Help understanding solar batteries charge and load.

Hey everyone I am confused and need some help understanding something. Autonomous robots use solar panels and batteries to operate 24 hours a day, but I am confused because lets say I have a

4S Li-ion battery, rated for 40amp continual discharge and 2amp maximum charge rating.

And a solar panel that puts out 16.8v@8a

And finally a load that draws 2 amps

1. If the solar panel is connected to the battery and the load, so during day hours it can charge the battery and provide power for the load, the 2amp load would draw 1a from the solar panel and 1a from the battery this would mean the battery would be getting charged at 6amps, how do the designers get around this problem?

I have seen Mppt charge controllers that have 3 terminals, solar, battery and load. Do these devices provide power to the load from the solar panel, whilst charging and keeping the battery charged up, and then when it senses the solar panel drops below a certain voltage, (night time) switch to providing power for the load from the battery instead?

Statement:
" 1. If the solar panel is connected to the battery and the load, so during day hours it can charge the battery and provide power for the load, the 2amp load would draw 1a from the solar panel and 1a from the battery this would mean the battery would be getting charged at 6amps, how do the designers get around this problem? "

is incorrect. The load will take power from the highest voltage source. Thus, if the solar panel is charging the battery, it will have the higher voltage, in theory. Thus, the solar panel will provide the load with 2A, and the remaining power will go to charging the battery with whatever potential is left.
If, for some reason the battery has a higher voltage (solar is not charging much or not at all), the load will take all its power from the battery. As battery voltage drops, the solar will supply power if it is higher than the battery. Eventually, at night, the battery will supply all the power as solar will provide nothing.
Bottom line, you cannot arbitrarily assume a 50/50 split for the power source for the load.

sagor1
But the battery is wired inbetween the solar panel and the load, is this irrelevent, because the path of least resistence i.e. higher voltage, will always supply the load.

So is it as simple as having an mttp attached to the solar panel set at 16.8v output, and a (li-ion 4s 16.8v) battery in between the mppt and the load, I fear that the load WILL take some draw from the battery, and that the batteries bms will allow the battery to charge to 16.2v (for example) whilst under load (voltage sag), and then raise to 18v etc, if the load terminates.

1.Are you saying this is impossible because voltage sag couldnt be happening on the battery because if the batt is charging i.e. the voltage coming from the mttp is higher than that of the battery, then all the load will be being drawn from the solar/mttp?

I think im being dumb... is this just a simple case of 2 voltage sources in paralell? I.e. the higher voltage source will feed the lower voltage until they reach the same potential.

The battery is functionally "between" the external supply and load, but not literally.
It's a very common application, the same as for things that use external charging power while they are operating.

Example - excuse the back-of-a-beermat quality diagram!

This is a general style of such a configuration, that I've used many times in both projects and products. With no external power, the battery feeds the load (or more likely load voltage regulator) via the lower right diode.

When external power is present at the input, that is routed directly to the output via the upper diode and the battery out diode is reverse biassed so nothing is drawn from the battery.

At the same time the charge control is powered to recharge the battery. That would also have a ground and possibly other battery connections, and the diode in line with that could be at the output rather than the input - but you can hopefully see the principal.



And this project-in-progress uses that type of setup - the main power is from 20V or 40V packs, but it also has a 10V / 12V (three cell lithium) battery to maintain power to the electronics if the main batteries are disconnected.

One of the regulator boards at the top right reduces the input to 15V which feeds a configuration similar to that above, except the charge diode is at the output of the charge controller.

The lower right heatsink & components on the matrix board are the charge controller - an LM1086CT set to get the correct voltage after the diode, with a current limit added using the ceramic resistors for sensing.

The electronics battery pack is below that, with a protection / balance board attached.

The "diodes" ae actually a power bridge rectifier just below & right of the upper voltage regulators. The output of the battery circuit feeds another regulator which provides 5.1V to the Jetson Nano at the top left.

The third regulator is for other 5V parts such as interfaces for the BLDC drivers at the bottom left, still to be built and fitted.

rjenkinsgb Thanks for the in depth explanation, I have a question about microcycling, so lets say I have a 16.8v input (from the mppt controller) with a 16.8v max (4s li-ion) in parallel, would I literally just have to set my HV cutoff voltage on the BMS to 16.4v for example to prevent microcycling, by doing so the battery would stay at 16.4v and all the load would be drawn from the solar>mppt>16.8v until this voltage sharply falls off (once the sun goes in), and the load would then be taken from the battery.

1. I am building an autonomos robot that I want to run 24h perday, the problem that I see with this setup is that, using an Mppt controller, (to which my understanding varies current to maintain a voltage depending on the input?) Is that, the mppt output could and will drop from 16.8v@8A to 16.8v@0.5A during cloudy moments. Now even if my battery is charged to 16.4v@200Ah the load will still draw from the low amperage mppt input correct? Which means my ESCs will be running at very low amperage (which is not deseriable).

2. If my logic is correct, and that is how an mppt works and the load prefers voltage>current, what would be the most effecient way to switch the load to the lower voltage but higher amperage battery. Could I place a contacter between the mppt and the battery, that is usually open and requires for example 12-20v@2A to be closed, and have the mppt output as the switches input, so when it falls below 12-20v@2A it disconnects itself and the battery takes over? Or is my understanding of mppts incorrect and it doesnt switch current to maintain voltage?

3. If my understanding of mppt is wrong and it works opposite, i.e. switches voltage to maintain current, in this case would a simple PWM controller be prefferable with a contactor still in place as per the previous example.

4. Would it be possible to provide 4a max charge to the battery without altering the potential current to the load.

Thank you.

1 - 3: With the type of setup in my drawing, there is no "switching" - if the external supply cannot give enough current for the load at any instant, it's voltage will be pulled down until the battery output diode starts to conduct and the remainder of the load current drawn from the battery.

And the reverse - as the external power increases (eg. from the solar panels, in your case, as the sun comes out) the load on the battery would gradually reduce until the external power supplied it all, then the excess would start to be used to charge the battery.

4: If the input power is adequate for the load with 4A spare and the charge controller can handle 4A, there is no reason why not.

The solar regulator needs enough voltage out to feed whatever battery charge controller you use; something over the full-charge battery voltage. I use around 2V more in the setup in the picture, so the LM1086 always has more than its minimum voltage, allowing for the extra 0.6V diode on its output.

In your system I'd suggest suitably high rated schottky diodes to minimise the voltage drops (and power loss).

It should be possible to shunt the diodes that feed the load with power FETs, controlled by voltage comparators, to virtually eliminate those diode losses.

Thanks rjenkinsgb you are helping me understand a few things, I dont think I phrased question 4 correctly, there will be times that the input will be providing 8A and the load will be at 2A meaning 6A will be going to the battery (too much current) is there a way I can regulate 4A max to the battery regardless of input amperage, WHILST allowing 8A to the load if and when it calls for it, what if I put the battery on a ring circuit with rectifier diodes and some kind of pentiometer facing toward the battery, and the return from the battery with just recifying diodes... would this work? Im guessing it would work but drive down the voltage to the battery which is undesirsble.

I guess the simple solution is to find a battery that can be charged at 6a, I guess this is the reason standard batteries for solar are not li-ion, and also why most systems step doen to 12v so they have lots of headroom to vary the current for charging purposes correct?

The solar controller will limit the voltage to whatever it is set to, at anything below the current the solar panels can give at that instant (after voltage conversion). The voltage will drop if the solar panels cannot support the load.

There is no "excess current" to be concerned about.

Think of it a bit like the solar panel and its regulator are a PSU, on during the day and off at night.
It has a set voltage when working and you can use whatever current you need, up to the maximum available.

There is no fundamental difference.

rjenkinsgb said:

The solar controller will limit the voltage to whatever it is set to, at anything below the current the solar panels can give at that instant (after voltage conversion). The voltage will drop if the solar panels cannot support the load.

There is no "excess current" to be concerned about.

Think of it a bit like the solar panel and its regulator are a PSU, on during the day and off at night.
It has a set voltage when working and you can use whatever current you need, up to the maximum available.

There is no fundamental difference.

Click to expand...

rjenkinsgb I understand, but a battery will suck the 6amps that are going spare, is there a way to limit the battery to receiving 4 amps, without limiting the load to 4amps (when it wants more, i.e. from the battery at night time)

The battery cannot "suck" anything!

The input to the battery must be restricted by a charge controller that limits the battery voltage and current, to prevent the battery overcharging and exploding!

That has no effect on either the output from the battery, or the direct supply from the input (solar regulator) to the load.

Look where the charge control block is on my diagram.

(And note a "protection board" is not a charge controller - and note you must use a combined protection and balance board with a multi-cell pack; that forms part of the battery assembly and the overall unit is the "battery" in my diagram - and likewise the lump in the bottom right corner of the photo, with the cells (green) just visible around the edges of the protection/balance board & two wires connecting it).

rjenkinsgb said:

The battery cannot "suck" anything!

The input to the battery must be restricted by a charge controller that limits the battery voltage and current, to prevent the battery overcharging and exploding!

That has no effect on either the output from the battery, or the direct supply from the input (solar regulator) to the load.

Look where the charge control block is on my diagram.

(And note a "protection board" is not a charge controller - and note you must use a combined protection and balance board with a multi-cell pack; that forms part of the battery assembly and the overall unit is the "battery" in my diagram - and likewise the lump in the bottom right corner of the photo, with the cells (green) just visible around the edges of the protection/balance board & two wires connecting it).

Click to expand...

rjenkinsgb I understand exactly what you are saying and maybe I am not literating myself properly. The mppt is providing 16.8v@8A the load is 16.8v@2A, this will mean 16.8v@6amps will be going to the battery (too high). I UNDERSTAND I could limit the mppt output to 6A but this will obviously change the voltage from the mppt and thus not charge the 4s battery to capacity, long story short.

I want 16.8v@8amps available from the mppt, but the battery never to charge above 4amps BUT ALSO 8amps available for the load when the battery is at capacity when thr solar is on or off, is thid possible?

Romadon said:

The mppt is providing 16.8v@8A

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WRONG. You do not understand!

It provides 16.8V AT UP TO 8A

That is a limit, a maximum value. The load can be any current up to that; you can use eg. something that takes half an amp if you wish and the voltage should stay at 16.8V

rjenkinsgb said:

WRONG. You do not understand!

It provides 16.8V AT UP TO 8A

That is a limit, a maximum value. The load can be any current up to that; you can use eg. something that takes half an amp if you wish and the voltage should stay at 16.8V

Click to expand...

rjenkinsgb Sorry again, I understand what you are saying completely I guess I just cant explain my problem properly.

Sometimes the load will be 8a, I want a max of 4a available for the battery at anytime, I cant set the max potential output at the mppt to 8a, because then the battery will use 8a when the load is not drawing anything from the mppt. I cant set the mppt to a 4a max potential because then the load will not have 8a available when it wants it. How do I circumvate this problem, or is it simply not possible in this circumstance, the only way I can see is to put the battery on a ring with rectifyers, and drop the voltage (amperage) potential to the battery which is no good for me? I am going to draw a diagram to help explain myself.

Romadon said:

rjenkinsgb Sorry again, I understand what you are saying completely I guess I just cant explain my problem properly.

Sometimes the load will be 8a, I want a max of 4a available for the battery at anytime, I cant set the max potential output at the mppt to 8a, because then the battery will use 8a when the load is not drawing anything from the mppt. I cant set the mppt to a 4a max potential because then the load will not have 8a available when it wants it. How do I circumvate this problem, or is it simply not possible in this circumstance, the only way I can see is to put the battery on a ring with rectifyers, and drop the voltage (amperage) potential to the battery which is no good for me? I am going to draw a diagram to help explain myself.

Click to expand...

Good, because what you say makes no sense at all?.

MPPT simply optimises the available power - so will provide less current with less light, while maintaining the voltage output of the panel. As far as I'm aware you don't (and can't) set current on MPPT, it simply varies depending on what's available from the incoming source. Charging current is something entirely different, and is set by the charging circuit.

Nigel Goodwin
Mppt set at 16.8v@8A potential
4s (16.8v max) li-ion battery in parallel
Motor escs (load)

The battery will charge @16.8v 8a when there is no load(this is undesirable).

I want to limit the batteries max charge potential to 16.8v@4a BUT still have 8a available for the load, from BOTH the solar panel (when the batteries topped up) AND/OR the battery (at night time etc).

I understand I could have something inbetween the mppt and the battery to limit charging to (16.8v@4a Max) but this would then limit the max potential current of 8a between the mppt(solar panels) and the load. Am I making sense here.

To put simply, any circuit I put between the mppt and the battery to limit current, is going to also limit current between the mppt and the load. Is there a way around this, whilst the battery is wired parallel in the circuit.

Romadon said:

Sorry again, I understand what you are saying completely

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No you do not, otherwise you would not keep repeating stuff about the battery having too much current.

Does eg. your phone charger take 15A or 20A from a wall socket just because that is what the socket is capable of supplying? no, it takes a fraction of an amp.

There is no difference with the solar controller. The output current will only be what the loads (including the battery charge controller) require - whether the solar controller can give 8A or 80000A - as long as the voltage remains the same.

Re. your new post - do you know what a diode or rectifier is?

Look at the diodes and current paths in my diagram. The battery charge path is totally separate to the load path when adequate power is available at the input.

The charge current and load current are completely independent.

Romadon said:

Nigel Goodwin
Mppt set at 16.8v@8A potential
4s (16.8v max) li-ion battery in parallel
Motor escs (load)

The battery will charge @16.8v 8a when there is no load(this is undesirable).

Click to expand...

You're still talking complete rubbish, so I don't see as you have any chance of making it work? - your entire premise is wrong, and you won't listen to anyone.

Nigel Goodwin rjenkinsgb Ok sorry all, I was always under the beleif that a Li-ion batt will charge at a much higher rate than is safe, if the current source can provide enough current to do so, hence why chargers are full of mosfets resistors and caps?

So thanks for the help, I can just put my li-ion batt in parallel with the mppt, and regardless of potential max current from the source, the battery will just charge at a safe rate, sounds like this is what you are trying to say?

Romadon said:

I can just put my li-ion batt in parallel with the mppt, and regardless of potential max current from the source, the battery will just charge at a safe rate

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NO

Look at the **********£*^$&$%****** (many expletives) diagram!

I'd suggest you learn basic electronics before attempting to use any lithium batteries; no one on here wants to be responsible for another user blowing themselves up or burning their house down, which is what will happen if you do not learn.

I am not responding further to this subject.