Any Value in Tail Current setting on Victron MPPT

[crayiii]

I'm trying to get our Victron solar chargers setup to be as effective as possible. At the moment, I'm focusing on the absorption settings.

I'm leaning on a setting of "adaptive" a 9-hour limit (effectively daylight limited) and the tail current being disabled.

I'm just not seeing a way to use the tail current setting effectively since the controllers tend to drop into float depending more on changes to house load or cloud cover (multiple controllers).

However, today I believe I've noticed a potential issue with this setup. We've had sustained 20-30 knots of winds for the last 12-hours so our two wind turbines had our house bank almost fully charged by the time solar kicked in this morning.

We've been sitting at absorption volts for hours while actual tail current has been below full for these batteries.

I turned the tail current setting on today and now both controllers are in float. I'm wondering if that was necessary or if I could have just let the controllers do their thing and sit at absorption volts for as long as they would have (no idea how long that would have been).

Is tail current in the Victron MPPT chargers even useful for a house bank that is always in use? Is there a concensus on enabling or not?

Our house bank are Firefly batteries.

[smac999]

Unless it can read tail current from a bmv. I’m not sure I’d use it. Because it doesn’t know the load otherwise. And how much left is actually charging.

You can connect them to a bmv. But I’m not sure what is shared or used.

[crayiii]

Originally Posted by smac999

Unless it can read tail current from a bmv. I’m not sure I’d use it. Because it doesn’t know the load otherwise. And how much left is actually charging.

You can connect them to a bmv. But I’m not sure what is shared or used.

They are connected to a Victron BMV and a Smart Sense but they aren't seeing amps from the BMV.

[noelex 77]

The tail current setting via the solar controllers is largely useless on most marine installations. The exceptions are when leaving the boat with no (or minimal) load.

Generally the feature on the solar controllers is better disabled, especially when multiple solar controllers are used. Fortunately this is easily done via the software.

Tail current information via a battery monitor is different. This indicates the current directly entering the batteries so is quite different to the tail current measured by the solar controllers. Tail current (or battery return current) measured by a shunt located close to the batteries is the ideal way to terminate the absorption stage. If your system has this ability use this information don’t disable this feature.

[BillKny]

As has already been pointed out, tail current as measured by a charger is useless as a criteria for ending charging. On almost all boats the normal loads are larger than the tail current and a charger has no way of knowing the difference.

In most cases, on most boats that are being actively used, solar panels aren't likely to reach a real 100% SOC so early in the day that there is a serious risk of ever over-charging the batteries.

What I do is just set the solar float voltage equal to the absorption voltage. That way I get as much power out of them as possible without worrying about them prematurely dropping down to float.

If you have a (relatively) large solar farm, and a (relatively) small battery bank and you reach 100% SOC by noon, this might not be a good plan for you.

[noelex 77]

Originally Posted by BillKny

In most cases, on most boats that are being actively used, solar panels aren't likely to reach a real 100% SOC so early in the day that there is a serious risk of ever over-charging the batteries.

What I do is just set the solar float voltage equal to the absorption voltage. That way I get as much power out of them as possible without worrying about them prematurely dropping down to float.

If you want to supply most or all of your power via solar, reaching 100% SOC early in the solar day is inevitable for much or at least part of the year. Disabling the float voltage setting is not sensible in these circumstances.

If the output routinely exceeds the solar input and you rely on generator charging then effectively disabling the float setting on the solar controllers can be quite appropriate and safe, but otherwise it is not a good idea.

In many cases, solar charging is at relatively low currents (compared to other sources such as the alternator and generator) and therefore needs quite short absorption times to correctly charge the batteries.

Most marine batteries have a terrible electical systems and die from undercharging, but unfortunately this has created a culture where significant overcharging is routinely advocated. Batteries will have the longest lifespan when charged correctly. Slightly overcharging (for lead acid batteries) is better than slightly undercharging but don’t overdo this principal.

[Jim Cate]

In many cases, solar charging is at relatively low currents (compared to other sources such as the alternator and generator) and therefore needs quite short absorption times to correctly charge the batteries.

Could you elaborate on this idea, please? It seems counter to intuition for me.

Jim

[noelex 77]

Originally Posted by Jim Cate

Could you elaborate on this idea, please? It seems counter to intuition for me.

Jim

Yes, it is counterintuitive and this causes many incorrect charging settings.

Frequently it is assumed that high charge rates would require a short absorption time and low charge rate requires a long absorption time.

In fact the opposite is true.

The charging of lead acid batteries should drop to a float voltage once the charging current drops to a set value. Battery manufacturers quote this number, but typically 1% is an average value. So for a 400Ahr battery bank, once the charge current at the absorption voltage drops to 4A, the charger (solar, alternator or shore power) should drop to the float voltage.

So the lower the charge current, the shorter the absorption time.

For example, if the charge current was only 4A, the absorption time would ideally be zero minutes. Once the absorption voltage is reached, the correct requirement to drop to float voltage is fulfilled.

In practice the current from solar panels varies considerably, nevertheless it is on average generally lower than from the alternator or shore power. It is commonly assumed that this should result in a longer absorption time, but the opposite is true.

Absorption times as short as 15 minutes are not unusual for solar systems, but the battery return amps are not difficult to measure and this is the best method for setting your charging system.

[Jim Cate]

Originally Posted by noelex 77

Yes, it is counterintuitive and this causes many incorrect charging settings.

Frequently it is assumed that high charge rates would require a short absorption time and low charge rate requires a long absorption time.

In fact the opposite is true.

The charging of lead acid batteries should drop to a float voltage once the charging current drops to a set value. Battery manufacturers quote this number, but typically 1% is an average value. So for a 400Ahr battery bank, once the charge current at the absorption voltage drops to 4A, the charger (solar, alternator or shore power) should drop to the float voltage.

So the lower the charge current, the shorter the absorption time.

For example, if the charge current was only 4A, the absorption time would ideally be zero minutes. Once the absorption voltage is reached, the correct requirement to drop to float voltage is fulfilled.

In practice the current from solar panels varies considerably, nevertheless it is on average generally lower than from the alternator or shore power. It is commonly assumed that this should result in a longer absorption time, but the opposite is true.

Absorption times as short as 15 minutes are not unusual for solar systems, but the battery return amps are not difficult to measure and this is the best method for setting your charging system.

I'm missing something here... The absorbtion cycle begins when the bulk charging raises the system voltage to the absorbtion level, right? At that time the charge rate (amps) is determined by both the acceptance rate of the battery and the capability of the charging system to supply current. With a high current capable charger the current is limited only by the acceptance rate of the battery and the state of charge will rise faster and the 1% tail current reached sooner. At that point, it should go to float. If the charger can not supply much current, it will take longer to reach the SOC where the 1% rate is reached.

Or that's how it seems to me... where have I gone wrong?

Jim

[rslifkin]

With a high current charger, the switch from bulk to absorb happens early, as voltage comes up fast and then current starts to drop. With a slow charge, by the time you get to absorb voltage you're already down to a fairly low number of amps, meaning closer to ready to drop to float.

[noelex 77]

Originally Posted by Jim Cate

Or that's how it seems to me... where have I gone wrong?

Imagine if you had a large charger, say one capable of delivering 300A, and this is used to charge a 400Ahr lead acid battery bank. The battery battery SOC starts at 50%.

With such a large charger the battery voltage would rise very rapidly to the absorption voltage. The charger would almost immediately be in the absorption phase with the battery still at a SOC barely above the starting value of 50%. Despite having a very large charger, the current delivered is dependent on the acceptance rate of the battery. As you know, it takes over 5 hours to fully charge a lead acid battery from a 50% SOC so the absorption time would need to be over 5 hours.

Now let’s take the opposite extreme of a very small charger only capable of delivering 3A. The charging would take much longer, almost 3 days, but when the voltage reaches the absorption voltage the tail current is already below the 1% level, as the charger is only producing 3A. So the absorption time is zero.

So in these extreme examples the very large charge source needs an absorption time of over 5 hours and the very small charge source needs a zero minute absorption time.

So a larger charge source will charge the battery quicker, but it will spend longer in the absorption phase. So to correctly charge the battery with a large charge source we need to increase the absorption time and with a small charge source the absorption time needs to shortened.

Most people get it the wrong way around.

[Dockhead]

Originally Posted by noelex 77

Imagine if you had a large charger, say one capable of delivering 300A, and this is used to charge a 400Ahr lead acid battery bank. The battery battery SOC starts at 50%.

With such a large charger the battery voltage would rise very rapidly to the absorption voltage. The charger would almost immediately be in the absorption phase with the battery still at a SOC barely above the starting value of 50%. Despite having a very large charger, the current delivered is dependent on the acceptance rate of the battery. As you know, it takes over 5 hours to fully charge a lead acid battery from a 50% SOC so the absorption time would need to be over 5 hours.

Now let’s take the opposite extreme of a very small charger only capable of delivering 3A. The charging would take much longer, almost 3 days, but when the voltage reaches the absorption voltage the tail current is already below the 1% level, as the charger is only producing 3A. So the absorption time is zero.

So in these extreme examples the very large charge source needs an absorption time of over 5 hours and the very small charge source needs a zero minute absorption time.

So a larger charge source will charge the battery quicker, but it will spend longer in the absorption phase. So to correctly charge the battery with a large charge source we need to increase the absorption time and with a small charge source the absorption time needs to shortened.

Most people get it the wrong way around.

Wow, that's just extremely useful knowledge. Learn something every day. I've been messing with lead-acid batteries since childhood, and am amazed every time when I learn something I never knew. Thanks.

[rgleason]

Originally Posted by Dockhead

Wow, that's just extremely useful knowledge. Learn something every day. I've been messing with lead-acid batteries since childhood, and am amazed every time when I learn something I never knew. Thanks.

Using Noelex 77's example, but for LiFePo, the principles would be the same, but because of the near horizontal charge (and discharge) slope in the middle, wouldn't the difference not be as large between the high volt and low volt charging? Perhaps this is another way of saying that the absorption stage for LiFePo is much shorter (and if run for too long will destroy the batteries)?

[sailcrazy]

Following...all new info for me.

[Jim Cate]

Originally Posted by noelex 77

Imagine if you had a large charger, say one capable of delivering 300A, and this is used to charge a 400Ahr lead acid battery bank. The battery battery SOC starts at 50%.

With such a large charger the battery voltage would rise very rapidly to the absorption voltage. The charger would almost immediately be in the absorption phase with the battery still at a SOC barely above the starting value of 50%. Despite having a very large charger, the current delivered is dependent on the acceptance rate of the battery. As you know, it takes over 5 hours to fully charge a lead acid battery from a 50% SOC so the absorption time would need to be over 5 hours.

Now let’s take the opposite extreme of a very small charger only capable of delivering 3A. The charging would take much longer, almost 3 days, but when the voltage reaches the absorption voltage the tail current is already below the 1% level, as the charger is only producing 3A. So the absorption time is zero.

So in these extreme examples the very large charge source needs an absorption time of over 5 hours and the very small charge source needs a zero minute absorption time.

So a larger charge source will charge the battery quicker, but it will spend longer in the absorption phase. So to correctly charge the battery with a large charge source we need to increase the absorption time and with a small charge source the absorption time needs to shortened.

Most people get it the wrong way around.

OK, I'm slowly understanding your argument. The way I see it, the difference in absorbtion times comes from a shorter bulk time with the high rate charger, not so much an "earlier" change into float, but the result is the same: more absorbtion time in a 50% to ~100% charge.

And FWIW, your hypothetical setup is similar to mine: 400ah house bank, 150 amp alternator. I've never seen more than 85 amps into the bank which tallies with the generic 0.2C max charge rate often quoted, and with those conditions, it stays in bulk for some while and doesn't quickly change to absorbtion as in your example... but I do understand the gist of what you have described.

In practice (as you state) when relying upon solar charging simply setting the float voltage to be approximately equal to the absorbtion voltage is a convenient hack that eliminates the worry about tail current controlled switching.

An interesting discussion, Nolex. Thanks for bringing it up.

Jim