solar system excess energy use

bjornderianbjornderian Registered Users Posts: 1
Hi! I'm new here.

I have a PV system and I'm planning to use my excess energy during day to charge a lithium ion battery. i already have a lead acid battery bank connected to the charge controller.

Is it possible to charge directly from DC current to reduce efficiency losses ? What would I need?


  • BB.BB. Super Moderators, Administrators Posts: 29,359 admin
    The easiest is to find chargers that are designed to charge your specific Lithium battery configuration... Are you looking for single cell chargers (18650 or other cylindrical batteries) or for charging 100 AH @ 12 volt prismatic batteries? Protected or unprotected cells, etc.

    There are charge controllers designed for larger solar power systems, such as the Midnite Classic that has a 12 volt signal output that can be programmed to control optional loads and other options. There are battery monitors that can be programmed to "turn on" when >90% state of charge and turn off at <80% state of charge (or whatever you wish). Take the 12 volt contact controller output and control your load AC/DC supply.

    Details matter here, and without details it is not really possible to give a good answer.

    12 VDC vs 120 VAC power... Most modern chargers are pretty efficient (90% or better efficiency)... The 10% loss, just make the system 1/90% or 1.11x larger (hardly worth worrying about--10% change in system size is "lost in the noise" of all of the other variables.

    There can be issues with 120 VAC systems... You have an AC inverter that has its own losses just from being turned on... 6 Watts is typical for a smaller inverter (300 Watt or so), and 10-40+ Watts is typical for larger inverters (1 kWatt and larger). So, it does depend on what other loads you may have (and some inverters have a "sleep mode" where the are in standby when they have no loads).

    Another issue that can happen with charge controllers--If you supply them with "interrupted/variable" power source, it causes the charge controllers to reset and start the charging cycle over... For Li Ion batteries, generally not an issue (they just charge to 4.20 Volts per cell, then stop). For other chemistries, such as NiMH, they need to monitor charging current/voltage profile and even temperature. For Lead Acid, once 14.75 volts is reached, then it needs to be held for ~2-6 hours (depending on how deep the discharge was). So for many chargers, it is not a great idea to interrupt their charging cycles based on available sun/etc.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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