Charging AGM battery with constant voltage 14.4 Volt?

[hyperknot]

Hi,

I just bought a solar panel for my car which has a regulated 14.4 V output, allowing me to directly charge the car's AGM battery from it. It's a 60 W panel, so realistically it might output say 50 W => max. 4 Amp under 14.4 Volt.

My question is, is it OK to simply connect it to the AGM battery like this? I mean it's a CV charging with 14.4 Volt, limited to max 4 Amp. Is this OK for the AGM battery? Or I need to buy a PWM / MPPT solar charge controller (heavy and big) which supports the 3 phase charging.

[jwet]

AGM batteries generally charge at 14.6-14.8V, not the 13.6 - 14.4 of standard lead acid.  Charging at 14.4v will undercharge the battery- not a terrible thing but it has to have some kind of termination mechanism, you don't want to float charge it at 14.4v if its full.  You don't necessarily need a 3 phase MPPT charger but you do need some kind of charger/battery management, you want to terminate and have under-voltage that won't let the battery go below 12.5v or so for longevity.  The nice thing about a 14.4v output cell is you can use it without a battery.  A bare solar cell into a light load can go up to 20V.  I've built remote systems with solar chargers for biology research and field cameras- it takes a bit of care to not ruin batteries and get something like their full capacity and life.

[hyperknot]

I've been reading about it and came to the conclusion that the standard chargers of the cars almost never do any of these advanced charging tricks. If I drive 10 hours on a highway then it's 10 hours at 14.8-14.9 V, with I guess much higher current then 4 A.

So if I'm camping and I put this solar panel for a day then it should be OK for the battery - or at least not worse than how the car is charging it.

On the other hand, leaving it constantly connected when not using it for weeks might be bad, for that behaviour I'd need to buy a solar controller, if I understand correctly. Is this logic right?

[radiolistener]

My question is, is it OK to simply connect it to the AGM battery like this? I mean it's a CV charging with 14.4 Volt, limited to max 4 Amp. Is this OK for the AGM battery? Or I need to buy a PWM / MPPT solar charge controller (heavy and big) which supports the 3 phase charging.

It depends on the state of the battery. Usually there are two charge mode for lead acid batteries:
1) Standby charge: it needs 13.8 V, but the battery can stay under charger all time, even if it's chaged to 100%
2) Cycle charge: it needs 14.4 V, but the battery needs to be disconnected when current drops down bellow 0.01C

Regarding to the current, safe current for charge is 0.1C, where C is a capacity of your battery.

For example, if battery capacity is 60 Ah, then safe max current is 0.1*60 = 6 Amps and battery needs to be disconnected at 0.01*60 = 0.6 Amps for cycle charge mode.

You can use smaller current, but small charge current is not good for everyday usage, because it leads to a battery sulfation.

Also, there is a new type Ca/Ca battery, they need a little more voltage for cycle use, about 15V, but I'm not sure about exact value, because I don't use such batteries.


You can test current charge of the battery by leave it to stay 2 days with no load and no charge at temperature +25°C then measure it's voltage. This way you get the EMF of the battery. Note, that EMF needs to be measured after long stay with no load and no charge and it needs to be measured at battery temperature +25°C.

The fully charged battery has EMF = 12.72 V
The fully discharged battery has EMF = 11.76 V

In short you can just substract 11.7 from voltage and then multiply by 100. This way you get battery charge in percents. 

Note, that the lead acid battery should be charged and measured when it has temperature +25°C. Also note that if current stops to drop down for a long time during charge, the charge should be stopped. If this happens before 0.01C it means that something is wrong with the battery and it is recommended to replace it.

[hyperknot]

Thanks for the explanation, I'll check it like that. In the meantime I've ordered a PWM controller from Epever, which has configurable Sealed / Flooded / Gel profile. Only 20 EUR but it gives me a good feeling to know that the battery is charged properly.

[floobydust]

What make/model is the solar module?  It's important to have a back-feed diode somewhere so the battery does not discharge at night and leave you stranded.
I would not worry about overcharging because the car's battery is never 100% charged - car makers are keeping them ~80% charged in order to get better fuel economy (ratings).
They undercharge and the ECU does regen charging as you coast down a hill/or decel. If you don't believe me, look at your car's voltage over a few days. Of course this algorithm fails in very cold weather or with an old battery, so a top-up charge does really well.

[hyperknot]

This is the panel I bought: https://www.amazon.com/TP-solar-Portable-Foldable-Charger-Generator/dp/B07SNP8XZ1

It doesn't say anything about a diode, it's true. But if I use it with the 19V port + PWM controller then I'm definitely safe I guess.

[floobydust]

Topsolar SolarFairy 60W Portable Foldable Solar Panel Charger Kit... That is a solar array and electronics - outputs for 19V laptop power (raw panel to 24V apparently), 5V USB, 14.4V that has some kind of output switch.
Lots of youtube reviews on it. People get around 40W out of it. But no teardowns to know how good the charger is.

[hyperknot]

I see, so the DC 19 is also PWM-ed? Basically I'd get the best performance if I'd cut off the PWM box and run the wires directly to the Epever controller?

[jwet]

The idea of chargers is that you don't want to tend them- they take what's available, look at the battery conditions and environment and take care of the battery.  They don't cost much at these low power levels.  You don't want to be thinking about your batteries and solar cells.

[Juan]

I have been using a big AGM battery to run a winch for firewood collecting for some years now and I use a smart charger designed for AGM batteries . It charges fast then equalizes the cells then changes over to a trickle standby charge . You can by solar charge controllers for AGM batteries . Don' t connect the battery directly as some solar panels put out a much higher voltage than is required .

[radiolistener]

I've ordered a PWM controller from Epever, which has configurable Sealed / Flooded / Gel profile

battery type selection in the charger is just a selection charge voltage, which may be a little different from 14.4 V for different type of batteries. But it's better to have voltage selection instead of battery type, in such way you can enter exact voltage which is specified in the manual for your battery.

Solar controller goal is just to stop charging battery when it's charged to 100%, but many chinese solar controllers just monitoring voltage on the battery which is not good way to determine charge complete event. The right way is to check charge current and stop charging when the current drops down below 0.01C.

There is also MPPT solar controllers, they allows to get max power from solar panel by varying the load impedance. Max power from solar panel allows to charge battery more faster, but it doesn't related with battery charge process, it's just about on how to get max power from solar panel (to make solar panel more efficiency).

[radiolistener]

Don' t connect the battery directly as some solar panels put out a much higher voltage than is required .

If solar panel power is not so high and battery is discharged this is not a problem, because voltage will be dropped down due to low internal resistance of the battery. But it is important to check voltage because voltage will rise up during charge and you're needs to disconnect battery when voltage rise above 14.4 V.

You can even put a battery into the mains through rectifier diode and proper current limiting resistor, with no voltage regulator. You're just needs to check the current through a battery. The charge current should not exceed a battery limit.

But if solar panel is power enough, it can put a lot of current through battery, it may exceed maximum charge limit for the battery and the battery can be damaged. Usually AGM battery has max limit for charge current is about 0.3C, but it may be very different for different batteries, so you're needs to check it in the battery manual.

I recommend to not exceed 0.1C for charge current. Higher current may lead to a battery damage, lower current may lead to a battery sulfation.

[MrAl]

Hi,

I just bought a solar panel for my car which has a regulated 14.4 V output, allowing me to directly charge the car's AGM battery from it. It's a 60 W panel, so realistically it might output say 50 W => max. 4 Amp under 14.4 Volt.

My question is, is it OK to simply connect it to the AGM battery like this? I mean it's a CV charging with 14.4 Volt, limited to max 4 Amp. Is this OK for the AGM battery? Or I need to buy a PWM / MPPT solar charge controller (heavy and big) which supports the 3 phase charging.

Hi,

60 watts is a lot for a single battery unless you dont get much sunlight during a typical day or you  get a lot of clouds over several days.
20 watts would do it and then probably never overpower the battery.

What you really have to do is monitor at least the voltage and see what it goes up to and what it goes down to at night before the next morning when the sun starts to energize the panel again.  What can happen is the voltage can ratchet up over days and weeks and go too high.
For example, on the first day it might take it up to 14.5v and then at night till the next morning drop down to 13.5v, then the next day it might take it up to 14.6v then at night till the next morning drop down to 13.6v, then the next day up to 14.7v and then 13.7v, then 14.8v then 13.8v, etc., etc.
This would mean that it is starting to over charge.
If it goes up to 14.5v the first day then 13.4 at night, then 14.4v the second day and 13.3 at night, then 14.3v the third day and 13.2 at night, you know it is undercharging.
It wont happen that fast though it could happen over two weeks where you will see a difference like that, so you might have to compare week by week.
When it is regulated properly, the average will keep it at relatively constant up and down levels, like 14.4 and 13.4v, and that will repeat with the same amount of sunlight each day.
Monitoring like this will give you a feel for what is going on.
With a 60 watt panel i would say it will over charge at some point without a controller that limits the upper end voltage.  For some batteries even 14.0v is high enough, and for yours probably 14.2v to 14.4v would be high enough, but like i mentioned above keep an eye on the voltage levels during the peak sunlight of the day and in the morning just before sunrise.

The charge for a battery is measured over time and the current level.  4 amps is pretty high when it charges over a full day of sunlight.  If you have a 60 ampere hour battery and you charge it at 4 amps for 8 hours (for example) that's already 32 ampere hours, which is half the capacity of the battery, which should never be required.  The normal self discharge of a 60 ampere hour battery per day would probably be around less than 3 ampere hours, period, which means 4 amps for LESS than ONE hour, per day.  It would really need only about 1 amp for 3 hours for example, or 1/2 amp for 6 hours.
So you see why 60 watts is a lot.  20 watts would probably do it but i dont know if you have any issues like shading during part of the day.  For that or any other problems you have to determine by way of the monitoring process over weeks.  It's not hard to do, just keep checking it until you know what works.  That's the only way to be sure though as no amount of theory will ever get you there without measurements.

[radiolistener]

60 watts is a lot for a single battery

It depends on the battery capacity. For 60 Ah charge current should be 0.1*60 = 6 Amps, 6 * 14.4 = 86.4 W source is required for a good charge.

60 Watts leads to a lower charge current, about 60 / 14.4 = 4.2 Amps. It's okay.

But of course battery needs to be disconnected when charge is complete to avoid overcharge, because it leads to electrolyte boil-off and battery damage. This why charge controller is needed, to do all things automatically.

[MrAl]

60 watts is a lot for a single battery

It depends on the battery capacity. For 60 Ah charge current should be 0.1*60 = 6 Amps, 6 * 14.4 = 86.4 W source is required for a good charge.

60 Watts leads to a lower charge current, about 60 / 14.4 = 4.2 Amps. It's okay.

But of course battery needs to be disconnected when charge is complete to avoid overcharge, because it leads to electrolyte boil-off and battery damage. This why charge controller is needed, to do all things automatically.

Hi,

Perhaps, but that would be for a full charge or just a significant charge in a shorter time.  With a solar panel the charge goes on all day long some days so if you charged at 6 amps the whole day even a 60 ampere hour battery would eventually launch into outer space :-)

It does depend on how you can control it too though.  If you had a 500 watt panel you could charge the battery if you had a way to cut off the charge current at the right times.  A 60 watt panel is ok i think if you have a charge controller, but the reason i brought this up was that you dont really need that big of a panel a 20 watt panel will keep the battery topped off unless you have some really extreme circumstances like shade for 7 hours a day and only 1 hour of sunlight (or something like that).

Just to note, the amount of charge the battery needs when recharging is the amount of charge that was taken out multiplied by a factor of the battery which is greater than one and is related to the charge efficiency.  For example, if you take 6 amps out for 1 hour you probably need to put in 6 amps for 1.5 hours (or something like that) but if you put 6 amps in for 8 hours that's going to be an extreme overcharging.  You can get away with 1 amp for maybe 8 or 9 hours, and if that occurs EVERY day it can still overcharge unless there is a way to limit it, or you take more out.

[radiolistener]

yes, 6 Amps for 60 Ah battery is required when it is discharged more than about 30%. When battery charge is about 70% the current starts to drop down.

But if you're planning to keep your battery on charge everyday then cycle charge mode (14.4 V) is not suitable for that. In such case you're needs to use standby charge mode which require 13.8 V for charge. All things regarding current still applied for standby mode, but you can leave it under voltage when the current drops down below 0.01C.

Note that standby charge mode is more aggressive and battery life will be shortened.

[Juan]

These  are good battery chargers and it would run fine on 230v in Hungry just cut the plug off and fit new one or get a converter plug . Good to top the battery up if the solar is not working good.
https://durst.com.au/product/bcs-a1206-smartcharger/

[hyperknot]

OK, my TopSolar SolarFairy panel arrived. The first thing from the user manual I noticed is that even the 19V output is PWM controlled, there is no output for just getting the panels's open circuit voltage directly. They state something like 21V for open circuit voltage.

The good news is that it does have some mechanism for disconnecting at night, it just starts charging whenever I guess the voltage gets over 14.4 V, right? I mean these simple circuits don't boost voltage probably right?
I imagine the following:
Cloudy day - only USB port works
Bit better day - DC 14.4 V port works but DC 19V doesn't yet
Sunny day - DC 19V works as well.

I'll need to measure this with a multimeter to make sure though.

[radiolistener]

hyperknot

there is no need to know open circuit voltage from solar panel. The goal of solar panel controller is to obtain max power from solar panel. The power is P = I * U = U^2 / R. I and U dependency for solar panel is not linear, so the solar panel controller should vary R load on solar panel output in such way to get the best match and obtain maximum efficiency from solar panel.

This is how MPPT solar controller works. But more cheap solar panel controllers don't care about max efficiency, it just trying to keep charge current for the battery and limit the max voltage on the battery.

[hyperknot]

But if I use a controller, isn't it sub-optimal to get the output from the panel which goes through a PWM to make DC 19V? I mean I'm thinking getting the direct "pure" output from the panel should be more efficient, shouldn't it?

[radiolistener]

I mean I'm thinking getting the direct "pure" output from the panel should be more efficient, shouldn't it?

Sometimes it may be more efficient, sometimes not, but if you're using MPPT controller, it should be more efficient in most cases than direct connection.

See solar panel curve to understand why:


MPPT controller trying to keep solar panel at P_MP point, in such way it get maximum power from it. The trick here is not to get max current or max voltage (because it may lead to a low efficiency), but get max power from solar panel and then using PWM convert that power to desired current or voltage.

[hyperknot]

I meant that the PWM controller is built into the panel's output box in my case.

So I have two option:
1. Panel - built-in PWM - external controller
2. Panel - (open up and solder the wires directly) - external controller

I mean that there is no "open circuit" output from the panel, it only has USB, DC 14.4V and DC 19V outputs. So I was thinking that some kind of lost must be happening inside the panel's output box. And that basically opening it up and trying to get the direct output from the panel would be more efficient, wouldn't it? I'd always use an external controller (Victron, Epever, etc.) in both cases.