question

agim avatar image
agim asked

Charging problems

I have problem to charge six 2v batteries connected serial that give 12 v . There are 270Ah batteries. I have MPPT 75\15 charger. Since I am from Montenegro we have a plenty of Sun. OK not this year but still in cloudy weather I get 15.16v from solar panel on sunny it goes up to 20v . I use this for charging batteries for sea light that has 20w light. Still my battery drains. This sea light is not close to me so I can not check it regularly but it happens on 15-16 v from solar to not have charging at all. What is the problem. I do not have a bluetoothe dongle. Please help I am tired of driving to sea light once a week to charge drained batteries with generator

battery charging
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3 Answers
ejrossouw avatar image
ejrossouw answered ·

Hi @Agim

"The controller will operate only if the PV voltage exceeds battery voltage (Vbat).

● PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V."

In light of this you need 17V to get the charging started. Best option is to add another panel in series or if you have 2 already in parallel, connect them series.

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I'll add - minimum 17v to start charging... in reality, a 12v nominal battery will usually be between 12.2 - 13.2v, so it'll require 17.2-18.2v from PV to start charging.

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agim answered ·

If I understand well practically I need 17.5v every day to charge battery so if I have 16v on cloudy day with battery 11.5 v it will not be charged that day. If it is like that all stories about better harvesting with mppt solar charger a just marketing because my solar panel will charge battery on 14v without mppt charger if the battery is for example at 11v. I just need mppt charger for protecting battery. Am I wright

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Not exactly. It only requires the initial 17.2V to "start up" the MPPT and thereafter 13.2V min to keep it going. Overall the MPPT is much better at harvesting energy and definitely a good investment, but PWM still has it's place depending on the requirement. Just add another panel if feasible and you will never look back.

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Not really that much better as can be seen in this case of a small panel. His system obviously worked OK as it was, now it doesn't unless he gets another panel. The +5V limit to start up sort of negates the possible advantage of MPPT in low light. In the middle of a sunny day there's not going to be much of a difference between any type of regulator.

I don't quite see the point of the arbitrary +5V but I guess there might be a practical reason for the design.

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"It is generally accepted that MPPT will outperform PWM in a cold to temperate climate, while both controllers will show approximately the same performance in a subtropical to tropical climate."

Solar and batteries are typically more complex than just the headline grabbing features. The above extract is a from a Victron document, which could have helped, although may not have flagged the +5V startup aspect. From it you will see MPPT favours higher voltage than PWM for various reasons. Not sure where the protection idea came from. Why did you want to upgrade in the first place? IMHO, PWM will continue to serve people well so no reason to not revert back if another panel is not an option.

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Idea of protection came from summer solar power which practically destroys batteries. That is the reason why we do not use bigger solar panels. It happened on 75Ah batteries to have explosion. So this system will be OK in summer time when we have 85 from 90 clear days to protect overcharging batteries but in winter I have problems because especially this winter we have a lot of rainy days. Can you tell me please does pwm system needs 5v difference to start ? Do cheap chinese products has this difference limit? You said that it needs initially 5v and then it will work till for example 13.2V . What about this situation I have in the morning 17.2V then it come heavy cloud and it fall down to 8V but afer that it rises till 16 because of light clouds I will not have harvest anymore because it needs maybe 17,9 V now ? And at the end why they need to put this 5v limits at all ?

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Let us start with the most basic principle. If you have two exact same sized glasses but filled to different water levels, the water will always flow from highest to the lowest lever until equal. Consider the lower level your battery. So in order to charge a battery, the input voltage always has to be higher than the battery voltage to get energy flowing into it so no matter which charger, the voltage always has to be greater than the battery's to charge it.

MPPT uses the solar panel voltage which can be much higher than the battery as where PWM charges at the battery voltage level which is much lower and therefore explains in part the differences.

More about that here https://www.victronenergy.com/upload/documents/White-paper-Which-solar-charge-controller-PWM-or-MPPT.pdf

A poorly designed system can destroy LA batteries easily by overcharging and overheating as could equipment without monitoring features of e.g. battery temperature.

Also look at the Victron https://www.victronenergy.com/solar-charge-controllers/bluesolar-pwm#manuals PWM chargers with optional temperature monitoring features which can prevent exploding batteries due to overcharging and overheating.

As for Chinese products, they are today the world leading manufactures and implementers of solar by Gigawat measure. Also just about every solar component of many reputable brands are manufactured there for the equipment we use. The important aspect is to make sure you buy a tried and tested product with the necessary features and compliment this with a well designed and well thought through system to ensure you don't blow up anymore batteries up. Typically you will find products certified compliant with European standards to be safe to buy and the same OEM item will actually be adopted many companies with maybe some customisation.

It will ultimately come down to your requirement and cost differences between your options. Happy reading and good look. Hopefully you can go forth and share safe the world from more dangerous exploding batteries with this in mind.

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agim avatar image
agim answered ·

@ejrossouw thanks for your help. Thanks to you I understand where is my problem and that unfortunately I have wasted money by buying 4 mppt 75/15 . You have written a long story but I still did not catch do pwm charger has 5v difference for intial starting of charging and do the other Chinese products have the same. Just short please . It will help me decide to buy new chargers or to continue like this. About explosions it happen two times in 5 year experience and far away from people. We saw it after sealite stoped working mostly just batteries stop working faster.

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Has no one mentioned adding another (similar speced) solar panel in series with your existing panel?

That will give you enough voltage for the mppt to start reliably, and give you twice the charging input for really cloudy days.

Much cheaper than changing the regulator.

Don't go over the mppt voltage limit of 75 volts. (pv input)

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In the first answer @ejrossouw already mentioned to use two panels in series.

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Can you please just answer my question ? Its much easier to put new charge controller then to take sollar panel on the island and to mount it on the top of sealite.

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No, PWM does not have the vbat +5v startup requirement. It will however depending on the device specification typically charge at 13.6v if charge values are not adjustable, but I cannot comment on budget devices otherwise and suggest you youtube pwm reviews.

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Thank you very much sir ! I will by quality one but I planned to by cheap one if quality one still has 5v initial difference . Thank you very much once again . This topic can be closed. :)


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