Input AC-DC charger and Solar microinverter parallel connect

Welcome to the forum Rosy_Posy,

It is difficult to understand what you want to do here... With electrical/solar/engineering questions, the details matter--A Lot!

Can you post brand/model numbers/links to the micro-inverters / charge controller / AC inverter you currently have installed and/or thinking of using? I am not quite sure what you are talking about here (Grid Tied/Grid Interactive solar panel=>AC power? Or what)?

Also--Can you tell us more about your battery bank? A 96 VDC lithium battery bank + BMS is not common for non-commercial installations. Typically, for residential solar systems, they tend to be 12/24/48 VDC based battery buses. 96 to 380 VDC power systems tend to be in large data centers or similar locations that need backup power systems.

Are you using shore power / Genset / Solar? And/or/all?

What is your "need"? More charging power? Where is that power coming from (shore or what)?

-Bill

Kuz,

In general, you can parallel multiple/different battery chargers to one battery bank. You need to program each charger "correctly" for your battery bank... And the total charging cannot exceed the maximum charging current for the bank (you need to check the battery and BMS specifications for those specs).

Looking at your solar panels... Several issues here. Most solar charge controllers are rated for a maximum battery bank voltage of around 48 to 60 VDC nominal. So "standard" solar charge controllers will not work as is--You can take two 48 VDC chargers, one for the "low group" of cells, and the second for the "high group" of cells. You can run into issues if you try to network them (the "high charger" will have its "ground" at +48 VDC--And you cannot connect the two controllers in parallel with "ground referenced" communications links (RS 232, some RS 422, etc. unless you use optical isolators or other comm. channel).

Another issue is almost all solar charge controllers are "buck type"--I.e., they take higher voltage (and lower current) and "buck"/down convert to lower voltage (and higher current) to charge the battery bank.

You need to look at the Vmp of your solar panel(s)... Most run in the range of 30-40 VDC or so (at 25C/75F). So, they would not charge a 96 VDC bank, unless you put (for example) 3x 36 VDC Vmp panels in series for a 112 Vmp array.

It sounds like you want to take the solar panel(s) and drive AC inverter(s) to power your 120 VAC input for your AC battery charger. While it is possible--You run into a bunch of issues...

One is the panel Wattage should be higher than the load Wattage... Or the load will "collapse" the AC voltage. 3.3 kWatt battery charger an 400-800 Watts of AC load--I would not expect to work/be stable.

If you want to parallel off grid AC inverters--Most do not parallel AC output and will not function correctly or possibly damage each other.

Most off grid AC inverters are expecting stable DC Voltage bus... Connecting solar panels to "typical" AC inverter DC input causes issues (many inverters "shut down" if battery bus voltage falls below cutoff voltage and you need to turn off/on the DC voltage again to reset (inverters are different--And there are some larger AC inverters that can run from a solar array only--Most seem to be >>400 Watts).

There are Grid Tied (Utility interactive) AC micro inverters... But they do need a separate AC off grid inverter to set grid voltage and frequency (may be a couple companies that can run GT micro inverters "off grid"--Still the issue of 400 Watt array and 3.3 kWatt AC battery charger).

Probably the simplest setup... Just as a starting point--Setup your Vmp-array voltage to be at least 1.25x your battery charging voltage set-point (Vmp falls as solar panels get hot). And use a simple relay to cut off the solar power when Vbus>charging set point. You can get more complicated--But I am not sure I would try... Simple 120 AC power to charge at dock. Solar panels in series with a DC relay charge cutoff at camp.

-Bill

Good morning Kuz,

That sounds like a "reasonable" charge controller... But look at the size of your solar array vs the capacity of the charge controller.

Some large charge controllers are pretty inefficient at lower loads (i.e., "wasting" 50 Watts on a 400 Watt array is "a lot", "wasting" 50 watts on a 4,000 Watt array is not much. (no idea of the specifications on the controller you are looking at--Just an exaggerated example).

Another issue to watch for on "portable solar charging systems". Most solar charge controllers boot and configure from battery bank power. If you connect the solar panels first, then the battery bank second, in some cases that can cause the charge controller to boot incorrectly/in a confused state, or even damage the charge controller if hit with solar panel energy first.

The typical solution is to always connect battery first and solar panels second... And when shutting down, disconnect panels first, then the battery bank.

Have fun!
-Bill

Lots of ways of looking at the issues there...

Simple, many times, means more reliable. Complex, means more ways to fail (and many times, less obvious ways to fail).

A basic BMS is there, in my humble opinion, as the last line of defense to protect the battery bank against anything else that may fail. I would suggest that the BMS operates (disconnects the battery/cells) only in exceptional situations. I would not suggest that the BMS over voltage cutoff be used like a "battery charger/controller".

There are also some BMS systems that "balance" cell voltages to keep them nearly identical... That way, the charge controller setting to XX.X volts for the whole bank will keep the bank between 20% and 90% (or your choices) state of charge. You do have to look at the "balancing" BMS design to ensure your charging set-points are correct.

There is/was at least one major BMS design that used a large fuse on the output bus of the battery bank. If anything went wrong (say a solar or inverter-charger was overcharging the battery bank), there was a "crowbar" relay that literately shorted out the battery bus and would pop the (large) battery bank fuse (as I understand from reading here).

For lead acid batteries--The controller needs to plus width modulate to vary the average charging current to the battery bank (hold the set-point voltage for 2-6 hours). A more "complex" charging system vs the "typical" lithium charging system (no temperature compensation, charge 100% until "set-point", then stop charging.

If I was going to inexpensive and "simple"--Let the BMS do its job (monitor per cell voltage, bank temperature, max current, etc.) and warn/shutdown if anything is exceeded. And a simple solar charger/controller that keeps the solar panel connected until the battery is full. That also lets you run DC loads (radio, GPS, lights, etc.) while charging with solar... Instead having the BMS shutdown the entire battery bus as you are charging/running DC gear on the boat.

You also need to look at the cost of failure... With LiFePO4 and similar chemistries--The cost of failure (over/under charging) is possibly a new battery bank (no fire/explosion). With other common lithium rechargeable chemistries--The cost of failure (over/under charging, charging near/below freezing, etc.) can be a major explosion and/or fire--With possible toxic waste cleanup.

I am certainly not an Li Ion expert... Just suggestions on how to look at the problem.

-Bill