AGM Battery voltage - confused

vaggianak · December 2014

Hello. I really need to understand the voltage readings in my batteries. Please, be patient, I'm new at this

12 batteries GFM 500 (2V / 500 Ah each), system in 24 V
4 kW of solar panels, divided in groups. Each one uses its own MPPT charge controller.
Gasoline generator 3.5 kVA
Inverter/charger 3000 W

Facts
When the sun shines, battery voltage is 28.8 V and the solar chargers indicating that batteries are full.
If I disconnect the solar chargers at this time, and cut off the loads, the voltage drops at 25.2-25.4 V. Then I turn on the power generator in order to charge the batteries, the generator works and charges the batteries at a slow rate. If then gradually I reconnect the solar chargers, voltage rises until it gets at 28.8 and the inverter/charger indicates that charging by the generator is at its final stage. And it goes this exact way, if I disconnect the solar chargers, charge using the generator etc.

1.I don't understand why the generator charges the batteries if they are supposed to be already fully charged by the sun. Generally I don't understand this rapid rise of voltage when i gradually reconnect the solar panels. Does it really correspond to charging or not?

2.If an electric consumption is on, let's say that it draws nominally 2 A AC, it draws 2x24=48 A from the batteries (DC), am I right? So I'm supposed to read a voltage reduction of (48/500)*100% =9.6% or (25.6-23)*0.096 = 0.25 V? I know that this calculation is way too rough to be confirmed, but I 'm trying to understand how it works.

Thank you in advance

Cariboocoot · December 2014

Re: AGM Battery voltage - confused

The important thing to realize here is that the batteries may charge at 28.8 Volts, but that is not the battery Voltage; it is the charging Voltage. The at rest Voltage (no current in or out) of 25.4 is battery Voltage, or what would be normal for a fully charged battery on its own.

On its own the generator inverter/charger has no way of knowing the batteries have been charged by the solar, so it will try a new charge cycle. Thus it will turn the Voltage up to 28.8 again and go through the whole Bulk/Aborb/Float regime according to its parameters and what it senses as the batteries' needs. If the batteries are not in fact discharged significantly this Voltage rise will occur rapidly.

2 Amps AC is at 120 Volts. At 24 Volts DC it would be a draw of 10 Amps plus a bit for the inverter and the conversion loss, possibly 11-12 Amps. Somewhat less at the higher charging Voltage. This is the basic math:

2 Amps * 120 VAC = 240 Watts
240 Watts / 24 Volts = 10 Amps 240 Watts / 28 Volts = 8.5 Amps
The missing factor is inverter power and conversion efficiency.

There will be no change in the Voltage if the batteries and/or charge source can handle this additional current demand.

vaggianak · December 2014

Re: AGM Battery voltage - confused

Thank you. So when the generator charges the batteries, it's going to try to raise the charging voltage at 28.8V? Because when the generator is on and connected with the inverter/charger, i get the measurement of 25.2 V, though the charger indicates that it's charging.

Here in Greece, we've got 230 V for single phase AC voltage, but I get your calculations, thank you

H2SO4_guy · December 2014

Re: AGM Battery voltage - confused

A battery monitor like a Trimetric would really help you to determine how full your batteries really are. It tells you how many amp hours have been used from the pack and that is a huge help to determine the state of charge. A bit pricy, $155.00 from NAWS (That's where I purchased mine) but the battery packs are a LOT more and if it makes the pack live longer it is well worth the money. A good tool that I use to make decisions based on charge like if I am going to heat by electricity or propane.

Good luck and let us know how it works out.

Skip

BB. · December 2014

Re: AGM Battery voltage - confused

Charging for a lead acid (including AGM) is a process.

You start with a discharged battery (less than ~80% State of charge):

Bulk--Battery below ~28.8 volts when charging (charge controller limits to maximum available charging DC current), May take minutes to hours--depending on state of charge and charging current
Absorb--Battery >~85% state of charge. Charge controller is "holding" charging set point of ~28.8 volts (you program for most controllers). Usually from 2-6 hours (~2 hours for shallow discharge, ~4-6 hours for 50% or more discharge).
Float--Controller drops back to ~26.4 volts after absorb. Battery will be kept charged and solar power can supply extra energy to other DC loads.

Obviously, for solar charge controllers, as the sun goes down, these phases can "Terminate" at any time (no sun, no charging current).

-Bill

mike95490 · December 2014

Re: AGM Battery voltage - confused

How does your generator based charging work? Does the generator feed the inverter, and the inverter acts as the charger ?

What is the brand of the charger ?
Does it need to be configured for your battery array?

vaggianak · December 2014

Re: AGM Battery voltage - confused

mike95490 wrote: »

How does your generator based charging work? Does the generator feed the inverter, and the inverter acts as the charger ?

Yes

mike95490 wrote: »

What is the brand of the charger ?

It's a Powerstar W7 3000/9000 W 24V . Max. charging current 35 A

Does it need to be configured for your battery array?

There is a simple configuration with a dip switch. It's already set in "AGM battery 28,2V - 26,8V". The other possible settings are GEL, LiFePO4, GEL2. My batteries are these

mike95490 · December 2014

Re: AGM Battery voltage - confused

You look at your battery specs (site is in a language I don't read so I won't guess as to what is what) and chose the closest charge regime. With AGM you never want to over charge or you pop the vents, loose gas, which looses capacity.

westbranch · December 2014

Re: AGM Battery voltage - confused

Mike here is an English link
http://en.naradapower.com/Products/SLA/battdata/pdffiles/LSA/GFM-500.pdf

mike95490 · December 2014

Re: AGM Battery voltage - confused

STOP !

These are not deep cycle batteries, they are standby batteries, and will not last for deep cycle use.
http://en.naradapower.com/Products/SLA/battdata/pdffiles/LSA/GFM-500.pdf excerpt:

The products are used as standby power for communication,
power, military and broadcast and television system.
They possess precise ABS heat seal technology between
container and lid and patented post seal structure.
The design float life is 15 years.

If you do use them for cycle service:
Equalize and Cycle Service 2.35V~2.40V@25 (77F ) Which in a 24V system (x 12 cells) would give you 28.2v ~ 28.8v for charging.

Now look at the "Charge Characteristic" table / curve, you have to hold the charge voltage for 4 to 6 hours to fully charge them. Some areas of earth get 5 hours of useable sunshine, but many don't.

A characteristic of damages batteries, is that they charge up quickly, but then also discharge quickly.

Something else that happens, is that any daytime loads consume some of the amps coming from the panels, and reduce the amps for charging the batteries.

vaggianak · December 2014

Re: AGM Battery voltage - confused

Well this is the exact datasheet of my batteries, and it doesn't include the charge characteristic. Cycle Voltage 2.30 - 2.35 V and Stand-by Voltage 2.22 - 2.26 V

Possible configurations in my charger are the following

1.GEL1 Fast V 14.0 - Float V 13.7
2.AGM Fast V 14.1 - Float V 13.4
3. LiFEPo4 Fast V 14.6 - Float V 13.7
4. Sealed Lead Acid Fast V 14.4 - Float V 13.6
5. Gel 2 Fast V 14.4 - Float V 13.8
6. Desulphation 15.5 V for 4 hours

Which one should I use?

I'm searching through datasheets to understand if TOYO GFM 500 and this Naradapower GFM 500 are the same, and if these batteries are deep cycle or not

Cariboocoot · December 2014

Re: AGM Battery voltage - confused

vaggianak wrote: »

Well this is the exact datasheet of my batteries, and it doesn't include the charge characteristic. Cycle Voltage 2.30 - 2.35 V and Stand-by Voltage 2.22 - 2.26 V

Possible configurations in my charger are the following

1.GEL1 Fast V 14.0 - Float V 13.7
2.AGM Fast V 14.1 - Float V 13.4
3. LiFEPo4 Fast V 14.6 - Float V 13.7
4. Sealed Lead Acid Fast V 14.4 - Float V 13.6
5. Gel 2 Fast V 14.4 - Float V 13.8
6. Desulphation 15.5 V for 4 hours

Which one should I use?

I'm searching through datasheets to understand if TOYO GFM 500 and this Naradapower GFM 500 are the same, and if these batteries are deep cycle or not

Of these, based on Voltage alone, #2 "AGM fast" is the closest to the battery specs given.

Frankly neither the charger nor the batteries look like good choices to me, but you deal with what you've got right?

AGM Battery voltage - confused

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