House Load Bias(Priority)?

ligwyd · September 2019

I came across this ad, that may be up for a while, and found one paragraph in the ad particularly interesting.

https://buyselltrade.ca/kaleden/amazing-portable-solar-generator-off-grid-system/classified/739150

"Solar panels and generator have a house load bias, meaning that house is priority. Surplus goes to batteries to prevent unnecessary battery cycling"

I always wondered that, with my off grid systems, when the sun is shining, and there is opportunity to run some large loads, must all power generated from the Sun or Generator go through the batteries or can the power go directly to the loads and leave the batteries alone so to speak? Maybe this is exactly what high end inverters do by design. I would think so but I am asking here anyway

I realize that my XW+ is a battery based inverter and also that I may be not understanding the basics in current flow through these inverters. I'll look and see if my manual has any info on this but just thought I'd post this question as it never really dawned on me until I read the above paragraph from the ad.

Thanks to all who may be able to add some clarity to this topic.

Estragon · September 2019

In general, to the extent loads (including related inverter and wire losses) are less than available solar (or generator) power, they will be powered by solar, not through the batteries. This would be the case even with no inverter involved (ie only DC loads).

Some inverters can use battery power to supplement generator power to the extent loads exceed generator input limit, others just pass generator current through, and will fault if generator input is exceeded. All will use solar for loads to the extent it's available.

In terms of current flow, the solar charge controller will try to reach or keep battery voltage according to it's program. In float, with full 48v nominalbatteries, for example, it may draw ~1% of battery capacity to hold ~54v. If battery capacity is 500ah, it would use (and the pv would produce) ~5a x 54v = 270w. When an inverter load is added, the battery voltage would otherwise drop below the target voltage, so the controller increases pv current (to the extent available) to keep voltage on target. The battery is still the same 270w load. The inverter load isn't going through the battery, it's just an additional load on the controller.

Grid-tied inverters add a bit of a twist in that some can be programmed to sell/draw from grid or battery based on various conditions (eg time of day, battery state of charge, etc). In this case, the grid is conceptually like a second battery, and the programming determines which "battery" has preference in terms of charging and discharging.

mcgivor · September 2019

Current will take the path of least resistance, when the batteries are charged and there are no loads the current from the array will be minimal, just enough to overcome self discharge along with the inverters self consumption. When a load is applied to the inverter the current from the array will increase and divert to the inverter as the batteries still only require a minimal amount, this assuming the load is not greater than the arrays capacity, so in this case the batteries do not discharge.

Should the loads be greater than the array capacity, or when a cloud passes, then the batteries will provide makeup current to supplement the array. As long as the loads are within the arrays capacity by say 20% the batteries will remain at a high state of charge and would not be considered as "passing through" the batteries.

What's undesirable is to get to a high state of charge, then discharge to a point where a rebulk is required as this would technically be counted as another cycle. Many users use opportunity loads during absorption when current is low and sun is plentiful, the array should able to support both load and battery needs in this case.

With lead acid it's more challenging than lithium due to the batteries need for an absorption along with their inability to remain at partial states of charge, but with careful monitoring it is possible to keep things in harmony.

The same theroy applies with generator charging, this can be problematic if the charger has a limited capacity and not part of the inverter as the batteries may need to support most of the load and possibly damage the charger.

The inverter charger systems, like the XW+ will pass AC current to the loads and provide battery charging similtaniously, which makes them so versitile.

ligwyd · September 2019

Appreciate the info guys........ Just read your two replies and still "In Absorb"

Re:

"What's undesirable is to get to a high state of charge, then discharge to a point where a re-bulk is required as this would technically be counted as another cycle."

If I am understanding correctly:

First off, I have not yet checked to see what the "delay time to trigger bulk charge" is set to and do not yet know if it is adjustable although I'm sure it is, as everything is adjustable with these hybrid inverters. Amazing equip!

When a large load is applied for only a short time, in sun or not, and batteries voltage temporarily dips below re-charge voltage, and a bulk charge is possibly un-necessarily triggered, when in fact, when the large load is no longer present, the batteries voltage would have simply recovered above re-charge voltage on their own.

It would seem that a bulk charge did not need to be triggered. Or is this a good thing so that a large load is compensated by a short triggered bulk charge of higher voltage, that will not last long anyway as the batteries are still nearly fully charged?

I guess a guy could simply lower the re-charge voltage, or is adjusting the "time to trigger bulk" also to be adjusted?

Estragon · September 2019

It's a bit of a balancing act, with values depending on your specific load patterns, bank size, etc. You generally want to set to avoid a rebulk if, for example, a well pump running for a minute or two to fill pressure tank, or running a microwave drags bank voltage down. As you note, voltage will mostly recover on it's own.

OTOH, if voltage is dropping because of a heavy tool use day, you may want to rebulk to avoid being at a low state of charge overnight.

ligwyd · September 2019

What about in a weekend use cabin, if a system is in standby but operational with no loads? Each morning, a quick bulk, absorb is triggered. Can this slowly leed to overcharge conditions damaging positive plates reducing battery life? Is it better to just shut a system down when not in use and initiate a charge cycle monthly followed by 1 hour EQ (XW EQ default) to mix up electrolyte?

Estragon · September 2019

My Midnite Classic charge controllers have a "skip days" setting, so they go straight to float for 'X' days unless batteries are drawn down to a given rebulk value. Doing daily absorb cycles on an idle bank is harder on batteries than floating with periodic absorbs IMHO.

As for shutting down, it depends on the length of shutdown, battery type, and temperature. Monthly is probably fine for flooded lead acid, but many months could be an issue, especially if warm. Cold batteries have a lower self-discharge rate than warm.

Another option would be to set absorb voltage lower when idle so the cycle isn't as hard on batteries and water loss is minimized. When occupied, set back to normal Vabs. I'm doing this for small array controllers to keep banks floated over the winter when main arrays are prone to get snow covered for long periods.

An absorb cycle should be mixing electrolyte without EQing. If there's no bubbling and mixing toward the end of absorb, this suggests too low asbosrb voltage and/or too short time. An EQ should be done to get cell capacity as equal as possible.

ligwyd · September 2019

When EQ ing, as long, as the 3 strings are all within tolerance of each-other would it be unwise to EQ(1 hour only) as a bank or should EQ in (only when necessary) always be done each string separately?

Estragon · September 2019

What does "strings all within tolerance of each other" mean?

ligwyd · September 2019

Sorry about that. I mean all 3 strings equal voltages and S.G. (or as close as possible after activating/ equalizing each string separately)

Estragon · September 2019

To me, EQing is done to solve a problem with weaker cells having diverged enough in SG to require intervention. If SG is close (which IMHO would be say a high of 1.27 vs a low of 1.26 or 1.255) between cells, EQ isn't needed. If SG variance is higher, EQing can raise the weaker cells, but at a cost of overcharging already fully charged cells. Once the problem cell(s) stop rising, that's about as good as it gets. It might take an hour to get there, or it might be a multi-day process. I do it to solve the problem, not to follow an arbitrary schedule.

Ideally, you'd EQ only the low cell(s), but as a practical matter, 2v cells are usually packaged in higher voltage blocks (making cell level EQ nearly impossible), and blocks are assembled into series strings, which are then paralleled. It's better to EQ strings individually to better target the low cell(s), but practically may not be worth the effort if the cell(s) aren't all that low.

I'd expect resting voltages between paralleled strings to be very close. What would be interesting to me would be is if they varied much with significant charge or discharge current flowing, or if one battery in a series string had a significantly different resting voltage than the rest of the string.

ligwyd · September 2019

Thanks for the feedback. Great information. Really appreciate the time people put in!

I'll keep a close on everything and take er from there:)

I watched a you tube vid of solar guy that had a shunt on each of three strings using lithium. Great way to keep an eye on exactly what is going on at all times. Very clean install!!!

https://www.youtube.com/watch?v=Zy69NRFh34I

House Load Bias(Priority)?

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