Battery lifespan and winter

Hi Guys, two questions. In joburg winter, my batteries only charge to max 80%, even with all-day sun. Is that to be expected? (I have 4 X 1. 4.8KWH lithium ion batteries). Or should I reconsider the positioning of the panels? Secondly, how important is it for the long-term lifespan of the battery that they charge to 100% on a fairly regular basis? Any guidance would be very much appreciated.

It is normal that there be less sunlight in winter. I don’t know what system you have, but in a Victron ESS System we have a setting called BatteryLife, which will automatically increase the minimum state of charge limit to ensure the battery is fully charged every day. In other words, it will automatically use more energy from the grid and less from the battery.

Lithium batteries don’t care about being left at half charge for longer periods. The reason you want to fully recharge (at least twice a month or so, preferably weekly) is for two reasons.

  1. Most Lithium balancers only balance above a certain voltage. If the battery is not charged more than 85%, the balancer doesn’t activate. This is not too much of an issue: If the battery is already balanced, the balancer doesn’t do much anyway. It can go a long time at lower states of charge before this becomes a problem.
  2. This is the bigger one. SOC drift. The State of Charge of the battery is an estimate. The electronics measures the current, multiplies if by time (Amps times hours = Amp Hours, times voltage is Watt Hours), and that is how it calculates it. The measurements are never completely accurate, especially at lower power levels, and because it does this a few times per second (a process we call numerical integration), there is always an error. This error is corrected when a higher voltage is observed, because we know the cells will rise to above 3.5V when they are full. A full charge is therefore required occasionally to reset your SOC estimate.

Hence why I have a BMV with the BMS.

The two are close as can be … sometimes not … them I trust the BMV.

The BMV has the same problem, although admittedly, when compared to a cheaper BMS, it’s rather excellent!

On that topic. Did you know there are BMSes without SOC measurement? Batrium is one. Unless you buy the additional ShuntMon accessory, it cannot do SOC, or current readings. It still does everything else you need (cell balancing, protection), but no SOC or power values. Of course, this is why in Venus 3.00, you can select a different battery monitor and controlling BMS now. Plus, Batrium has a feature in the latest firmware where a BMV/SmartShunt can be used to communicate the missing info to the Batrium BMS. There is truly no end to what can be done when people work together.

(Yeah I know that is a little OT, but also right in the general ballpark).

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Must say, that is great.

Like how this is all progressing. A brand name, I forgot, recently added the cell voltages to the data given/shown.

THAT, being my personal bear bug, is also slowly coming through …

Thanks, that’s extremely helpful

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If you have a Victron inverter with a GX I would recommend making use of BatteryLife especially if you can’t get to 100% after a couple of consecutive sunny days. That’ll just ensure you get to 100% at least once a week ensuring the balancers does its thing like it was mentioned.

If I have 2x sunny days in a row in Joburg I generally get mine back to 100% provided it’s not laundry day and we don’t use the oven too much (of course today is the 2nd sunny day and it’s also laundry day, sooo hopefully tomorrow stays this clear).

What I have is an alarm set on my VRM (Victron inverter) so that I get an alert if the battery didn’t reach 99% for 6 days. It hasn’t happened yet, but the plan is that if that alarm is triggered then I will just charge the batteries up from the grid (if it’s available of course) and then go back to cycling it like normal.

That will be my way of getting it to 100% once a week, but maybe using a little less grid than BatteryLife would after a couple of cloudy days. Knowing myself I’m just overcomplicating it in any case though.

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Bad weather for days … I set to Keep Charged, or like 80% … and let it all be.

I thought it was as simple as a 20% min SoC and you’re good to go, maybe add BatteryLife to get it up to 100% a week. Buuuuut then I remembered that when I designed this install in my head it was all based around loadshedding being that annoying thing for 2 hours a day for a couple of weeks during winter.

Oh how things have changed.

Hence me wanting to play with Node-RED and the EskomSePush node just to build a loadshedding buffer, but then you also have bad weather days etc. etc.

I might end up giving up on tweaking it, but I’m sure you still remember how equally fun and annoying it was when you just started.

Sounds like you need more panels!