Two battery sources, one DC circuit

stmoloud
stmoloud Registered Users Posts: 111 ✭✭
in Solar Beginners Corner #1
I'm sure this has been answered elsewhere on the forum so easy answer would be if someone could paste a relevant link. I did try. 
I have a power switch which I switch off when I exit the RV for any prolonged time. This cuts all DC, but unfortunately also to the radio presets. 
I have a spare 85 ah battery which I believe I can use to back up the radio preset memory so have tapped into the radio DC input with a diode on the positive feed.
I have a voltage on the 'backup' battery of 12.6. If my main supply of 225 ah drops below this 12.6 v is it possible then that everything on the DC circuit is then supplied by the backup and not the main? At that point the backup will have a higher voltage of the two but I'm not sure if available amps will have an effect.

Thanks
760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    Not sure how the diode fits in.  Depending on the diode, there may be a material voltage drop across it.  You can get relays (ACRs) which will combine batteries/banks in parallel for charging, and separate for running loads.

    On my boat, the radio +ive bypasses the switch and goes directly to house bank.  As such, it keeps presets during summer use season.  I lift all circuits at the battery for winter, so no parasitic load from the radio during winter storage.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited November 2018 #3
    The current needed to keep the presets is probably miniscule, leaving the radio connected directly to the bank would probably have no significant drain, over a short period of time.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #4
    On mine, the load for the radio is okay for a few weeks, but gets to be significant over many weeks or months.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #5
    IMO, you want a float charger (could be solar) on flooded lead acid batteries anyway.  AGM would be different.

    I am available for custom hardware/firmware development

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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,101 ✭✭✭✭✭
    #6
    Having a hard time finding one, but they make a small back up power supply that plugs into the cig lighter/power port that is powered by a 9 volt battery. It's supposed to be used to save presets and such when removing the battery. Might work for you, provided there are no other loads than the radio's clock and presets. I believe I found one in a auto supply like Pep Boys.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • stmoloud
    stmoloud Registered Users Posts: 111 ✭✭
    edited December 2018 #7
    @littleharbor2 Yes have heard of these, they are supposedly not very long-lasting.
    Anyway, having some sleep on this I have managed to find a solution to the radio preset problem and also now have an emergency back-up system using the 85 ah. 
    The radio itself requires only about 500 mA so I have a 1 amp trickle charger on the battery. The voltage to the radio is routed via a switch, one side of which is a diode prior to the radio input of the 225 ah source. The voltage from the 85 ah supplies the radio  but goes no further into the dc system. The presets then are always preserved whatever is the voltage source.
    Furthermore, the other side of the switch is used to bypass the diode so if need be the 85 ah can supply the whole dc system. 
    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,020 ✭✭✭✭
    #8
    Ha!  I put a dedicated on/off switch on my rv radio circuit.  Even turned off it was the most annoying night light. Usually where we camp there's no radio I want to listen to either.  A gorilla tape flap covers the led's on the fridge...I like dark for sleep.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Watch the "cheap" 1 amp trickle chargers... They (for me) have a tendency to boil the batteries dry (over charge, over gas) if I don't check the electrolyte levels every few months.

    Another issue is I had a dead battery and used a cheap trickle charger (1 amp @ 12 volts)--And while the battery did charge (to move the car), the trickle charger overheated and popped the case apart (dead battery drew more current than charger could safely supply)...

    Instead, I now use the more expensive "float chargers" and never have had to add battery water (I use them for cars/trucks that are not driven very often)...

    https://www.amazon.com/gp/product/B005EKY1EM

    BatteryMinder and BatteryTender both seem to be good brands:

    http://www.batterytender.com/
    http://www.batteryminders.com/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • stmoloud
    stmoloud Registered Users Posts: 111 ✭✭
    #10
    @Ralph Day Me too I try to avoid led's in the bedroom. But outside of there in other areas it reminds me a lot of Christmas. There's so many they do concern me what their cumulative draw might be. 
    @BB. If drawing off at the same time of charging the tendency would be for the charger to always to be on charge mode and maybe never get to the float of of 14.4 volts?  At the moment the charger is putting out 13.55 v and it is quite warm.
    I was considering hooking the battery up to my 10 watt solar panel but it putting in  580mA  max would probably not be sufficient.

    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
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