Using only the surplus PV Power

[Cynic]

Hi.

I have been reading the threads on this forum - they are very informative - thank you all for your wisdom so freely offered. In many threads there is discussion regarding how the deeper Lead Acid batteries are drained the sooner they have to be replaced and that this is a major maintenance expense in a PV based power system.

Is it possible, to configure a solar MPPT controller based charger system so that only the surplus current not going into charging the batteries is used to power a low-priority item such as a water heater. Is there a technical term for such a system or device (so I can search further). I am thinking that the surplus power could be used to power a water heater or well pump to a tank while the batteries remain topped up waiting to kick in to provide backup power for a freezer etc. In this way the batteries would see almost no cycling.

One way I could think of doing it is detecting that the MPPT controller is not sending amps to the batteries and then flipping a relay to activate the additional circuit. Do MPPT controllers typically output such signals? Or would I have to monitor the DC charging current out of the MPPT controller?

Another idea might be to let the MPPT controller charge the batteries and then simply monitor the charge level and flip a the second circuit in and out directly off the batteries - all the while only permitting a very nominal amount of discharge from the batteries. Does this seem a better idea from the perspective of battery life?

Any ideas on a ballpark estimate of how long a lead acid battery might remain viable if used in the above scenarios?

I am aware that the economic math doesn't add up if buying a system such as I describe from new. However, I have some of the equipment already (got it very cheaply second hand), I like doing stuff like this, and am exploring interesting ideas.

As a side note, I'm completely new to PV/solar/batteries but do have knowledge of electrics/electronics and hence am comfortable with DC/AC volts/amps Watts and VA and the associated math - so no need to sugar coat things.

I appreciate your advice.

[inetdog]

Hi.

I have been reading the threads on this forum - they are very informative - thank you all for your wisdom so freely offered. In many threads there is discussion regarding how the deeper Lead Acid batteries are drained the sooner they have to be replaced and that this is a major maintenance expense in a PV based power system.

Is it possible, to configure a solar MPPT controller based charger system so that only the surplus current not going into charging the batteries is used to power a low-priority item such as a water heater. Is there a technical term for such a system or device (so I can search further). I am thinking that the surplus power could be used to power a water heater or well pump to a tank while the batteries remain topped up waiting to kick in to provide backup power for a freezer etc. In this way the batteries would see almost no cycling.

One way I could think of doing it is detecting that the MPPT controller is not sending amps to the batteries and then flipping a relay to activate the additional circuit. Do MPPT controllers typically output such signals? Or would I have to monitor the DC charging current out of the MPPT controller?

Another idea might be to let the MPPT controller charge the batteries and then simply monitor the charge level and flip a the second circuit in and out directly off the batteries - all the while only permitting a very nominal amount of discharge from the batteries. Does this seem a better idea from the perspective of battery life?

Any ideas on a ballpark estimate of how long a lead acid battery might remain viable if used in the above scenarios?

I am aware that the economic math doesn't add up if buying a system such as I describe from new. However, I have some of the equipment already (got it very cheaply second hand), I like doing stuff like this, and am exploring interesting ideas.

As a side note, I'm completely new to PV/solar/batteries but do have knowledge of electrics/electronics and hence am comfortable with DC/AC volts/amps Watts and VA and the associated math - so no need to sugar coat things.

I appreciate your advice.

Some Charge Controllers like the Midnight Classic have an output that tells you that the batteries are in Float and the panel power is all available to drive a load. Others can add that as an option. Most do not, but may offer a LOAD terminal that is energized only when the battery voltage is above some threshold. But even that is not going to tell you that there is left-over current available from the panels while the batteries are in Absorb or Bulk or how much extra is available.
And since a standard water heater is a very large on-off load, it will be difficult to use it for this purpose unless you add a "dimmer" or substitue a lower wattage element for one of the standard ones.
Such a control could be designed into an integrated battery and load management system that had monitor inputs for Vpanel, Ipanel, Vbattery and CC status, but I am not aware of a commercial offering.
With a smart management system, you could turn on some level of load and see whether it starves the batteries of needed panel current, and then leave it on or try a smaller load, for example.

[billvon]

Is it possible, to configure a solar MPPT controller based charger system so that only the surplus current not going into charging the batteries is used to power a low-priority item such as a water heater. Is there a technical term for such a system or device (so I can search further).

It is called "load diversion." Most older controllers (like the Trace C40) can be set up to do this. You generally need a second controller. You can also use the Midnight Solar Classic plus the Clipper to do this. (You'd have to replace the resistive load in the Clipper with your own load but this shouldn't be hard since it is user replaceable.) This is intended for wind systems; it loads down either the DC or AC side of the system after charge has completed to prevent the turbine from overspeeeding.

I am thinking that the surplus power could be used to power a water heater or well pump to a tank while the batteries remain topped up waiting to kick in to provide backup power for a freezer etc. In this way the batteries would see almost no cycling.

Water heater - should work OK

Pump - probably not with the Classic plus Clipper; the pump's voltage/current would be set by how much power it has to absorb, not by what's ideal to start and run the pump. Most pumps will be very unhappy in this mode. If you wanted to do this you'd need to roll your own. You could probably set a Classic to output "Opportunity Hi" on one of the aux channels and use that to drive a relay. (The Classic allows you to set minimum cycle times which would be important with pumps and the like.)

Backup power for a freezer - can't see how that would work, since the freezer is not a reliable load, can't handle quick disconnects and reconnects and is sort of important to maintain power to. Unless you are OK with the freezer defrosting regularly.

One way I could think of doing it is detecting that the MPPT controller is not sending amps to the batteries and then flipping a relay to activate the additional circuit. Do MPPT controllers typically output such signals?

Good ones have aux outputs that let you trip on things like high voltage, or float phase.

Any ideas on a ballpark estimate of how long a lead acid battery might remain viable if used in the above scenarios?

If you could do a true PWM system (like the Clipper) then the batteries wouldn't see any life reduction; they wouldn't be cycling at all. With a bang-bang controller (like the Opportunity Hi mode) you'd see some reduction in life, since you would be cycling the batteries very often - but very shallowly.

[Sunking]

Cynic as mentioned some controllers have a Dump Load terminal to divert power to after the batteries are fully charged. It is mostly used to put a load on wind turbines to prevent them from over speeding and destroying themselves. With that said I don't see any way for it to extend battery life. The dump load is just a way to use some of the energy that is otherwise wasted in a battery system and typically is dumped to a heating element to be burned off as heat and some like in Marine applications use to heat water while they can. The energy cannot be stored any other way that I know of.

[Bala]

the OP is talking about using excess? power which is a different thing to minimizing battery depth of discharge.

I would think the cycling of batteries in a stand alone system with PV charge only is unavoidable unless there is no load?

If you have reliable wind or hydro then then this can reduce cycling.

Also my understanding is that a charge controller in float mode does not mean the batteries are fully charged so care needs to be taken when using excess? power.

[inetdog]

the OP is talking about using excess? power which is a different thing to minimizing battery depth of discharge.

I would think the cycling of batteries in a stand alone system with PV charge only is unavoidable unless there is no load?

If you have reliable wind or hydro then then this can reduce cycling.

Also my understanding is that a charge controller in float mode does not mean the batteries are fully charged so care needs to be taken when using excess? power.

The usual distinction made is that when a battery is in float mode, very little if any charging is taking place. The current going into the battery is primarily going to replace self-discharge losses. An FLA battery should be completely charged and gassing at the end of the Accept period, while an AGM should be gassing and recombining without excess pressure being generated to cause venting.
A trickle charger, on the other hand, is forcing a constant current into the battery and guaranteeing that it will overcharge slowly. In an FLA or AGM battery, a low level of overcharge will not harm the battery. But too high a trickle charge current could.

[Sunking]

the OP is talking about using excess?

Yes that is the way I read it. But the challenge is that excess power has to go somewhere and be utilized. That is just the Laws of Physics. The problem is the excess power is dynamic meaning changes every second and is unknown from minute to minute. The only things that I can think of at the minute that works with a dynamic source is a heating element, grid tied inverter, and some DC motors.

[Sunking]

A trickle charger, on the other hand, is forcing a constant current into the battery and guaranteeing that it will overcharge slowly.

Sorry but I strongly disagree. A Trickle Charger is a constant voltage float charger with very low current capacity used to overcome the self discharge rate of the battery.

[Bala]

Yes that is the way I read it. But the challenge is that excess power has to go somewhere and be utilized.

I use my excess power when ever possible,

but my point is that using excess power during the day has nothing to do with preventing battery cycling, which is what I understand the OP is talking about.

[inetdog]

Sorry but I strongly disagree. A Trickle Charger is a constant voltage float charger with very low current capacity used to overcome the self discharge rate of the battery.

I strongly agree and yet disagree at the same time.

It looks like the usage of the two terms trickle charger and float charger is not well standardized. Some sources (http://www.autos.com/car-maintenance...-float-charger for example) distinguish a trickle charger as one which cannot be left connected to a battery indefinitely without damaging it while a float charger has accurate output voltage control and can safely be left connected to a battery indefinitely. I have seen circuits offered as "trickle chargers" which are actually constant current. But float chargers seem universally to supply a regulated voltage (although the available current may also be limited in the case that the battery is discharged below the float level.)
Small solar panels which have an output voltage well above the float voltage but have a low power output which limits the available current are advertised as trickle chargers.
Other sources use the two terms interchangeably.
In some cases trickle chargers are described as constant current chargers which exactly compensate the batteries self-discharge current. But the only way to do that without knowing the exact self-discharge versus temperature curve of a particular battery is to use a (hopefully temperature compensated) fixed voltage output instead.

The best thing to do in the case where usage varies so widely is to describe which you mean unambiguously by specifying whether you are describing a low capacity charger which is constant current or one which is constant voltage.

[Naptown]

The Tristar Mppt CC can be configured to do this but it takes at least two. They can be programmed to be a charge controller diversion controller and one other function that escapes me at the moment. However they will only perform one function depending on what they are programmed to do.

[billvon]

I would think the cycling of batteries in a stand alone system with PV charge only is unavoidable unless there is no load?

In an off grid system - yes. However, depending on the implementation of the surplus-power diversion load, you might _increase_ battery cycling above and beyond what is already occurring. This is undesirable.

[Sunking]

I strongly agree and yet disagree at the same time.

It looks like the usage of the two terms trickle charger and float charger is not well standardized.

A Float Charger is both a battery charger and Trickle Charger. A TC is just a very low power charger designed to provide a maximum charge rate of C/100 which is not capable of charging a battery in any meaningful amount of time.

Telephone, CATV, POCO, Rail Road, Mining, Utilities all use rectifier battery plants which are float chargers with EQ capability. They are used to charge the batteries plus supply all power for the equipment. Float Chargers give the battery maximum battery life and a best used where battery power is only used for emergency operation. The down side of a float charger is it does take longer to fully recharge a battery verse say a 3-stage charger that has Bulk, (constant current), Adsorb (constant voltage-current taper), and Float (constant voltage -current taper). As you can see Absorb and Float are virtually the same thing, the only difference is Absorb is a higher voltage.

A Float and Trickle use exactly the same algorithm. They are both constant voltage current taper. If connected to a discharged battery both will supply constant current until the battery terminal voltage reaches the SET POINT. When it reaches the Set Point the current tapers off but voltage remains constant as the battery internal resistance raises to the full charge point. When fully charged will only demand about 1% of the Battery C rating. Th edifference as I said is the TC only can supply about C/100, where a float charger can be as high a 5C but typically C/10 to C/2.

[inetdog]

A Float Charger is both a battery charger and Trickle Charger. A TC is just a very low power charger designed to provide a maximum charge rate of C/100 which is not capable of charging a battery in any meaningful amount of time.
...
Telephone, CATV, POCO, Rail Road, Mining, Utilities all use rectifier battery plants which are float chargers with EQ capability. They are used to charge the batteries plus supply all power for the equipment. Float Chargers give the battery maximum battery life and a best used where battery power is only used for emergency operation.
...
A Float and Trickle use exactly the same algorithm. .....

I appreciate the in-depth description of the difference, as used by industry professionals. This is clearly the way we should be using the terms on this forum. My point is that in advertising, product descriptions, general audience articles, discussions in other forums, and even in questions from new users in this forum, it is not valid to make the assumption that they are being correctly used that way. Sometimes we get too comfortable with language which is specific to the subject and short-cut the explanations we give by using specific terms which may not convey the necessary information to a casual visitor.

[Naptown]

I appreciate the in-depth description of the difference, as used by industry professionals. This is clearly the way we should be using the terms on this forum. My point is that in advertising, product descriptions, general audience articles, discussions in other forums, and even in questions from new users in this forum, it is not valid to make the assumption that they are being correctly used that way. Sometimes we get too comfortable with language which is specific to the subject and short-cut the explanations we give by using specific terms which may not convey the necessary information to a casual visitor.

Yes we do tend to do that. I am as guilty as anyone.