Maximum charging voltage for 8 bateries 12v200ah from 8 solar panels

PaneleB · March 2017

I need to post my experience but advises are needed as well.
Last week we successfully installed an off-grid system for a farm in a remote area where there was no electricity.
So we installed 8 solar panels 250 watt and 8 amp each.
Inverter offgrid mppt 4 kw with integrated charge controller
8 bateries 12vx200ah each, bateries are leed acid vrla deep cycle.
It was all set to 48 volt. And this is to run a milking pump 1.5 kw for 2 hours a day, one hour in the evening and one hour in the morning, and lighting also up to 50wat/h all night.
Problem is that since the next morning of the first day we installed it, batteries will drop to 46 volt and inverter will shut down (it was set to 46v and not less in order to protect batteries) and charging is set to 54v and then it will stop charging.

My questions:

What is the maximum voltage i should let the batteries be charged, 54v (13.5v) or 57.6v (14.4v)? For the moment that we have set the charging limit of 54volt, after the sunset, the evening milking process will start and batteries will drop to 50.7volt. Next early morning we see them 50volt we start next milking process and it will not finish it but inverter will shut down when reaching 46v.

What should be the minimum voltage i need to set the inverter to stop in order to protect batteries from low voltage?

We have 8 batteries, and in total it makes 19.2 kw (12vx200ahx8), how come they cannot afford up to 4kw energy needed?

Is there any other way we can do to provide both milking process evening and morning with already installed batteries?

Many thanks in advance:)

mcgivor · March 2017

Without knowing the manufacturer or their recommendations, using basic figures the charge voltage should be close to 56.6V for VRLA batteries, bulk/absorb and 53.6V float These values are for 25 Deg C and must be compensated for at higher or lower temperatures with a remote temperature sensor, or manually by adding 0.60V for every 5 degrees rise, or subtracting for lower.

The low voltage shut down setting for the inverter is set a little high in my opinion, somewhere in the region of 44V would be more appropriate, the voltage will sag under load but recover as soon as it is disconnected. Best to measure the voltage after load, with no charging input and adjust accordingly, a value of around 49.5V five minutes later would be reasonable. Cable size and length between battery and inverter are critical to reduce voltage drop and nuisance low voltage shut downs.

Please note the figures are generic and personal opinions, manufacturer's recommendations should be followed if available.

littleharbor2 · March 2017

How are the solar panels wired? With 8 250 (typical Vmp. of 30 volts) watt panels the only configuration that may work would be 2 strings of 4 in series. Todays 60 cell 250 watt panels aren't 24 volt nominal voltage. Will your inverter accept up to 150 Voc? You may be better off using an independent MPPT charge controller.

Estragon · March 2017

As noted, in your application a lower low battery cut-off may work better. Your pump likely draws a very high current for a short time to get started, then drop off to the rated current as it continues to run. Voltage may sag a lot during the start phase and recover. This isn't so bad for batteries.

Leaving them in a low voltage (discharged) state for long periods is a killer. You have the charging voltage set so low the batteries probably aren't getting fully charged (check mfg specs), so this damage process is likely underway.

Another factor is you are running a fairly big inverter for your small lighting load overnight. The inverter may be drawing quite a bit more than the 5w lighting load estimate.

2000w of panel is on the low side for 400ah of battery. Do you have a generator to charge if needed?

PaneleB · March 2017

Thanks a lot for your replies.
Solar panels are connected 2 in series and all 4 strings in parallel, suitable for 48volt connection, and amp should be aprox 32 at maximum.

To Estragon
"a lower low battery cut-off may work better" you may be right I guess i will have to try it also inverter was set to 42volt (not acceptable I guess) but i did increase it to 46volt. If batteries are drained in the morning there is no chance they will remain drained for more than half day, sun starts rising in an hour or less and they will start charging.

Charging Voltage has been set to 50 amp. 30 amp should come from solar panels.

We need inverter ON all the time so there is not any need to have a separate Mppt charge controller.

Batteries get full by the end of the day and they stop charging when 54 volt by 2 or 3 pm every day. There is a sunny weather all the time. There is a generator but we did not connect it, maybe a bigger one is needed.

My question again is: can i increase the limit from 54v to 56v or 57v is there any harm on batteries?

Thank you

PaneleB · March 2017

mcgivor said:

Without knowing the manufacturer or their recommendations, using basic figures the charge voltage should be close to 56.6V for VRLA batteries, bulk/absorb and 53.6V float These values are for 25 Deg C and must be compensated for at higher or lower temperatures with a remote temperature sensor, or manually by adding 0.60V for every 5 degrees rise, or subtracting for lower.

The low voltage shut down setting for the inverter is set a little high in my opinion, somewhere in the region of 44V would be more appropriate, the voltage will sag under load but recover as soon as it is disconnected. Best to measure the voltage after load, with no charging input and adjust accordingly, a value of around 49.5V five minutes later would be reasonable. Cable size and length between battery and inverter are critical to reduce voltage drop and nuisance low voltage shut downs.

Please note the figures are generic and personal opinions, manufacturer's recommendations should be followed if available.

Thank you for your opinion that might be helpful as well.

I have not checked it into so many details, temperature is not considered, as all the batteries are indoor and temperature may vary from 10 to 28 degrees Celsius, Winter-Summer. Cables used are 25mm, that might be an issue to because when i compared battery voltage measured at inverter and Battery poles, there was a diference, it was showing 0.2-0.3v higher at batteries poles.
Thank again

littleharbor2 · March 2017

PaneleB said:

Thanks a lot for your replies.
Solar panels are connected 2 in series and all 4 strings in parallel, suitable for 48volt connection, and amp should be aprox 32 at maximum.

Thank you

Not necessarily, MPPT controllers want at least 1 1/2 to 2 times the charging voltage to work effectively. Your panels likely aren't 24 volt nominal panels. If your panels have a Vmp. of 30.xx volts that isn't high enough to charge a 24 volt battery or a 48 volt battery in a 2in series configuration, especially when the panels get warm/hot.

mike95490 · March 2017

If the panels are not well aimed at the sun, or there are not enough solar hours in your day, the batteries will not get fully charged, regardless of the voltage.
VRLA batteries are particularity troublesome, because you there is no way to check their state of charge. Only the Mfg can give you the spec for Cycle Usage Charging Voltage and End amps Termination. That's a mouth-full. But it's the only way to start to narrow down the charging settings for your site. You could use flooded batteries and test their charge state with a Hydrometer, but you already bought the other batteries.
Sadly, many chargers that are called mppt. are not really MPPT and you may be loosing a lot of power in that.
Your PV array should be feeding at least 90V to the Charger, or it may not actually work well. it has to have a voltage differential to perform the MPPT functionally.
To monitor/measure end amps, you need to remove all loads, and measure the amps going into the batteries with a clip-on ampmeter,
or have a SHUNT installed to use with a meter to measure amps.

Raj174 · March 2017

PaneleB,
Please post the battery make and model. It would help determine charge settings .

mcgivor · March 2017

Raj174 said:

PaneleB,
Please post the battery make and model. It would help determine charge settings .

Make and model of the inverter /charger would also be helpfull, to better understand the whole picture .

westbranch · March 2017

Most batteries are rated using a temp of ~ 25*C, if you are going down to 10* you really need a Temp sensor and a Chjarge controller that uses one ... and also 2 panels in series is probably not making enough volts to do a "full" charge now that the batteries are under charged as the early shutdown is showing... Please tell us the make and model of your charger /inverter and batteries...

Maximum charging voltage for 8 bateries 12v200ah from 8 solar panels

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