Multiple lithium battery banks? Options?
Howdy all. I have an off-grid 3000W array and a Midnite Classic CC, that I am currently looking into getting some batteries for. I am intrigued with DIY lithium batteries. I would be building a 48V battery bank from 16 x 3.2V lithium batteries @ 100ah. Here's where I am confused:
Since lithium batteries have such a high charge rate (C/2), they charge relatively fast (say 2 hours with a high DOD). This means that you have a smaller battery bank than a lead-acid bank (fewer days of backup power). This also means that you top off your batteries relatively quick, which means that you're not using all your solar potential. If the sun is out for more than 2 hours, then your panels are just sitting and doing nothing. Here are some ideas to overcome this:
- Add a "opportunity" circuit which is flipped on when float is reached (programmable on the Midnite Classic). This circuit would have any of my battery-powered items that would charge once the battery bank is full. This would be power tool batteries, electronic devices, computer, etc. The limitation here is just how little power these items' batteries actually hold.
- Add a secondary lithium battery bank. How to charge this though? I thought perhaps a second charge controller for this bank. A 5-pin relay would be flipped by the first CC once float has occurred. The relay would redirect solar power coming in from the first to the second charge controller. I would also have to use a battery selector switch to choose which battery bank my inverter was running off of.
- A combination of the two above: Add an opportunity circuit that has a 48V battery charger to charge the secondary battery bank. I haven't found any 48V to 48V DC-DC chargers for this, so it would run through the inverter. It would charge the second battery bank once the first is full. Again, I would use a battery switch for choosing which bank my inverter is running from.
Finally, would it simply be better to double the lithium battery bank and simply charge at a lower-than-recommended charge rate (C/4 in my case)? Is this bad for lithium batteries?
Any thoughts on this? I can't be the only one to think about the fact that lithium batteries charge fast and leave our solar panels sitting idle.
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