85AH or 105AH

cizzi
cizzi Solar Expert Posts: 109 ✭✭✭✭
I know there's math involved in this based on load usage but would I be better off with an 85AH,105 AH AGM or 112 AH AGM battery for an 85W panel installation?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,398 admin
    Re: 85AH or 105AH

    Roughly,

    85 watts / 17 volts (~Vmp) = 5 amps (~Imp)

    Assume minimum ~5% (5-13% is rule of thumb range) of battery 20 hour Amp rating for charging current:

    5 amps / 0.05 = 100 amps (for 20 hour rated battery)

    So--any of those battery ratings will be OK... AGM's have very low drain--so a low charge rate should be fine.

    Of course, you have to size the panels to any load so you don't drain the battery over the long term...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cizzi
    cizzi Solar Expert Posts: 109 ✭✭✭✭
    Re: 85AH or 105AH

    Hey BB thanks.. I've been researching most of the night on this.. to see exactly what I'll be ordering.. the Kyocera 85W panel is for sure.. the battery would be in the 100AH range for my purposes... (dc lights and light loads at night). I decided to build a second system in parallel since my first one only has 2x15 amorphous panels and 31AH battery.. so im gonna run a bigger gauge wire along with a new charge controller and keep my existing system seperate.. (right now I only have a 2/14 AWG exterior wire running to my roof), maybe i'll run a 2/10 for the new system.. im researching mounting options right now, i like the simple z brackets i found out about tonight!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,398 admin
    Re: 85AH or 105AH

    Depending on your $$$ and needs over time... You could easily start with a single 85 watt panel and add a second 85 watt panel in series with the first if you use an MPPT type charge controller (the Morning Star MPPT is a very good to start with if you plan to expand--because of series connection, don't need to add more copper cable later).

    The MPPT controller can really help you on cold/clear/snow on the ground days in your area. Also, if possible, make sure you order the Remote Battery Temperature sensor--practically required for good battery performance.

    Your system is probably too small to justify--but check out a battery monitor to watch how well your batteries (load/charging) are doing would be nice too... The Trimetric would be a good entry level model for your system (hey--this is easy--isn't my money ;) ). About the only way to really know if you are killing your batteries or not--especially useful if you are not the only user of your system (presumably--given that it is your money, you will be more careful than others to ensure the batteries are properly maintained).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cizzi
    cizzi Solar Expert Posts: 109 ✭✭✭✭
    Re: 85AH or 105AH

    Well the prostar series of charge controllers have the metrics on the unit themselves. Can't I simply get the 30A model? right now I have the 15A.. so I wouldnt have to buy the trimetric, although knowing how many AH are in the battery is pretty neat.. and the prostar series are PWM and not MPPT.. will I benefit alot more with MPPT ? Another thing, my battery is stored inside so the temperature is pretty constant, why would I need a remote temperature sensor? I thought that was for when the batteries are physically located further than the charge controller. Back to your comment on the 85W panels in series, you're saying that so I get 24V and I can go a further distance over the same gauge copper wire correct? Just brainstormining with you here.. I want to make educated decisions as money doesnt come easy for me either!

    oh yeah how about them tristar series from morningstar.. they have rs232 capabilities with msview software to monitor data directly from my computer, is that a viable option for me?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,398 admin
    Re: 85AH or 105AH

    In cold weather, yes, you can benefit--probably on the order of ~30% or so in very cold/clear weather. I will let somebody with experience in cold areas verify.

    By adding panels in series, you up the voltage, but keep the current the same--so the losses to wire heating (I^2 * R) remain the same too...

    The Morning Star MPPT controller should be able to take up to 3 "12 volt" panels in series (you need to check the Voc and your low temperature to validate).

    The remote temperature sensor is really required for some controllers (like the Morning Star MPPT). In some case, the internal temperature sensor does not appear to be well calibrated (although, you could probably just "adjust" your absorb and float voltage settings to compensate)--but the bigger is that charge controller get warm when charging--and therefor their internal temperature sensor gets warm too--and you end up with a dramatically undercharged battery in many cases.

    For example, the controller is set for -5mV/°C per cell--or -30mV/°C for a 12 volt battery... If you are off by 10°C (18°F), your will be off by 0.300 volts or loosing around ~20-40% of your total battery's capacity because of under charging by the controller being warm (dropping output voltage) and the battery is cool (needing higher voltage to complete charging) (example for a flooded cell 90% is 12.5 volts, 60% is 12.2 volts)...


    AGM's are very sensitive to overcharging so you don't want the controller to not charge the battery to the correct voltage (if you overcharge and generate too much hydrogen gas--it will vent the gases and water--and you cannot add water/acid back into an AGM).

    Normally, I like computer monitoring for anything--but in reality, the amount of power a computer takes to run will swamp the power collected by a "small" system (my little old laptop takes 20 watts).

    Some of them (Like the Morning Star SunSaver MPPT controller) save the last 30 days of data... I have not looked at the controllers--so I don't know what they save--but having a computer connection also lets you program the settings more accurately (if the default settings do not meet your needs). The list price for the meter to PC adapter appears to be ~$39... You will have to call your local supplier (or Morning Star) to find out more.

    The Morning Star MPPT seems to have a pretty nice 30 day summary of data (click on Meter Map?). Remember though--to a degree it is hard to estimate the maximum solar pv power (in Watt*Hours) available from your system by looking at the logs. Because, the charge controller throttles back on the voltage/current as the batteries complete their charge cycle... So, you will have how many Amp*Hours it took to recharge your batteries back to full charge (if able to do it in one day). But, it is a good start for learning about your system.

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