How low can I discharge my 48 volt off grid battery system safely on a daily basis?

Offgridpower · March 2

Can someone explain to me in baby language, like a 4 year old, how come a 48V system cannot be drained to 24V, which would be a 50% discharge and why my generator should kick in at 46V or 44V? Much appreciated. Just moved into an off grid house, with 64 batteries. My Xantrex readout is 51-52V once the generator stops or the sun goes down, then the drain down starts and by middle of night my generator kicks in again to charge up. Then the generator usually goes for 2-3 hours for the full cycle, bulk - float - absorb, before it shuts down. It seems its charging nothing and just wasting propane as the battery sometimes already fully charged. So, I was thinking about lowering where the generator kicks in to about 40V, so that the generator kicks in less and when it does kick in, charges up the battery more. Thanks for the input.

706jim · March 2

Draining a 48 volt battery to 24 volts is akin to killing it. At 24 volts it will be dead dead dead, not 50% discharged. You could consider stopping the generator when the batery reaches 80% SOC to save fuel, but dont' run it to 24 volts.

BB. · March 2

Welcome to the forum OGP.

The answer is that batteries store electrical energy based on the chemical properties of the materials that the battery(ies) are constructed from...

To keep things simple, lets look at a 12 volt automotive Flooded Cell Lead Acid Battery (note: These batteries are of slightly different construction vs Flooded Cell Deep Cycle Lead Acid batteries--But are close enough for our discussion).

More or less, your car battery is "full" it measures 12.8 volts (~100% State of charge, not charging, not discharging--Rested for 3+ hours). And more or less, is "mostly empty" at 11.8 volts (basically around 20% state of charge).

https://footprinthero.com/lead-acid-battery-voltage-charts

And is pretty much "dead" at 11.5 (unloaded) to 10.5 volts (with some electrical loads). Most rechargeable batteries taken to "dead" will not recover when recharged.

Some articles to read:

https://www.solar-electric.com/learning-center/deep-cycle-battery-faq.html/
https://batteryuniversity.com/
https://www.rollsbattery.com/wp-content/uploads/2018/01/Rolls_Battery_Manual.pdf

Batteries are "pretty simple" at a surface level... But the details can get very deep when you do more research. Add different designs, constructions, materials, etc... And it is very easy to get lost in the details.

For lead acid batteries, more or less you want to run them from 50% to 100% state of charge --And you can go down towards 20% State of charge--If you quickly recharge the battery bank (if you let them sit at less than 75% SoC for days/weeks/months--You will end up with a sulfated battery that has almost no useful storage capacity).

Also, the deeper you cycle the batteries, the less cycle life they will have... For example Rolls Surette's deep cycle batteries may have around 1,400 cycle life if discharged to 50% SoC and recharged the next day or so... If you discharge to 20% SoC, perhaps 700 Cycle Life--And discharge to 75% SoC--Perhaps a 2,800 Cycle life. The deeper you cycle, the faster the battery bank will wear out.

Note that measuring resting voltage for Lead Acid batteries is not a very accurate method to determine State of Charge... Better is to use a hydrometer to measure the Specific Gravity of the electrolyte of each cell--And monitor a single "pilot cell" for quick checks.

At this point, we don't know very much about your system (brand/model of battery bank, Amp*Hour Capacity, age, etc.). We also do not know how much energy you used per day (Watt*Hours, Amp*Hours @ xx Volts), the size and location (nearest city is a good start) of your solar array, generator+charger setup/specifications, etc.

The battery bank is the "heart" of your system. You need to understand how much you discharge/recharge your battery bank on a daily basis. How well is your system working (is the solar array working as expected or do have a failed panel or wiring connection)? Are you using more power than your system can provide (wear out batteries quicker, use more fuel to keep the battery bank in a good operational range, etc.).

The battery bank is the first thing that usually gets "murdered" when things are not going "right". And batteries are not cheap these days--So taking care of them is usually a top priority.

When you "inherit" an off grid home--You need to quickly get control of the situation. Usually that means ensuring that you have energy conservation in mind. LED Lighting, an efficient refrigerator, an efficient well pump, an energy efficient laptop computer, (if you have these), and typically use other fuels (propane and such) for cooking, Hot water, space heating, etc.
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Generally, when designing an off grid system--Our rule of thumb to get started is to plan on using 1/4 of battery bank AH capacity per day (total energy usage) and have the solar array recharge the battery bank the next day. With solar, that usually means the solar array is almost 2x your daily loads (allows for winter sun, occasional guests, running a clothes washer a couple times a week, etc.).

The idea is that you have enough solar and battery bank such that, in general, you only need to run the Genset during stormy weather or in deep winter areas (more northern areas, a few days of dark/stormy weather, etc.). In many reasonably sunny areas--You might be able to get away with almost zero Genset usage. In Alaska, Canada, etc.--In winter you are stuck with using a Genset during those long dark winter days.

Do you have anybody nearby that can help you that has off grid power system experience (installer, handy neighbor)? Did you get any information from the previous owner?

In any case--Need to get started with the basics. What and where is your system? What is your energy usage/exceptions? Do you have a hydrometer and a good quality voltmeter? Also very nice for debugging and understanding your system, an AC+DC Current Clamp DMM (digital multi-meter) is very helpful...

https://www.solar-electric.com/midnite-solar-battery-hydrometer.html
https://www.amazon.com/Battery-Testing-Hydrometer/s?k=Battery+Testing+Hydrometer (with glass construction)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid priced AC+DC Current Clamp DMM)

At this point, you are probably behind the curve here... If the battery bank is charged during the day, and you are drawing a "reasonable" (i.e., not very much) amount of power overnight, the generator should not be starting the Genset. There is a good chance the battery bank is failing--But it is also possible that your system is not property charging (solar, Genset, etc.).

Your system is probably on the "larger size" (48 volt Xantrex units are not "small", and should have a pretty substantial battery bank running it. And a larger (8-12 kW or so?) Gen$et will $uck down the fuel pretty quickly too.

At this point, at least you have a working (if not working well) battery bank. This gives you a few days/weeks to measure/understand your daily energy needs, how well the solar array is working, and double check the programming & operation of your Genset+Xantrex inverter-charger.

The Xantrex XW and related hardware is a complex piece of kit... You do not want to make assumptions that it is setup and running correctly--And at the same time, you do not want to just start changing settings--It is very easy to end up in the dark if you program the system incorrectly.

You are diving into the deep end of the solar pool... And this can cost you (very likely multiple) $10,000's to get the system fully operational for your needs.

Batteries (depending on brand/model/chemistry/how well maintained/how they are cycled) may last 5-8 years (FLA). Electronics (inverter, charger, etc.) 10+ years. And solar array should last 20+ years (assuming no rocks/branches breaking panels).

Your thoughts, information, etc.?

-Bill

littleharbor2 · March 2

My $0.02, With 64 batteries in your bank You have a very large, and possibly questionably wired bank. I'm going to risk a guess that your batteries are toast. You should not be going into auto gen charging unless you have had a few back to back cloudy days.
Need all details of your system, including your daily Kw consumption. I predict you are going to find yourself in the market for batteries very soon. Actually it sounds like you are beyond that date.

Photowhit · March 3

Being dropped into an off-grid system is difficult. Take your time to figure it out BEFORE throwing money at it!!! It may well be a poorly designed system as I agree with Littleharbor.

Just some basics about lead acid charging cycle I wrote years ago. It might help understand the difference between system voltage and battery voltage;

To run through your original post, The voltage you are seeing is the system voltage and not the battery voltage. If you are connected to charging or a load it will effect the system voltage.

During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

BULK;

First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

ABSORB;

Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries.

The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

FLOAT;

Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

The system voltage drop you see at night when the sun goes down is the charge controller moving into a resting mode with no energy to contribute to the system.

The morning voltage may reflect a load present that is effecting the voltage level. With sealed batteries, you would want to disconnect the battery from the system and allow it to 'rest' for a while to get an accurate idea of it's SOC (State Of Charge) from the voltage

How low can I discharge my 48 volt off grid battery system safely on a daily basis?

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