using solar to supplement charging capability

slyttle
slyttle Registered Users Posts: 9
Hi there,

Currently I have a small 200 Ah battery backup system that powers a few lights and computer equiptment during blackouts.

I have a small 20amp stepped plugin charger that charges and floats my two deep cycle batteries.

I am thinking of slowly expanding my setup one battery (100Ah / battery) setup but realize that to do so I need to increase my charging capability instep with the size of my bank.

What I am wondering is whether, rather than buying a bigger charger, could I add about 200 Watts of solar and 'blend' the output from the solar charge controller and the plugin charger.

My hope (albeit uneducated) is that the output from the solar charge controller will combine with the plugin charger to bring the total charging capacity up to the necessary level.... Then each time I add another battery I just add another ~200W of solar. (btw the 200W solar figure came from scanning a few RV boards and reading how large a panel is necessary to charge a 100Ah deep cycle batter within about a day and a half ... I am not sure of the exact calculation here)

Any advice you have would be appreciated.

-Sheldon

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: using solar to supplement charging capability

    Yes, you can charge from more than one source at the same time. Whether or not it's worth the investment is another story.

    For example, a "200 Watt" panel is likely to be of the GT type - meaning its Vmp will be around 30 and thus require an expensive MPPT type charge controller to make maximum use of the panel's power. So for the money invested you might be better off buying the bigger AC charger.

    With 300 Amp hours of battery your existing 20 Amp charger would still manage a 6% peak charge rate, so you might not actually need any increase in charging capacity (depending on load draw at the time of charging).

    Solar charging would benefit you most when you don't have any AC source to charge from. If the outage lasts long, for example.
  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: using solar to supplement charging capability
    slyttle wrote: »
    My hope (albeit uneducated) is that the output from the solar charge controller will combine with the plugin charger to bring the total charging capacity up to the necessary level.... Then each time I add another battery I just add another ~200W of solar.

    In general this will work. During constant current charging (i.e. "bulk" charging when the battery is low) the powers will add. If solar can provide 10 amps and your charger can provide 20, then total charge rate will be 30.

    However during constant voltage charging (absorb and float) you will NOT add powers, since the battery can no longer absorb full power. This isn't a big deal since the battery is largely full at that point, but this results in a lot of questions here - "my solar panel should be giving me 20 amps but I am only reading 4! What's wrong?"

    A second problem is that the controllers may 'fight' as they get to full charge, since they will be seeing the voltage of the output of the other charger rather than the voltage of the battery. This won't damage anything but it may result in very long (for example) absorb times.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: using solar to supplement charging capability
    slyttle wrote: »
    Then each time I add another battery I just add another ~200W of solar.

    Solar power systems do not upgrade very well. At some point you need to start over with a higher system voltage.

    As far as upgrading your battery bank one battery at a time: That's usually a big mistake in renewable energy (RE) systems. The first problem is that when mixing batteries you should not mix old and new... the new ones will be quickly dragged down to the level of the old ones. The other problem with adding batteries is that you are doing this in parallel. Parallel batteries are problematic for a number of reasons, a good discussion is here: http://forum.solar-electric.com/showthread.php?14674

    That said, there is one application where parallel batteries are not so bad: UPS systems where the batteries are seldom cycled. In RE systems the daily cycling is what makes parallel batteries so problematic. A UPS battery is designed differently from a RE battery. UPS batteries are designed to be constantly floated and they have a low cycle life. A RE battery is actually harmed by constant floating and infrequent cycling.

    If your system is really only used for occasional blackouts, you can probably get away with a few batteries in parallel. You should probably use a busbar in the battery box to connect the parallel strings (see the smartgauge site referenced in the link above). Also, it is a good idea to put fuses in series with each battery when you start paralleling more than a couple of batteries.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i