I'm seeing a bit of a problem with Re-Charging the battery bank

KnowledgeSponge
KnowledgeSponge Solar Expert Posts: 173 ✭✭✭
Maybe I'm at another precipice where I'm about to learn something else very important.
Maybe you guys can point me in the right direction.

Yesterday, I ran one of my 121 Ah (12v) batteries down to about 50%

Then immediately put it on charge to bring it back up. (Automotive battery charger, 6 Amp / 12v)

I was surprised to see how long it would take to charge. Actually, I shouldn't have been.

121Ah battery and discharged down to approximately 60Ah

so.....

I need to replace 60Ah in the battery at 6 amps. Simple....Charge time = Ah needed / Charge Amps

So, I will need to charge this battery for 10 hours at 6 amps to replenish the charge.....and therein lies the problem.

If I discharge TWO 121 Ah batteries to 50% I will need 20 hours of charge time at 6A

I plan to have at least 4, 121 Ah batteries

So, let's say I discharge all 4 to 50% each night......I will need 40 Hours @ 6 amps (or 240 Ah total) to re-charge those batteries.

The only way to offset this is to add charging amps (MORE Panels)....right?

But you can only charge a battery so fast before you begin to heat it up and boil off electrolyte. right?

What do you consider the maximum safe charging rate for Marine Deep Cycle 121 Ah batteries?

I am using 100W, 5.68 A solar panels. Am I correct to assume I will need somewhere around 2 panels PER battery
to charge them daily? (depending on sunlight hours and intensity of course)
Where I live and how my location is arranged, I can get about 6-7 hours of direct, strong sunlight daily.

So, with two panels charging One battery, that would be around 11 amps over 6-7 hours or approximately or 66 - 77 amp hours back to the battery.

Is that a good rough estimate? 10A Per battery for charging? In my case (with 100Watt, 5.68A panels) 2 panels per battery?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Re: I'm seeing a bit of a problem with Re-Charging the battery bank

    Yep--And you should add some time for "absorb" charging (when the battery is at 14.5 volts and the charging current slowly ramps down to ~1% or so rate of charge).

    And, you should probably not plan on recharging to 100% SOC every night--Just above 90% a couple times a week (charging to 100% every day can be hard on batteries too).

    You are using Marine Batteries for Deep Cycle application (50% discharge per night)?... They are not going to hold up well for that type of application.

    Our normal recommendation (always read the battery manuals and spec. sheets) is 5-13% rate of charge (100 AH battery, 5-13 Amp rate of charge). You can go higher--upwards of 25% rate of charge, but you should use a remote battery temperature sensor to ensure that the battery does not do "thermal run away" (batteries get hot, their charging voltage drops, charge controller thinks battery needs more charging, battery gets hotter, you may get battery guts/acid splattered around your room). A controller with RBTS will reduce the charging voltage set point as the batteries get hot.

    Batteries also are most efficient when charging below 80% SOC--Most of the energy goes into the the chemical storage of electrical energy. As batteries charge above ~80% SOC, more and more of the energy is turned into heat--And near 100% SOC, all of the energy is being turned into heat and Hydrogen/Oxygen gases. Equalization (charging at basically 100% SOC, you should limit current to ~2.5-5% rate of charge and terminate equalization charging as soon as all cells are "equalized"--the individual cells stop rising specific gravity over a 1 hours period between measurements).

    There are many variations in equipment, batteries, charging profiles. AGM's can take very high charging currents but should not have high charging voltages. Flooded cell batteries can take more charging current when less than 80% full and less current when 90%+ full.

    If your plan is to charge the batteries from the grid during off peak times and use the power during peak times (for me, I can "buy power" at $0.09 per kWH and avoid/sell power at $0.30 per kWH) using batteries+chargers+inverters... From my back of the envelope calculations, it will cost you in the range of $0.45 per kWH (very roughly, do your own math) to "time shift" energy usage. Heavily used batteries will not last that long, or you have to buy fork lift batteries (which are less efficient, so cost more in electricity+distilled water to operate).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: I'm seeing a bit of a problem with Re-Charging the battery bank

    Problems listed in order:

    #1). How do you know you were at 50% SOC? A properly programmed battery monitor could tell +/- a bit of tolerance, or temp compensated hydrometer. But this issue is minor.

    #2). It is not as simple as 6 Amps * 10 hours = 60 Amp hours back in the battery. Batteries do not charge linearly. As it charges the current drops off (unless you have a constant current charge source, which doesn't apply to solar) and so as time goes on less and less power is returned to the battery. Also, the darn things aren't 100% efficient: at least some of the power (about 20%) is ultimately wasted on the charging process (heat).

    #3). Yes you have to increase the current in order to get the charging done in the "window of opportunity" (i.e. hours of sunlight). And yes there is an upper limit for the rate or else the batteries will suffer. Once in a while going to high current is one thing; consistently charging at a high rate is another.

    #4). Not all batteries are the same.

    And so we come to the tried-and-true rule-of-thumb of 5% to 13% of the battery capacity at the 20 hour rate being the desired peak current. 5% (net) is a minimum recommended by most makers for FLA's or AGM's. Usually. 13% tends to be the upper practical limit, beyond which you end up buying a really big array that probably won't be utilized most of the time. On top of that each particular battery will have its own specific maximum after which the current can cause thermal runaway aka "burned up battery".

    In the middle is 10% peak charge current with a 25% maximum DOD which works about 90% of the time.

    So a 121 Amp hour (odd number, that) battery could take about 15 Amps of current. Two of them in parallel = 30 Amps.

    How to get it from 100 Watt panels with Imp of 5.68 and PWM type controller: three panels in parallel per battery, six in total. That might hit 34 Amps, but you will probably never see that. Keep in mind that any concurrent loads will detract from the net charge rate. You could snake it down to 4 panels if you are worried about too much current and/or have no loads to run while charging.

    How to get it with an MPPT type charge controller (since you're looking at over 400 Watts): About the same number of panels. The difference comes in how you get that 400-600 Watts and what you pay for it, as the "GT panels" come at a lower cost per Watt than "OG panels" and this can make up for he additional cost of the MPPT controller.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: I'm seeing a bit of a problem with Re-Charging the battery bank

    it is true you need much more than the face value of 6a x 10hrs. as coot pointed out the charge efficiency for the battery can be at 80% for fla type batteries like yours. this translates into the same straight up 60ah plus and additional 20% or 12ah for a total of 72ah. over 5hrs this appears to be 72ah/5hrs=14.4a. bb is also correct that the absorb stage stretches it out longer with a smaller ending current, but we'll forgo that in this explanation. that 14.4a represents an 11.9% charge rate 14.4a/ 121ah=11.9% and is quite doable. now there are losses from the pv system as well and stc is not always the condition you will see to get the constant 14.4a from. this efficiency is generally around 77% give or take so to insure the 14.4a divide it by the 77% for 18.7a from the pvs. this will push a higher % of charge if the factors go more favorable and less if not, but 18.7a represents a 15.45% max charge rate if it ever reaches it due to better efficiency. doable with extra maintenance i think. and is per 121ah battery.