what happens in the absorption stage if I ..........

Nabiru3
Nabiru3 Registered Users Posts: 13 ✭✭
in Off Grid Solar & Battery Systems #1
Hey everyone, I have a pretty simple Q but it has been bothering me for quite time.

In an off grid solar system where the solar array produces around 750w at max, I have a 24v system where the batteries reaches the absorption stage early in the day, if the batteries are in the absorption stage and I connect loads equal to or less to the current provided by the solar array will the battery remain at the absorption stage and charges at a lower rates or will it simply stop charging the battery and focuses solely in powering your loads ?

Thanks in advance.
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited June 2018 #2
    In the absorption stage the current will be high at first gradually dropping as full charge nears, the surplus can be used but it's best to do in the latter stages to ensure the batteries needs are satisfied. This takes a little time experimenting to understand what size loads can be used and when they can be applied, the amount of time in absorption could be extended as a compensation, if your controller allows, in float the loads could be close to what the array can provide, but care must be taken to not overextend the duration then pull from the batteries as the sun drops closer to the horizon. It really is a learning process applicable  to your particular system, so you are correct, just be careful not to overestimate the amount of use available. Additionally as seasons change so dose the available production, so it takes at least a year to fine tune.
    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.  
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  • mevenable
    mevenable Registered Users Posts: 72 ✭✭
    #3

    Once you get into absorption, look to see how many kW are being used as time goes on to charge your batteries - for my 48 V system, it starts with over 2 kW to keep it at the requisite 56.6 V for absorption, but within about half an hour it is down to about 1 kW.  Since on a nice sunny day my system can do much better than that, I am able to run all kinds of other things at the same time.  That being said, the time it takes to go through the bulk stage also has an influence on how much it needs to stay in absorption.  If you do a nice slow bulk on a cloudy day, it needs less when you got to absorption, if it pushed through early with bring sun, so it had more kW in the bulk, it will need more in the absorption.  In any case, what happens if you take out to much of the pv input during absorption, it just drops back into bulk, and you see the voltage go down.  If it is still getting some power from the pvs for charging, then it will eventually bulk back up and start absorbing.  you just need to look and make sure that the system stays in absorption, if it does not, you are taking out too much for your load

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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #4
    You could also check the panel voltage - if it's above Vmp, then you have excess power available.

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