Battery Bank / Inverter Conundrum

[staticGenerator]

Hello,

New to forum, hope to get some advice. Sorry for the long introduction but want to provide as much info as possible to bring to light what I’ve considered and to help provide a solution. I have some background in electronics, but definitely a novice.

TLDR; I have a battery bank that I cannot find a good inverter voltage to match to it and need advice on how I can match an inverter to the battery bank.

I purchased qty 12 of these battery modules here: **broken link removed** hoping to create a portable 110-120V AC system to power my AC tools when I’m remote. They would be connected in parallel to add enough current while retaining the 28.8 voltage. My goal is to find an inverter that will provide ~1500W+ of power. I can add more of these battery packs as needed to meet that output.

The pack output is listed as 28.8V DC. However, that is at the nominal cell voltage of 3.6V. The pack could output as low as 22V at lowest battery charge of 2.75V and as high as 33.6V at the full charge of 4.2V (Samsung’s specs).

The issue is finding a match with an inverter that will work well in this voltage range (22-33V). I’ve looked at hundreds of inverters - literally. As most inverters are based on 12V battery increments, a fully charged battery bank will cause an over-voltage fault with a 24V inverter. A decline in charge approaching 30V would cause an under voltage fault in a 36V inverter. To try and resolve this, here are some options I’ve considered so far and the potential drawbacks of each one.

Option A-keep battery pack and BMS intact and find an inverter with wider range of voltage input.

Inverter Category I - Very expensive inverters (>$2K) that accommodate a wider voltage tolerance.
Pros: high quality industrial build quality
Cons: expense not feasible

Railway Inverters
Input Voltage24Vdc (17 – 34V)

IPS Inverters
Input Voltage 20~40 Volts DC

Inverter category II - Solar grid tie inverters, stacked (in parallel) as needed
Pros: less expensive, handle the range of input voltage
Cons: many users indicate cannot be used for this purpose because the unit will drain batteries quickly even though no load is applied to inverter?

1000W Grid Tie Inverter

Micro Inverter

Option B-Connect step up/step down modules in series with each battery pack, then connect them in parallel to achieve the optimum voltage and current requirements for the inverter.

The recommended solution here Is to place a zener diode in series with each of the DC to DC converter to create an ‘OR’ gate for each channel: https://www.researchgate.net/post/c...rent_Also_can_anybody_suggest_a_better_method

Option C-Abandon the supplied battery pack configuration, redesigning the system to accommodate a more common voltage configuration, rearranging the battery configuration to align with a more ideal 12V battery multiple, replacing the BMS with one that will work with the new voltage layout.

My thought was to attempt Option B, wiring in parallel with step up or step down modules and Zener diodes in the output to protect the modules from over current draw on any module. This would then allow a voltage ideal for an inverter in multiples of 12V. The problem has been that I’ve been unable to find a zener diode capable of 50V and 10A (that figure is higher than needed and we could use a 32V 5A diode, but over engineering for safety). Also, the Zener diodes in the higher wattage range appear to be more expensive than than the DC to DC converter that would be needed.

1. Does anyone have a recommendation for a zener diode that would work for the voltage and current requirements?
2. Are any of the above options a better route?
3. Would you offer advice for a completely different approach?

Thanks for taking the time to review and offer any suggestions!

 

[KeepItSimpleStupid]

You should have asked before buying everything. 20/20 hindsight.

Batteries are wierd and they don;t like being put in parallel the same way you can;t parallel a generator and the utility at he same time. In a grid-tie, simplisticly, the voltage of the inverter is set a little higher than the utility, so the battery attempts to deliver mostly current.

You need a way to put batteries/power supplies in parallel. The hP power supply handbook mentions what' called a master.slave arrangement. The scnereo for 2 supplies isn;t too bad to understand. lets say they are a 0-12V, 0-5A identical supplies. One supply is set to deliver 12V with CC mode set at whatever/2. The other supply is constantly set slightly higher voltage or the than the master's voltage with a CL set at whatever/2, so each supply delivers 1/2 the load current.

To get your packs to run in parallel, you need a way to make that happen.

 

[rjenkinsgb]

Simple.
You need SEVEN lithium cells in series to match gear intended for 24V lead-acid systems. Remove one set of three from each module.

You can parallel similar cells, as long as they are all at the same charge voltage when you do it - like the three cells parallel in each position in those modules.

I'd rebuild the whole battery set as 13 cell modules in parallel, 7 series.

That gives you 29.4V maximum and down to around 21..22V flat.

You will need a new BMS / balance system, but that is far cheaper than any other approach.

 

[staticGenerator]

Simple.
You need SEVEN lithium cells in series to match gear intended for 24V lead-acid systems. Remove one set of three from each module.

You can parallel similar cells, as long as they are all at the same charge voltage when you do it - like the three cells parallel in each position in those modules.

I'd rebuild the whole battery set as 13 cell modules in parallel, 7 series.

That gives you 29.4V maximum and down to around 21..22V flat.

You will need a new BMS / balance system, but that is far cheaper than any other approach.

Thank you. This was one of the options I considered. Just hate to waste the existing BMS. But the batteries are in good shape and reasonably priced either way so looks like this is the path I will take.