake of argument, how long could I safely use the battery for hoovering between 10% and 20%?

For some time, months perhaps, however since the voltage falls very quickly below 15%, it may be more practical running at slightly higher states of charge. What you will find is that lithium charges faster and very efficiently compared with led acid, thus the daily yield will be greater.
One issue that has been reported is that short cycling the battery may introduce a memory effect, temporally reducing the effective capacity, this is removed and full capacity restored by a full charge.
The chart shows the SOC working area that will give the greater service life, it may be over optimistic, but you get the idea,


Mike

 

[jt_retro]

The chart shows the SOC working area that will give the greater service life

I'm not sure I understand the chart. Eg For the top row (100%), is that 100% discharge to 0%, then back up to 100%?

Interesting to note the 15% self discharge threshold. I'll try to stay above that. I'm only using extreme numbers as worst case. Realistically, I see the SOC hoovering between 50% and 70%.

Thanks

 

[jt_retro]

I would think that putting your panels in a 2S2P configuration, you would double the array voltage and start charging earlier, finish charging later, and charge more during cloudy or otherwise low light conditions.

Thanks for this! I had never considered 24v would improve yield due to start/stop times. I may just give that a go! My Epsolar Tracers do support a higher input voltage.

One thing that's given me pause is the question on if I should parallel 2 panels horizontally or vertically. My house is in the way (array is ground mounted) causing poorer yield during low sun. So maybe parallelling vertically would be better.

Cheers

 

[MisterSandals]

So maybe parallelling vertically would be better.

When 2 panels are in series, and one is shaded, it drastically reduces the 2 panel string output. So the thought is to keep panels in series with equal sun.
If you have 2 that get sun first (or most) for example, they would be a good candidate to have in series.

 

[Bobert]

My experience won’t parallel exactly with your situation because I was using flooded lead acid batteries and agm batteries behave better at charging. I found that the lifepo4 batteries that I switched to transformed the performance of my solar panels. With lead most of the day was spent only using a fraction of my available solar to top off my batteries even so I seldom reached full charge. With lithium the batteries charge at full blast until nearly full and since it is not necessary to fully charge them it’s no big deal if you don’t have enough solar to make it to %100. The faradaic efficiency is so much better than lead acid batteries it felt like I had added some extra solar panels. I think you will be very happy with the switch.

 

[rmaddy]

When 2 panels are in series, and one is shaded, it drastically reduces the 2 panel string output.

Bypass diodes should greatly limit that reduction.

 

[jt_retro]

Folks just to sum up, if I used my lifepo4 from 20% to 50% for all eternity, I'll get good cycle life still? I'm still unclear if permanently only using part-capacity is bad for it.

Thanks

 

[rmaddy]

Folks just to sum up, if I used my lifepo4 from 20% to 50% for all eternity, I'll get good cycle life still? I'm still unclear if permanently only using part-capacity is bad for it.

That would actually be bad since the BMS would never be able to properly balance the cells. Use your LiFePO₄ from 5% SOC to 99.9% SOC and they will be happy longer than you'll be able to use them. The "trick" is to set the absorption voltage just high enough to fully charge them but not high enough to stress them each time. 14.4V seems to be a reasonable absorption voltage with a float voltage of 13.0-13.2V. And of course be sure to use a LiFePO₄ friendly charger.

 

[upnorthandpersonal]

That would actually be bad since the BMS would never be able to properly balance the cells.

He can just bottom balance if he's at low state of charge this often. For the cells themselves it's not an issue to be at 20%-50% in normal use.

 

[rmaddy]

He can just bottom balance if he's at low state of charge this often. For the cells themselves it's not an issue to be at 20%-50% in normal use.

I can't imagine spending the money on LiFePO₄ batteries and then only using 30% of the capacity. Seems like a huge waste. And why keep the SOC at 20%-50%? Why not 70%-100% or 65%-95% or some other arbitrary 30% range?

 

[jt_retro]

That would actually be bad since the BMS would never be able to properly balance the cells

Does this imply that a lifepo4 battery suffers from the same issue as LA, in that I have to make sure I charge up the lifepo4 to 100%?

The reason for the low state of charge, is due to the sun. During the summer months, I anticipate higher SOC. Winter....that's a different story.