Question about battery size and charge rate

whackamolewhackamole Registered Users Posts: 18
Can one of you experts clarify the rule of thumb I've read about having a charge rate of 5 - 13% of battery ah capacity? Here is how I am interpreting this:

Given 120 watts of solar panels (~5+ amps) does this mean I should have a battery no larger than 100 ah? (100 x .05 == 5)?

I was considering grabbing 2 trojan T105's, which would be around 200ah. If my understanding of this general rule is correct, I would need more panels?

Comments

  • westbranchwestbranch Solar Expert Posts: 5,153 ✭✭✭✭
    Re: Question about battery size and charge rate

    yes you have it right, a 5 amp panel is recommended for a 100 aH battery. this will help prevent sulphation when a deep drawdown occurs say below 75% SoC.

    The 5% RoT will enable your battery to be quickly recharged.

    A qualifier is that the type and amount of use made of the system can have an impact, remember it is a RoT and ther are other variables.

    Search for a string questioning the 3 day RoT. there is a good discussion around this topic

    see http://forum.solar-electric.com/showthread.php?t=3527

    HTH

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mike95490mike95490 Solar Expert Posts: 8,429 ✭✭✭✭✭
    Re: Question about battery size and charge rate
    westbranch wrote: »
    The 5% RoT will enable your battery to be quickly recharged.

    for the life of me, I can't figure out RoT ??? Help !

    Reduction of Trouble ? Rate of Time ? Recycle or Transport ? Recharge or Terminate ?
    Ratio of Trains ?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Super Moderators, Administrators Posts: 29,639 admin
    Re: Question about battery size and charge rate

    "Rule of Thumb"???

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Question about battery size and charge rate

    on the minimum charge, one has to be careful here because the difference between 5a into a 104ah battery as opposed to into a 100ah battery is negligable. the 5% figure is the 20hr rate of a battery and when dealing with solar 20hrs could equate to 4 sunny days give or take a few hours during the summer for many. we arbitrarilly used this as the minimum, although in some cases we've said lower %s could be used, but one has to be cautious going that low in that the losses with charging a battery and the natural shelf losses batteries experience don't defeat the charge going into the battery or slow it down too excessively. long time intervals without reaching a full charge allows sulphation to take place and will do damage to the capacity and lifespan of a battery. this could also take place with higher charge rates if the power is being used at the time making the net charge rate the difference between that which is generated and that which is used. imo you should opt for around 10-13% for better overall charging of those trojan batteries.
    i better add here that if i remember rightly trojan does not recommend going over a 13% charge rate, not that you'd do such a thing with 5a.
  • whackamolewhackamole Registered Users Posts: 18
    Re: Question about battery size and charge rate

    So, even though the trojan t105 pair puts me below the 5% mark in terms of total battery amperage of 200ah and 120 watts of panel, it sounds like I could still use this combination as long as I manage to only use enough battery AH that my panels can supply in, say, 3 days at 5-6amps during full sun?

    Also, could this be considered a good thing since I would not be drawing down the battery very much, thus extending the battery's life?

    (Trying to justify using the trojans because they are less expensive, but seem to be recommended for first time projects.)
  • BB.BB. Super Moderators, Administrators Posts: 29,639 admin
    Re: Question about battery size and charge rate

    When you "equalize" charge the batteries, you actually stir the electrolyte to prevent stratification. Also, as the batteries age, they get less efficient/more self discharge.

    It would be better if you added some panels to better charge the batteries (unless you will be using a generator once or twice a month to equalize the set).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Question about battery size and charge rate
    BB. wrote: »
    When you "equalize" charge the batteries, you actually stir the electrolyte to prevent stratification. Also, as the batteries age, they get less efficient/more self discharge.

    It would be better if you added some panels to better charge the batteries (unless you will be using a generator once or twice a month to equalize the set).

    -Bill

    This brings up a point I have. Can you equalize with solar panel output? I have a prostar CC and it "equalizes" at 14.9/15.1. Isn't that below the recommended voltage? And doesn't equalization take more than the amount sun hours that you'll have in a day? Sorry for being off topic.
  • mike95490mike95490 Solar Expert Posts: 8,429 ✭✭✭✭✭
    Re: Question about battery size and charge rate
    Tim F. wrote: »
    This brings up a point I have. Can you equalize with solar panel output? I have a prostar CC and it "equalizes" at 14.9/15.1. Isn't that below the recommended voltage? And doesn't equalization take more than the amount sun hours that you'll have in a day? Sorry for being off topic.


    If you have enough solar PV, you can EQ from solar. The voltage depends on your battery mfg suggested. Most charge controllers, have a "generic" EQ voltage set, but nearly all can be fine tuned. Your batteries have to start off at a full charge, before you start EQ cycle. Otherwise, you are just charging the batteries. The EQ cycle is only 2 or 3 hours. You may have to run your genset in the AM, to bulk charge the batteries, before you can complete the charge, and start the EQ.
    If you don't have enough solar, you have to run a generator, or provide an AC power charging source . 15.1 is an average EQ voltage, sometimes battery mfg's want 16V.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Super Moderators, Administrators Posts: 29,639 admin
    Re: Question about battery size and charge rate

    You can read the Battery FAQ and check the vendor's information too.

    The float levels are generally adjustable--and also very with battery temperature (cold batteries require higher voltage to charge).

    Once a battery is charged--then equalization generally is only done for a limited amount of time (couple hours?).

    So, you can charge your battery bank in the morning (before sun), and let the solar finish the charge and equalize). The smaller the panels (vs battery bank), the less well you will probably be able to equalize without using a generator.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Question about battery size and charge rate

    timf,
    the eq v may be slightly lower than trojan's recommendation of 15.5v, but in the case of using a slightly lower eq v you may opt to add a bit more time for it to stir things up if you feel it didn't quite do it the first time. though you could kick the batteries in the pants will full pv output, the timeframe to do this would be very short seeing as how there's no regulation on the pv voltages and that means little stirring of the electrolyte. with full pv output you'll minumumly be adding much more maintenance on the batteries and on the other end of the spectrum you're lowing the lifespan and risking damaging the batteries not to mention voiding any warranty. bottom line here is don't exceed their eq v limit.

    whackamole,
    consider buying the marine batteries at walmart or someplace similar for a 5a charge as they used to carry batteries in the 100-125ah range. a 120w pv will be more than 5a and may be closer to 7a, but under as 120w/17.4v=6.9a. 6.9a/225ah=3.07% for t105s. this is most likely too low for t105s given their higher shelf losses and charging efficiency. for some marine batteries you may see something like this---6.9a/115ah=6.0%.
  • System2System2 Posts: 6,290 admin
    Re: Question about battery size and charge rate

    Just so I'm clear...When you are speaking of 3% or 5% of rated AH, are you speaking of the total AH rating of the battery bank? Or are you speaking of the 50% rate of discharge? For example, I have (4) 6v GC's rated at 210AH(ea). So in series/para. I get 420AH @12v. 50% DOD = 210AH (the max I would use.) So, am I calculating the 3%-13% of 420AH or of 210AH? If the latter, then couldn't I simply keep my DOD down to say 25% and calculate my PV needs from there? If the former, then does it really matter how much I use except with regards to battery health?
  • BB.BB. Super Moderators, Administrators Posts: 29,639 admin
    Re: Question about battery size and charge rate

    The 3% to 13% is of the 20 Hour full battery capacity (in amps)... Below 3%, difficult to equalize properly. Above 13%, less efficient and could overheat the batteries (and cause thermal run-a-way if a remote battery temperature sensor is not used--as batteries get warm, their voltage drops, causing more current flow, hotter battery, etc.).

    So, 3% would take ~33 hours to charge a 100% discharged bank. 13% would take about 8 hours to fully charge a 100% discharged bank (1/0.xx).

    Several issues with battery life--discharging a standard deep cycle (wet cell) storage battery below 50% will shorten its cycle life (see battery FAQ).

    Another issue, when discharging below 75% state of charge (25% usage), the sulfates ("discharged" material on plates) starts to harden with in hours... If left for a period of time (day-week-months) the sulfates harden and will not return to their "charged" state when the battery is recharged. The hardened sulfates then represent the "lost" capacity of the battery (and eventually lead to battery replacement because of loss of capacity).

    Concord AGM (if I recall correctly) claim that they do not suffer sulfate hardening and can be cycled down to 20% state of charge without damage... AGM's, however, cannot take overcharging at all (no equalization required) and cost 2x-3x as much as wet cell batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Question about battery size and charge rate
    Tim F. wrote: »
    Just so I'm clear...When you are speaking of 3% or 5% of rated AH, are you speaking of the total AH rating of the battery bank? Or are you speaking of the 50% rate of discharge? For example, I have (4) 6v GC's rated at 210AH(ea). So in series/para. I get 420AH @12v. 50% DOD = 210AH (the max I would use.) So, am I calculating the 3%-13% of 420AH or of 210AH? If the latter, then couldn't I simply keep my DOD down to say 25% and calculate my PV needs from there? If the former, then does it really matter how much I use except with regards to battery health?


    timf,
    it is the total battery bank ah that i speak of when i say 5% to 13% for charging. in your case it would be of the 420ah or at 5%=21a and 13%=54.6a. if you had another arrangement for the same batteries that upped the voltage to 24v then the current of the battery bank will be halved or 210ah and the percentages will be applied to that and would be 1/2 of the amps i stated for those same batteries at 12v with 10.5a and 27.3a respectively.
  • System2System2 Posts: 6,290 admin
    Re: Question about battery size and charge rate
    niel wrote: »
    timf,
    it is the total battery bank ah that i speak of when i say 5% to 13% for charging. in your case it would be of the 420ah or at 5%=21a and 13%=54.6a. if you had another arrangement for the same batteries that upped the voltage to 24v then the current of the battery bank will be halved or 210ah and the percentages will be applied to that and would be 1/2 of the amps i stated for those same batteries at 12v with 10.5a and 27.3a respectively.

    Thanks. That's exactly what I needed to know. :D
  • westbranchwestbranch Solar Expert Posts: 5,153 ✭✭✭✭
    Re: Question about battery size and charge rate

    Niel, a simple question, re making the battery a 24 V instead of 12v, Yes amperage goes down... by 1/2... but...

    would this not also require the panels to be rewired in series to up the Voltage 2x and halving the total Amps (x / 2) ?:confused:

    As I see it, the end result being you need the same number of panels,

    Parallel 2 @ 12 v 6 a = 12 v 12 a
    Series 1 @ 24v 6 a = 24 v 6 a

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Question about battery size and charge rate

    yes, in many cases the pvs would have to be rewired to match up with the battery voltage requirements. we were talking of batteries and their charge rates here in this thread so i didn't ramble on that far.:roll: seeing as how you are asking i put to you then why would you need to rewire pvs if they are already outputting say over 34vdc voc, as many pvs do, into a previously 12v battery system that may now be 24v? there may or may not be an advantage to rewiring them as you can leave it as it is in such a case, right? in your example you would be correct to say it needs rewired. does this answer your concern on the pvs?
  • westbranchwestbranch Solar Expert Posts: 5,153 ✭✭✭✭
    Re: Question about battery size and charge rate

    thanks Niel, I was just worried that we seemed to jump over the voltage/amperage relationship in 12 and 24 volt configurations if/when the panels and batteries could be set up either way. I had not thought about that option (24v) ... I was still thinking in a '12 volt' mode.

    Eric;)
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • whackamolewhackamole Registered Users Posts: 18
    Re: Question about battery size and charge rate

    Does the use of a MPPT (i.e. sunsaver) affect this calculation (the original thread topic's calculation, that is)? Hope this is clear:

    If I use the sunsaver MPPT and wire the panels (6 x 20watt panels 1.19a) as 3 24v strings, I presumably eke out more usable power because the MPPT "converts" any volts over nominal 12v battery voltage into amperage into the battery, right? So rather than my 5-6 amps of power getting to the battery, I get more than that, getting me to the magical minimum power needed for charging?

    Sorry I'm obtuse on this, sometimes I think I've figured it out, and then it goes cloudy...
  • BB.BB. Super Moderators, Administrators Posts: 29,639 admin
    Re: Question about battery size and charge rate

    If you have "12 volt panels" (really ~16-18 volt Vmp -- maximum power voltage) connected directly to either a PWM or MPPT controller on a standard day (not cold)--then the two controllers will output about the same amount of Imp current to charge the batteries.

    If, you have a very cold day, the Vmp of the panels rise, and because P=V*I, then the total power rises too... The MPPT controller can take advantage of the increased voltage and provide more current at the Vbattery voltage.

    The PWM controller (on a cold day) will not increase power output--it will just output Imp of the panels to the batteries (Imp does not change much with respect to temperature).

    In your case, you are using another property of the MPPT charge controller--to act like a transformer--take the high voltage/low current of the series connected solar panels and transform that (efficiently) down to the low voltage/high current for the battery.

    1. Using high voltage panels (vs low voltage batteries) reduces wiring resistance loss (lower current on the solar panel wiring).
    2. MPPT controller does get the "optimum" or maximum power from the solar panels no matter the panel voltage (power available from panels will go up as temperature drops).
    3. Allows you to use non "12 volt panels" on 12 volt battery bank (don't have to "match" panel Vmp to Vbatt for best efficiency.
    4. Allows more "headroom" to run in hot climates (where panel voltage falls) to charge cold batteries (where battery voltage rises).

    So, the short answer is you will not have to "adjust" your panel power calculations because of the series/parallel panel connections. And, if you have very cold weather (freezing and below), you will get a bit more energy (plus you saved money on the copper wiring for the solar panel to charge controller run).

    In any case, once the battery has finished "bulk charging" (~80-90% charged), either PWM or MPPT controller will start to reduce current to the battery bank as they finish the charging cycle (absorb through float charging cycles)... In these last two states, neither PWM or MPPT controllers will have a power advantage (since the controller is limiting/regulating current/power from the solar panels to the batteries to finish the charge cycle).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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