Adding a Backup Generator

AYHSMB

My off grid system consists of a Midnite Classic 150, a 2500w PSW inverter/charger, and a 24v, 440ah battery bank. I'd like to add a generator for backup charging when needed. My question is should I hook up to the charge controller or inverter/charger? It seems a bit more difficult to hook up to the charge controller. If I go through the inverter, will the CC still read the bank voltages/charge state correctly? 

mike95490

You hook the generator (I'm assuming that it's a 120Vac, >3kw generator) to the AC INPUT of the inv/chg, and you may have to fiddle with it's settings to allow charging to start. You would want to set the charging rate to around 40 - 45A DC, and if you can monitor the charging, when you drop to 15A or so, the batteries should be full enough and you can stop burning fuel.
Battery charging is very demanding from a generator because the load is solid and continuous and often the charger is a poor Power Factor (PF) . After you stop charging, let the generator run with no load for a minute to cool down before shutting off.

BB.

Are you looking to automate the generator (auto start/stop)?

The inverter-chargers seem to be much more programmable in terms of voltage and current limits. And you can have some very neat functions like generator support (a 4 kW genset will power loads + battery charging up to genset limit; then if loads > 4 kWatt, the inverter-charger will actually start taking energy from the battery bank to support the genset up to, for example, 8 kWatt load--4kw from genset + 4 kw from inverter+battery bank)... Really neat if you are in a far north location that gets dark in winter and you rely on the genset for much of your power during winter (you are somewhere in Canada?).

If you just want a manual genset and separate battery charger--Genset + charger that can output somewhere between 10% and 20% of your battery 20 Hour AH capacity. For example, such a system may look like (using 10% rate of charge in example. For 20%, you just double the answers):

• 440 AH battery bank * 10% rate of charge = 44 amps (@ 24 volts)

Use a 40 Amp battery charge (example) some nice chargers that include remote battery temperature sensors (important in areas with wide operating temp ranges):

https://www.solar-electric.com/samlex-sec-2440ul-40a-24vdc-battery-charger.html (40 amp @ 24 volt--Seems OK for application)
https://www.solar-electric.com/xantrex-804-2430-truecharge2-30-amp-battery-charger.html (30 amp @ 24 volts)

The genset for chargers would be (roughly):

• 40 amps * 30 volts charging * 1/0.80 AC charger eff * 1/0.80 genset derating = 1,875 Watt / VA genset minimum rated
• 30 amps * 30 volts charging * 1/0.80 AC charger eff * 1/0.80 genset derating = 1,406 Watt / VA genset minimum rated

The above AC chargers have remote battery temperature sensors (seems to be rare for AC chargers). The 30 amp charger is small enough to run on a Honda eu2000i genset. The 30 amp Xantrex charger is also PFC (power factor corrected)--That allows a much smaller Watt / VA genset vs the plain old big charger with a big internal transformer and diodes--Or some of the older electronic power supply designs.

If you want to use the 2,500 Watt inverter-charger--You need to review the program settings and generator requirements.

Using a 10% rate of charge from 50%->90% charging is going to take ~3-4 hours in bulk (maximum charge rate) and another 2-6 hours of "absorb" charging (if you choose to do this with genset in deep winter).

Most gensets + chargers are efficient near 100% rated output--But when you drop from 40 amps charging and slowly downward ramping to ~1% Bank AH capacity (440AH * 1% = ~4.4 amps) charge termination for "full charge" (>90% SoC)--Big gensets+charge controllers are not going to be very fuel efficient.

Most gasoline/propane gensets drop from 100% to 50% fuel flow for 100% to 50% of rated load, and run near 50% or a bit less for 50% to 0% rated load.

So using smaller gensets take longer go charge (10% vs 20% rate of charge), but during ~1/2 the time, the smaller genset use less fuel to go from 80%-90%+ SoC.

If you are cycling daily and using the genset a lot--Cycling the battery bank between 50% to 80% is actually not a bad thing... Lead Acid batteries are very efficient at that SoC cycling--Vs 90%-100% which is very inefficient when gassing/EQ'ing. You can cycle to 90%+ once a week (and one vendor suggested that cycling to 90%+ once a month is OK too).

And there is the old question of what happens if something fails... If your inverter-charger fails--You are out of luck until you can get it repaired or a replacement (can't charge, can't invert).

Vs separate Inverter and Charger... If one fails, the other is still available.

Since (I guess) you already have the 2,500 Watt PSW inverter-charger--I would look at its specifications and see how you can program it and what genset (or if your present genset work) is needed.

The other question--Auto vs manual genset operation... I am a big believer in the KISS principle (Keep It Simple Stu....).

Trying to get a full auto genset + solar power system is fairly complex, and you still need the remote monitoring/somebody to come on site to figure out the problem, repair, and get running again.

If your loads are only used when somebody is there--I would suggest that you use manual control... And setup a simple instruction sheet with a battery monitoring system (BMS) that people other than you can follow (i.e., start genset at 50% SoC (if no sun in forecast), turn off genst at 80% SoC. And once a week (during winter) run to 90%+ SoC (on Saturdays) when needed.

There are simple SoC meters (voltage only) and more complex SoC meters that even have programmable alarm outputs (i.e., turn on Genset required light from 50% to 80% State of Charge)...

https://www.solar-electric.com/midnite-solar-mnbcms.html (simple voltage monitor + warning  LEDs if not >90% SoC in the last 7/14 days)
https://www.solar-electric.com/bogart-engineering-tm-2030-a-battery-monitor.html (full BMS with battery shunt)
https://www.solar-electric.com/victron-energy-bmv-712-smart-battery-monitor.html (full BMS with alarm + Bluetooth)
https://www.solar-electric.com/midnite-solar-whiz-bang-jr-current-sense-module.html (BMS sensor, need shunt, for Midnite Charge controller)

The problem with shunt based BMS systems... They are the most capable and "accurate" to a degree... But they can and do drift over time (i.e., the reset to 100% full only when the battery bank is fully recharged--And will drift if you cycle below full charge for days/weeks at a time).

Even if you have a Midnite BMS for your Classic controller, getting a second voltage only like the Midite (first link) is not bad... It gives you two separate methods to monitor your battery bank so if one "lies", you have a second backup.

Also--If you are using flooded cell batteries... You have a hydrometer?

https://www.solar-electric.com/search/?q=hydrometer

And, I highly suggest a DC Current Clamp DMM (really AC/DC current clamp DMM)--Here is a mid-priced one. They will really help you diagnose and understand your off grid power system better (and more safely--The current clamp does not require you to touch any exposed copper connections):

https://www.amazon.com/gp/product/B019CY4FB4

You can find DC current clamp meters down toward $50 that will work for our needs. Make sure the current clamp is DC (and AC) rated... There are many AC/DC meters that measure voltage, but only measure AC current with the clamp.

Your thoughts?

-Bill