Looking for help understanding maximum charging current...

Welcome to the forum Asinnobooks,

GEL batteries are (many brands/models) are not great for solar power systems... They are great for sitting unused and during discharge. However during charging, many (some, many, most?) have issues with high charging current/higher charging voltages).

GEL batteries have "gelled" type electrolyte (basically sulfuric acid+water+silica gel)--the downside being at high charging currents/voltage the GEL battery can gas which forms gas bubbles in the electrolyte--And this can permanently cause a loss in capacity of the GEL battery.

AGM batteries (absorbed glass mat) batteries are very similar to GEL batteries, without the problem of forming gas bubbles in the GEL electrolyte.

Always start with the settings in the Battery Specifications--But the numbers are roughly (12 volt bank, 24 bolt bank is 2x higher):

• 14.75 volts set point for Flooded Cell batteries
• 14.40 volts set point for AGM
• 14.20 volts set point for AGM

And dropping to around ~13.6 volts or so for "float charge" set point.

Charging current wise--For "longest" life, around 10% to 13% rate of charge for Lead Acid type batteries is recommended. And if your controller has the option, use a remote temperature sensor to monitor battery bank temperature (higher temperature lead acid battery, lower charging voltage required).. For 13%+ rate of charge, highly recommend a RBTS (remote battery temperature sensor) to ensure a happy and healthy battery (and for RVs that run from deserts to snow country, RBTS are really helpful).

For GEL batteries--Many recommend a 5% rate of charge (which is really too low for "optimum" off grid use--The sun is just not in the sky that many hours per day for fully recharge a 50% discharged GEL battery).

The caveat here is that there are many different GEL battery designs/manufactures--Again fall back to the manual and make sure you meet their requirements vs the above generic recommendations.

For MPPT controllers--The typical "max current" calculation for charging current (the most current you will see for a few hours on a cool/clear day during solar noon, a few times a year):

• 400 Watt array * 0.77 controller+panel deratings * 1/28.4 volts battery charging voltage = 10.85 Amps usual "max" current for MPPT system
• 200 AH * 0.05 (5%) suggested rate of charge = 10 amps "generic" max charging current

So, your 10.85 amps is certainly close enough for "solar work"...

-Bill

Thereafter, I am slightly confused (sorry). Here's what I've understood:
-The battery charging voltage (28.4V) comes from two 12V, 200Ah batteries wired in parallel and their volt set points.
-The 0.77 is the efficiency of the 400W solar array.
-The 5% rate of charge is the rate at which a GEL battery can safely be charged at (I'm seeing that figure at 20% for AGMs?)

I had guessed you were doing a 24 volt battery bank (2x batteries in series). In fact, you are doing 2x batteries in parallel. So the correct equation for a 12 volt battery bank would be:

• 400 Watt array * 0.77 controller+panel deratings * 1/14.2 volts battery charging voltage = 21.7 Amps usual "max" current for MPPT system
• 400 AH * 0.05 (5%) suggested rate of charge = 20 amps "generic" max charging current (for US GEL batteries)

Mcgivor does a good job of explaining... 2x batteries in series, the voltage adds (12+12=24volt). 2x batteries in parallel, the AH capacity adds (200+200=400AH).

Note that the energy stored from two batteries series vs parallel connected store the same amount of energy. Just deliver it at different battery bus voltages:

• 2 * 12 volt series batteries * 200 AH capacity = 24 volts * 200 AH = 4,800 Watt*Hours (stored energy)
• 12 volt battery * 2 parallel * 200 AH capacity = 12 volts * 400 AH = 4,800 Watt*Hours (same stored energy)

The ~77% derating is because "Hot" solar panels have lower Vmp (voltage maximum power). And MPPT controllers are "constant power devices (ignoring losses):

• Parray = Vmp * Imp (solar array voltages) = Vbatt * Ibatt (battery side of controller)

Since Vmp falls almost 19% (or ~79% of rated Vmp under "standard test conditions" with "cool" solar panels/cells), that means the harvest from the array also falls almost 19% too (the other major losses are the 5% electrical losses of the MPPT controller).

We use rules of thumbs here a lot... This is to make the design process much simplier and quicker to a reasonable design without going through each of the details.

If you wish to better understand the details, we can certainly go into them.

-Bill

Starting with the "technical questions".... Solar cells are NOT solar batteries.

We are all pretty familiar with what a battery is... More or less a battery holds 12 volts from zero to maximum current (Amperes). A "constant voltage" source.

Solar panels, for the most part, are "constant current sources". If you have an Imp = 10 amps solar panel (current maximum power)... Under full sun the panel will (for a first approximation) output 10 amps from zero volts to Vmp (voltage maximum power).

Say you have a Vmp=35 VDC for your panel (we need to know the details, Vmp and Imp at least for your panel(s)). Then we look at the equation for power:

• Power = Voltage * Current

If your load is at 35 volts, then the panel will output (under full noontime sun on a subfreezing day):

• Power panel = Vmp * Imp = 35 VDC * 10 Amp = 350 Watt "rated panel"

HOWEVER, if your load is a "dead short" or a 12 volt battery (no MPPT charge controller, just wired to battery or using a PWM type controller), the power equation looks like this:

• Power from panel = Vmeasured * Imeasured = 12 volt battery * 10 amps = 120 Watts
• Power from panel = 0 volts (dead short) * 10 amps = 0 Watts from panel

Panel temperatures can get very hot on a nice sunny summer / windless day... Upwards of 80C... At that point, Vmp (standard temperature rating and sun @ 25C) falls by 80% or so:

• Panel output hot summer day = Vmp * temp derating * Imp (Imp does not change much with temperature
• Panel output hot summer day = 35 Vmp(std) * 0.80 temperature derating * 10 amps = 280 Watts from panel to MPPT controller (derated for "normal operation" conditions)

Think of an MPPT (maximum power point tracking) Solar charge controller as being similar to the automatic transmission on a car... It takes the V and I of the solar panel and "matches" the conditions of the load V and I of battery bank (like matching the engine Torque and RPM to wheel RPM and Torque required).

So... when in "bulk charge" mode (the MPPT controller is outputting maximum power to a discharged battery), the Power into the MPPT controller is about the same as the power out... Say 280 Watts is available from my imaginary solar panel, then charging a 12 volt battery bank:

• Power = Voltage * Current
• Current = Power / Voltage
• Current into battery = Psolar / Vbatt
• Ibatt = 280 Watts (hot panel) / 12 volts charging "very discharged" battery = 23.3 amps into battery (less than 50% charged)
• Ibatt = 280 Watts (hot panel) / 14.2 volts charging "80% full" battery = 19.73 amps into 80% charged battery

Above numbers are just rough examples--Exact numbers will vary.

Here is a thread that discusses about understanding solar panels:

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

And, an FAQ that has lots of links to various solar/battery/conservation discussions. See if there is anything of interest:

https://forum.solar-electric.com/discussion/4426/working-thread-for-solar-beginner-post-faq/p1

-Bill