so i bought 2 "marine deep cycle" batteries, now what?

EverlongEverlong Solar Expert Posts: 36 ✭✭
My setup consists of 2x235w, 24v mppt, and two batteries. Unfortunately i cant return them, i am planning on getting two trojan batteries next(12v).

Can i mix them wih the other batteries? I understand that trojans require different voltages to charge.

I doubt i will be depleting them more than 80% since i will be using it to power 5-8 strings of LEDs and 6 10w par20 s trough the night.
Should i just get two more of the DC el cheapos? My life does not depend on ths, its more an experiment than anything permanent/functional.


  • BB.BB. Super Moderators, Administrators Posts: 30,526 admin
    Re: so i bought 2 "marine deep cycle" batteries, now what?

    Do you mean you will not deplete them below 80% State of Charge (20% discharged)?

    Assuming your batteries are around 100 AH each (20 hour rate), the recommended rate of charge would be 5-13%:
    • 200 AH * 14.5 volts * 1/0.77 panel+controller derating * 0.05 rate of charge = 188 watt array minimum
    • 200 AH * 14.5 volts * 1/0.77 panel+controller derating * 0.10 rate of charge = 377 watt array nominal
    • 200 AH * 14.5 volts * 1/0.77 panel+controller derating * 0.13 rate of charge = 490 watt cost effective maximum

    So, your 470 Watt Array would be a very healthy rate of charge for the battery bank.

    I would probably just keep the batteries (assuming you don't mean 80% depth of discharge), and use them/experiment with them.

    Many (most) of us have "killed" our first set or two of batteries anyway. So, why not see how much power you use, and what you would do next time (larger battery bank, etc.).

    Can you mix batteries--Yes they will work but they will not charge/discharge as evenly as if they where matched sets. Trying different batteries types together would be an interesting experiment in itself.

    The issue may end up that true Deep Cycle batteries can use higher charging voltages. If you have the charger programmed for lower charging voltage, you may end up sulfating the D.C. batteries faster. If you program for the higher charging voltage, you may end up damaging the Marine Batteries a bit and limiting their life somewhat.

    I am not sure there is a handy formula that you can plug in here. One of the sets may die in a few years, or they may last farily well together for 3-5+ years--and any early aging effects may not be obvious.

    If I was doing an experiment, I would use a DC Current Clamp meter to measure the current into the two strings (type A and Type B battery banks). And see if they charge/discharge sharing the current, or if one set takes/supplies more current than the other.

    If they share current--then you can probably mix without too much concern (just keep an eye on the water levels--The marine type batteries may be "maintenance free" type and who knows what the water levels will do when mixed with deep cycle).

    If your LED lighting loads are not large--You could back the charging voltage down a bit (towards 14.2 volts or so) and see if the batteries all reach "full charge" or not.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: so i bought 2 "marine deep cycle" batteries, now what?

    Do not mix them with other batteries. Use them 'til they croak, then replace them. Since this is not a critical application it doesn't really matter when they give up, does it?
  • EverlongEverlong Solar Expert Posts: 36 ✭✭
    Re: so i bought 2 "marine deep cycle" batteries, now what?

    Yes and no ;) they cost me 270$ and they are brand new. Regret not asking for advice sooner, but as BB said i should be OK if i only use 20% of their capacity between charges. Well, today i will be going to the store to get a kill-a-watt meter and will keep you posted!
    Thanks guys
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