Energy Matters Forums

[Al510]

Today i have been playing with a smartsolar 100/30 and a solar king 100 ah lithium battery. With the custom settings, i set the absorption voltage at 13.8v. When in bulk mode the voltage quite quickly rises to 13.8v , at which point the controller switches to absorption. The battery is nowhere near fully charged at this point, and depending on the absorption time setting, switches to float.
The longest possible absorption time you can set is 2 hours, as the wake up battery voltage will be higher than 12.6v which triggers a 1/6 multiplication of the absorb time setting, the maximum being 12 hours.
Once the absorb time is up it switches to float, when set at 13.5v only delivers about 3 to 4 amps.
So, based on a 200w panel giving a max of about 10 amps, if the controller switches to absorption at the 50% mark, then getting the full 10 amps for 2 hours of absorption, at 3 to 4 amps charging, it will be lucky to get back to 100% by night fall. Infact it won't get to full as its only charging at 13.5v.
The voltages i have used are based on what i have read, being safe for long term lithium use. I could increase them but at a possible cost to battery cycle life?
I have bought these controllers to use on a large lithium bank, thinking they are fully configurable but appears they are not, unless i am missing something.

[davef]

What percentage charge do you think the batteries get to? I have seen 3.4V or 13.6V as recommended for better cycle life. I charge mine to 3.55V or 28.2V, maybe 90-95%.

I am still following a "recommendation" not to float them. Again, for cycle life it appears that you want to keep the number of "microcycles" as low as possible. I do one major charge in the morning then switch off about 75% of the panels then 3-5 minor charge cycles during the day. I need to go into the evening well-charged as I have a rather small battery bank (24V/ 5KWH). Typical on-site house draw is 2KWH, so at times it is a struggle keeping them above 50%.

Do you have a BMS on that battery? There was a lot of debate years ago about whether or not you needed one. No idea what the current thought is on that. I use a Electrodacus SBMS60 which does all the balancing for me. I had to run the battery system without balancing for 1 month and only had about 2mV between cells.

It sounds to me that that batteries are still charging at a good rate when set to "float". I would expect the current to drop from absorb (13.8V, 3-4A) to float.

Had a real quick look at their datasheet and can't find what type of batteries they meant to charge. Is LiFePO4 mentioned anywhere?

Dave

[davef]

What is your typical discharge rate?

[Al510]

Hi Davef
At a guess i would think that when the controller switched to absorption, it would have only been about 50%. I will have to charge the battery with my charger until the current flow drops to 2ah or so and hook up my bmv712 and start with a known state of charge, discharge to 20% and then try it again.
When this battery voltage rose to 13.8v to trigger the absorb cycle, if i set a short cycle to get it to float, with the float set at 13.3v, the battery quickly drops back to that voltage.
The controller does have a lithium setting, 14.2v absorption, 13.5v float, with bulk being contsant current until the battery gets to absorption voltage.
I have also read that lithium doesn't like to float, so i thought a low setting, 13.3v would reduce the effect, but as the battery voltage then needs to drop below that voltage to trigger another absorption cycle, it will not go back to absorption.
Maybe it is just a quirk of this battery that it's voltage rises so fast when in bulk, i don't know. Its a cheapish drop in type with a built in bms that i bought to run my fridge in my landcruiser.
I was just playing with it to get a better idea of the controller settings, and i guess i was just a bit dissapointed in the available user definable settings when they are advertised as being able to set a custom charge algorithm.
I bought these controllers to charge a 10kw 24v battery in a caravan that i plan to live in full time after xmas.

[davef]

Found this in my Battery stuff:
https://www.technomadia.com/2012/09/a-year-on-lithium-rv-batteries/
I noticed that it mentioned a LiFePO4 setting for your controller.

See attached file for a typical day here, notice that that battery slowly ramps up to about 27.2V then it starts to ramp up much quicker. The charging stopped at 27.7V, which is a bit early I think due to the relatively high charge current of 25A. At this point the PV panel size is to reduced to about 25% . The first charge cycle is then finished off with this lower current at 28V.

In the afternoon notice that it spends a lot more time at 27.1V (charging about 4-5A) but then still climbs just as rapidly to 28V as it did on the higher current.

So, I think we are seeing similar behaviour.

I'd suggest having a look at the SBMS120 at http://www.electrodacus.%20com

Dave

[davef]

A bit of information that might be more helpful ... at 27.2V (2 * 13.6V) when the voltage starts its rapid increase the SOC is 95%. 100% is 3.55V/cell (2 * 14.1V)

[Al510]

Thanks Dave,
I have seen the technomadia posts before. There is an update, i think 5 years from memory, might even be a video. They have lost over 20% capacity, i think they were down to 360ah of the 500. I would have to find it again to be sure on the numbers. They put the loss down to the hot space the battery was kept in.
I have put this battery on charge on my projecta charger this afternoon. It was at 13.21v before, it has been taking over 10amps for 1.5 hours and still going, at a constant voltage of 13.8v.

[Al510]

The electrodacus looks interesting. I've had a look through the manual, its a bit hard to tell how the charge regime works. Is it still bulk,absorption and float? Or can you switch off float if no loads are present?
This battery is still taking 10 amps at 2.5 hours, so was definitely not full. Seems strange to me that the charge voltage would climb so fast as to put the charger into absorption . Any one have an explanation ?

[davef]

I apply heat if below 15C and alarms go off at 35C. I think Electrodacus had about 1%/year for A123 cells, so 5% a year is going to hurt the pocket!

At a 1/10C charge rate you should get more than 100Amp/Hour on a new set of cells.

Back to your voltage question. I tried reducing mine from 3.55V/cell to 3.45V/cell as I had read that one might get longer cycle life. Unfortunately the SOC algorithm requires the cell voltage to get near 3.55V/cell for the SOC to reset itself to 100%. So, it is back to 3.55V/cell.

Maybe, try allowing them to charge up to 14.1V and see what their behaviour is like. I found some specs:

Charge Voltage 14.4V - 14.6V

which isn't much higher than my 28.2V

[davef]

The SBMS series are just ON/OFF chargers. The theory is that there is no absorption and definitely no float. For PV panels with a Vmp of 30V charging a LiFePO4 to 26/27V there is little benefit with MPPT.

But ... at 13.8V I would say you are hitting the area where the voltage will ramp up quickly. The issue that needs clarification is what SOC you are really at when it switches to absorption.

What voltage is the battery at?