charging and running a load simultaneously

Solar Expert Posts: 65 ✭✭✭✭
Hi there,

what happens in your system when you charge with say 20 amps a 24V/24V system and use at the same time a 24V load that draws 20 amps (assuming a perfect world w/out losses)?

does all the current pass the battery? (guess so...) ... and does this add to the bat's deterioration - as in a "charge-discharge-cycle"?

If I could run the 20A load *consistently* (perfect weather) run every day for 4 hrs, could I theoretically get away with a smaller bank?

just trying to figure it all out :-)

cheers
al

Comments

• Solar Expert Posts: 10,300 ✭✭✭✭
Re: charging and running a load simultaneously

when drawing all of the current the pvs are presenting to the battery, the battery does not charge. when the battery and the controller voltages are identical no current goes to the battery and it all goes to the load, but it will maintain the current battery soc because any voltage drop as witnessed by storage losses would be stopped as a small voltage difference would be present enough to send a small current to the battery to maintain its soc. if this no charge situation is on a full battery it works fine as any idle discharge will be met with a maintenance charge, but if not at 100% soc sulphation will take place in the long term for the battery. what happens when the small maintanence charge is present is a slight lowering of the voltage to the load until the battery is back at its voltage. i think i covered this right.
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: charging and running a load simultaneously

Neil covered it pretty well, except for the size of the battery bank.

Clearly, if you present a scenario as you describe, my intuition would be that you size the battery for the expected net load, that is the loads required from it when it doesn't get perfect solar charging, then indeed you should be able to downsize the bank. For example if you used (assuming perfect conditions as you describe) 20 amps for 4 hours during the day = 80 amp hours and ALL of that is produced by the sun, and then you only use 2 amps for say 6 hours the rest of the day = 12 amp hours, you would size the battery for the 12 amp hours plus what ever reserve you desire.

We do much the same thing. We charge all the small batteries, (lap tops, printer etc) pump water, do our internet stuff in the morning when our sun is the best. Most afternoons our batteries get back to full charge, leaving us to draw them down over night with lighting and the radio etc.

I'm sure Bill or Crewser or others smarter than I will chime in if my intuition is wrong.

Tony
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: charging and running a load simultaneously

because you never wish to exceed 50% dod the example you give of 12ah would mean at least a battery of 24ah and then you can add reserve.
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: charging and running a load simultaneously

Good point,,, though in my world I never want to get below 20% dod. (Use no more than 20 % of capacity on a regular basis, in case I am stating dod backwards!)

Tony
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: charging and running a load simultaneously

dod= depth of discharge. 0% dod=fully charged. 100% dod=fully discharged.
this could've been said in terms of soc as well which is the opposite of dod. soc=state of charge. 100% soc=fully charged. 0% soc=fully discharged.
the interesting thing here is that 50% dod and 50% soc are the same whether you are an optimist or a pessimist.:p:roll: your 20% dod means that .2 x ah battery capacity has been used. an example for any not still understanding is that given a 100ah battery that is 20% dod or .2 x 100 = 20ah deficit from being full. for those that would go to the 50% dod point and are at the 20% dod point would mean there's still another 30ah that can be used. for soc that would be 80% or 80ah with 50% equaling 50ah. it's so simple it's confusing. here on the forum ilike to list dod just to avoid even more confusion.
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: charging and running a load simultaneously

I suspect that battery mfts call for 50% dod, or soc just to avoid confusion!

T
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