Multiple lithium battery banks? Options?

Guisepi · March 2020

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.

Dave Angelini · March 2020

My thought is use an established battery make that supports network control of the power system. In this way you can add different banks of batteries and each has it's own BMS talking to the system on a network. This is what I do for clients.

The other way for DIY is to just build independent systems that each manage their own battery.

The real problem here for DIY is one DC buss (+48v) and the charge set points being able to run on different battery requirements, and do this safely.

mcgivor · March 2020

Assuming you intend to replace an existing battery bank use the same capacity if that was working in terms of average consumption, yes LFP can certainly be cycled to greater DOD but being conservative will increase life expectancy.

Keeping it simple would be, in my opinion, the way to go, having a single larger bank rather than the complexity of two, a single BMS would be needed.

The opportunity loads concept is a way to use lost potential energy heating or cooling are high consumers so depending on needs, I use air conditioning and water cooler in the tropics. Having excess PV is normal in off-grid, very useful for those periods of poor weather, that's where the added battery capacity comes into play and where LFP shines being insensitive to partial states of charge.

Guisepi · March 2020

Thanks for the responses.

Just to clarify, this solar system is a hydraulic deploy, fold-out array mounted on a flatbed trailer. I purchased it without good batteries, which is why I’m adding some in. It originally had four 100ah AGM batteries in series for a 100ah 48V battery bank. The charge rate for this array is just over 50A, meaning the battery bank was receiving a C/2 charge rate. Needless to say, that is too high of a charge rate for AGMs. Hence why I would like to reconfigure it.

While I would normally design a system around usage, focusing on battery bank capacity before moving onto array size, this system has a set array size — hence why I am building a battery bank around the charge rate of the array. I’ll be using this trailer as an initial off-grid setup for powering tools and some home stuff while building/remodeling a house. Usage will vary, and I will likely use it opportunistically for my larger tools.

mike95490 · March 2020

Many controllers can limit the max charge current to the batteries, when you get a couple cloudy days, you will be glad for the extra capacity

Multiple lithium battery banks? Options?

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